The ball mill design places the most efficient cutting area on the side of the ball rather than at the end. Different mill sizes are used to enter small areas where thick coatings need to be removed (ROUTED). Rubberized abrasives of the proper grade and grit are ideally suited for removing thin, hard coatings from flat surfaces; soft coatings adhere to and coat the abrasive causing it to become ineffective. Rotary bristle brushes work better than rubberized abrasives on contoured or irregular surfaces, such as soldered connections, because the bristles conform to surface irregularities. Ball mill routing and abrasion removal are shown in figure 3-3.
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CUT AND PEEL.—Silicone coatings (also referred to as RTV) can easily be removed by cutting and peeling. As with all mechanical removal methods, care must be taken to prevent damage to either components or boards.
CHEMICAL REMOVAL.—Chemical removal uses solvents to break down the coatings. General application is not recommended as the solvent may cause damage to the boards by dissolving the adhesive materials that bond the circuits to the boards. These solvents may also dissolve the POTTING COMPOUNDS (insulating material that completely seals a component or assembly) used on other parts or assemblies. Only thin acrylic coatings (less than 0.025 inch) are readily removable by solvents. Mild solvents, such as ISOPROPYL ALCOHOL, XYLENE, or TRICHLOROETHANE, may be used to remove soluble coatings on a spot basis.
Evaluations show that many tool and technique combinations have proven to be reliable and effective in coating removal; no single method is the best in all situations. When the technician is determining the best method of coating removal to use, the first consideration is the effect that it will have on the equipment.
Once the required repairs have been completed the conformal coating must be replaced. To ensure the same protective characteristics, you should use the same type of replacement coating as that removed.
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Conformal coating application techniques vary widely. These techniques depend on material type, required thickness of application, and the effect of environmental conditions on curing. These procedures cannot be effectively discussed here.
Q1. What material is applied to electronic assemblies to prevent damage from corrosion, moisture, and stress?
Q2. What three methods are used to remove protective material?
Q3. What chemicals are used to remove protective material?
Q4. Abrasion, cutting, and peeling are examples of what type of protective material removal?
Q5. Why should the coating material be replaced once the required repair has been completed?
To properly perform the required repair, the 2M technician must be knowledgeable of the techniques used by manufacturers in the production of electronic assemblies. The techniques, materials, and types of components determine the repair procedures used.
Assemblies may range from simple, single-sided boards with standard-sized components to double- sided or multilayered boards with miniature and microminiature components. The variations in component lead termination and mounting techniques used by manufacturers present the technician with a complex task. For example, the 2M technician is concerned about the type of solder joints on the module. To determine the solder joint type, the technician must consider the board circuitry, hole reinforcement, and lead termination style.
Recall the discussion from topic 1 on printed circuit board construction and the types of interconnections used. Single-sided and some double-sided boards have UNSUPPORTED HOLES where component leads are soldered to the pad. The clearance-hole method is also an interconnection with no hole support. SUPPORTED HOLES are those that have metallic reinforcement along the hole walls.
In addition to the plated-through hole you studied earlier, EYELETS, shown in figure 3-4, view (A), view (B), and view (C), are also used in both manufacturing and repair. These hole-reinforcing devices are usually made of pure copper, but are often plated with gold, tin, or a tin-lead alloy. The copper-based eyelet is pliable; when set, it reduces the possibility of circuit board damage. Eyelets may be inserted into single-sided or double-sided boards and are of three different types - ROLL SET, FUNNEL SET, and FLAT SET. All three are types referred to as INTERFACIAL CONNECTIONS. Interfacial connections identify the procedure of connecting circuitry on one side of a board with the circuitry on the other side.
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As you can see, the flat-set eyelet actually provides reinforcement for the pads on both sides of the circuit board and reinforces the hole itself. The design of the roll-set eyelet (which may trap gasses, flux, or other contaminants, and obscures view of the finished solder flow) is not acceptable as a repair technique. The funnel-set eyelet does not provide as much pad reinforcement as the other types. However, it provides better "outgassing" of flux, moisture, or solvents from the space between the eyelet and the hole wall. It also provides a better view of the finished solder connection than the roll-set eyelet.
The finished circuit board consists of conductive paths, pads, and drilled holes with components and/or wires assembled directly to it. Leads and wires may terminate in three ways: (1) through the hole in the board, (2) above the surface of the board, or (3) on the surface of the board.
THROUGH-HOLE TERMINATION.—This style provides extra support for the circuit pads, the hole, and the lead by a continuous solder connection from one side of the circuit board to the other. Three basic variations of through-hole termination are the CLINCHED LEAD (two types), STRAIGHT- THROUGH LEAD, and OFFSET PAD.
Clinched Lead.—The clinched-lead termination is usually used with unsupported holes, but is found with supported holes as well. Both clinched-lead types, FULLY CLINCHED and SEMICLINCHED (figure 3-5), provide component stability. Like the fully clinched lead, the semi-clinched lead also provides stability during assembly. However, this termination can be easily straightened to allow removal of the solder joint should rework or repair be required. Note that the fully clinched lead is bent 90 degrees while the semiclinched lead is bent 45 degrees.
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Straight-Through Lead.—Straight-through terminations (figure 3-6) are used by manufacturers when the termination stability is not a prime consideration. This termination type may also be used with unsupported holes. The through-hole termination provides a better, solder-joint contact area and more solder support; the solder runs from the component side to the conductor. The straight-through termination is the easiest to remove and rework.
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The Offset-Pad Termination.—This termination, shown in view (A) of figure 3-7, is a variation of clinch-lead termination. The pad is set off from the centerline of the hole. The lead clinch is also offset from the hole centerline so that it may contact the pad [view (B)].
ABOVE-THE-BOARD TERMINATION.—Above-the-board termination is accomplished through the use of terminals or posts. Terminals are used for a variety of reasons. The type of terminal depends on its use. Although many configurations are used, all terminals fall into one of the five categories covered in this section [figure 3-8, views (A) through (E)].
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• PIN TERMINALS AND TURRET TERMINALS [view (A)] are single-post terminals, either insulated or uninsulated, solid or hollow, stud or feed-through. Stud terminals protrude from one side of a board; feed-throughs protrude from both sides.
• BIFURCATED OR FORK TERMINALS [view (B)] are solid or hollow double-post terminals.
• HOOK TERMINALS [view (C)] are made of cylindrical stock formed in the shape of a hook or question mark.
• PERFORATED OR PIERCED TERMINALS [view (D)] describe a class of terminals that uses a hole pierced in flat metal for termination (e.g., terminal lugs).
• SOLDER CUP TERMINALS [view (E)] are a common type found on connectors.
Turret and bifurcated terminals are used for interfacial connections on printed circuit boards, terminal points for point-to-point wiring, mounting components, and as tie points for interconnecting wiring. Hook terminals are used to provide connection points on sealed devices and terminal boards.
Terminals used for wire or component lead terminations are normally made of brass with a solderable coating. Uninsulated terminals may be installed on an insulating substrate to form a terminal board. They may also be added to a printed circuit board or installed on a metal chassis. Insulated terminals are installed on a metal chassis.
ON-THE-BOARD TERMINATION.—On-the-board termination (figure 3-9) is also called LAP FLOW termination. In a lap flow solder termination, the component lead does not pass through the circuit board. This form of planar mounting may be used with both round and flat leads.
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Q6. What term is used to identify the procedure of connecting one side of a circuit board with the other?
Q7. Name two types of through-hole termination.
Q8. Turret, bifurcated, and hook terminals are used for what type of termination?
Q9. When a lead is soldered to a pad without passing through the board, it is known as what type of termination?
Most of the damage in printed circuit board repair occurs during disassembly or component removal. More specifically, much of this damage occurs during the desoldering process. To remove components for repair or replacement, the technician must first determine the type of joint that is used to connect the component to the board. The technician may then determine the most effective method for desoldering these connections.
Three generally accepted methods of solder connection removal involve the use of SOLDER WICK, a MANUALLY CONTROLLED VACUUM PLUNGER, or a motorized solder extractor using CONTINUOUS VACUUM AND/OR PRESSURE. Of all the extraction methods currently in use, continuous vacuum is the most versatile and reliable. Desoldering becomes a routine operation and the quantity and quality of desoldering work increases with the use of this technique.
SOLDER WICKING.—IN this technique, finely stranded copper wire or braiding (wick) is saturated with liquid flux. Most commercial wick is impregnated with flux; the liquid flux adds to the effectiveness of the heat transfer and should be used whenever possible. The wick is then applied to a solder joint between the solder and a heated soldering iron tip, as shown in figure 3-10. The combination of heat, molten solder, and air spaced in the wick creates a capillary action and causes the solder to be drawn into the wick.
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This method should be used to remove surface joints only, such as those found on single-sided and double-sided boards without plated-through holes or eyelets. It can also remove excessive solder from flat surfaces and terminals. The reason is that the capillary action of the wicking is not strong enough to overcome the surface tension of the molten solder or the capillary action of the hole.
MANUALLY CONTROLLED VACUUM PLUNGER.—The second method of removing solder involves a manually controlled and operated, one-shot vacuum source. This vacuum source uses a plunger mechanism with a heat resistant orifice. The vacuum is applied through this orifice. Figure 3-11 shows the latest approved, manual-type desoldering tool. This technique involves melting the solder joint and inserting the solder-extractor tip into the molten solder over the soldering iron tip. The plunger is then released, creating a short pulse of vacuum to remove the molten solder. Although this method offers a positive vacuum rather than the capillary force of the wicking method, it still has limited application. This method will not remove 100 percent of the solder and may cause circuit pad lifting because of the extremely high vacuum generated and the jarring caused by the plunger action.
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Because 100 percent of the solder cannot be removed, the extraction method is not usually successful with the plated-through solder joint. The component lead in a plated-through hole joint usually rests against the side wall of the hole. Even though most of the molten solder is removed by a vacuum, the small amount of solder left between the lead and side walls causes a SWEAT JOINT to form. A sweat joint is a paper-thin solder joint formed by a minute amount of solder remaining on the conductor lead surfaces.
MOTORIZED VACUUM/PRESSURE METHOD.—The most effective method for solder joint removal is motorized vacuum extraction. The solder extractor unit, described in topic 2, is used for this type of extraction. This method provides controlled combinations of heat and pressure or vacuum for solder removal. The motorized vacuum is controlled by a foot switch and differs from the manual vacuum in that it provides a continuous vacuum. The solder extraction device is a coaxial, in-line instrument similar to a small soldering iron. The device consists of a hollow-tipped heating element, transfer tube, and collecting chamber (in the handle) that collects and solidifies the waste solder. This unit is easily maneuvered, fully controllable, and provides three modes of operation (figure 3-12): (1) heat and vacuum (2) heat and pressure, and (3) hot-air jet. Some power source models provide variable control for pressure and vacuum levels as well as temperature control for the heated tubular tip. The extraction tip and heat source are combined in one tool. Continuous vacuum allows solder removal with a single heat application. Since the slim heating element allows access to confined areas, the technician is protected from contact with the hot, glass, solder-trap chamber. Continuous vacuum extraction is the only consistent method for overcoming the resweat problem for either dual or multilead devices terminating in through-hole solder joints.
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Motorized Vacuum Method.—In the motorized vacuum method, the heated tip is applied to the solder joint. When melted solder is observed, the vacuum is activated by the technician causing the solder to be withdrawn from the joint and deposited into the chamber. If the lead is preclipped, it may also be drawn into a holding chamber. To prevent SWEATING (reforming a solder joint) to the side walls of the plated-through hole joint, the lead is "stirred" with the tip while applying the vacuum. This permits cool air to flow into and around the lead and side walls causing them to cool.
Motorized Pressure Method.—In the pressure method, the tip is used to apply heat to a pin for melting a sweat joint. The air pressure is forced through the hole to melt sweat joints without contacting
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the delicate pad. This method is seldom used because it is not effective in preventing sweating of the lead to the hole nor for cooling the workpiece.
Hot-Air Jet Method.—The hot-air jet method uses pressure-controlled, heated air to transfer heat to the solder joint without physical contact from a solder iron. This permits the reflow of delicate joints while minimizing mechanical damage.
When the solder is removed from the lead and pad area, the technician can observe the actual condition of the lead contact to the pad area and the amount of the remaining solder joint. From these observed conditions, the technician can then determine a method of removing the component and lead.
With straight-through terminations, the component and lead may be lifted gently from uncoated boards with pliers or tweezers. Working with clinched leads on uncoated boards requires that all sweat joints be removed and that the leads be unclinched before removal.
The techniques that have been described represent the successful methods of desoldering components. As mentioned at the beginning of this section, the 2M technician must decide which method is best suited for the type of solder joint. Two commonly used but unacceptable methods of solder removal are heat-and-shake and heat-and-pull methods.
In the heat-and-shake method, the solder joint is melted and then the molten solder is shaken from the connection. In some cases, the shaking action may include striking the assembly against a surface to shake the molten solder out of the joint. This method should NEVER be used because all the solder may not be removed and the solder may splatter over other areas of the board. In addition, striking the board against a surface can lead to broken boards, damaged components, and lifted pads or conductors.
The heat-and-pull method uses a soldering iron or gang-heater blocks to melt individual or multiple solder joints. The component leads are pulled when the solder is melted. This method has many shortcomings because of potential damage and should NOT be attempted. Heating blocks are patterned to suit specific configurations; but when used on multiple-lead connections, the joints may not be uniformly heated. Uneven heating results in plated-through hole damage, pad delamination, or blistering. Damage can also result when lead terminations are pulled through the board.
When desoldering is complete, the workpiece must undergo a careful physical inspection for damage to the circuit board and the remaining components. The technician should also check the board for scorching or charring caused by component failure. Sometimes MEASLING is present. Measling is the appearance of light-colored spots. It is caused by small areas of fiberglass strands that have been damaged by epoxy overcuring, heat, abrasion, or internal moisture. No cracks or breaks should be visible in the board material. None of the remaining components should be cracked, broken, or show signs of overheating. The solder joints should be of good quality and not covered by loose or splattered solder, which may cause shorts. The technician should examine the board for nicked, cracked, lifted, or delaminated conductors and lifted or delaminated pads.
Q10. When does most printed circuit board damage occur?
Q11. What procedure involves the use of finely braided copper wire to remove solder?
Q12. What is the most effective method of solder removal?
Q13. When, if at all, should the heat-and-shake or the heat-and-pull methods of solder removal be used?
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The 2M technician should restore the electronic assembly at least to the original manufacturer's standards. Parts should always be remounted or reassembled in the same position and with termination methods used by the original manufacturer. This approach ensures a continuation of the original reliability of the system.
High reliability connections require thoroughly cleaned surfaces, proper component lead formation and termination, and appropriate placement of components on the board. The following paragraphs describe the procedures for properly installing components on a board including the soldering of these components.
The termination areas on the board and the component leads are thoroughly cleaned to remove oxide, old solder, and other contaminants. Old or excess solder is removed by one of the desoldering techniques explained earlier in this topic. A fine abrasive, such as an oil-free typewriter eraser, is used to remove oxides. This is not necessary if the area has just been desoldered. All areas to be soldered are cleaned with a solvent and then dried with a lint-free tissue to remove cleaning residue.
Component leads are formed before installation. Both machine- and hand-forming methods are used to form the leads. Improper lead formation causes many repairs to be unacceptable. Damage to the SEALS (point where lead enters the body of the component) occurs easily during the forming process and results in component failure. Consequently, lead-forming procedures have been established. To control the lead-forming operation and ensure conformity and quality of repairs, the technician should ensure the following:
1. The component is centered between the holes, and component leads are formed with proper bend-radii and body seal-to-bend distance.
2. The possibility of straining component body seals during lead forming is eliminated.
3. Stress relief loops are formed without straining component seals while at the same time providing the desired lead-to-lead distances.
4. Leads are measured and formed for both horizontal and vertical component mounting.
5. Transistor leads are formed to suit standard hole spacing.
Lead-Forming Specifications.
Component leads are formed to provide proper lead spacing.
• The minimum distance between the seal (where the lead enters the body of the component) and the start of the lead bend must be no less than twice the diameter of the lead, as shown in figure 3-13.
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• Leads must be approximately 90 degrees from their major axis to ensure free movement in hole terminations, as shown in figure 3-14.
• In lead-forming, the lead must not be damaged by nicking.
• Energy from the bending action must not be transmitted into the component body.
COMPONENT PLACEMENT.—Where possible, parts are remounted or reassembled as they were in the original manufacturing process. To aid recognition, manufacturers use a coding system of colored dots, bands, letters, numbers, and signs. Replacement components are mounted to make all identification markings readable without disturbing the component. When components are mounted like the original, all the identification markings are readable from a single point.
Component identification reads uniformly from left to right, top to bottom, unless polarity requirements determine otherwise, as shown in figure 3-15. To locate the top, position the board so the part number may be read like a page in a book. By definition, the top of the board is the edge above the part number.
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When possible, component identification markings should be visible after installation. If you must choose between identification and electrical value markings, the priority of selection is as follows: (1) electrical value, (2) reliability level, and (3) part number.
Components are normally mounted parallel to and on the side opposite the printed circuitry and in contact with the board.
FORMATION OF PROPER LEAD TERMINATION.—After component leads are formed and inserted into the board, the proper lead length and termination are made before the lead is soldered. Generally, if the original manufacturer clinched (either full or semi) the component leads, the replacement part is reinstalled with clinched leads.
When clinching is required, leads on single- and double-sided boards are securely clinched in the direction of the printed wiring connected to the pad. Clinching is performed with tools that prevent damage to the pad or printed wiring. The lead is clinched in the direction of the conductor by bending the lead. The leads are clipped so that their minimum clinched length is equal to the radius of the pad. Under no circumstances does the clinched lead extend beyond the pad diameter. Natural springback away from the pad or printed wiring is acceptable. A gap between the lead end and the pad or printed wiring is acceptable when further clinching endangers the pad or printed wiring. These guidelines ensure uniform lead length.
Q14. To what standards should a technician restore electronic assemblies?
Q15. How is oxide removed from pads and component leads?
Q16. Leads are formed approximately how many degrees from their major axis?
Q17. When you replace components, identification marks must meet what requirements?
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Q18. In what direction are component leads clinched on single- and double-sided boards?
The fundamental principles of solder application must be understood and observed to ensure consistent and satisfactory results. As discussed in topic 2, the soldering process involves a metal-solvent action that joins two metals by dissolving a small amount of the metals at their point of contact.
SOLDERABILITY.—As the solder interacts with the base metals, a good metallurgical bond is obtained and metallic continuity is established. This continuity is good for electrical and heat conductivity as well as for strength. Solderability measures the ease with which molten solder wets the surfaces of the metals being joined. WETTING means the molten solder leaves a continuous permanent film on the metal surface. Wetting can only be done properly on a clean surface. All dirt and grease must be removed and no oxide layer must exist on the metal surface. Using abrasives and/or flux to remove these contaminants produces highly solderable surfaces.
HEAT SOURCE.—The soldering process requires sufficient heat to produce alloy- or metal-solvent action. Heat sources include CONDUCTIVE, RESISTIVE, CONVECTIVE, and RADIANT types. The type of heat source most commonly used is the conductive-type soldering iron. Delicate electronic assemblies require that the thermal characteristics of a soldering iron be carefully balanced and that the iron and tip be properly matched to the job. Successful soldering depends on the combination of the iron tip temperature, the capacity of the iron to sustain temperature, the time of iron contact with the joint, and the relative mass and heat transfer characteristics of the object being soldered.
SELECTION OF PROPER TIP.—The amount of heat and how it is controlled are critical factors to the soldering process. The tip of the soldering iron transfers heat from the iron to the work. The shape and size of the tip are mainly determined by the type of work to be performed. The tip size and the wattage of the element must be capable of rapidly heating the mass to the melting temperature of solder.
After the proper tip is selected and attached to the iron, the operator may control the heat by using the variable-voltage control. The most efficient soldering temperature is approximately 550 degrees Fahrenheit. Ideally, the joint should be brought to this temperature rapidly and held there for a short period of time. In most cases the soldering action should be completed within 2 or 3 seconds. When soldering a small-mass connection, control the heat by decreasing the size of the tip.
Before heat is applied to solder the joint, a thermal shunt is attached to sensitive component leads (diodes, transistors, and ICs). A thermal shunt is used to conduct heat away from the component. Because of its large heat content and high thermal conductivity, copper is usually used to make thermal shunts. Aluminum also has good conductivity but a smaller heat content; it is also used to conduct heat, especially if damage from the physical weight of the clamp is possible. Many types, shapes, and sizes of thermal shunts are available. The most commonly used is the clamp design; this is a spring clip (similar to an alligator clip) that easily fastens onto the part lead, as shown in figure 3-16.
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APPLICATION OF SOLDER AND SOLDERING IRON TIP.—Before solder is applied to the joint, the surface temperature of the parts being soldered is increased above the solder melting point. In general, the soldering iron is applied to the point of greatest mass at the connection. This increases the heat in the parts to be soldered. Solder is then applied to a clean, fluxed, and properly heated surface. When properly applied, the solder melts and flows without direct contact with the heat source and provides a smooth, even surface that feathers to a thin edge.
Molten solder forms between the tip and the joint, creating a heat bridge or thermal linkage. This heat bridge causes the tip to become part of the joint and allows rapid heat transfer. A solder (heat) bridge is formed by melting a small amount of solder at the junction of the tip and the mass being soldered as the iron is applied. After the tip makes contact with the lead and the pad and after the heat bridge is established, the solder is applied with a wiping motion to form the solder bond. The completed solder joint should be bright and shiny in appearance. It should have no cracks or pits, and the solder should cover the pad. Examples of preferred solder joints are shown in figure 3-17. They are referred to as full fillet joints.
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When a solder joint is completed, solvent must be used to remove all flux residue. The two most highly recommended solvents, in the order of their effectiveness, are 99.5 percent pure ethyl alcohol and 99.5 percent pure isopropyl alcohol.
Q19. What is solderability?
Q20. What is the most common source of heat in electronic soldering?
Q21. What determines the shape and size of a soldering iron tip?
Q22. What term describes a device used to conduct heat away from a component?
Q23. What is the appearance of a properly soldered joint?
In topic 1 you learned the advantages of DIPs. They are easily inserted by hand or machine and require no special spreaders, spacers, insulators, or lead-forming tools. Standard hand tools and soldering equipment can be used to remove and replace DIPS.
DIPs may be mounted on a board in two ways: (1) They may be mounted by plugging them into DIP mounting sockets that are soldered to the printed circuit boards or (2) they are soldered in place and may or may not be conformally coated. Although plug-ins are very easy to service, they lack the reliability of soldered-in units, do not meet MILSPECS, and are seldom used in military designed equipment. They are susceptible to loosening because of vibration and to poor electrical contact because of dust and dirt and corrosion.
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To remove plug in DIPs, use an approved DIP puller, such as the one shown in figure 3-18. The puller shown is a plastic device that slips over the ends of the DIP and lifts the DIP evenly out of the socket. Before the DIP is removed, the board is marked or a sketch is made of the DIP reference mark location; then the reference mark for the replacement part will be in the proper position. The DIP is grasped with the puller and gently lifted straight out of the socket. Lifting one side or one end first results in bent leads. If the removed DIP is to be placed back in the circuit, particular care is taken in straightening bent leads to prevent breaking. To straighten bent leads, the technician grasps the wide portion of the lead with one pair of smooth-jaw needle nose pliers; with another pair, the technician then bends the lead into alignment with the other leads. Tools used for lead straightening should be cleaned with solvent to remove contaminants.
To replace a plug-in DIP, the technician should clean the leads with solvent and then check the proper positioning of the reference mark. To do this, the technician holds the DIP body between the thumb and forefinger and places the part on the socket to check pin alignment. The pins are not touched. If all pins are properly aligned, the technician presses the part gently into the socket until the part is firmly seated. As pressure is applied, each pin is checked to ensure that all pins are going into the socket. If pins tend to bend, the part is removed and the pins are straightened. The socket is then inspected to make sure the holes are not obstructed. Then the process is repeated. After a thorough visual inspection, the card should be ready for testing.
The removal of soldered-in DIPs without conformal coatings is essentially the same as the removal of discrete components, except that a skipping pattern is always used. A skipping pattern is one that skips from pad to pad, never heating two pads next to each other. This reduces heat accumulation and reduces
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the chance of damage to the board. Of course, many more leads should be desoldered before the part can be removed. Special care must be exercised to make sure all leads are completely free before an attempt is made to lift the part off the board. If the part is known to be faulty, or if normal removal may damage the board, then the leads should be clipped. Once this has been done, desoldering can be done from both sides of the board. After the clipped leads have been desoldered, they can be removed with tweezers or pliers.
The removal of DIPs from boards with conformal coatings should be completed in the same manner as for other components. The coating should be removed using the preferred method of removal for that particular type of material. The coating should be removed from both sides of the board after masking off the work area. Particular care should be taken when removing the material from around the delicate leads. If the part is to be reused, as much of the coating should be removed from the leads as possible. As with DIPs without conformal coatings, if the part is known to be bad or if the possibility of board damage exists, the leads are clipped; the part and leads are then removed as described earlier in this section. Once the part has been removed, the work area should be completely cleaned to remove any remaining coating or solder.
The steps for replacing a soldered-in DIP are similar to those for replacing a plug-in DIP. Once the part is in position, it is soldered using the same standard used by the manufacturer, or as close to that standard as is possible with the available equipment. The joints should be soldered as quickly as possible using only as much heat as is necessary using a skipping pattern. The repaired card should then be visually inspected for defects in workmanship, and testing of the card should take place. Once the successful repair has been accomplished, a conformal coating should be applied to the work area.
You should recall from chapter 1 of this module, that TO packages are mounted in two ways— plugged-in or embedded. The term plug-in, when referring to TOs, should not be confused with DIP plug-ins. TOs are normally soldered in place. You will come across sockets for TOs, but not as frequently as for DIPs. Figure 3-19 shows the methods of mounting TOs. Notice that plug-ins may either be mounted flush with the board surface or above the surface with or without a spacer. The air gap or spacer may be used by the manufacturer for a particular purpose. This type of mounting could be used for heat dissipation, short circuit protection, or to limit parasitic interaction between components. The spacer also provides additional physical support for the TO. The technician is responsible for using the same procedure as the manufacturer to replace TOs or any other components.
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The procedure for removal of plug-in TOs (with or without conformal coatings) is the same as that used for a similarly mounted DIP or discrete component. The conformal coating is removed if required. Leads are desoldered and gently lifted out of the board. Then board terminals and component leads are cleaned.
In some plug-ins, the leads must be formed before they are placed in a circuit. Care should be taken to ensure that seal damage does not occur and that formed leads do not touch the TO case. This would result in a short-circuit.
When the new part or the one that was removed is installed, the leads are slipped through the spacer if required, and the part is properly positioned (reference tab in the proper location). The leads are aligned with the terminal holes and gently pressed into position. The part is soldered into place and visually inspected. Then the card is tested and the conformal coating is replaced if required.
The removal of an imbedded TO package varies only slightly from the removal of other types of mountings. First, the work area is masked and the conformal coating is removed if required. Then the desoldering handpiece is used to remove the solder from each lead. When all leads are free, the TO is pushed out of the board. If all the leads are free, the TO should slip out of the board easily. The package should not be forced out of the board. Excessive pressure may cause additional damage. If the leads are not completely free, the leads must be clipped and removed after the package is out of the board. This process is shown in figure 3-20.
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The most critical part of replacing an imbedded TO is the lead formation. The leads are formed to match the original part as closely as possible. Once the body and leads are seated, the leads can be soldered and the board inspected.
Up to this point, all of the components discussed have had through-the-board leads. In addition, the removal and replacement of discrete components, DIPs, and TOs have been similar.
Different techniques are used in the removal and replacement of flat packs and devices with on-the- board terminations. Lap-flow solder joints require that the technician pay particular attention to workmanship. Some of the standards of workmanship will be discussed later in this section.
Prior to the removal of a flat pack, as with other ICs, a sketch should be prepared to identify the proper positioning of the part. The conformal coating should be removed as required.
To remove the flat pack, the 2M technician carefully heats the leads and lifts them free with tweezers. If the part is to be reused, special care is taken not to damage or bend the leads. The work area around the component should then be thoroughly cleaned and prepared for the new part.
Flat packs attached to boards normally have formed and trimmed leads. Manufacturers form and trim the leads in one operation with a combination die. However, most replacement flat packs are received in a protective holder (figure 3-21) and the leads must be formed and trimmed by hand. Cost prevents equipping the repair station with the variety of tools and dies to form leads because of the variety of component configurations.
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LEAD-BENDING TECHNIQUES.—The 2M technician learns several methods of lead forming that will provide proper contact for soldering and circuit operations. The techniques used to bend leads include the use of specialized tools and such common items as flat toothpicks, bobby pins, and excess component leads. Care is taken not to stress the seal of the component during any step of the lead forming. Figure 3-22 illustrates two views, view (A) and view (B), of properly formed flat-pack leads.
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Because most replacement flat packs come with leads that are longer than required, they must be trimmed before they are soldered. The removed part is used as a guide in determining lead length. Surgical scissors or scalpels are recommended for use in cutting flat-pack leads. Surgical scissors permit all leads to be cut to the required lead length in a smooth operation with no physical shock transmitted to the IC.
LAP-SOLDERING CONNECTIONS.—Before a connection is lap-soldered, the solder pads are cleaned and pretinned and the component leads are tinned. This is particularly important if they are gold plated. The IC is properly positioned on the pad areas, and the soldering process is a matter of "sweating" the two conductors together. When multilead components, such as ICs, are soldered, a skipping pattern is used to prevent excessive heat buildup in a single area of the board or component. When soldering is completed, all solder connections are thoroughly cleaned. All joints should be inspected and tested. The standards of workmanship are more specific for flat-pack installation.
Q24. When removing the component, under what circumstances may component leads be clipped?
Q25. How are imbedded TOs removed once the leads are free?
Q26. How is a flat pack removed from a pcb?
Q27. How do you prevent excessive heat buildup on an area of a board when soldering multilead components?
Q28. What are the two final steps of any repair?
Removal and replacement of components on boards and circuit cards are, by far, the most common types of repair. Equally important is the repair of damaged or broken cards. Proper repair of damaged boards not only maintains reliability of the board but also maintains reliability of the system.
Cards and boards may be damaged in any of several ways and by a number of causes. Untrained personnel making improper repairs and technicians using improper tools are two major causes of damage. Improper shipping, packaging, storage, and use are also common sources of damage. The source of damage most familiar to technicians is operational failure. Operational failures include cracking caused by heat, warping, component overheating, and faulty wiring.
Before attempting board repairs, the technician should thoroughly inspect the damage. The decision to repair or discard the piece depends on the extent of damage, the level of maintenance authorized, operational requirements, and the availability of repair parts and materials. The following procedures will help you become familiar with the steps necessary to repair particular types of damage. Remember, only qualified personnel are authorized to attempt these repairs.
Conductor (run) and pad damage is very common. The technician must examine the board for nicks, tears, or scratches that have not broken the circuit, as well as for complete breaks, as shown in figure 3- 23. Crack damage may exist as nicks or scratches in the conductor. These nicks or scratches must be repaired if over one-tenth of the cross-sectional area of the conductor is affected as current-carrying capability is reduced. Cracks may also penetrate the conductor.
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CRACK REPAIR.—Four techniques are used to repair cracks in printed circuit conductors. One method is to flow solder across the crack to form a solder bridge. This is not a high-reliability repair since the solder in the break will crack easily.
The second method is to lap-solder a piece of wire across the crack. This method produces a stronger bond than a solder bridge; but it is not highly reliable, as the solder may crack.
A third repair technique is to drill a hole through the board where the crack is located and then to install an eyelet in the hole and solder it into place.
The fourth method is to use the clinched-staple method, shown in figure 3-24. It is the most reliable method and is recommended in nearly all cases.
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Pads or conductor runs may be completely missing from the board. These missing pads or runs must be replaced. Also included in this type of damage are conductors that are present but damaged beyond repair.
REPLACING DAMAGED OR MISSING CONDUCTORS.—The procedures used to replace damaged or missing conductors are essentially the same as using the clinched-staple method of conductor repair.
REPLACING THE TERMINATION PAD.—Many times the termination pad, as well as part of the conductor, is missing on the board. In these cases, a replacement pad is obtained from a scrap circuit board. Refer to figure 3-25 as you study each step.
The underside of the replacement pad and the area where it will be installed is cleaned. An epoxy is used to fasten the replacement pad to the board. An eyelet is installed to reinforce the pad before the epoxy sets and cures. This ensures a good mechanical bond between the board and pad and provides good electrical contact for components. After the epoxy cures, the new pad is lap-soldered to the original run.
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REPAIRING DELAMINATED CONDUCTORS.—DELAMINATED CONDUCTORS (figure 3- 26) are classified as conductors no longer bonded to the board surface. Separation of the laminations may occur only on a part of the conductor. Proper epoxying techniques ensure complete bonding of the conductor to the circuit board laminate. The following procedures are used to obtain a proper bond:
1. A small amount of epoxy is mixed and applied to the conductor and the conductor path; no areas are left uncoated.
2. The conductor is clamped firmly against the board surface until the epoxy has completely cured.
REPLACING EYELETS.—Eyelets have been referred to in several places in this topic. Not only are they used for through-the-board terminations, but also to reinforce some types of board repairs. As with any kind of material, eyelets are subject to damage. Eyelets may break, they may be installed improperly, or they may be missing from the equipment. When an eyelet is missing or damaged, regardless of the kind of damage, it should be replaced. The guidelines for the selection and installation of new eyelets are far too complex to explain here. However, they do comprise a large part of the 2M technician's training.
When boards are cracked, the length and depth of the cracks must be determined. Also, the disruption to conductors and components caused by cracks must be determined by visual inspection. To avoid causing additional damage, the technician must exercise care when examining cracked boards and
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must not flex the board. Rebuilding techniques must be used to repair damage, such as cracks, breaks, and holes that extend through the board. The following steps are used to repair cracks:
1. Abrasive methods are used to remove all chips and fractured material.
2. The edges of the removed area are beveled and undercut to provide bond strength.
3. A smoothly surfaced, nonporous object is fastened tightly against one side of the removed area.
4. The cutaway area is filled with a compound of epoxy and powdered fiberglass (figure 3-27). Extreme care is exercised to prevent the formation of voids or air bubbles in the mixture.
5. The surface of the filled area is smoothed to make it level with the surface of the original board.
6. The board is cured, smoothed, redrilled, and cleaned.
Broken boards should be examined to determine if all parts of the board are present and if circuit conductors or components are affected by the break. They are also examined to determine if the broken pieces may be rejoined reliably or if new pieces must be manufactured.
Breaks and holes are repaired in the same manner as cracks unless broken pieces are missing or the hole exceeds 1/2 inch in diameter. In such cases, the following repair steps are used:
1. The same technique used in repairing cracks is used to prepare the damaged edge.
2. A piece as close in size to the missing area as possible is cut from a scrap board of the same type and thickness. The edges of this piece are prepared in the same manner as the edges of the hole.
3. A smooth-surfaced object is tightly fastened over one side of the repair area, and the board is firmly clamped in an immovable position with the uncovered area facing up.
4. The replacement piece is positioned as nearly as possible to the original board configuration and firmly clamped into place.
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5. The repair is completed using the same epoxy-fiberglass mixture and repair techniques used in the patching repair method discussed in the following section on burned board repair.
Scorched, charred, or deeply burned boards should be inspected to determine the size of the discolored area and to identify melted or blackened conductors and burned, melted, or blackened components. The depth of the damage, which may range from a slight surface discoloration to a hole burned through the circuit board, should also be determined. Damage not extending through the board may be repaired by patching (figure 3-28). The following procedure is used in the repair of these boards.
1. If the board is scorched, charred, or burned, all discolored board material is removed by abrasive methods, as shown in figure 3-29. Several components in the affected area may have to be desoldered and removed before the repair is continued.
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2. Repairable delaminations not extending to the edge of the circuit board should be cut away by abrasive methods until no delaminated material remains.
3. Delaminated material is not removed if it is repairable.
4. After all damaged board material is removed, the edge of the removed area is beveled and undercut to provide holding points for the repair material.
5. Solvent is used to clean thoroughly and to remove all loose particles.
6. A compound of epoxy and powdered fiberglass is mixed and used to fill the cutaway area.
7. The epoxy repair mixture is cured according to the manufacturer's instructions.
8. The surface of the filled area is leveled after the compound is cured.
9. If delaminations extend to the edge of the board, the delaminated layers are filled completely with the repair mixture and clamped firmly together between two flat surfaces.
10. After the cure is completed, abrasive methods are used to smooth the repaired surface to the same level as the original board.
11. If necessary, needed holes are redrilled in the damaged area, runs are replaced, eyelets and components are installed, and the area is cleaned. Figure 3-30 shows the repaired area ready for components.
Q29. List three causes of damage to printed circuit boards.
Q30. What is the preferred method of repairing cracked runs on boards?
Q31. Damaged or missing termination pads are replaced using what procedure?
Q32. How is board damage caused by technicians?
Q33. What combination of materials is used to patch or build up damaged areas of boards?
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Safety is a subject of utmost importance to all technical personnel. Potentially hazardous situations exist in almost any work area. The disregard of safety precautions can result in personal injury or in the loss of equipment or equipment capabilities.
In this section we will discuss two types of safety factors. First, we will cover damage that can occur to electronic components because of electrostatic discharge (ESD) and improper handling and stowage of parts and equipment. Second, we will cover personal safety precautions that specifically concern the technician.
Electrostatic discharge (ESD) can destroy or damage many electronic components including integrated circuits and discrete semiconductor devices. Certain devices are more susceptible to ESD damage than others. Because of this, warning symbols are now used to identify ESD-sensitive (ESDS) items (figure 3-31).
Static electricity is created whenever two substances (solid or fluid) are rubbed together or separated. This rubbing or separation causes the transfer of electrons from one substance to the other; one substance then becomes positively charged and the other becomes negatively charged. When either of these charged substances comes in contact with a conductor, an electrical current flows until that substance is at the same electrical potential as ground.
You commonly experience static build-up during the winter months when you walk across a vinyl or carpeted floor. (Synthetics, especially plastics, are excellent generators of static electricity.) If you then touch a door knob or other conductor, an electrical arc to ground may result and you may receive a slight shock. For a person to experience such a shock, the electrostatic potential created must be 3,500 to 4,000 volts. Lesser voltages, although present and similarly discharged, normally are not apparent to a person's nervous system. Some typical measured static charges caused by various actions are shown in table 3-2.
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Table 3-2.—Typical Measured Statics Charges (in volts)
WALKING ACROSS CARPET 35,000 1,500 WALKING OVER VINYL FLOOR 12,000 250 WORKER AT BENCH 6,000 100 VINYL ENVELOPES FOR WORK INSTRUCT. 7,000 600 POLY BAG PICKED UP FROM BENCH 20,000 1,200 WORK CHAIR PADDED WITH URETHANE FOAM 18,000 1,500
Metal oxide semiconductor (MOS) devices are the most susceptible to damage from ESD. For example, an MOS field-effect transistor (MOSFET) can be damaged by a static voltage potential of as little as 35 volts. Commonly used discrete bipolar transistors and diodes (often used in ESD-protective circuits), although less susceptible to ESD, can be damaged by voltage potentials of less than 3,000 electrostatic volts. Damage does not always result in sudden device failure but sometimes results in device degradation and early failure. Table 3-2 clearly shows that electrostatic voltages well in excess of 3,000 volts can be easily generated, especially under low-humidity conditions. ESD damage of ESDS parts or circuit assemblies is possible wherever two or more pins of any of these devices are electrically exposed or have low impedance paths. Similarly, an ESDS device in a printed circuit board, or even in another pcb that is electrically connected in a series can be damaged if it provides a path to ground. Electrostatic discharge damage can occur during the manufacture of equipment or during the servicing of the equipment. Damage can occur anytime devices or assemblies are handled, replaced, tested, or inserted into a connector.
Technicians should be aware of the many sources of static charge. Table 3-3 lists many common sources of electrostatic charge. Although they are of little consequence during most daily activity, they become extremely important when you work with ESD material.
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Table 3-3.—Common Sources of Electrostatic Charge
Certified 2M technicians are trained in procedures for reducing the causes of ESD damage. The procedures are similar for all levels of maintenance. The following procedure is an example of some of the protective measures used to prevent ESD damage.
1. Before starting to service equipment, the technician should be grounded to discharge any static electric charge built up on the body. This can be accomplished with the use of a test lead (a single-wire conductor with a series resistance of 1 megohm equipped with alligator clips on each end). One clip end is connected to the grounded equipment frame, and the other clip end is
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touched with a bare hand. Figure 3-32 shows a more refined ground strap which frees both hands for work.
2. Equipment technical manuals and packaging material should be checked for ESD warnings and instructions.
3. Prior to opening an electrostatic unit package of an electrostatic sensitive device or assembly, clip the free end of the test lead to the package. This will cause any static electricity which may have built up on the package to discharge. The other end remains connected to the equipment frame or other ESD ground. Keep the unit package grounded until the replacement device or assembly is placed in the unit package.
4. Minimize handling of ESDS devices and assemblies. Keep replacement devices or assemblies, with their connector shorting bars, clips, and so forth, intact in their electrostatic-free packages until needed. Place removed repairable ESD devices or assemblies with their connector shorting bars/clips installed in electrostatic-free packages as soon as they are removed from the equipment. ESDS devices or assemblies are to be transported and stored only in protective packaging.
5. Always avoid unnecessary physical movement, such as scuffing the feet, when handling ESDS devices or assemblies. Such movement will generate additional charges of static electricity.
6. When removing or replacing an ESDS device or assembly in the equipment, hold the device or assembly through the electrostatic-free wrap if possible. Otherwise pick up the device or assembly by its body only. Do not touch component leads, connector pins, or any other electrical connections or paths on boards, even though they are covered by conformal coating.
7. Do not permit ESDS devices or assemblies to come in contact with clothing or other ungrounded materials that could have an electrostatic charge. The charges on a nonconducting material are not equal. A plastic storage bag may have a −10,000 volt potential 1/2 inch from a +15,000 volt potential, with many such charges all over the bag. Placing a circuit card inside the bag allows the charges to equalize through the pcb conductive paths and components, thereby causing failures. Do not hand an ESD device or assembly to another person until the device or assembly is protectively packaged.
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8. When moving an ESDS device or assembly, always touch (with bare skin) the surface on which it rests for at least one second before picking it up. Before placing it on any surface, touch the surface with your free hand for at least one second. The bare skin contact provides a safe discharge path for charges accumulated while you are moving around.
9. While servicing equipment containing ESD devices, do not handle or touch materials such as plastic, vinyl, synthetic textiles, polished wood, fiberglass, or similar items which create static charges; or, be sure to repeat the grounding action with the bare hands after contacting these materials. These materials are prime electrostatic generators.
10. If possible, avoid repairs that require soldering at the equipment level. Soldering irons must have heater/tips assemblies that are grounded to ac electrical ground. Do not use ordinary plastic solder suckers (special antistatic solder suckers are commercially available).
11. Ground the leads of test equipment momentarily before you energize the test equipment and before you probe ESD items.
Work benches on which ESDS items will be placed and that will be contacted by personnel should have ESD protective work surfaces. These protective surfaces should cover the areas where ESD items will be placed. Personnel ground straps are also necessary for ESD protective work bench surfaces. These straps prevent people from discharging a static charge through an ESDS item to the work bench surface. The work bench surface should be connected to ground through a ground cable. The resistance in the bench top ground cable should be located at or near the point of contact with the work bench top. The resistance should be high enough to limit any leakage current to 5 milliamperes or less; this is taking into consideration the highest voltage source within reach of grounded people and all parallel resistances to ground, such as wrist ground straps, table tops, and conductive floors. See figure 3-33 for a typical ESD ground work bench.
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Energized equipment provides protection from ESD damage through operating circuitry. Circuit cards with ESD sensitive devices are generally considered safe when installed in an equipment rack; but they may be susceptible to damage if a "drawer" or "module" is removed and if connector pins are touched (even putting on plastic covers can transfer charges that do damage). There must not be any energized equipment placed on the conductive ESD work surface. An ESD work area is for "dead" equipment ONLY.
ESD protection is critical. If you should be assigned to 2M repair school, your education in ESD prevention will be quite extensive.
Throughout your career you will be aware of emphasis placed on safety. Safety rules remind you of potential dangers in work. Most accidents are preventable. Accidents don't happen without a cause. Most accidents are the result of not following prescribed safe operating procedures.
This would be a good time to review the safety section in topic 5 of NEETS, Module 2, Introduction to Alternating Current and Transformers. That section covers the basics of electrical shock and how to prevent it.
The 2M technician should be aware of other potential dangers in addition to the dangers of electrical shock. These dangers are discussed in the following paragraphs.
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Hazards associated with the use of power tools include electrical shock, cuts, and particles in the eye. Safe tool use practices reduce or eliminate such accidents. Listed below are some of the general safety precautions that you should observe when your work requires the use of power tools.
• Ensure that all metal-cased power tools are properly grounded.
• Do not use spliced cables unless an emergency warrants the risks involved.
• Inspect the cord and plug for proper connection. Do not use any power tool that has a frayed cord or broken or damaged plug.
• Make sure that the on/off switch is in the OFF position before inserting or removing the plug from the receptacle.
• Always unplug the extension cord from the receptacle before the portable power tool is unplugged from the extension cord.
• Ensure all cables are positioned so they will not constitute a tripping hazard.
• Wear eye protection (goggles) in work areas where particles may strike the eye.
• After completing a task requiring a portable power tool, disconnect the power cord as described above and store the tool in its assigned location.
When using a soldering iron, remember the following:
• To avoid burns, always assume that a plugged-in soldering iron is HOT.
• Never rest a heated iron anywhere but in a holder provided for that purpose. Faulty action on your part could result in fire, extensive equipment damage, and/or serious injuries.
• Never use an excessive amount of solder. Drippings can cause serious skin or eye burns and can cause short circuits.
• Do not swing an iron to remove excess solder. Bits of hot solder can cause serious skin or eye burns or may ignite combustible material in the work area.
• When cleaning an iron, use a natural fiber cleaning cloth; never use synthetics, which melt. Do not hold the cleaning cloth in your hand. Always place the cloth on a suitable surface; then wipe the iron across it to avoid burning your hand.
• Hold small soldering jobs with pliers or a suitable clamping device to avoid burns. Never hold the work in your hand.
• Do not use an iron that has a frayed cord or damaged plug.
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• Do not solder electronic equipment unless the equipment is electrically disconnected from the power supply circuit.
• After completing a task requiring a soldering iron other than the iron that is part of a work station, disconnect the power cord from the receptacle. When the iron has cooled, store it in its assigned stowage area.
The technician who smokes while using a cleaning solvent is inviting disaster. Unfortunately, many such disasters have occurred. For this reason, the Navy does not permit the use of gasoline, benzine, ether, or like solvents for cleaning since they present potential fire or explosion hazards. Only nonvolatile solvents should be used to clean electrical or electronic apparatus.
In addition to the potential hazard of accidental fire or explosion, most cleaning solvents can damage the human respiratory system where the fumes are breathed for a period of time.
The following positive safety precautions should be followed when performing cleaning operations.
• Use a blower or canvas wind chute to blow air into a compartment in which a cleaning solvent is being used.
• Open all usable port holes and place wind scoops in them.
• Place a fire extinguisher nearby.
• If it can be done, use water compounds instead of other solvents.
• Wear rubber gloves to prevent direct contact with solvents.
• Use goggles when a solvent is being sprayed on surfaces.
• Hold the nozzle close to the object being sprayed.
Where water compounds cannot be used, inhibited methyl chloroform (1.1.1 trichloroethane) should be used. Carbon tetrachloride is not used. Cleaning solvents that end with ETHYLENE are NOT safe to use. Methyl chloroform is an effective cleaner and is as safe as can be expected when reasonable care is exercised, such as adequate ventilation and the observance of fire precautions. When using inhibited methyl chloroform, avoid direct inhalation of the vapor. It is not safe for use, even with a gas mask, because its vapor displaces oxygen in the air.
A 2M technician will encounter several uses for aerosol dispensers. The most common type is in applying conformal coatings.
Specific instructions concerning the precautions and procedures that must be observed to prevent physical injury cannot be given in this section because of the many available industrial sprays. However, all personnel concerned with handling aerosol dispensers containing volatile substances must clearly understand the hazards involved. They must also understand the importance of exercising protective measures to prevent personal injury. Strict compliance with the instructions printed on the aerosol
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dispensers will prevent many accidents that result from misapplication, mishandling, or improper storage of industrial sprays.
The rules for safe use of aerosol dispensers are listed below:
• Carefully read and comply with the instructions printed on the container.
• Do not use any dispenser that is capable of producing dangerous gases or other toxic effects in an enclosed area unless the area is adequately ventilated.
• If a protective coating must be sprayed in an inadequately ventilated space, either an air respirator or a self-contained breathing apparatus should be provided. However, fresh air supplied from outside the enclosure by exhaust fans or portable blowers is preferred. Such equipment prevents inhalation of toxic vapors.

• Avoid skin contact with the liquid. Contact with some liquids may cause burns, while milder exposure may cause rashes. Some toxic materials are actually absorbed through the skin.
• Do not puncture the dispenser. Because it is pressurized, injury can result.
• Keep dispensers away from direct sunlight, heaters, and other heat sources.
• Do not store dispensers in an environment where the temperature exceeds the limits printed on the can. High temperatures may cause the container to burst.
Q34. List two causes of damage to ESD-sensitive electronic components.
Q35. What is the purpose of the wrist ground strap?
Q36. What is the cause of most accidents?
This topic has presented information on miniature and microminiature (2M) repair procedures and 2M safety precautions. The information that follows summarizes the important points of this topic.
CONFORMAL COATINGS are protective materials applied to electronic assemblies to prevent damage caused by corrosion, moisture, and stress.
CONFORMAL COATINGS REMOVAL is accomplished mechanically, chemically, or thermally, depending on the material used.
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Component LEADS are terminated either through the board, above the board, or on the board.
SOLDER may be removed by wicking, by a manual vacuum plunger, or by a continuous vacuum solder extractor.
ELECTRONIC ASSEMBLIES should be restored to the original manufacturer's standards using the same orientation and termination method.
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A GOOD SOLDER JOINT is bright and shiny with no cracks or pits.
When REPLACING DIPs, TOs, AND FLAT PACKS, make certain that pins are placed in the proper position.
COMPONENT LEADS may be clipped prior to removal only if the part is known to be bad or if normal removal will result in board damage.
The technician must determine through INSPECTION what method of repair is necessary for the board.
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ELECTROSTATIC DISCHARGE (ESD) can damage or destroy many types of electronic components including integrated circuits and discrete components.
Special handling is required for ELECTROSTATIC-DISCHARGE-SENSITIVE (ESDS) devices or components.
USE PRESCRIBED SAFETY PRECAUTIONS when you use power tools, soldering irons, cleaning solvents, and aerosol dispensers.
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ANSWERS TO QUESTIONS Q1. THROUGH Q36.
A1. Conformal coating.
A2. Chemical, mechanical, and thermal.
A3. Solvents or xylene and trichloroethane.
A4. Mechanical.
A5. To ensure protective characteristics are maintained.
A6. Interfacial connections.
A7. Clinched lead, straight-through, and offset pad.
A8. Above-the-board termination.
A9. On-the-board termination.
A10. During disassembly or repair.
A11. Wicking.
A12. Continuous vacuum.
A13. These methods should not be used.
A14. Manufacturer's standards.
A15. A fine abrasive.
A16. 90 degrees.
A17. They should be readable from a single point.
A18. In the direction of the run.
A19. The ease with which molten solder wets the surfaces of the metals to be joined.
A20. Conductive-type soldering iron.
A21. The type of work to be done.
A22. A thermal shunt.
A23. Bright and shiny with no cracks or pits.
A24. If the component is known to be defective or if the board may be damaged by normal desoldering.
A25. By pushing it gently out of the board.
A26. Heat each lead and lift with tweezers.
A27. Use a skipping pattern.
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A28. Inspect and test.
A29. Operational failures, repairs by untrained personnel, repair using improper tools, mishandling, improper shipping, packaging, and storage.
A30. Clinched staple.
A31. Epoxy a replacement pad to the board, set an eyelet, and solder it.
A32. Repairs by untrained personnel and technicians using improper tools.
A33. Epoxy and fiberglass powder.
A34. Esd, improper stowage, and improper handling.
A35. To discharge any static charge built up in the body.
A36. Deviation from prescribed safe operating procedures.
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ALLOWANCE PARTS LIST (APL)—Repair parts required for unit having the equipment/ component listed.
ALLOWANCE EQUIPAGE LIST (AEL)—Equipment requirements for a unit having the exact equipment/component listed.
BEAM-LEAD CHIP—Semiconductor chip with electrodes (leads) extended beyond the wafer. BONDING WIRES—Fine wires connecting the bonding pads of the chip to the external leads of the
package.
BUILT-IN TEST EQUIPMENT (BITE)—Permanently mounted to the equipment for the purpose of testing the equipment.
CABLE HARNESS—A group of wires or ribbons of wiring used to interconnect electronic systems and subsystems.
CATHODE SPUTTERING—Process of producing thin film components.
CERMET—A combination of powdered precious-metal alloys and an inorganic material such as alumina. Used in manufacturing resistors, capacitors, and other components for high-temperature applications.
CORDWOOD MODULE.—A method of increasing the number of discrete components in a given space. Resembles wood stacked for a fireplace.
CRYSTAL FURNACE.—Device for artificially growing cylindrical crystals for producing semi- conductor substrates.
DEPOT-LEVEL MAINTENANCE (SM&R Code D)—Supports S&R Code I and SM&R Code O activities through extensive shop facilities and equipment and more highly skilled personnel.
DICE—Uncased chips.
DIE BONDING—Process of mounting a chip to a package.
DIFFUSION—Controlled application of impurity atoms to a semiconductor substrate.
DISCRETE COMPONENTS—Individual transistors, diodes, resistors, capacitors, and inductors.
DOPING—See Diffusion.
DUAL IN-LINE PACKAGE (DIP)—IC package having two parallel rows of preformed leads. ENCAPSULATED—Imbedded in solid material or enclosed in glass or metal.
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EPITAXIAL PROCESS—The depositing of a thin uniformly doped crystalline region (layer) on a substrate.
EUTECTIC ALLOY—An alloy that changes directly from a solid to a liquid with no plastic or semiliquid state.
EUTECTIC SOLDER—An alloy of 63 percent tin and 37 percent lead. Melts at 361º F.
FILM ICs—Conductive or nonconductive material deposited on a glass or ceramic substrate. Used for passive circuit components, resistors, and capacitors.
FLAT PACK—IC package.
FLIP CHIP—Monolithic IC packaging technique that eliminates need for bonding wires.
FLUX—Removes surface oxides from metals being soldered.
GENERAL PURPOSE ELECTRONIC TEST EQUIPMENT (GPETE)—Multimeters, oscilloscopes, voltmeters, signal generators, etc.
GROUND PLANES—Copper planes-used to minimize interference between circuits and from external sources.
HYBRID ICs—Two or more integrated circuit types, or one or more integrated circuit types and discrete components on a single substrate.
INTEGRATED CIRCUIT (IC)—Elements inseparably associated and formed on or within a single substrate.
INTERMEDIATE-LEVEL MAINTENANCE (SR&R Code I)—Direct support and technical assistance to user organizations. Tenders and shore-based repair facilities.
ISOLATION—The prevention of unwanted interaction or leakage between components.
LANDS—Conductors or runs on pcbs.
LARGE SCALE INTEGRATION (lsi)—An integrated circuit containing 1,000 to 2,000 logic gates or up to 64,000 bits of memory.
MASK—A device used to deposit materials on a substrate in the desired pattern.
MICROCIRCUIT—A small circuit having high equivalent-circuit-element density, which is considered as a single part composed of interconnected elements on or within a single substrate to perform an electronic-circuit function.
MICROELECTRONICS—That area of electronics technology associated with electronic systems built of extremely small electronic parts or elements.
MICROCIRCUIT MODULE—An assembly of microcircuits or a combination of microcircuits and discrete components that perform one or more distinct functions.
MODIFIED TRANSISTOR OUTLINE (TO)—IC package resembling a transistor.
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MODULAR PACKAGING—Circuit assemblies or subassemblies packaged to be easily removed for maintenance or repair.
MODULE—A circuit or portion of a circuit packaged as a removable unit. A separable unit in a packaging scheme displaying regularity of dimensions.
MILITARY STANDARDS (MILSTD)—Standards of performance for components or equipment that must be met to be acceptable for military systems.
MINIATURE ELECTRONICS—Modules, packages, pcbs, and so forth, composed exclusively of discrete components.
2M—Miniature/Microminiature repair program.
MONOLITHIC IC—ICs that are formed completely within a semiconductor substrate. Silicon chips.
OFF-LINE TEST EQUIPMENT—Tests andisolates faults in modules or assemblies removed from systems.
OHMS PER SQUARE—The resistance of any square area of thin film resistive material as measured between two parallel sides.
ON-LINE TEST EQUIPMENT—Continuously monitors the performance of electronic systems. ORGANIZATIONAL-LEVEL MAINTENANCE (SM&R Code O)—Responsibility of the user
organization.
PACKAGING LEVELS—System developed to assist maintenance personnel in isolating faults.
PHOTO ETCHING—Chemical process of removing unwanted material in producing printed circuit boards.
POINT-TO-POINT WIRING—Individual wires run from terminal to terminal to complete a circuit.
PRINTED CIRCUIT BOARD (peb)—The general term for completely processed printed circuit or printed wiring configurations. It includes single-layered, double-layered, and multi-layered boards.
SCREENING—Process of applying nonconductive or semiconductive materials to a substrate to form thick film components.
SHIELDING—Technique designed to minimize internal and external interference.
SOURCE, MAINTENANCE, AND RECOVER-ABILITY CODES (SM&R CODES)—Specify maintenance level for repair of components or assemblies.
SUBSTRATE—Mounting surface for integrated circuits. May be semiconductor or insulator material depending on type of IC.
THICK FILM COMPONENTS—Passive circuit components (resistors and capacitors) having a thickness of 0.001 centimeter.
AI-4
THIN FILM COMPONENTS—Passive circuit elements (resistors and capacitors) deposited on a substrate to a thickness of 0.0001 centimeter.
VACUUM EVAPORATION—Process of producing thin film components.
VERY LARGE SCALE INTEGRATION (vlsi)—An integrated circuit containing over 2,000 logic gates or 64,000 bits of memory.
WAFER—A slice of semiconductor material upon which monolithic ICs are produced.
AII-1
Linear Integrated Circuits, Basic Electricity and Electronics Course, Module 34, CANTRAC A-100- 0010, Naval Education and Training Program Development Center Detachment, Great Lakes, III., 1981.
Technical Manual, Miniature/Microminiature (2M) Electronic Repair Program, Vols. I, II, and III, NAVSEA TE000-AA-HBK-010/020/030/2M, Naval Sea Systems Command, Keyport, Wash., 1982.
Technical Manual, Miniature/Microminiature (2M) Electronic Repair Program, Vols. I, II, and III, NAVSEA TE000-AA-HBK-010/020/030/2M, Naval Sea Systems Command, Keyport, Wash., 1982.
General Maintenance Handbook, Electronics Installation and Maintenance Books, NAVSEA SE000-00- EIM-160, Naval Sea Systems Command, Washington, D.C., 1981.
Technical Manual, Miniature/Microminiature (2M) Electronic Repair Program, Vols. I, II, and III, NAVSEA TE000-AA-HBK-010/020/030/2M, Naval Sea Systems Command, Keyport, Wash., 1982.
INDEX-1
INDEX-2
INDEX-3
Information: The text pages that you are to study are provided at the beginning of the assignment questions.
1
Textbook assignment: Chapter 1, “Microelectronics,” pages 1-1 through 1-56. Chapter 2, “Miniature/Microminiature (2M) Repair Program and High-Reliability Soldering,” pages 2-1 through 2-22. _____________________________________________________________________________________
1-1.
1. Microelectronics 2. Modular packages 3. Integrated circuits 4. Solid-state technology
1-2. During World War II, which of the
following limitations were considered unacceptable for military electronics systems?
1. Large size, heavy weight, and wide
bandwidth 2. Excessive power requirements, large
3. Large size, heavy weight, and excessive power requirements
4. Heavy weight, complex circuits and limited communications range
1-3. The development of which of the following types of components had the greatest impact on the technology of microelectronics?
1. Vacuum tubes and resistors 2. Transformers and capacitors 3. Vacuum tubes and transistors 4. Transistors and solid-state diodes
1-4. For a vacuum tube to operate properly in a variety of different circuit applications, additional components are often required to "adjust" circuit values. This is because of which of the following variations within the vacuum tube?
1. Element size 2. Warm-up times 3. Plug-in mountings 4. Output characteristics
1-5. Point to point wiring in a vacuum tube circuit often caused which of the following unwanted conditions?
1. Heat interactions 2. Inductive interactions 3. Capacitive interactions 4. Both 2 and 3 above
1-6. Functional blocks of a system that can easily be removed for troubleshooting and repair are called
1. sets 2. chassis 3. modules 4. vacuum tubes
2
1-7. Which of the following characteristics of a printed circuit board (pcb) is NOT an advantage over a point-to-point wired tube circuit?
1. The pcb weighs less 2. The pcb eliminates the need for
point-to-point wiring 3. The pcb eliminates the need for a
heavy metal chassis 4. The pcb contains a limited number
1-8. A module in which the components are supported by end plates is referred to as
1. a pcb 2. cordwood 3. a substrate 4. encapsulated
1-9. A module which is difficult to repair because it is completely imbedded in solid material is one which has been
1. balanced 2. enveloped 3. integrated 4. encapsulated
1-10. All components and interconnections are formed on or within a single substrate in which of the following units?
1. Cordwood 2. Integrated circuit 3. Equivalent circuit 4. Printed circuit board
1-11. Monolithic integrated circuits are usually referred to as
1. hybrids 2. substrates 3. silicon chips 4. selenium rectifiers
1-12. In integrated circuits, a conductive or nonconductive film is used for which of the following types of components?
1. Capacitors and diodes 2. Transistors and diodes 3. Resistors and capacitors 4. Resistors and transistors
1-13. Which of the following types of electronic circuits is NOT a hybrid integrated circuit?
1. Thick film and transistors 2. Thin film and silicon chips 3. Transistors and vacuum tubes 4. Silicon chips and transistors
1-14. What maximum number of logic gates should be expected in a large-scale integration circuit?
1. 20 2. 200 3. 2,000 4. 20,000
3
1-15. Integrated circuits containing more than 64,000 bits of memory are referred to as
1. hybrid integration 2. large-scale integration 3. small-scale integration 4. very large-scale integration
1-16. Which of the following pieces of equipment is used to prepare component layout in complex ICs?
1. A mask 2. A camera 3. A computer 4. A microscope
1-17. A device that allows the depositing of material in selected areas of a semiconductor substrate, but not in others, is known as a
1. blind 2. screen 3. filter 4. wafer mask
1-18. Which of the following types of material is preferred for film circuit substrates?
1. Silicon 2. Ceramic 3. Germanium 4. Fiberglass
1-19. A typical silicon wafer has approximately (a) what diameter and (b) what thickness?
1. (a) 2 inches (b) 0.01 to 0.02 inches 2. (a) 2 inches (b) 0.21 to 0.40 inches 3. (a) 3 inches (b) 0.21 to 0.40 inches 4. (a) 3 inches (b) 0.01 to 0.20 inches
1-20. Artificially grown silicon or germanium crystals are used to produce substrates for which of the following types of integrated circuits?
1. Hybrid 2. Thin-film 3. Thick-film 4. Monolithic
1-21. Elements penetrate the semiconductor substrate in (a) what type of IC but (b) do NOT penetrate the substrate in what type of IC?
1. (a) Diffused (b) thin-film 2. (a) Diffused (b) epitaxial 3. (a) Thick-film (b) epitaxial 4. (a) Thick-film (b) thin-film
1-22. Pn junctions are protected from contamination during the fabrication process by which of the following materials?
1. Oxide 2. Silicon 3. Germanium 4. Photoetch
1-23. The prevention of unwanted interaction or leakage between components is accomplished by which of the following techniques?
1. Isolation 2. Insulation 3. Integration 4. Differentiation
4
1-24. Vacuum evaporation and cathode sputtering are two methods used to produce which of the following types of components?
1. Diodes 2. Thin-film 3. Thick-film 4. Transistors
1-25. To deposit highly reactive materials on a substrate, which of the following methods is used?
1. Photoetching 2. Photolithography 3. Cathode sputtering 4. Vacuum evaporation
1-26. To produce thin film resistors, which of the following materials is/are used?
1. Nichrome 2. Tantalum 3. Titanium 4. Each of the above
1-27. Which of the following is a major advantage of hybrid ICs?
1. Ease of manufacture 2. Ease of replacement 3. Design flexibility 4. Easy availability
1-28. IC packaging is required for which of the following reasons?
1. To dissipate heat 2. For ease of handling 3. To increase shelf life 4. To meet stowage requirements
_______________________________________ IN ANSWERING QUESTIONS 1-29 AND 1-30, MATCH THE IC PACKING EXAMPLES IN THE QUESTIONS TO THE PACKAGING DESCRIPTIONS IN FIGURE 1A.
A.
B. DIP C. Flatpack D. Hybrid
Figure 1A.
Packaging descriptions. _______________________________________
1-29.
1. A 2. B 3. C 4. D
1-30.
1. A 2. B 3. C 4. D
5
1-31. Which of the following types of DIPs are most commonly used in the Navy's microelectronics systems?
1. Glass 2. Metal 3. Ceramic 4. Plastic
1-32. In IC production, gold or aluminum bonding wires are used for which of the following purposes?
1. To bond the chip to the package 2. To provide component isolation 3. To connect the package to the
circuit board 4. To connect the chip to the package
1-33. IC packages that may be easily installed by hand or machine on mounting boards fall into which of the following categories?
1. TO 2. DIP 3. Flatpack 4. Each of the above
1-34. The need for bonding wires has been eliminated by which of the following production techniques?
1. LSI 2. Beam lead 3. Flip chip 4. Both 2 and 3 above
THIS SPACE LEFT BLANK INTENTIONALLY.
6
______________________________________ IN ANSWERING QUESTIONS 1-35 THROUGH 1-37, MATCH THE LETTER IN EACH OF THE FIGURES THAT IDENTIFIES PIN 1. ______________________________________
1-35.
1. A 2. B 3. C 4. D
1-36.
1. E 2. F 3. G 4. H
1-37.
1. J 2. K 3. L 4. M
1-38. Letters and numbers stamped on the body of an IC serve to provide which of the following types of information?
1. Use 2. Serial number 3. Date of manufacture 4. Applicable equipment
1-39. Descriptive information about a particular type of IC may be found in which of the following documents?
1. The manufacturer's data sheet 2. The equipment Allowance Part List
(APL) 3. The National Stock Number (NSN) 4. The IC identification number list
1-40. Assemblies made up EXCLUSIVELY of discrete electronic parts are classified as
1. vacuum-tube circuits 2. microcircuit modules 3. hybrid microcircuits 4. miniature electronics circuits
7
1-41. An assembly of microcircuits or a combination of microcircuits and discrete components is referred to as a
1. mother board 2. microprocessor 3. miniature module 4. microcircuit module
1-42. A technician has isolated a problem to a plug-in module on a printed circuit board. What is this level of system packaging?
1. Level O 2. Level I 3. Level II 4. Level III
1-43. A faulty transistor would be identified as what level of packaging?
1. Level O 2. Level I 3. Level II 4. Level III
1-44. A chassis located in a radar antenna pedestal would be identified as what level of system packaging?
1. Level I 2. Level II 3. Level III 4. Level IV
1-45. Which of the following characteristics is NOT an advantage of multilayer printed circuit boards?
1. Allows greater wiring density on
boards 2. Provides shielding for a large
number of conductors 3. Eliminates complicated wiring
harnesses 4. Reduces the number of components
1-46. Which of the following circuit connection methods is NOT used in making interconnections on a multilayer printed circuit board interconnection?
1. Terminal lug 2. Clearance hole 3. Layer build-up 4. Plated-through hole
1-47. The most complex to produce and difficult to repair printed circuit boards are those made using which of the following methods?
1. Layer-buildup 2. Clearance-hole 3. Step-down-hole 4. Plated-through-hole
1-48. Environmental performance requirements for ICs are set forth in which of the following publications?
1. 2M repair manual 2. Military Standards 3. System maintenance manuals 4. Manufacturer's data sheet
8
1-49. Ground planes and shielding are used to prevent which of the following electrical interactions?
1. Cross talk 2. External interference 3. The generation of rf within the
system 4. All of the above
1-50. Training requirements for miniature and microminiature (2M) repair personnel was established by which of the following authorities?
1. Chief of Naval Education and
Training 2. Chief of Naval Technical Training 3. Chief of Naval Operations 4. Commander, Naval Sea Systems
1-51. The standards of workmanship and guidelines for specific repairs to equipment are contained in which of the following Navy publications?
1. Introduction to Microelectronics 2. NAVSHIPS Technical Manual 3. Electronics Installation and
Maintenance Books (EIMB) 4. Miniature/Microminiature (2M)
1-52. A technician is authorized to perform 2M repairs upon satisfactory completion of which of the following types of training?
1. A 2M training class 2. On-the-job training 3. NEETS, Module 14 4. Any electronics class "A" school
1-53. Repairs that are limited to discrete components and single- and double- sided boards are classified as what level of repairs?
1. Intermediate 2. Organizational 3. Miniature component 4. Microminiature component
1-54. To ensure that a 2M technician maintains the minimum standards of workmanship, the Navy requires that the technician meet which of the following requirements?
1. Be licensed 2. Be certified 3. Be experienced 4. Be retrained
1-55. If a technician should fail to maintain the required standards of workmanship, the technician's certification is subject to what action?
1. Cancellation 2. Recertification 3. Reduction to next lower level 4. Withholding pending requalification
1-56. The most extensive shop facilities and highly skilled technicians are located at what SM & R level of maintenance?
1. Depot 2. Operational 3. Intermediate 4. Organizational
9
1-57. SM & R code D maintenance facilities are usually located at which of the following activities?
1. Shipyards 2. Contractor maintenance
organizations 3. Shore-based facilities 4. All of the above
1-58. Direct support to user organizations is provided by which of the following SM & R code maintenance levels?
1. Depot 2. Operational 3. Intermediate 4. Organizational
1-59. Inspecting, servicing, and adjusting equipment is the function of which of the following SM & R code maintenance levels?
1. Depot 2. Operational 3. Intermediate 4. Organizational
1-60. The maintenance level at which normal 2M repairs are performed is set forth in the maintenance plan and specified by the
1. NAVSEA 2M Repair Program 2. Source, Maintenance, and
Recoverability (SM & R) code 3. Chief of Naval Operations 4. Equipment manufacturers'
1-61. Boards or modules that are SM & R code D may be repaired at the organizational level under which of the following conditions?
1. On a routine basis 2. When parts are available 3. To meet an urgent operational
commitment 4. When code D repair will take six
1-62. Source, Maintenance, and Recovery (SM & R) codes that list where repair parts may be obtained, who is authorized to make the repair, and the maintenance level for the item are found in which of the following documents?
1. Allowance Equipage Lists (AEL) 2. Allowance Parts Lists (APL) 3. Manufacturer's Parts List 4. Navy Stock System
1-63. Test equipment that continuously monitors performance and automatically isolates faults to removable assemblies is what category of equipment?
1. On-line 2. Off-line 3. General purpose 4. Fault isolating
1-64. A dc voltmeter that is permanently attached to a power supply for the purpose of monitoring the output is an example of what type test equipment?
1. General Purpose Electronic Test
Equipment (GPETE) 2. Built in Test Equipment (BITE) 3. Off-line test equipment 4. Specialized test equipment
10
1-65. Which of the following types of test equipment is classified as off-line automatic test equipment?
1. Centralized Automatic Test System
(CATS) 2. Versatile Avionic Shop Test System
(VAST) 3. General Purpose Electronic Test
Equipment (GPETE) 4. Test Evaluation and Monitoring
1-66. Fault diagnosis using GPETE should only be attempted by which of the following personnel?
1. Officers 2. Technician strikers 3. Experienced technicians 4. Basic Electricity and Electronics
1-67. During fault isolation procedures, a device or component should be desoldered and removed from the circuit only at which of the following times?
1. After defect verification 2. For out-of-circuit testing 3. During static resistance checks 4. At any time the technician desires
1-68. 2M repair stations are equipped according to the types of repairs to be accomplished. The use of microscopes and precisions drill presses would be required in which of the following types of repair?
1. Miniature 2. Microminiature 3. Both 1 and 2 above 4. Emergency
1-69. In the selection of a soldering iron tip, which of the following factors should be considered?
1. The complexity of the pcb 2. The composition of the pcb 3. The area and mass being soldered 4. The type of component being
1-70. The handpiece that can be used for the greatest variety of operations is the
1. solder extractor 2. rotary-drive tool 3. resistive tweezers 4. lapflow and thermal scraper
1-71. Regardless of location, 2M repair stations require adequate work surface area, lighting, power, and what other minimum requirement?
1. Heat source 2. Ventilation 3. Illumination 4. Dust-free space
1-72. Solder used in electronics is an alloy composed of which of the following metals?
1. Tin and zinc 2. Tin and lead 3. Lead and zinc 4. Lead and copper
1-73. A roll of solder is marked 60/40. What do these numbers indicate?
1. 60% tin, 40% lead 2. 60% tin, 40% copper 3. 60% lead, 40% tin 4. 60% lead, 40% copper
11
1-74. Which of the following alloys will melt directly into a liquid and have no plastic or semiliquid state?
1. Metallic alloy 2. Eutectic alloy 3. Zinc-lead alloy 4. Copper-zinc alloy
1-75. The PREFERRED solder alloy ratio for electronic repair is 63/37. Which of the following alloy ratios is also ACCEPTABLE for this type of repair?
1. 30/70 2. 50/50 3. 60/40 4. 70/30
12
Textbook assignment: Chapter 3, “Miniature and Microminiature Repair Procedures,” pages 3-1 through 3-51.
2-1. Which of the following requirements must be met by a 2M technician to be authorized to perform repairs at a particular level?
1. Have knowledge of that level 2. Be certified at that level 3. Be experienced at that level 4. Be licensed at the next higher level
2-2. Protective materials applied to electronic assemblies to prevent damage caused by corrosion, moisture, and stress are called
1. conformal coatings 2. isolation materials 3. electrical insulation 4. encapsulation coatings
2-3. Before working on a pcb, the conformal
coating should be removed from what part of the board?
1. The entire board 2. The component side 3. The area of the repair 4. The side opposite the component side
2-4. What are the approved methods of
conformal coating removal?
1. Peeling and abrading 2. Stripping and heating 3. Mechanical and thermal only 4. Mechanical, thermal, and chemical
2-5. Most methods of conformal coating
removal are variations of which of the following types of removal?
1. Thermal 2. Chemical 3. Mechanical 4. Electrical
2-6. What is the preferred method of removing epoxy conformal coatings?
1. Solvent 2. Ball mill 3. Hot-air jet 4. Thermal parting tool
2-7. A conformal coating is considered to be
thin if it is less than what thickness?
1. 0.025 inches 2. 0.040 inches 3. 0.050 inches 4. 0.250 inches
2-8. Of the various mechanical methods of
conformal coating removal, physical contact with the work piece is NOT required using which of the following methods?
1. Cutting 2. Grinding 3. Hot-air jet 4. Thermal parting
2-9. Cutting and peeling is an easy method of
removing which of the following types of coatings?
1. Epox 2. Varnish 3. Parylene 4. Silicone
13
2-10. Thin acrylic coatings are readily removed in which of the following ways?
1. Routing 2. Hot-air jet 3. Cut and peel 4. Mild solvents
2-11. When applying an application of
conformal coating, which of the following conditions is true?
1. The board should be clean and moist 2. The coating should be applied only to
the component replaced 3. The coating should be the same type as
that used by the manufacturer 4. The coating should be applied only to
2-12. The procedure of connecting circuitry on one side of a board with the circuitry on the other side is known as
1. mounting 2. termination 3. hole reinforcement 4. interfacial connections
2-13. Reinforcement for circuit pads on both
sides of the board is provided by which of the following types of eyelets?
1. Flat-set 2. Roll-set 3. Funnel-set
2-14. The manner in which wires and leads are
attached to an assembly is described by which of the following terms?
1. Termination 2. Solder joints 3. Lead formation 4. Interfacial connections
2-15. Clinched leads, straight-through leads, and offset pads are variations of what type of termination?
1. Solder cup 2. On-the-board 3. Above-the-board 4. Through-hole
2-16. What total number of degrees of bend are
(a) fully clinched leads and (b) semiclinched leads?
1. (a) 45 (b) 90 2. (a) 45 (b) 45 3. (a) 90 (b) 45 4. (a) 90 (b) 90
2-17. Which of the following types of terminals
are used as tie points for interconnecting wiring?
1. Pins 2. Hooks 3. Solder cups 4. Turrets
2-18. During assembly, component stability is
provided by which of the following types of lead termination?
1. Hook terminal 2. Clinched lead 3. Turret terminal 4. On-the-board (lap-flow)
2-19. Which of the following types of lead
terminations is the easiest to remove and rework?
1. Offset-pad 2. Semiclinched 3. Fully clinched 4. Straight-through
14
2-20. Turret, fork, and hook terminals are examples of what type of termination?
1. Off-set 2. On-the-board 3. Above-the-board 4. Through-the-board
2-21. When a lead is soldered to a pad without
passing it through the board, what type of termination has been made?
1. Lap-flow 2. Off-set pad 3. Clinched lead 4. Straight-through lead
2-22. Most damage to printed circuit boards
occurs at which of the following times?
1. During trouble shooting 2. During component removal 3. During component replacement 4. During normal system operations
2-23. Of the solder removal methods listed
below, which one is most versatile and reliable?
1. Wicking 2. Heat and shake 3. Motorized solder extractor 4. Manually controlled vacuum plunge
2-24. When removing solder with a solder wick,
where should the wick be placed in relation to the solder joint and the iron?
1. Below both the joint and the iron 2. Between the joint and the iron 3. Above the joint and the iron
2-25. The motorized solder extractor may be operated in three different modes. Which of the following is NOT one of those modes?
1. Hot-air jet 2. One-shot vacuum 3. Heat and vacuum 4. Heat and pressure
2-26. Stirring the lead during desoldering
prevents which of the following unwanted results?
1. Sweat joints 2. Overheating 3. Cold solder joints 4. Pad delamination
2-27. Of the following solder removal methods,
which one is acceptable for removing solder?
1. Hot-air jet 2. Heat and pull 3. Heat and shake 4. Heat and squeeze
2-28. Which of the following examples
represents properly formed leads?
15
2-29. A 2M technician is repairing a board that is manufactured with semiclinched leads. What type of termination should the technician use in replacing components?
1. Lap flow 2. Clinched lead 3. Semiclinched lead 4. Straight-through-lead
Figure 2A.Component mounting.
IN ANSWERING QUESTION 2-30, REFER TO FIGURE 2A. 2-30. What component on the board is NOT
properly mounted?
1. A 2. B 3. C 4. D
2-31. What type of heat source is a soldering
iron?
1. Radiant 2. Resistive 3. Conductive 4. Convective
2-32. The shape and size of the soldering iron tip to be used is determined by which of the following factors?
1. The type of flux to be used 2. The type of work to be done 3. The type of solder to be used 4. The voltage source for the iron
2-33. A thermal shunt is attached to the leads of
a transistor prior to applying solder to the joint. This shunt serves what purpose?
1. Prevents short circuits 2. Retains heat at the joint 3. Conducts heat away from the
component 4. Physically supports the lead during
soldering 2-34. What is the appearance of a good solder
joint?
1. Dull gray 2. Crystalline 3. Bright and shiny 4. Shiny with small pits
2-35. What is the most efficient soldering
temperature?
1. 360 °F 2. 440 °F 3. 550 °F 4. 800 °F
16
Figure 2B.
Solder joints. IN ANSWERING QUESTION 2-36, REFER TO FIGURE 2B.
2-36. All the solder joints in the figure have two things in common. The first is that all are through-the-board terminations. The other is that they are all what type of joint?
1. Sweat 2. Full-fillet 3. Unacceptable 4. Clinched-lead
2-37. Plug-in DIPs are mounted by using which
of the following aids/parts?
1. Insulators 2. Special tools 3. Clinched leads 4. Mounting sockets
2-38. Plug-in DIPs are susceptible to loosening
because of which of the following causes?
1. Heat 2. Stress 3. Warpage 4. Vibration
2-39. Component leads may be clipped to aid in their removal under which of the following conditions?
1. When the component is conformally
coated 2. When the component is known to be
defective 3. When board damage may result from
normal removal methods 4. Both 2 and 3 above
2-40. Visual inspection of a completed repair is
conducted to evaluate which of the following aspects of the repair?
1. Workmanship quality 2. Component placement 3. In-circuit quality test 4. Conformal coating integrity
2-41. When speaking of TO mounting
techniques, the term "plug-in" refers to the same technique as is used with DIPs.
1. True 2. False
2-42. In addition to heat dissipation and physical
support, which of the following needs might justify the use of a spacer with a TO mount?
1. Vibration elimination 2. Proper lead formation 3. Proper lead termination 4. Short-circuit protection
2-43. For the removal of imbedded TOs in
which all the leads are free, which of the following methods is recommended?
1. Push out gently 2. Pull out with pliers 3. Pull out with fingers 4. Tap out with soft mallet
17
2-44. What is the most critical step in replacing an imbedded TO?
1. Seating the leads 2. Forming the leads 3. Soldering the leads 4. Seating the body of the TO
2-45. When a TO or a DIP is replaced on a
printed circuit board, what type of termination is normally used?
1. Lap flow 2. On-the-board 3. Above-the-board 4. Through-the-board
2-46. What type of termination is used in the
replacement of a flat pack?
1. Lap flow 2. Solder cup 3. Solder plug 4. Full fillet
2-47. Heating the leads and lifting them free
with tweezers is the preferred method of removing which of the following components?
1. TOs 2. DIPs 3. Flat packs 4. Transistors
2-48. Which of the examples shows the correct lead formation for a flat pack?
2-49. Use of a skipping pattern when soldering
1. cold solder joints 2. excessive heat buildup 3. the component from moving 4. the need to visually inspect the piece
2-50. Cards and boards may be damaged under which of the following conditions?
1. When unauthorized repairs are
attempted by untrained personnel 2. When technicians use improper tools 3. When improperly stored 4. Each of the above
2-51. DS3 Spark is preparing to repair a cracked
conductor on a card. For this type of damage, which of the following repair methods is preferred?
1. Solder bridge 2. Clinched staple 3. Install an eyelet at the crack and solder
in place 4. Lap-flow soldered wire across the
18
2-52. To ensure a good mechanical bond between the board and replacement pad and to provide good electrical contact for components, which of the following procedures is used?
1. Epoxying the pad to the run 2. Electroplating the repair area 3. Installing an eyelet in the pad 4. Lap-flow soldering the repair area
2-53. Breaks, holes, and cracks in pcbs are
1. fiberglass and rosin 2. epoxy and powdered carbon 3. conformal coating and RTV 4. epoxy and powdered fiberglass
2-54. What is the first step in the repair of
burned or scorched boards?
1. Filling the burned area with epoxy and fiberglass
2. Removing all discolored material 3. Removing all delaminated conductors 4. Cleaning all surfaces with solvent
2-55. Which of the following statements is
correct concerning the repair of repairable delaminated conductors?
1. All delaminations are removed 2. Repairable delaminations are not
removed 3. All delaminations are epoxied to the
board 4. All delaminations are replaced with
insulated wire 2-56. Damage to some electronic components
can occur at what minimum electrostatic potential?
1. 14 volts 2. 35 volts 3. 350 volts 4. 3,500 volts
2-57. Electrostatic discharge (ESD) has the greatest effect on which of the following devices?
1. Silicon diodes 2. Selenium rectifiers 3. Germanium transistors 4. Metal-oxide transistor
Figure 2C. Symbol.
IN ANSWERING QUESTION 2-58, REFER TO FIGURE 2C. 2-58. The symbol shown in the figure is found
on a component package. What does it indicate about the component?
1. It is a high cost item 2. It is a bipolar device 3. It is an integrated circuit 4. It is electrostatic discharge sensitive
2-59. Electrostatic charges may develop as high
as which of the following voltages?
1. 3,000 volts 2. 15,000 volts 3. 20,000 volts 4. 35,000 volts
19
2-60. To prevent an electrostatic charge built up on the body of the technician from damaging ESDS devices, the technician should take which of the following precautions?
1. Be grounded 2. Wear gloves 3. Ground the device 4. Handle the device with insulated tools
2-61. The best source of information concerning the application, handling, and storage of any aerosol dispensers may be found in/on which of the following sources?
1. NAVSEA 2M manual 2. NEETS, Module 14, Topic 3 3. On the aerosol dispenser 4. Applicable military standards