Miniature and microminature repair procedures

Miniature and microminature repair procedures Removal and replacement of discrete components Above-the-board termination Component Desoldering Installation and soldering of printed circuit components Soldering of PCB Components Removal and replacement of dips Repair of printed circuit boards and cards Safety Grounded Work Benches Summary Answers

MINIATURE AND MICROMINIATURE REPAIR PROCEDURES

LEARNING OBJECTIVES

Upon completion of this topic, the student will be able to:

INTRODUCTION

As you progress in your training as a technician, you will find that the skill and knowledge levels required to maintain electronic systems become more demanding. The increased use of miniature and microminiature electronic circuits, circuit complexity, and new manufacturing techniques will make your job more challenging. To maintain and repair equipment effectively, you will have to duplicate with limited facilities what was accomplished in the factory with extensive facilities. Printed circuit boards that were manufactured completely by machine will have to be repaired by hand.

To meet the needs for repairing the full range of electronic equipment, you must be properly trained. You must be capable of performing high-quality, reliable repairs to the latest circuitry.

MINIATURE AND MICROMINIATURE ELECTRONIC REPAIR PROCEDURES

As mentioned at the beginning of topic 2, 2M repair personnel must undergo specialized training. They are trained for a particular level of repair and must be certified at that level. Also, recertification is required to ensure the continued high-quality repair ability of these technicians.

THIS SECTION IS NOT, IN ANY WAY, TO BE USED BY YOU AS AUTHORIZATION TO ATTEMPT THESE TYPES OF REPAIRS WITHOUT OFFICIAL 2M CERTIFICATION.

In the following sections, you will study the general procedures used in the repair, removal, and replacement of specific types of electronic components.By studying these procedures, you will become familiar with some of the more common types of repair work. Before repair work can be performed on a miniature or microminiature assembly, the technician must consider the type of specialized coating that usually covers the assembly. These coatings are referred to as CONFORMAL COATINGS.

CONFORMAL COATINGS

Conformal coatings are protective material applied to electronic assemblies to prevent damage from corrosion, moisture, and stress.These coatings include epoxy, parylene, silicone, polyurethane, varnish, and lacquer. Coatings are applied in a liquid form; when dry, they exhibit characteristics that improve reliability. These characteristics are:

Conformal Coating Removal

Because of the characteristics that conformal coatings exhibit, they must be removed before any work can be done on printed circuit boards. The coating must be removed from all lead and pad/eyelet areas of the component. It should also be removed to or below the widest point of the component body. Complete removal of the coating from the board is not done.

Methods of coating removal are thermal, mechanical, and chemical. The method of removal depends on the type of coating used. Table 3-1 shows suggested methods of removal of some types. Note that most of the methods are variations of mechanical removal.

Table 3-1. - Conformal Coating Removal Techniques

The coating material can best be identified through proper documentation; for example, technical manuals and engineering drawings. If this information is not available, the experienced technician can usually determine the type of material by testing the, hardness, transparency, thickness, and solvent solubility of the coating. The thermal (heat) properties may also be tested to determine the ease of removal of the coating by heat. The methods of removal discussed here describe the basic concept, but not the step-by-step 'how to' procedures.

THERMAL REMOVAL. - Thermal removal consists of using controlled heat through specially shaped tips attached to a handpiece. Soldering irons should never be used for coating removal because the high temperatures will cause the coatings to char, possibly damaging the board materials. Modified tips or cutting blades heated by soldering irons also are not used; they may not have proper heat capacity or allow the hand control necessary for effective removal. Also, the thin plating of the circuit may be damaged by scraping.

The thermal parting tool, used with the variable power supply, has interchangeable tips, as shown in figure 3-1, that allow for efficient coating removal. These thin, blade-like instruments act as heat generators and will maintain the heat levels necessary to accomplish the work. Tips can be changed easily to suit the configuration of the workpiece. These tips cool quickly after removal of power because their small thermal mass and special alloy material easily give up residual heat.

Figure 3-1. - Thermal parting tips.

The softening or breakdown point of different coatings vary, which is a concern when you are using this method. Ideally, the softening, point is below the solder melting temperature. However, when the softening point is equal to or above the solder melting point, you must take care in applying heat at the solder joint or in component areas. The work must be performed rapidly to limit the heating of the area involved and to prevent damage to the board and other components.

HOT-AIR JET REMOVAL. - In principle, the hot-air jet method of coating removal uses controlled, temperature-regulated air to soften or break down the coating, as shown in figure 3-2. By controlling the temperature, flow rate, and shape of the jet, you may remove coatings from almost any workpiece configuration without causing any damage. When you use the hot-air jet, you do not allow it to physically contact the workpiece surface. Delicate work handled in this manner permits you to observe the removal process.

Figure 3-2. - Hot air jet conformal coating removal.

POWER-TOOL REMOVAL DESCRIPTION. - Power-tool removal is the use of abrasive grinding or cutting to mechanically remove coatings. Abrasive grinding/rubbing techniques are effective on thin coatings (less than 0.025 inch) while abrasive cutting methods are effective on coatings greater than 0.025 inch. This method permits consistent and precise removal of coatings without mechanical damage or dangerous heating to electronic components. A variable-speed mechanical drive handpiece permits fingertip-control and proper speed and torque to ease the handling of gum-type coatings. A variety of rotary abrasive materials and cutting tools is required for removal of the various coating types. These specially designed tools include BALL MILLS, BURRS, and ROTARY BRUSHES.

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.

Figure 3-3. - Rotary tool conformal coating removal.

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.

Conformal Coating Replacement

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.

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.

Q.1 What material is applied to electronic assemblies to prevent damage from corrosion, moisture, and stress?
Q.2 What three methods are used to remove protective material?
Q.3 What chemicals are used to remove protective material?
Q.4 Abrasion, cutting, and peeling are examples of what type of protective material removal?
Q.5 Why should the coating material be replaced once the required repair has been completed?

REMOVAL AND REPLACEMENT OF DISCRETE COMPONENTS

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.

Interconnections and Assemblies

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.

Figure 3-4A. - Eyelets (interfacial connections). ROLL SET

Figure 3-4B. - Eyelets (interfacial connections). FUNNEL SET

Figure 3-4C. - Eyelets (interfacial connections). FLAT SET

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.

Lead Terminations

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.

Figure 3-5A. - Clinched leads. FULLY CLINCHED

Figure 3-5B. - Clinched leads. SEMICLINCHED

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.

Figure 3-6. - Straight-through termination.

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)].

Figure 3-7A. - Offset pad termination SIDE VIEW

Figure 3-7B. - Offset pad termination TOP VIEW

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)].

Figure 3-8A. - Terminals. PIN AND TERMINALS

Figure 3-8B. - Terminals. HOLLOW

Figure 3-8C. - Terminals. HOOK TERMINALS

Figure 3-8D. - Terminals. PIERCED TERMINALS

Figure 3-8E. - Terminals. SOLDER CUP

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.

Figure 3-9. - On-the-board termination.

Q.6 What term is used to identify the procedure of connecting one side of a circuit board with the other?
Q.7 Name two types of through-hole termination.
Q.8 Turret, bifurcated, and hook terminals are used for what type of termination?
Q.9 When a lead is soldered to a pad without passing through the board, it is known as what type of termination?

Component Desoldering

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.

Figure 3-10. - Solder wicking.

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.

Figure 3-11. - Manual desoldering tool.

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.

Figure 3-12A. - Motorized vacuum/pressure solder removal. VACUUM MODE

Figure 3-12B. - Motorized vacuum/pressure solder removal. PRESSURE MODE

Figure 3-12C. - Motorized vacuum/pressure solder removal. HOT AIR JET MODE

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 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.

Q.10 When does most printed circuit board damage occur?
Q.11 What procedure involves the use of finely braided copper wire to remove solder?
Q.12 What is the most effective method of solder removal?
Q.13 When, if at all, should the heat-and-shake or the heat-and-pull methods of solder removal be used?

INSTALLATION AND SOLDERING OF PRINTED CIRCUIT COMPONENTS

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.

Termination Area Preparation

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 Lead Preparation

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:

The component is centered between the holes, and component leads are formed with proper bend-radii and body seal-to-bend distance. The possibility of straining component body seals during lead forming is eliminated. Stress relief loops are formed without straining component seals while at the same time providing the desired lead-to-lead distances. Leads are measured and formed for both horizontal and vertical component mounting. 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.

Figure 3-13. - Minimum distance lead bend to component body.

Leads must be approximately 90 degrees from their major axis to ensure free movement in hole terminations, as shown in figure 3-14.

Figure 3-14. - Ideal lead formation.

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.

Figure 3-15. - Component arrangement.

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.

Q.14 To what standards should a technician restore electronic assemblies?
Q.15 How is oxide removed from pads and component leads?
Q.16 Leads are formed approximately how many degrees from their major axis?
Q.17 When you replace components, identification marks must meet what requirements?
Q.18 In what direction are component leads clinched on single- and double-sided boards?

Soldering of PCB Components

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.

Figure 3-16. - Thermal shunt.

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.

Figure 3-17. - Preferred solder joint.

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.

Q.19 What is solderability?
Q.20 What is the most common source of heat in electronic soldering?
Q.21 What determines the shape and size of a soldering iron tip?
Q.22 What term describes a device used to conduct heat away from a component?
Q.23 What is the appearance of a properly soldered joint?

REMOVAL AND REPLACEMENT OF DIPS

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.

Removal of Plug-In DIPs

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.

Figure 3-18. - Typical DIP puller.

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.

Removal and Replacement of Soldered-In DIPs

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 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.

REMOVAL AND REPLACEMENT OF TO PACKAGES

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.

Figure 3-19. - TO mounting techniques.

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.

Figure 3-20. - Imbedded TO removal.

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.

REMOVAL AND REPLACEMENT OF FLAT PACKS

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.

PLANAR-MOUNTED COMPONENTS (FLAT-PACKS)

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.

Flat-Pack Removal

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-Pack Replacement

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.

Figure 3-21. - Flat pack in protective holder.

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.

Figure 3-22A. - Properly formed flat pack leads.

Figure 3-22B. - Properly formed flat pack leads.

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.

Q.24 When removing the component, under what circumstances may component leads be clipped?
Q.25 How are imbedded TOs removed once the leads are free?
Q.26 How is a flat pack removed from a pcb?
Q.27 How do you prevent excessive heat buildup on an area of a board when soldering multilead components?
Q.28 What are the two final steps of any repair?

REPAIR OF PRINTED CIRCUIT BOARDS AND CARDS

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.

Repair of Conductor and Termination Pads

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.

Figure 3-23. - Pcb conductor damage.

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.

Figure 3-24. - Clinched-staple repair of broken conductor.

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.

Figure 3-25. - Replacement of damaged termination pad.

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.

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:

Figure 3-26. - Delaminated conductors.

A small amount of epoxy is mixed and applied to the conductor and the conductor path; no areas are left uncoated. 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.

Repair of Cracked Boards

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 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:

Figure 3-27. - Repair of cracked pcbs.

The surface of the filled area is smoothed to make it level with the surface of the original board. The board is cured, smoothed, redrilled, and cleaned.

Broken Board Repair

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:

The same technique used in repairing cracks is used to prepare the damaged edge. 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. 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. The replacement piece is positioned as nearly as possible to the original board configuration and firmly clamped into place. 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.

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.

Figure 3-28. - Repair of surface damage.

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.

Figure 3-29. - Repair of burned boards.

Repairable delaminations not extending to the edge of the circuit board should be cut away by abrasive methods until no delaminated material remains. Delaminated material is not removed if it is repairable. 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. Solvent is used to clean thoroughly and to remove all loose particles. A compound of epoxy and powdered fiberglass is mixed and used to fill the cutaway area. The epoxy repair mixture is cured according to the manufacturer's instructions. The surface of the filled area is leveled after the compound is cured. 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. After the cure is completed, abrasive methods are used to smooth the repaired surface to the same level as the original board. 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.

Figure 3-30. - Repaired board ready for components.

Q.29 List three causes of damage to printed circuit boards.
Q.30 What is the preferred method of repairing cracked runs on boards?
Q.31 Damaged or missing termination pads are replaced using what procedure?
Q.32 How is board damage caused by technicians?
Q.33 What combination of materials is used to patch or build up damaged areas of boards?

SAFETY

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

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).

Figure 3-31. - Warning symbols for ESDS devices.

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.

Table 3-2. - Typical Measured Statics Charges (in volts)

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.

Table 3-3. - Common Sources of Electrostatic Charge

Prevention of ESD Damage

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.

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 touched with a bare hand. Figure 3-32 shows a more refined ground strap which frees both hands for work.

Figure 3-32. - ESD wrist strap.

Equipment technical manuals and packaging material should be checked for ESD warnings and instructions. 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. 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. 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. 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. 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. 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. 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. 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). Ground the leads of test equipment momentarily before you energize the test equipment and before you probe ESD items.

Grounded Work Benches

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.

Figure 3-33. - Typical ESD ground work bench.

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.

PERSONAL SAFETY

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.

Power Tools

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.

Soldering Iron

When using a soldering iron, remember the following:

Cleaning Solvents

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.

Aerosol Dispensers

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 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:

Q.34 List two causes of damage to ESD-sensitive electronic components.
Q.35 What is the purpose of the wrist ground strap?
Q.36 What is the cause of most accidents?

SUMMARY

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.

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.

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.

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, cleANSWERS 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.
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

aningsolvents, and aerosol dispensers.