Par 805 Page 169 torn if care is not taken with them. Fortunately, most vent screens are heavy stainless steel and not easily damaged.
(2) Deformation. Connector receptacles attached to structure and brackets may be deformed, or the brackets deformed, if the mounting screws are over-tightened, or if the bracket is not thick enough. Beryllium copper spring fingers can also be bent or deformed such that they do not contact the structure with enough pressure to seal the joint against EMI. This can also be true for bulb seals and conductive elastomer gaskets if they are pressed past the point where they can spring back (compression set).
(3) Corrosion. Corrosion is one of the most common problems associated with EMI protection for two reasons: finishes over metal are often removed during bonding preparation to ensure good electrical conductivity, and many conductive materials used for EMI protection system are dissimilar to the aircraft structure. The two types of corrosion which can occur are electrolytic and galvanic. Chapter 2 provides a discussion on these types of corrosion.
Electrolytic corrosion occurs when a net direct current (DC) is applied between metals in the presence of a conducting fluid (electrolyte). The rate of corrosion depends upon the amount of current and the nature of the electrolyte. Galvanic corrosion occurs when a potential difference exists between two dissimilar metals in the presence of an electrolyte causing a current to flow.
The current is made up of electrons sacrificed from one of the metals. The rate of corrosion depends on the electrochemical potential between the two metals and the conditions under which contact is made. Most corrosion associated with EMI protection is galvanic since dissimilar metals are used. Table 2-3 lists materials with respect to galvanic corrosion potential. The closer together the materials are on the list, the less potential difference can develop between them when they are placed together with an electrolyte. Consequently, corrosion will occur slowly.
Materials at the extremes of the list will develop a high potential difference when placed together. The materials at the top of the list will be the ones to sacrifice electrons and will exhibit corrosion. Unfortunately, aluminum, which is used most often in aircraft structure, is higher on the list than materials used in EMI protection devices. Consequently, it is important to inspect areas where EMI protection is installed to ensure structural components are not corroding. In some cases, to protect the aluminum aircraft structure from corrosion, a sacrificial material such as tin/zinc is applied to the structural side of the EMI joint. The EMI gasket or spring fingers contacting this material make the required metal-to-metal contact for EMI protection and at the same time protection is provided to the aluminum structure. Consequently, corrosion at the EMI joint will be slower and the sacrificial material can be replaced before corrosion attacks the aircraft structure. Because an electrolyte is required for corrosion to occur, environmental seals are often used in conjunction with EMI protection to prevent moisture from contacting the metal-to-metal EMI joint. Conductive coatings on aluminum, such as alodine coatings, should be used, instead of nonconductive anodize coating.
b. Inspection Procedures. Inspection procedures for EMI protection devices and associated corrosion include visual inspections and EMI testing.
(1) Visual Inspections. The condition of avionics enclosures, bonding straps, shields, shield terminations, structural joints, gaskets, spring fingers, and conductive coatings on composites may be assessed during visual inspections. Wherever EMI protection is installed, it is imperative that periodic visual corrosion inspections be performed. Where sacrificial coatings are
Par 805 Page 170 used, inspection and re-application will be necessary on a periodic basis as well. It may be necessary to remove some EMI protection to inspect aircraft structure for corrosion if there is a history of it or if the aircraft has been exposed to salt spray. Electronic equipment bonding pressure-wiper fingers and bonding straps should be visually inspected when the equipment is removed and replaced. Broken or damaged bonding devices should be replaced. Metal overbraid on wiring in external areas such as wheel wells should be periodically inspected for corrosion.
The limits for corrosion on EMI protection features is very dependent on the aircraft and system design, and should be specified in the aircraft and component maintenance manuals.
(2) EMI Testing. Some EMI protection failures cannot be detected through visual inspection. An example is wire overbraid which is covered with an opaque jacket. Shield corrosion under the jacket cannot be detected visually. Compression set may occur in conductive elastomer gaskets, on avionics enclosures, where the gasket has been deformed to the point that it no longer seals against EMI. Structural bonding for P-static and lightning protection cannot be visually inspected without major aircraft disassembly. High-impedance bonds between electronic equipment and aircraft structure also cannot be visually detected. Circuit components such as filters should be tested to ensure correct operation. Electromagnetic vulnerability testing, in which the aircraft is radiated with EMI and aircraft electronic systems are monitored for failures, is costly and unlikely to be conducted unless major problems are suspected. Even simpler EMI testing requires some specialized equipment. Consequently, unless in-flight EMI problems are reported, testing will probably not be conducted. The following describes some types of testing which may be conducted.
(a) DC Resistance Test. Bonding of electronic equipment to aircraft structure is commonly verified by measuring the DC resistance between the equipment case and the aircraft structure with a low resistance ohmmeter. In order to make an accurate measurement paint must be removed from the equipment case structure where the measurement is taken. These areas must then be refinished. DC resistance is not an accurate indication of how well the bond performs at alternating current (AC) frequencies. Rather than measuring DC resistance to verify a bond, it is recommended that the bond be cleaned of contaminants or corrosion, which may cause high resistance on a regular basis.
(b) Shield Continuity Tests. DC resistance measurements may be used to measure the resistance and continuity of wire shields and shield terminations. Again, in order to make an accurate measurement, the insulation or finishes over the shields and terminations must be pierced or removed. DC resistance measurements may not effectively detect high resistance shields or terminations if there are multiple conductive paths for the shield. This is particularly true for shielded wire bundles with multiple branches and connectors.
(c) Interfering Signal Analysis. For EMI emissions, many times the most effective means of determining the source of the emissions is by using a spectrum analyzer and appropriate probes. The spectrum analyzer is used to measure the frequency and amplitude of the interference. Clamp-on current probes may be used to detect conducted emissions on wires and wire bundles. Small receiving antennas may also be used to detect radiated emissions. The spectrum analyzer may also be connected to the antenna port at the susceptible RF receiver rack connector to measure the level and frequency of the interfering signal. Training is required to
Par 805 Page 171 properly set up and monitor the spectrum analyzer, and to pick out the EMI interference from normal internal and external electromagnetic fields.
(d) P-Static Test. Structural bonding degradation is usually suspected when there is static in communication systems. A P-static test can be conducted to determine where the degradation has occurred. This test deposits a simulated P-static charge on the aircraft using a handheld ion discharge wand. As the wand is moved slowly across the structure, a receiver detects the noise associated with degraded bonds. Once the area of the bond degradation is located and isolated by visual inspection, a reason for the problem should be determined. The P-static test is a high voltage test which requires special training to ensure aircraft and personnel safety.
(e) Transient Anomalies. Electronic equipment EMI protection circuit component failures usually cannot be isolated on the aircraft and may be transparent to equipment operation if no EMI is present. Periodic anomalies associated with electronic equipment operation may be EMI related, and the equipment should be replaced and sent for diagnostic testing and repair.
c. Repair Procedures. Repair procedures related to EMI protection and associated corrosion depend on the type of EMI protection involved, and the degree and type of corrosion.
Beryllium copper gaskets, spring fingers, and bonding pressure-wiper fingers that are broken or damaged beyond tolerable limits should be replaced. Damaged bulb seals and ground straps should also be replaced. Since metal-to-metal contact is required, surfaces should be cleaned carefully to remove any primer, paint, grease, or corrosion prior to replacement. In some applications, after the replacement of EMI protection, an environmental seal is installed to protect the metal-to-metal joint from corrosion. Torn screens may be repaired by stitching them together as long as none of the metal mesh is missing. Conductive elastomers may have damaged sections replaced without replacing the entire gasket. Sections of metal wiring overbraid can be replaced if corroded or damaged. Maintenance personnel should refer to the applicable Original Equipment Manufacturer’s (OEM) service directives for specific repair information. The following general instructions apply when corrosion is noted:
(1) When corrosion is observed, disassemble or remove only the affected area;
(2) Remove corrosion using the mildest available method; and
(3) Apply protective finishes, and assemble as required.
NOTE: See Chapter 4 for cleaning and surface preparation; Chapter 5 for corrosion removal, surface treatment, painting, and sealing; and Chapter 6 for treatment of specific installations.
806 through 900. RESERVED.
Par 901 Page 172 CHAPTER 9. EFFECT AND TREATMENT OF CORROSION ON ELECTROSTATIC
DISCHARGE SENSITIVE EQUIPMENT
901. DEFINITION/DESCRIPTION OF ELECTROSTATIC DISCHARGE (ESD). ESD is a transfer of an electrical charge between bodies of different electrostatic potentials. ESD is caused by direct contact or induced by an electrostatic field. Static electricity is a potential electrical charge at rest. Static electrical charges can accumulate on electrical insulators and ungrounded conductors. A static electrical charge will build up as a result of triboelectric (frictional) activity which transfers electrons from one material to another. When two materials are physically rubbed together or pass close to each other, or where one material flows relative to another (such as a gas or liquid over a solid), electrons are transferred between the materials. One of the materials gains electrons and becomes negatively charged while the other loses electrons and becomes positively charged. A potential difference exists between the negatively and positively charged objects and between the charged objects and ground. This potential difference may increase due to a change in capacitance as the objects are moved. Capacitance is defined as the ratio of the magnitudes of the total charge on either conductor to the potential differences between conductors. The equation, Charge = Capacitance x Potential Difference, illustrates this relationship. Since the charge is constant, the potential difference will increase as the capacitance decreases. For example, when a polyethylene bag is rubbed, the potential difference between it and another charged object or ground may be only a few hundred volts while it is laying on a bench. However, when it is picked up the potential difference may increase to several thousand volts due to a decrease in capacitance. As the capacitance of a charged object decreases, the potential difference between it and another object will increase until an electrostatic discharge occurs via an arc. Static charges discharged near sensitive electronic components may cause damage. The electrical potential generated during discharge can be as high as 25,000 volts; more than 1,000 times the minimum voltage required to damage the most sensitive electronic components. The threshold of sensitivity for a human to feel a static discharge is approximately 3,500 volts. Therefore, ESD-sensitive components can be damaged by maintenance personnel without their knowledge.
902. SOURCES OF STATIC CHARGE.
a. Triboelectric Series. A triboelectric series, as seen in Table 9-1, is a list of materials and substances in order of positive to negative charging as a result of the triboelectric effect. A substance listed higher on the triboelectric series list develops a positive charge when rubbed with the lower listed substance. This happens because the higher listed substances have more free electrons compared to lower listed substances. Electrons from those substances positioned higher on the list may be transferred to substances positioned lower on the list when the charge is high enough. The order of ranking in a triboelectric series is not always constant or repetitive and the degree of separation of the two substances in the triboelectric series does not necessarily indicate the magnitude of the charges created by triboelectric effect. The magnitude of the charge is dependent upon numerous properties and the nature of the material or substance. These properties can also be modified by other factors such as surface cleanliness, ambient environmental conditions, contact pressure, speed of rubbing or separation, lubrication, and the amount of surface area over which the rubbing occurs. Table 9-2 lists some common sources of static electricity.
Par 902 Page 173 TABLE 9-1. TRIBOELECTRIC SERIES (PARTIAL) Most Positively Charged Air Human hands Glass Mica Human Hair Nylon Wool Fur Lead Silk Aluminum Paper Cotton Steel Wood Hard Rubber Nickel Copper Brass Silver Gold Platinum Acetate Rayon Polyester Celluloid Orlon Polyurethane Polyethylene Polypropylene PVC (vinyl) Silicon Teflon More Negatively Charged
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.