Module 14 - Introduction to Microelectronics
Navy Electricity and Electronics Training Series (NEETS)
Chapter 3:  Pages 3-41 through 3-51

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.

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8.   When moving an ESDS device or assembly, always touch (with bare skin) the surface on which it rests for at least one second before picking it up. Before placing it on any surface, touch the surface with your free hand for at least one second. The bare skin contact provides a safe discharge path for charges accumulated while you are moving around.

9.   While servicing equipment containing ESD devices, do not handle or touch materials such as plastic, vinyl, synthetic textiles, polished wood, fiberglass, or similar items which create static charges; or, be sure to repeat the grounding action with the bare hands after contacting these materials. These materials are prime electrostatic generators.

10.   If possible, avoid repairs that require soldering at the equipment level. Soldering irons must have heater/tips assemblies that are grounded to ac electrical ground. Do not use ordinary plastic solder suckers (special antistatic solder suckers are commercially available).

11.   Ground the leads of test equipment momentarily before you energize the test equipment and before you probe ESD items.

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.

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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.

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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.

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·   Do not solder electronic equipment unless the equipment is electrically disconnected from the power supply circuit.

 
·   After completing a task requiring a soldering iron other than the iron that is part of a work station, disconnect the power cord from the receptacle. When the iron has cooled, store it in its assigned stowage area.

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

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dispensers will prevent many accidents that result from misapplication, mishandling, or improper storage of industrial sprays.

The rules for safe use of aerosol dispensers are listed below:

 
·   Carefully read and comply with the instructions printed on the container.

 
·   Do not use any dispenser that is capable of producing dangerous gases or other toxic effects in an enclosed area unless the area is adequately ventilated.

 
·   If a protective coating must be sprayed in an inadequately ventilated space, either an air respirator or a self-contained breathing apparatus should be provided. However, fresh air supplied from outside the enclosure by exhaust fans or portable blowers is preferred. Such equipment prevents inhalation of toxic vapors.

 
·   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?

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.

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Component LEADS are terminated either through the board, above the board, or on the board.

SOLDER may be removed by wicking, by a manual vacuum plunger, or by a continuous vacuum solder extractor.

ELECTRONIC ASSEMBLIES should be restored to the original manufacturer's standards using the same orientation and termination method.

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A GOOD SOLDER JOINT is bright and shiny with no cracks or pits.

When REPLACING DIPs, TOs, and FLAT PACKS, make certain that pins are placed in the proper position.

COMPONENT LEADS may be clipped prior to removal only if the part is known to be bad or if normal removal will result in board damage.

The technician must determine through INSPECTION what method of repair is necessary for the board.

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ELECTROSTATIC DIsCHARGE (ESD) can damage or destroy many types of electronic components including integrated circuits and discrete components.

Special handling is required for ELECTROSTATIC-DIsCHARGE-SENSITIVE (ESDS) devices or components.

USE PRESCRIBED Safety PRECautionS when you use power tools, soldering irons, cleaning solvents, and aerosol dispensers.

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Answers to Questions Q1. Through Q36.

A1.   Conformal coating.

A2.   Chemical, mechanical, and thermal.

A3.   Solvents or xylene and trichloroethane.

A4.   Mechanical.

A5.   To ensure protective characteristics are maintained.

A6.   Interfacial connections.

A7.   Clinched lead, straight-through, and offset pad.

A8.   Above-the-board termination.

A9.   On-the-board termination.

A10.   During disassembly or repair.

A11.   Wicking.

A12.   Continuous vacuum.

A13.   These methods should not be used.

A14.   Manufacturer's standards.

A15.   a fine abrasive.

A16.   90 degrees.

A17.   They should be readable from a single point.

A18.   In the direction of the run.

A19.   The ease with which molten solder wets the surfaces of the metals to be joined.

A20.   Conductive-type soldering iron.

A21.   The type of work to be done.

A22.   A thermal shunt.

A23.   Bright and shiny with no cracks or pits.

A24.   If the component is known to be defective or if the board may be damaged by normal desoldering.

A25.   By pushing it gently out of the board.

A26.   Heat each lead and lift with tweezers.

A27.   use a skipping pattern.

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A28.   Inspect and test.

A29.   Operational failures, repairs by untrained personnel, repair using improper tools, mishandling, improper shipping, packaging, and storage.

A30.   Clinched staple.

A31.   Epoxy a replacement pad to the board, set an eyelet, and solder it.

A32.   Repairs by untrained personnel and technicians using improper tools. A33.   Epoxy and fiberglass powder.

A34.   ESD, improper stowage, and improper handling.

A35.   To discharge any static charge built up in the body.

A36.   Deviation from prescribed safe operating procedures.

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