science universe

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, cleaning solvents, and aerosol dispensers.

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.

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.

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, cleaning solvents, and aerosol dispensers.

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.

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.

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.

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.

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.

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

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.

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.

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.

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.

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:

• · 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.
• · 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.

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:

• · 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.
• · Do not spray protective coating on warm or energized equipment because this creates a fire hazard.
• · 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?

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.

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.

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.

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

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.

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.

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.

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.

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:

• · 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.
• · 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.

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:

• · 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.
• · Do not spray protective coating on warm or energized equipment because this creates a fire hazard.
• · 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?

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.

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.

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:

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.

Figure 3-27.—Repair of cracked pcbs.

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 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:

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.

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.

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.

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.

Figure 3-29.—Repair of burned boards.

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.

Figure 3-30.—Repaired board 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?