Par 902 Page 174 TABLE 9-2. TYPICAL PRIME CHARGE SOURCES Object or Process Material or Activity Work Surfaces Waxed, painted, varnished surfaces, vinyl, or plastics Floors Sealed concrete, waxed or finished wood Clothes Clean room smocks, synthetic personnel garments, nonconductive shoes, virgin cotton Chairs Finished wood, vinyl, fiberglass, packaging and handling Plastic bags, wraps, envelopes, tape, bubble wrap, foam, plastic trays, totes, boxes, vials, parts bins Assembly, Cleaning, Test Spray Cleaners, and Repair Areas Plastic solder suckers, solder irons with ungrounded tips, solvent brushes (synthetic bristles), cleaning or drying by fluid or evaporation, temperature chambers, cryogenic sprays, heat guns and blowers, sand blasting, electrostatic copiers
b. People are Prime Sources of ESD. Electrostatic charges generated by rubbing or separating materials are readily transmitted to a person’s conductive sweat layer causing that person to be charged. When a charged person handles or comes in close proximity to an ESD-sensitive component, that component can be damaged by a direct discharge when it is touched or by subjecting it to an electrostatic field. Table 9-3 shows typical electrostatic voltages generated by personnel.
TABLE 9-3. TYPICAL ELECTROSTATIC VOLTAGES Means of Static Generation Electrostatic Voltages 10 to 20% Relative Humidity 65 to 90% Relative Humidity Walking across carpet 35,000 1,500 Walking over vinyl floor 12,000 250 Worker at bench 6,000 100 Vinyl envelopes for work instructions 7,000 600 Plastic bag picked up from bench 20,000 1,200 Chair padded with polyurethane foam 18,000 1,500
903. COMPONENT FAILURE MODES.
a. Intermittent Failures. Intermittent failures of electronic components can occur due to ESD. These upset-type failures are often characterized by loss of information or temporary distortion of functions. No apparent hardware damage occurs, and proper operation resumes automatically after the ESD exposure. In the case of some digital equipment, proper operation resumes after reentry of information or re-sequencing of the equipment. Parts susceptible to
Par 903 Page 175 intermittent operation due to ESD are those in any logic family that require small energies to switch states or small changes of voltage in high-impedance lines. For example:
(1) E-channel metal oxide semiconductor (NMOS),
(2) P-channel metal oxide semiconductor (PMOS),
(3) Complementary oxide semiconductor (CMOS), and
(4) Low power transistor-transistor logic (TTL) items.
b. Catastrophic Failures. Catastrophic failures due to ESD are characterized by permanent damage to components which prevents them from operating. Most, but not all, catastrophic failures occur immediately after the ESD event. Catastrophic failures can be subdivided into two different categories:
(1) Direct Failures. Direct catastrophic failures are the result of an electrical overstress of electronic parts from ESD, a low voltage discharge from a person or object, an electrostatic field, or a high voltage discharge through a spark or arc. In some cases, catastrophic failures may not occur until a period of time has passed after the ESD exposure. Such may be the case when normal operating stresses and time are required to sufficiently damage a component and cause it to fail completely. Experience has shown that certain components seem to be susceptible to this type of failure mode. For example, an ESD discharge could result in an aluminum short circuit of SiO2 in the dielectric layer of an integrated circuit. Subsequent use, in time, could allow high currents flowing through the circuit to vaporize the aluminum and block current flow.
(2) Latent Failures. Latent failures due to ESD occur when a component is sufficiently damaged which shorten its operational life. The component may only be marginally damaged by the ESD event and continue to operate for some time. Degradation continues due to the damaged condition of the component and ordinary operational stress. When the component does fail, it may not be obvious that ESD was the original cause. In some cases, intermittent failures in which a component exhibits degradation may in fact be a catastrophic failure due to ESD in which physical damage has caused it to fail to operate throughout its design range.
904. IDENTIFICATION OF ESD-SUSCEPTIBLE EQUIPMENT.
a. Classes of Devices. Any component which can be damaged by 16,000 volts or less is considered sensitive to ESD. These components include microelectronics devices, discrete semiconductors, film resistors, resistor chips, other thick and thin film devices, and piezoelectric crystals. The three ESD-sensitive classifications are as follows:
(1) Class 1: Extremely sensitive: Ranges from 0 to 2 kilovolts (kV).
(2) Class 2: Sensitive: Ranges from 2 to 4 kV.
(3) Class 3: Less sensitive: Ranges from 4 to 16 kV.
Par 904 Page 176
b. Circuit Cards. ESD-sensitive components installed on circuit cards are still susceptible to ESD. For that reason, circuit card assemblies are treated as ESD sensitive.
Equipment containing circuit cards with ESD-sensitive components, such as computers, receiver/transmitters, digital display units, encoder/decoders, etc., require special treatment to prevent ESD from entering through connector receptacles and damaging sensitive components.
905. ESD PROTECTION REQUIREMENTS.
a. General. Electronic components are classified into three groups depending on ESD-sensitivity. Each group has different handling requirements. In facilities where numerous types of electronic assemblies and circuit cards are repaired, it is not always evident to which ESD-sensitivity group a particular assembly belongs. Consequently, most repair facilities use standard ESD protection procedures based on the most susceptible device they expect to repair.
The ESD workstation is an essential part of ESD protection and is the only safe location to repair, package, or handle ESD-sensitive components or circuit cards. The purpose of the workstation is to keep potential differences below the level that can damage ESD-sensitive components. This is accomplished in several ways. The bench top, floor mat, and a personnel wrist strap are electrically connected through resistors to ground. In addition, the floor mat, bench top, chair, component containers, and all other materials in the area are made from static dissipative material. No static generators (e.g., such as plain plastic wrap, Styrofoam, plastic coffee cups, etc.) are allowed in the area. Humid air helps to dissipate electrostatic charges by keeping surfaces moist and increasing surface conductivity. The workstation and surrounding area should be kept between 40 and 60 percent relative humidity for this purpose. Ionized air generators which produce both positive and negative ions may be used at the ESD workstation to dissipate any static charge. Personnel are often required to wear static dissipative smocks and should avoid wearing synthetic clothing under the smock. Figure 9-1 shows an ESD workstation.
ESD workstations should be periodically monitored to ensure all components are functional.
Par 905 Page 177 FIGURE 9-1. TYPICAL ESD WORKSTATION
1. Wrist Strap with Resistor
2. Heel Straps/Grounders
3. Static-Dissipative Work Surface
4. Ionizer/Monitor
5. Bench
6. Ground Cords
7. Lined Antistatic Tote with Cover
8. Materials Drawers Labels, Pouches, Bags, Accessories
9. ESD- Protective Roll Materials
10. Static-Dissipative Floor Mat *Special chairs/stools and garments are not shown.
b. Packaging. Circuit cards and components should be packaged in ESD-protective packaging prior to leaving the ESD workstation. Static shielding bags which have a static-dissipative inner layer and a conductive outer layer are used for this purpose. They should be noncorrosive and should zip-lock or heat seal closed. All static shielding bags are identified with an ESD caution sticker over the closed seals, so that broken stickers will become indicators of opened bags. Figure 9-2 shows typical ESD caution labels. Cushion wrap (bubble wrap) used
Par 905 Page 178 around circuit cards should also be made of static-dissipative material. Circuit cards may be packaged in reusable ESD fast pack containers. At the equipment level, conductive connector receptacle dust caps are used to prevent ESD from entering the equipment through the connector receptacle and damaging sensitive components. In cases where conductive dust caps are unavailable, a conductive grid tape may be used to cover connector receptacles.
c. ESD Protection Guidelines. Procedures to prevent ESD damage should start at the time electronic equipment is removed from the aircraft and continue through the packaging, shipping, inspection, repair, testing, storage, and eventual reinstallation to the aircraft.
(1) When removing ESD-sensitive equipment from the aircraft, the aircraft should be grounded and power removed. Prior to disconnecting the cables from the equipment, personnel should touch the metal case of the equipment to equalize any electrostatic potentials. Once the cables are disconnected, conductive dust caps or conductive grid tape should be placed on the connector receptacles. The conductive dust caps or grid tape should not be removed until the equipment is at an ESD workstation or test station where ESD protection is in place. When installing ESD-sensitive equipment on the aircraft, the outer shell of the cable connector should be touched to the outer shell of the equipment mating connector to equalize electrostatic potentials. ESD-sensitive equipment should not be opened to expose circuit cards anywhere other than an ESD workstation.
(2) Testing and repair of ESD-sensitive circuit cards and equipment at an ESD workstation should include the following ESD protection procedures:
(a) Ensure work areas, equipment, and wrist strap assembly are grounded.
(b) Attach wrist strap and place metal tools and accessories on grounded bench surface.
(c) Place conductive container on bench. Remove components and circuit cards from ESD protective packaging by unzipping or heating to open. Remove shorting device if present. Avoid touching leads on components. Place components and circuit cards on conductive work surface/test fixture.
(d) Compressed gases will not be used to cool fixtures.
(e) Test through connectors or tabs only.
(f) After testing, replace shorting packages and protective packaging.
(g) Only high input impedance multimeters and test instruments should be used (to avoid sharp current transients).
(h) Dielectric strength tests are prohibited.
Par 905 Page 179 FIGURE 9-2. TYPICAL ESD CAUTION LABELS
(i) U se only static-dissipative type solvents.
(j) Heat guns for test or curing are prohibited.
(k) Drying lamps, photo spots, and thermal probes are allowed.
(l) Do not remove components or assemblies from their sockets with power applied.
(m) The use of air to clean equipment or circuit cards is prohibited unless a filtered ionizing air gun is used.
Par 905 Page 180
(n) Do not use a solvent ultrasonic cleaning bath for component assemblies.
(o) Cure conformal coating materials in accordance with OEM instructions either by normal ambient curing or in an oven that contains grounding provisions to prevent static charge buildup. Oven temperature adjustments should be precise and accurate to prevent needless eddy flow of coating during curing.
(p) Compressed carbon dioxide or nitrogen should not be used to cool the test chamber or oven without grounding provisions.
(3) ESD Packaging Guidelines. Correct packaging for shipment or storage of ESD-sensitive components and circuit card assemblies should be accomplished at the ESD workstation. A conductive tote may be used to carry ESD-sensitive components and assemblies from the repair workstation to the packaging workstation if required. At no time during shipment or storage should packaging identified by an ESD symbol be opened unless at an ESD workstation. ESD-sensitive equipment should be shipped or stored with conductive dust caps or conductive grid tape over connector receptacles.
(4) ESD Repair Personnel Guidelines. All personnel who repair, package or handle ESD-sensitive components, circuit card assemblies or equipment should have formal ESD training. The most extensive and costly ESD workstations will not provide protection if people are not properly trained in correct practices. Manufacturers and repair station facilities should require some type of ESD training certification for their personnel.
906. CORROSION CONTROL PRACTICES FOR ESD-SENSITIVE DEVICES.
a. General. Some types of ESD packaging can be corrosive and should be avoided. ESD dustcaps for connector receptacles made from carbon should be avoided due to corrosive tendencies. To identify these dustcaps, rub the dustcap on a piece of paper. If a mark is made on the paper, the dustcap contains carbon and should not be used.
b. Corrosion Inspection and Repair. Corrosion inspection and repair of circuit card assemblies and ESD-sensitive equipment should be conducted at an ESD workstation where proper ESD precautions may be taken. Corrective action depends on the size, the degree, and type of corrosion, in the damaged area. Maintenance personnel should refer to the applicable Original Equipment Manufacturer’s (OEM) service directives for specific repair procedures or the treatment of specific avionics equipment identified in Chapters 6 and 8.
907 through 1000. RESERVED.
Par 1001 Page 181 CHAPTER 10. EMERGENCY ACTION FOR SERIOUS CORROSION OF AVIONICS
EQUIPMENT 1001. GENERAL.
a. This chapter describes emergency corrosion cleaning and treatment procedures to be followed after aircraft or equipment accidents and incidents, particularly those involving exposure to large amounts of saltwater, fire extinguishing agents, industrial pollutants, soot, etc.
Immediate action must be taken to remove, clean, dry, and preserve all affected avionics equipment to reduce corrosion damage. When removal of avionics equipment is impractical, in-place cleaning, drying, and preserving should be accomplished.
b. The emergency action procedures outlined in the beginning of this chapter are normally used by the operator only to prevent further corrosion damage. Equipment that has received initial emergency treatment should be forwarded to an avionics repair station for cleaning, drying, inspection, operational checks, and preservation, or returned to the Original Equipment Manufacturer (OEM) for disposition as described later in this chapter.
1002. EMERGENCY RECLAMATION TEAM.
NOTE: In cases involving aircraft accidents, permission to remove equipment must be obtained from the senior Department of Transportation (DOT) member of the accident investigation team prior to the start of emergency reclamation procedures.
a. Goal of an Emergency Reclamation Team. Each organization that operates, stores, maintains, or repairs avionics equipment should have a team identified to handle emergency reclamation situations. The primary goal of the emergency reclamation team is to accomplish those tasks that are necessary to salvage the affected equipment and perform appropriate corrosion control efforts to minimize damage.
b. Emergency Reclamation Team Organization. An emergency reclamation team should consist of a senior team member whose responsibilities include directing the salvage, removal priority, and corrosion control efforts for the equipment involved. The size and composition of the remainder of the emergency reclamation team will depend upon the urgency of the situation, type of reclamation effort, and the size of the reclamation task.
1003. EMERGENCY PREPARATIONS.
a. Removal Priority. Each organization that operates, stores, maintains, or repairs avionics equipment should publish an instruction that defines the role and purpose of the emergency reclamation team. The instruction should provide general guidelines for the team to accomplish their task in a safe and efficient manner. It should be reviewed by the emergency reclamation team members on a regular basis, and should include a priority list for the removal and reclamation of equipment and a list of all anticipated tools and materials necessary to accomplish the reclamation task. Table 10-1 provides a recommended guide for a priority removal list.
Par 1003 Page 182 TABLE 10-1. PRIORITY GUIDE FOR EMERGENCY REMOVAL OF AVIONICS
EQUIPMENT Priority Type Equipment
1. Avionics Equipment (radios, computers, radar equipment, etc.)
2. Instruments (aircraft instruments, meters, etc.)
3. Electrical Equipment (switches, wiring, indicator light panels, etc.)
4. Miscellaneous Equipment (mounting racks, etc.)
b. Required Tools, Materials, and Equipment. Immediate availability of the necessary corrosion control tools, materials, and equipment will help to significantly reduce additional damage to the affected equipment. In addition to the corrosion control products listed in Appendix 1, other special support equipment that will be useful and should be readily available are:
(1) Dry nitrogen source,
(2) Dry air source,
(3) Vented drying oven (forced air),
(4) Vented drying oven (bulb-type),
(5) Hot air blowers,
(6) Pump, backpack-style, and
(7) Clean empty 55-gallon drums (removable lid).
c. Production Planning. Whenever possible, all salvaged components of the aircraft or equipment should be treated simultaneously. The most experienced personnel on the emergency reclamation team should be assigned to disassemble and process the affected equipment. This effort will reduce the corrosion potential and other damage to the equipment and ensure that the work is accomplished in a thorough and competent manner. Whenever possible, personnel that normally examine and evaluate this type of equipment should work closely with the disassembly and preservation personnel. This enables non-reclaimable items to be scrapped immediately and only usable components that were exposed to corrosive agents to be disassembled and treated.
The time saved by this procedure may be used in preserving salvageable components.
1004. EMERGENCY CLEANING PROCEDURES.
CAUTION: Cleaning compounds and solvents identified in Appendix 1 may react with some encapsulants or plastics used to form wire harness tubing, wire coatings, conformal coatings, gaskets, seals, etc. Test these compounds and solvents on a small area for softening or other adverse reactions prior to
Par 1004 Page 183 general application. See Chapter 4, Table 4-3, for further restrictions on these materials.
a. General. Where possible, the primary method of emergency cleaning described in subparagraph 1004b should be used. When a sufficient quantity of fresh water is not available, use one of the alternate emergency cleaning methods described in subparagraphs 1004c(1) through (3).
b. Primary Method. The primary method for the removal of saltwater, fire extinguishing agents, etc., should be used when a sufficient quantity of fresh water is available. The primary method procedures are:
(1) Flush all internal and external areas with clean fresh water. Whenever possible, units or subcomponents that have been removed should be immersed and flushed thoroughly in clean fresh water. A 55-gallon drum may be used for this purpose. Tilt the unit or subcomponent back and forth to aid in draining off the excess water.
(2) Blow off excess water with not more than 10 pounds per square inch (psi) air pressure or dry nitrogen. Deflect jet of air off interior, back, and sides of enclosure to diffuse.
(3) If there is any evidence of salt or fire extinguishing agents, a second cleaning action should be initiated using a solution of 1 part Aircraft Cleaning Compound to 10 parts of fresh water, conforming to MIL-PRF-85570, Type II. Scrub the affected areas with the cleaning solution using a brush. Flush thoroughly with fresh water and drain excess. The equipment may be immersed in fresh water to aid in removing hidden contaminants. Tilt the unit or equipment back and forth to aid in draining off the excess water.
(4) Blow off excess water as specified in subparagraph 1004b(2).
c. Alternate Methods. The following describes alternate cleaning methods when sufficient fresh water is not available.
(1) Solvent Method.
NOTE: Local air pollution regulations may restrict the use of this and other solvents. Comply with all local air pollution regulations.
(a) Clean exterior of equipment using a brush and dry cleaning solvent, MIL-PRF-680, Type II.
(b) Collect waste solvent from exterior cleaning and dispose of by recycling or as a hazardous waste.
CAUTION: Properly dispose of all hazardous waste in accordance with local regulations.
(c) Disassemble equipment as required. Clean and dry interior surfaces in accordance with instructions of subparagraphs 1004b(3) and (4).
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(2) Aircraft Cleaning Compound Method.
(a) Apply a solution of 1 part Aircraft Cleaning Compound to 9 parts of fresh water, conforming to MIL-PRF-85570, Type I. Scrub or wipe interior and exterior surfaces of equipment using a brush or clean cloth until contaminants become intermixed or emulsified.
Wipe off all surfaces thoroughly with a clean cloth removing contaminants and cleaner.
(b) Blow off excess solution with not more than 10 psi dry air pressure or dry nitrogen. Deflect the jet of air off interior, back, and sides of enclosure to diffuse.
(3) Water-Displacing Method. The water-displacing method should be used as the last choice of the alternate cleaning methods. Additionally, the water-displacing method is considered only temporary preservation until proper cleaning methods can be accomplished.
After application of the water-displacing compound, tag equipment with an appropriate marking indicating the component has been temporarily preserved with Water-Displacing Corrosion Preventive Compound, conforming to MIL-PRF-81309, Type II, but still requires cleaning.
(a) Blow off excess water and other contaminants from component with not more than 10 psi dry air pressure or dry nitrogen. Deflect the jet of air off interior, back, and sides of enclosure to diffuse.
(b) Totally immerse equipment in a 55-gallon drum of Water-Displacing Corrosion Preventive Compound, conforming to MIL-PRF-81309, Type II. Tilt the unit or equipment back and forth to aid in removing any water. Immerse the equipment a second time, repeating the tilting to thoroughly coat all surfaces. If total immersion is not practical, spray, brush, or wipe the interior and exterior of equipment with Water-Displacing Corrosion Preventive Compound.
(c) Blow off excess Water-Displacing Corrosion Preventive Compound from component with not more than 10 psi dry air pressure or dry nitrogen. Deflect the jet of air off interior, back, and sides of enclosure to diffuse.
1005. EMERGENCY DRYING AND PRESERVATION.
a. General. Drying and preservation are essential to eliminate any traces of water and to control corrosion until the equipment can be disassembled, inspected, and repaired at an authorized repair station.
b. Drying and Preservation Procedures. The following drying and preservation procedures should be used with the listed drying equipment after cleaning in accordance with subparagraph 1004b.
(1) Vented Drying Oven (Forced Air).
(a) Blow off excess liquid from component with not more than 10 psi dry air pressure or dry nitrogen. Deflect the jet of air off interior, back, and sides of enclosure to diffuse.
(b) Dry the equipment in a vented forced air oven at a temperature of not more than 130 °F (54 °C) for 1 to 2 hours.
Par 1005 Page 185
(c) For all avionics components and electrical connectors, apply (by spraying) a coating of Water-Displacing Corrosion Preventive Compound, Ultra-Thin Film, conforming to MIL-PRF-81309, Type III.
(2) Vacuum Oven Drying.
(a) Blow off excess liquid from component with not more than 10 psi dry air pressure or dry nitrogen. Deflect the jet of air off interior, back, and sides of enclosure to diffuse.
(b) Dry the equipment in a vacuum oven with a temperature of not more than 130 °F (54 °C) and a minimum pressure of 25 inches of Hg (inHg) for 1 to 2 hours.
(c) For all avionics components and electrical connectors, apply (by spraying) a coating of Water-Displacing Corrosion Preventive Compound, Ultra-Thin Film, conforming to MIL-PRF-81309, Type III.
(3) Vented Drying Oven (Bulb-Type).
(a) Blow off excess liquid from component with not more than 10 psi dry air pressure or dry nitrogen. Deflect the jet of air off interior, back, and sides of enclosure to diffuse.
(b) Dry the equipment in a vented bulb-type drying oven at not more than 130 °F (54 °C) for 4 to 6 hours.
(c) For all avionics components and electrical connectors, apply (by spraying) a coating of Water-Displacing Corrosion Preventive Compound, Ultra-Thin Film, conforming to MIL-PRF-81309, Type III.
(4) Hot Air Blower.
(a) Blow off excess liquid from component with not more than 10 psi dry air pressure or dry nitrogen. Deflect the jet of air off interior, back, and sides of enclosure to diffuse.
(b) Dry the equipment with a hot air blower until dry.
(c) For all avionics components and electrical connectors, apply (by spraying) a coating of Water-Displacing Corrosion Preventive Compound, Ultra-Thin Film, conforming to MIL-PRF-81309, Type III.
(5) Heated Compartment.
(a) Blow off excess liquid from component with not more than 10 psi dry air pressure or dry nitrogen. Deflect the jet of air off interior, back, and sides of enclosure to diffuse.
(b) Dry the equipment in a heated compartment with proper air circulation at a temperature between 100 °F (38 °C) and 130 °F (54 °C) until dry.