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(d) Sealing Compound, Silicone Rubber, (RTV), conforming to MIL-A-46146, Type II is used for sealing electrical connectors. This compound is a less viscous (flows easily), single-component (one-part) compound. The different types of this compound are defined in Subparagraph 505d, Sealants.
(e) Sealing Compound, Fuel Resistant, conforming to SAE-AMS-S-8802 is used to prevent entry of corrosive elements, and where fuel and oil may be present. This polysulfide sealing compound consists of a base and an accelerator (curing agent) in a kit. This sealing compound has a maximum temperature of 275 °F (135 °C). The different types of this compound are defined in Subparagraph 505d, Sealants.
(f) Sealing Compound, Corrosion-Inhibiting, conforming to MIL-PRF-81733 is used to prevent entry of corrosive elements, and where fuel and oils may be present. This polysulfide sealing compound contains corrosion inhibitors, and consists of a base and an accelerator (curing agent) in a kit. This sealant has a maximum temperature of 250 °F (121 °C).
The different types of this compound are defined in Subparagraph 505d, Sealants.
(g) Sealing Compound conforming to MIL-M-24041 is used for sealing and reinforcing electrical connectors, wiring, and components in a seawater environment. This sealing compound is a two-component polyether polyurethane system consisting of a prepolymer and a curing agent. This sealing compound has good cold-flow properties and will adhere to metal, rubber, and polyvinyl chloride (PVC). The temperature range of this material is -80 °F to +300 °F.
(2) Reverting Potting Compounds. Some potting compounds have a past history of reverting to a liquid after a year or two. This reversion is highly dependent on the environment of the potting compound. Compounds that revert exhibit a sticky, viscous, oozing consistency that flows out of the connector back shell. In some cases, the reverted potting compounds flow around and through pins and receptacles, insulating the connection where continuity is required.
(3) Precautions. When using potting compounds, the following precautions should be followed:
(a) Thoroughly clean the area to be potted using an approved solvent.
(b) Follow manufacturer’s instructions when mixing the base compound and accelerator. Substitutions, partial mixing, or incorrect proportions of the base compound and accelerator may produce a sealant with inferior properties.
(c) Do not mix base compounds and accelerator components of different batch numbers because substantial electrical properties may result.
(d) Potting compounds may contain small quantities of flammable solvents, and/or may release by-products on curing. Observe all warnings and cautions, and use personal protective gear identified in the MSDS and by the potting compound manufacturer.
(e) Potting compounds that have exceeded their listed shelf life should not be used unless tested and certified by an approved laboratory.
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(f) Avoid contamination of the potting compound. Do not use masking tape and fiberboard molds around the connector during the potting compound cure. If potting molds are not furnished with the connector or are not available, a plastic sleeve should be constructed. The plastic sleeve will aid in forming the potting compound around the connector during the cure.
(g) Allow potting compounds to cure until firm prior to installing connector or components in equipment.
(h) Frozen pre-mixed potting compounds should be used as soon as possible after their removal from the freezer or a significant (up to 50 percent) reduction in working life can be experienced.
(i) Remove reverting or reverted potting as soon as possible.
c. Fungus-Proof Coatings. Fungus-proof coatings should not be applied indiscriminately to all electrical components. Treat only those components that have a specific need or are designated in the applicable manufacturer’s service directives. Fungus-proof coatings can, in some instances, be detrimental to the function and maintenance of equipment. For example, fungus-proof coatings deteriorate wire insulation, and their removal for maintenance action is labor-intensive. A repair activity should re-coat an entire area only when a touch-up of the previously coated component will not provide the required protection. Many hours of repair time will be required if the fungus-proof coating is not maintained.
(1) Fungus-Proof Varnish. The authorized fungus-proof coating is a varnish conforming to MIL-V-173C. These varnishes should be thinned, if necessary, with an approved solvent per the manufacturer’s instructions or with a thinner conforming to A-A-3007.
(2) Items That Should Not Receive Fungus-Proof Varnish. Varnish should not be applied to any surface where its application will interfere with the operation or performance of the equipment. Those surfaces should be protected during application of fungus-proof varnish with pressure-sensitive tape. The following items should be protected from varnish application:
(a) Components and Materials.
1. Cable, wire, braids, and jackets that are flexed during operation of the equipment.
2. Cables where treatment would reduce the insulation resistance below, or increase the loss factor above, the acceptable service limits provided by the OEM.
3. Variable capacitors (air, ceramic, or mica dielectric).
4. High wattage and wire wound resistors.
5. Ceramic insulators that are subject to an operating voltage over 600 volts and in danger of flashover.
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6. Painted, lacquered, or varnished surfaces, unless otherwise specified by the OEM.
7. Rotating parts such as dynamotors, generators, motors, etc. Electrical components that are associated with the rotating component may be treated in accordance with the procedures outlined in this AC.
8. Wave guides (mating surfaces).
9. Electron tubes.
10. Tube clamps.
11. Miniature tube shields.
12. Plug-in relays.
13. Pressure-contact grounds.
14. Coaxial test points or receptacles.
15. Windows, lenses, etc.
16. Transparent plastic parts.
17. Plastic materials such as polyethylene, polystyrene, polyamide, acrylic, silicone, epoxy (other than printed wiring boards), melamine, fiberglass, fluorocarbon, vinyl, and alkyds.
18. Materials used for their specific arc-resistance properties and classified as such.
(b) Electrical contacts, contact portions, or mating surfaces of binding posts. Also connectors, fuses, jacks, keys, plugs, and relay sockets (including tube sockets, switches, and test points).
(c) Mechanical Parts.
1. Bearing surfaces (including bearing surfaces of gaskets and sliding surfaces).
2. Gear teeth and gear trains or assemblies.
3. Pivots and pivot portions of hinges, locks, etc.
4. Screw threads and screw adjustments (those moved in the process of operation or adjustment).
5. Springs, except at base of pile-up.
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(d) Surfaces which rub together for electrical or magnetic contact, such as bearings, contact fingers, potentiometers, shafts, shields, and variable auto-transformers.
(e) Surfaces whose operational temperatures exceed 266 °F (130 °C), or whose operating temperature will cause carbonization or smoking.
(f) The exterior or visible outside portion of indicating instruments (do not open or treat inside), control boxes, or other equipment mounted in the cockpits of aircraft.
(3) Methods of Application. The varnish coating may be applied by spraying, brushing, dipping, or any combination of these processes. The dried film should have a clear, smooth finish (free from bubbles, wrinkles, filaments, or spray dust). Runs, puddles, or gathering of the film into drops should be avoided. The dry film thickness should be at least 0.002 inch unless otherwise directed in the manufacturer’s service directives.
NOTE: All surfaces to be coated should be free of dirt, grease, and other foreign matter. Components that cannot be cleaned satisfactorily or that show evidence of corrosion should be cleaned and treated per the instructions in this AC or replaced by acceptable components. Local air pollution regulations may restrict the use of fungus-proof varnish. Comply with all local air pollution regulations.
CAUTION: Avoid varnish adhesion failures. Apply only on clean, dry surfaces with a temperature of less than 100 °F (38 °C).
(a) Spraying. For larger equipment, a pressure-pot spray gun system with the tip regulated to give a wet spray is recommended. For small, compact equipment, a pencil spray tip, regulated to give a narrow, wet spray, is recommended. The varnish should be applied in a continuous wet coat over all parts to prevent the formation of fuzz or filaments. Incomplete wetting of the component surface with varnish will produce a dry spray. The dry spray texture appears as dust on the surface of the component. Dry spray is an unacceptable finish. The equipment or individual assembly should be sprayed from as many angles as necessary to ensure complete coverage with a wet coat. If more than one coat of varnish is necessary or required by the manufacturer’s service directives, allow sufficient drying time between coats. Refer to the varnish manufacturer’s time/temperature instructions.
(b) Brushing. All parts which cannot be reached by spray may be coated with a brush. A brush may also be used to cover small areas not covered during the spraying process.
On components that require extensive masking, brush application may prove more efficient than spray application.
(c) Dipping. Subassemblies or components may be coated by dipping, provided all requirements of subparagraphs 505c(2) and 505c(5) are met.
WARNING: Components that are to be enclosed in airtight cases (hermetically sealed) should be allowed to air-dry for at least an additional 24 hours after the varnish dries. Fumes (outgassing) may accumulate to dangerous levels within the case and may be ignited by an ignition source.
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(4) Drying of Coated Equipment. Equipment coated with varnish should be dried by heating to 130 °F (54 °C). Heating should be gradual to prevent shrinking, cracking, warping, or other deterioration of the parts or materials. The drying temperature should be maintained for at least 1/2 hour but no longer than 3 hours. The drying process may be done in a vented oven, vacuum oven, or with a hot air gun.
(5) Special Precautions. When varnish is to be applied on certain types of equipment, special precautions are required. The following paragraphs list equipment and parts requiring special precautions.
(a) Radio Receivers and Transmitters. The application of varnish will cause changes in some of the circuit constants. These changes may be discernible only by electrical tests and measurements. A change in alignment may be noted immediately after application of the varnish. As the varnish dries and ages, further changes in circuit constants may take place.
The greatest change ordinarily will occur within 72 hours after treatment. The set should be completely realigned at the end of that period.
(b) Coil Shields. When coil shields are removed and replaced, they can be damaged and alter the tuning adjustments. If there is significant damage, proper alignment may be impossible. Extreme care should be exercised in removing and installing coil shields.
(c) Trimmer Capacitors. Avoid spraying or brushing varnish on the plates of trimmer capacitors. To minimize damage, all trimmer capacitors should be masked completely during varnish application. If the variable capacitor fails to operate satisfactorily after treatment, make a thorough inspection for varnish deposits.
(d) Tuning Slugs. If varnish is accidentally applied to a tuning slug, the tuning slug will normally require replacement. Extreme care should be taken in removing and replacing tuning slugs.
(e) Discriminator Circuits. Careful adjustment of discriminator circuits after applying varnish is essential, especially in the case of frequency-modulated receivers.
Discriminator circuits are more susceptible to varnish-induced changes in circuit constants than any other component.
(f) Tuned Circuits. Be especially careful during masking to ensure the wires associated with the tuned circuit are not moved. Movement of such wires may change the circuit value.
(g) Relays. Deposits of varnish on the armature, pivot, or similar components will cause the relays to bind. Mask the entire relay carefully prior to general component varnish application. After the masking has been removed, brush-apply varnish as required to coils and leads. Relays with palladium-tipped contacts should be removed before application of varnish.
(h) Meters. Since meters are easily damaged by the varnish spray treatment, all meters should be checked for accuracy before treatment. Some meters may be affected by the heat of the drying process. In other instances, meter magnets may be affected by magnetic fields
Par 505 Page 90 that exist around drying equipment. Refer to the OEM applicable service instructions for varnish treatment. If guidance is not available, do not apply varnish.
d. Sealants. Sealants are another type of protective film used in avionics equipment.
Sealants are usually provided in the uncured form as a liquid or paste which solidifies (cures) after application. Sealants can be applied by hand methods, such as brush or spatula, or by air or mechanically powered filleting/injection guns. Sealants form a flexible seal in gaps and depressions, preventing moisture intrusion at mechanical joints, spot welds, and threaded closures. In addition, they prevent entry of corrosive environments into faying surfaces, fastener areas, exposed landing gear switches, and other metal-encased avionics equipment. They function principally as waterproof barriers. Therefore, when an inspection notes damaged sealant, it should be removed, the area inspected for damage, and new sealant applied. The following describes various types of sealant available.
(1) Sealing Compound, Polysulfide, Conforming to MIL-PRF-81733. A two-part, dichromate cured, polysulfide sealant with added soluble chromates which inhibit corrosion.
This sealant is intended for use at faying surfaces subject to galvanic action and temperatures up to 250 °F (121 °C). This sealant has excellent resistance to shrinkage, aircraft fuels, and lubricating oils. It also resists deterioration from saturation with ester-type hydraulic fluids. The mixed sealant may be applied by brush or spatula. The sealant is provided in four types which allow long assembly times.
(2) Sealing Compound, Polysulfide, Low Temperature Curing, Conforming to SAE-AMS-S-83318. A two-part polysulfide sealant characterized by an excellent cure rate at low temperature. This sealant is intended for quick field repairs to other sealants. Apply with brush.
(3) Sealing Compound, Polysulfide, Conforming to SAE-AMS-S-8802, Type A. A brush-applied, room-temperature cured sealing compound that is used for sealing gaps, seams, and faying surfaces. This sealant has excellent adhesion to most aircraft materials; excellent resistance to water, aircraft fuels (aviation gas and jet fuel), and petroleum-based oils; good resistance to deterioration from saturation with diphosphate ester-type hydraulic fluids; and can withstand temperatures up to 250 °F (121 °C). Four different sealants are available with application life and cure times from 1/2 to 4 hours. Type B sealants are spatula-applied compounds with the same characteristics as Type A.
(4) Adhesive Sealant, Silicone, RTV, Noncorrosive, Conforming to MIL-A-46146, Type III. This one-part adhesive sealant is used for encapsulating and sealing electrical and electronic components where the avionics equipment will operate at temperatures between 250 °F (121 °C) and 350 °F (177 °C).
(5) Adhesive Sealant, Silicone, RTV, Conforming to MIL-A-46146, Type I. This general purpose, one-part adhesive sealant is used for encapsulating and sealing electrical and electronic components. This material has good resistance to oxidation, weathering, and water.
CAUTION: A large number of RTV silicone sealants contain an acetic acid curing agent. These sealants, in contact with metal, result in rapid corrosion.
Par 505 Page 91 RTV sealants that contain acetic acid are not authorized for use on electronic or electrical circuits. They may be identified in most cases by a vinegar odor from the dispenser tube or while curing.
(6) Sealants Containing Acetic Acid. Table 5-2 lists some of the RTV silicone sealants that are considered corrosive and should not be used in avionics equipment.
(7) Adhesion Promoters. Some sealing compounds may require the application of a special primer or adhesion promoter prior to the sealant application, in order to develop a good adhesive bond with the surface. Use only those primers or adhesion promoters recommended by the manufacturer for their product.
(8) Sealant Application Procedures.
(a) Cleaning. Preclean the surface where sealant is to be applied per Chapter 4, paragraph 405 to remove dirt, grime, etc. Perform a final wipe of the area with an approved solvent, recommended by the sealant manufacturer, using a clean cheesecloth. While the surface is still wet with solvent, wipe dry with a clean, dry piece of cheesecloth.
(b) Masking. To prevent sealant from contacting adjacent areas during application and to aid in post-application smooth out, mask the area to be sealed with pressure-sensitive tape.
(c) Primers or Adhesion Promoters. Apply sealant primer or adhesion promoter when recommended by, and in accordance with, the sealant manufacturer’s instructions. Allow the surface to air dry undisturbed for 30 minutes to one hour prior to sealant application.
(d) Sealant Mixing. The proper mixing of the base and accelerator sealant components is essential for proper curing and adhesion of the sealant. Mixing should be accomplished in a clean environment per the manufacturer’s instructions. The accelerator and base components should be added together after the proper amounts are determined by weight or volume, then mixed until a uniform color is obtained. Two-part sealants are chemically cured and do not depend on solvent evaporation for curing.
Par 505 Page 92 TABLE 5-2. CORROSIVE SILICONE SEALANTS, ADHESIVES, AND COATINGS RTV 102 RTV 192 RTV 999 RTV 92-055 RTV 103 RTV 198 RTV 1890 RTV 94-002 RTV 106 RTV 236 *RTV 3144 RTV 94-003 RTV 108 RTV 730 *RTV 20-046 *RTV 94-009 RTV 109 *RTV 731 *RTV 20-078 *RTV 94-034 RTV 112 RTV 732 RTV 30-079 *RTV 96-005 RTV 116 *RTV 733 *RTV 30-121 *RTV 96-080 RTV 118 RTV 734 *RTV 4-2817 RTV 96-081 *RTV 140 RTV 736 *RTV 90-092 RTV Q3-6069 *RTV 142 *RTV 780 *RTV 92-005 RTV Q3-6090 RTV 154 *RTV 781 RTV 92-007 RTV Q4-2817 RTV 156 RTV 784 RTV 92-009 RTV 92-010 RTV 157 *RTV 785 *RTV 92-018 SCS 100 RTV 158 RTV 786 *RTV 92-024 RTV 159 *RTV 891 *RTV 92-048 *Products discontinued by the manufacturer, but may still be available from approved sources.
NOTE: RTV 730 is a high temperature (550 °F to 600 °F) corrosive sealant that may be required to seal engine compartment electrical connectors.
RTV 730 should be used only when specified by the OEM or authorized engineering authority.
(e) Application Life. Once mixed, two-part sealants have an application life. The application life of a sealant is the length of time that a mixed sealing compound remains usable in standard conditions of 70 °F (24 °C) and 50 percent relative humidity. Generally, the dash number of the sealant manufacturer’s part number is the application life.
(9) Sealant Application. Apply sealant within the application life for the sealant being used. Thick sealants may be applied with a nonmetallic spatula or scraper. Avoid entrapment of air. Work sealant into recesses by sliding the edge of the spatula or scraper firmly back over the recesses. Smoothing of the sealant surface will be easier if the spatula or scraper is first dipped in water. Sealant applied with a brush is applied and smoothed by the brush until the desired thickness is reached. Sealant applied with an air or mechanically powered filleting/injection gun, depending on the application, may not require masking and is especially suited to filling seams, fay sealing, and creating form-in-place door seals. When applying a fay seal, ensure there is adequate sealant squeeze-out between the mating parts.
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e. Conformal Coatings. Conformal coatings are generally two-part coatings that are applied in a thin coat to and over PCBs and other components. Conformal coatings protect PCBs and components from moisture, fungus, and thermal shock. They also protect the components from fatigue failure by their encapsulating properties. The coatings are flexible, flame resistant, and are suitable for application to PCBs by dipping, brushing, spraying, or vacuum deposition.
The following types of conformal coatings are available: acrylic resin (AR), epoxy resin (ER), silicone resin (SR), polyurethane resin (UR), and paraxylylene (XY). Coating thickness is dependent upon the type of coating system. For AR, ER, and UR coatings, the proper thickness is 0.002 plus or minus 0.001 inch; for SR coatings, 0.005 plus or minus 0.003 inch; and for XY coatings, 0.0006 plus or minus 0.0001 inch.
(1) Conformal Coating Removal. Conformal coatings must be removed prior to component removal from the PCB. The following describes the degree of recommended coating removal and various removal methods:
(a) Removal Area. Remove only as much coating as is necessary to facilitate removal of the discrepant component. Remove the coating from all solder connection areas to allow for desoldering. Figure 5-1 shows a typical step-by-step process for the removal of a component from a conformal-coated PCB.
NOTE: Do not attempt to remove all the coating down to the laminate surface. A thin residual layer will not interfere with component removal.
Par 505 Page 94 FIGURE 5-1. CONFORMAL COATING REMOVAL C AUTION: Do not attempt component removal until the component leads are completely free of solder, as damage to the PCB could occur.
1. Prior to desoldering, remove the coating from along all sides of the affected component, below the widest profile of the component.
2. To debond the component, heat the component body to release it from any residual coating or bonding compound.
3. Remove any residual remaining coating that may interfere with the replacement component positioning, soldering, or post-soldering cleaning.
Par 505 Page 95 TABLE 5-3. REMOVAL METHOD PREFERENCE ON SPECIFIC COATINGS
REMOVAL METHODS Virgin Coating Type Chemical Controlled Heat Thermal Parting Controlled Heat Hot Air Abrasive Dental Burrs Abrasive Disc & Bullets Abrasive Rotary Brushes Abrasive Dental Tools Acrylic (AR) 1* 2 5# 4# 3 Epoxy (ER) 2# 1* 3 5# 4 Urethane (UR) 2# 1* 3 4# Silicone (SR) 3 2 Paraxylylene (XY) 2 1 5 4 3 *Denotes method that best identifies generic type of conformal coating.
#For thin coating only.
(b) Removal Methods. There are three basic methods to remove conformal coatings. There is no preferred method or combination of methods. The method(s) employed will depend upon coating thickness, circuit density, and coating transparency. Table 5-3 lists the suggested removal methods for the various type of conformal coatings.
1. Chemical. This method is effective on AR coatings (lacquers and varnishes) which may be dissolved by either an approved solvent recommended by the coating manufacturer; 1, 1, 1 Trichloroethane, MIL-T-81533; or Isopropyl Alcohol, TT-I-735. These solvents are also effective on some SRs. They may dissolve or swell the silicone resins, allowing removal. They should be applied with a solvent-saturated cotton swab or acid brush. Gently rub the affected area and blot the solvent residue with a clean cotton swab. Repeat as required until the coating is removed. The final step is to neutralize the area with distilled or deionized water and blot dry with disposable tissue.
NOTE: Local air pollution regulations may restrict the use of these solvents.
Comply with all local air pollution regulations.
WARNING: Solvents are flammable and toxic to the eyes, skin, and respiratory tract. Skin and eye protection is required. Avoid repeated or prolonged contact. Use only in well-ventilated areas. Keep away from open flames or other sources of ignition.
2. Controlled Heat. Heat is effective on all resin coatings which will soften or revert (over cure) when exposed to a controlled, localized, low temperature (300 °F to 400 °F) heat source. The heat source method may also be effective for the release of components from the conformal coating or adhesive bonding of compounds beneath component bodies. This method is recommended on thicker coatings (0.025 inch or greater) because thin coatings offer little thermal protection to the PCB laminate. The use of either low pressure hot air or thermal parting is recommended. The area that is heated should be held to a minimum. After the coating
Par 505 Page 96 has softened, gently push the residue aside with a plastic scraper or thermal parting tip. Scraping and peeling in conjunction with applied heat is effective on all resin coatings except silicone RTVs.
CAUTION: Excessive heat can cause damage to components and PCBs. Use only the minimum temperature which will assist in the removal of the component.
3. Abrasion. Abrasive removal is effective on all resin coatings. The type of abrasive removal tool and method (motorized or manual) is dependent upon the thickness, hardness, surface contour, and transparency of the coating. Motorized abrasive cutting with dental burrs is effective on transparent, thick, and hard coatings of PR and UR. Abrasive discs are effective on all transparent, thin, hard coatings with flat surfaces. Dental brushes are effective on soft to semi-hard coatings with irregular surfaces. Manual abrasive dental tools, carvers, knives, and chisels are effective on thin, soft to semi-hard coatings where the removal area is small.
CAUTION: Exercise extreme care when using abrasive methods. Abrasive tools have a high potential for causing damage.
(2) Conformal Coating Replacement. Preparation of the laminate PCB surface is the most important part of the conformal coating application process. Most adhesion failures are the result of improper cleaning and surface preparation.
(a) Cleaning. Prior to the replacement of the conformal coating, all repair actions residues (dirt, grime, soldering flux, etc.) should be removed. Clean the PCB laminate surface or component in accordance with the instructions in Chapter 4, paragraph 405.
(b) Drying. Dry the PCB or component in accordance with the instructions in Chapter 4, paragraph 406. Set temperature to 130 °F (54 °C) and dry the PCB or component for 1 hour.
(c) Stabilization. Allow the PCB or component to stabilize at room temperature, 77 °F (25 °C), for 1 hour. Coatings that are not recommended for accelerated curing above room temperature may be adversely affected by preheated laminates.
(d) Preservation. Cleaned and dried PCBs or components should be stored in a clean, static-free plastic bag if the PCB or component is not conformal coated within 30 minutes after reaching the stabilized room temperature. Seal the bag to prevent moisture from forming on the PCB or component.
(e) Pretreatment. Some conformal coatings require an additional treatment prior to coating application.
1. XY-coated assemblies require a pretreatment adhesion promoter to increase the bond strength of the replacement coating. Follow the coating manufacturer’s adhesion promoter and application instructions. Ensure a 0.25 inch (6 cm) overlap of any existing coating.