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2. SR-coated assemblies require a pretreatment primer to increase the bond strength of the replacement coating. Follow the coating manufacturer’s application instructions.
Ensure a 0.25 inch (6 cm) overlap of any existing coating.
(f) Selection of a Replacement Coating. The selection of a replacement coating is based on the compatibility of the replacement coating with that of the original coating on the PCB or component. Refer to the OEM drawings or repair manual.
(g) Conformal Coating Mixing. Mix the replacement conformal coating per the coating manufacturer’s instructions. Ensure the components are mixed in a clean container. Any air bubbles generated during the mixing must be removed. To remove the air bubbles, place the mixture in an anaerobic incubator (vacuum oven) at room temperature and 28 in/Hg for 30 minutes.
NOTE: Some two-part conformal coating resins require measurement by weight and not by volume.
CAUTION: Increasing or decreasing the mixture ratios of the conformal coating components will result in degradation of the coating properties.
(h) Application of Conformal Coating. The selection of a conformal coating will depend upon the area to be coated and any existing coating. The coating should be applied to a uniform thickness, with a 0.25 inch (6 cm) overlap of any existing coating, and no air bubbles.
1. Spraying. Spraying is one of the best methods when more than just a spot touch-up is required. Spraying is particularly useful when the surfaces are uneven or irregular, or there is a high density of components on the PCB.
2. Brushing. Brushing is one of the best methods for minor patching of existing conformal coating. It is preferable to flow the coating over the area and then brush it out to attain a uniform thickness.
3. Dipping. Dipping, like spraying, is one of the best methods when more than just a spot touch-up is required. Dipping is generally more suitable to production runs, where more than one PCB or component is involved. Dipping is particularly useful when the surfaces are uneven or irregular, or there is a high density of components on the PCB. Ensure proper coating thickness and avoid puddling of the coating by rotating the PCB or component in all axes during curing of the coating.
(i) Coating Thickness. All conformal coatings, with the exception of silicone RTVs, should have the coating thickness applied to 0.003 inch or less, unless directed otherwise by the OEM directive. Applications greater than 0.003 inch will not allow the solvents in the curing coating to evaporate or outgas, thereby trapping the solvents within the curing coating.
These trapped solvents can cause bubbles and pinholes in the coating. Refer to the OEM or maintenance directives for the proper coating thickness. Silicone RTVs may be applied in a thickness up to 0.008 inch. If multiple coats of any of the coatings are required by direction, ensure sufficient time for the solvent to outgas.
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(j) Conformal Coating Curing. Although the shortest cure time is desirable to speed the production process, the curing time is generally determined by the chemical characteristics of the coating. The curing temperature is limited by the PCB and its components.
There are two main types of curing systems used: moisture reactive and solvent based.
Conformal coating curing should always be accomplished in a dust-free environment. Elevated temperature curing should be accomplished in a forced-air drying oven at a temperature of not more than 130 °F (54 °C).
1. Moisture reactive curing systems use the relative humidity to form a gelatin of the compound and subsequent cure. These types of coating systems usually can be cured at room temperature, 77 °F (25 °C), but the curing process requires 24 hours.
2. Solvent based curing systems rely on elevated temperature to accelerate the chemical reaction. The chemical reaction among the ingredients produces a stable one-part solvent that is oxidized by the elevated temperature to cure the coating in as little as 30 minutes.
(k) Coating Inspection and Preservation. The final step in the process of conformal coating replacement is a thorough inspection using an ultraviolet (UV) light and a 10x to 15x magnifying lens. Visually inspect the new and original coating for defects such as charring, discoloration, cracking, delamination, solder flux residue, dry spots, foreign matter, air bubbles, and pinholes. If the inspection is satisfactory, the PCB or component should be placed in a polyethylene bag and taped closed.
506 through 600. RESERVED.
Par 601 Page 99 CHAPTER 6. TREATMENT OF SPECIFIC AVIONICS EQUIPMENT
601. GENERAL. All aircraft electrical/electronic equipment should be opened and inspected for evidence of internal moisture and corrosion on a scheduled basis as determined by the Original Equipment Manufacturer (OEM) or for cause. When corrosion is detected, prompt corrective action is required. Corrective action should include cleaning, corrosion removal, treatment, application of protective finish, and preservation where required. Maintenance personnel should always use the mildest method of cleaning and corrosion removal described in Chapters 4 and 5. The procedures and techniques for corrosion removal and the restoration of protective coatings described in this chapter are intended to aid the avionics technician in typical repair of specific equipment. In each case, some discretion on the part of the maintenance personnel is warranted. It is important that the maintenance personnel analyze the problem, select the appropriate corrective action, and confirm the effectiveness of their corrosion control.
It is recommended that maintenance personnel periodically review the conditions required for and the types of corrosion in avionics equipment described in Chapter 2.
602. REPAIR OF AVIONICS EQUIPMENT HOUSINGS, MOUNTING RACKS, AND STORAGE HARDWARE.
a. Bilge Areas. A common trouble spot on all aircraft is the bilge area. This is especially true for helicopters. These areas contain all types of avionics equipment and present a natural sump or collection point for all types of liquids. Accumulation of waste, hydraulic fluids, fuel, water, dirt, grime, loose fasteners, drill shavings, and other debris is typical. Sump liquids should be pumped or drained from the bilge area whenever discovered. Bilge areas should be cleaned using the procedures and materials in AC 43-4, Corrosion Control for Aircraft, Chapter 4, paragraph 4.13.3. Maintenance personnel should ensure, prior to installing any avionics equipment in the bilge area, that the area and equipment are cleaned and preserved.
b. Equipment Bays. Avionics equipment bays and installed equipment are highly susceptible to corrosion. This area is especially corrosion-prone in helicopters and other aircraft and equipment that are cooled with external ram air. Maintenance personnel should perform an inspection of all structures and equipment any time equipment bays are opened. The corrosion inspection and treatment processes are outlined in the following paragraphs.
(1) Visually inspect the structure, fixed mountings, installed equipment, hardware, and wiring for evidence of corrosion. Particular attention should be paid to dissimilar metal couples.
(2) Remove corrosion using 320-grit abrasive cloth, conforming to A-A-1048, or an abrasive nylon mat, conforming to A-A-58054, in accordance with the OEM’s maintenance instructions.
NOTE: Generally, corroded hardware is replaced.
(3) Clean, rinse, and dry the affected area in accordance with the instructions and materials detailed in Chapter 4, subparagraphs 405i and o.
(4) After the corrosion is removed, treat all affected aluminum surfaces with a chemical conversion coating in accordance with Chapter 4, subparagraph 405m.
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(5) Apply primer and topcoat as required in accordance with the OEM’s maintenance instructions or Chapter 4, subparagraph 405m, and Chapter 5, paragraph 503.
(6) Apply preservation as required or when environmental conditions dictate in accordance with Chapter 4, paragraph 407.
c. Engine Compartments. Inspect compartment hardware, electrical wire bundles and connectors, terminal boards, and junction boxes for evidence of corrosion and damage (see Figure 6-1).
(1) Repair or replace damaged components as required.
(2) Clean and treat corrosion in accordance with subparagraphs 602b(2) through (6) with the following exceptions:
(a) Clean and treat electrical connectors in accordance with subparagraph 603k.
(b) Clean and treat electrical terminal boards in accordance with subparagraph 602h.
d. Battery Compartments, Boxes, and Adjacent Areas. The battery, battery cover, battery box, and adjacent areas (especially areas below the battery compartment) are very susceptible to the corrosive action of spills and fumes of the battery and electrolyte. Figure 6-2 shows a typical battery compartment. Two different types of batteries are encountered on avionics equipment: lead acid (sulfuric acid electrolyte) and nickel-cadmium (potassium hydroxide electrolyte). Neutralize and clean the battery, battery cover, battery box, and adjacent areas as follows:
(1) Preparation of Solutions for Cleaning and Neutralizing Battery Electrolytes.
Indicating solutions are required for cleaning areas that are subject to battery electrolyte spills.
These solutions will determine the size and location of the spill and will indicate when the area has been completely neutralized. Use a 10% sodium bicarbonate (household baking soda) solution to neutralize sulfuric acid from lead acid batteries and a 3% boric acid solution to neutralize potassium hydroxide from nickel-cadmium batteries.
(2) Litmus Indicating Solution for Lead Acid Batteries. Litmus indicating solutions are used on lead acid battery electrolyte spills. Mix, by volume, 70% Isopropyl Alcohol, CAS 1393-92-6 Litmus Powder, and 30% distilled water in a plastic bottle with a hand squeeze pump in a 1 pint solution. Add 1 tablespoon of litmus powder to the liquid and mix thoroughly until a deep blue color is observed.
(3) Bromothymol Blue Indicating Solution for Nickel-Cadmium Batteries.
Bromothymol Blue indicating solutions are used on nickel-cadmium battery electrolyte spills.
Pour 1 pint of Bromothymol Blue indicating solution into a plastic bottle with a hand squeeze pump. Using an eyedropper, add 1 drop at a time of phosphoric acid into the solution, with subsequent mixing after each drop, until the color of the solution changes from blue to gold/amber.
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(4) Sodium Bicarbonate Neutralizing Solution for Lead Acid Batteries. Pour 1 pint of fresh water into a plastic bottle, add 2 ounces of sodium bicarbonate, O-S-576, and mix thoroughly.
(5) Boric Acid Neutralizing Solution. Pour 1 pint of fresh water into a plastic bottle, add 1/2 ounce of boric acid, A-A-59282, and mix thoroughly.
FIGURE 6-1. ENGINE COMPARTMENT ELECTRICAL CONNECTORS
Par 602 Page 102 FIGURE 6-2. BATTERY COMPARTMENT (
6) Cleaning and Neutralizing Procedures.
WARNING: Sulfuric acid and battery electrolytes are highly toxic to the eyes, skin, and respiratory tract. Avoid all contact. Skin and eye protection is required. Ensure adequate ventilation. If any acid or electrolyte contacts the skin or eyes, flood the affected area immediately with water and consult medical services. When working around batteries, always wear eye protection (face shield), an acid-resistant apron, and gloves.
(a) Remove any standing liquid or puddles of electrolyte with a squeeze bulb type syringe, absorbent cloth, or sponge. Place these items in a leak-proof container for removal from the work area. Properly dispose of or neutralize these items to prevent contamination of other areas.
(b) Using a pump spray bottle with the proper indicating solution, spray the entire area with the minimum amount of spray needed to wet the area.
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1. For lead acid battery spills, use the litmus solution, which will change in color from deep blue to red in any area that is contaminated with battery acid.
2. For nickel-cadmium battery spills, use the Bromothymol Blue Indicating Solution, which will change in color from amber or gold to a deep blue in any area that is contaminated with potassium hydroxide.
(c) Using a pump spray bottle with the proper neutralizing solution, spray the entire area where the indicating solution showed a change in color. Ensure the area is well saturated and that the stream is directed into all seams and crevices where the battery electrolyte could collect. Use care to prevent liquids from spreading to adjacent areas and ensure that bilge area drains are open to allow fluids to flow overboard. Allow the neutralizing solution to remain on the surface for at least 5 minutes or until all bubbling action ceases.
(d) Rinse the area thoroughly with a liberal amount of clean water and remove any standing liquid or puddles, as in subparagraph (a).
(e) Reapply the indicating solution as in subparagraph (b). If the solution does not change color, rinse the area as in subparagraph (d) and dry the area with a clean cloth or rags. If the solution changes color, repeat subparagraphs (c), (d), and (e).
(f) Corrosion Preventive Compound, as discussed in Chapter 4, paragraph 407, may be used to displace any moisture or temporarily preserve any bare metal until the area is properly protected.
(g) Repair or apply, as required, prepaint treatment, paint coatings, and sealant per Chapters 4 and 5. Special acid and/or alkali resistant coatings are usually required for battery compartments, boxes, and surrounding areas. Refer to the OEM’s maintenance instruction manual.
e. Frames, Mounting Racks, and Shock Mounts. Shock mounts and associated hardware on pod or airframe mounted equipment are usually the last items to be inspected for corrosion damage. Figure 6-3 shows evidence of a moisture entrapment area on the avionics shelf and corrosion on equipment racks and shock mounts. These inspections normally require the removal of shock mounts to facilitate a thorough examination. For this reason, shock mounts, their associated racks, and hardware should be preserved after inspection to ensure protection from the elements. For frames, mounting racks, and shock mounts that are not normally painted, remove corrosion and preserve as follows:
(1) Remove corrosion with 320-grit abrasive cloth, conforming to A-A-1048, or abrasive nylon mat, conforming to A-A-58054, Type I. Pay particular attention to dissimilar metal couples.
NOTE: The use of dissimilar metals in the selection of screws, washers, and nuts should be minimized wherever possible. See Chapter 7 for information on bonding and grounding hardware.
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(2) Clean the affected area with Isopropyl Alcohol, TT-I-735. Apply with a clean cloth or cheesecloth. Pay particular attention to the rubber shock absorbers of the shock mounts. The rubber may swell if saturated with Isopropyl Alcohol, TT-I-735. If the rubber shock mounts do swell, they should return to normal size in a short time after the Isopropyl Alcohol, TT-I-735, evaporates.
(3) For frames, mounting racks, and shock mounts that are normally painted, remove corrosion and clean as indicated in steps (1) and (2). Conversion coat, prime, and paint in accordance with the OEM’s maintenance instructions and the procedures of Chapter 5, paragraphs 503 and 504.
(4) Frames, mounting racks, shock mounts, and associated hardware should be preserved with a thin film of Water-Displacing Corrosion Preventive Compound, identified in Chapter 4, paragraph 407, and Table 4-4. Corrosion Preventive Compound, conforming to MIL-DTL-85054, is recommended for lasting protection.
FIGURE 6-3. MOISTURE ENTRAPMENT AREA AND CORRODED FRAME,
MOUNTING RACKS, AND SHOCK MOUNTS
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f. External Mounted Equipment. External mounted equipment is susceptible to the same corrosive environment as the airframe. Internal and external cleaning techniques are the same as for the airframe except for electromagnetic gaskets, shields, electrical connectors, and mating surfaces. Treat these areas as follows:
NOTE: See Chapter 8 and the OEM’s maintenance manual for information on electromagnetic gaskets and mating plating where the plating surfaces may have been removed.
(1) Remove corrosion and clean in accordance with subparagraph 602g.
(2) Conversion coat, prime, and paint in accordance with the OEM’s maintenance manual and the procedures of Chapter 5, paragraphs 503 and 504.
(3) Clean and preserve electrical connectors in accordance with subparagraph 603k.
(4) If the application of conversion coating, primer, and paint cannot be accomplished, temporarily preserve the external surfaces with a thin film of Water-Displacing Corrosion Preventive Compound, conforming to MIL-DTL-85054. For internal surfaces, apply a thin film of Water-Displacing Corrosion Preventive Compound, conforming to MIL-PRF-81309, Type II.
g. Cockpit and Control Boxes. Cockpits are susceptible to dirt, grime, and corrosive attack from cooling air and general human occupancy. The following inspection, cleaning, and treatment procedures should apply to the cockpit and avionics components.
(1) Use a vacuum cleaner to clean the cockpit area of dirt and dust. Pay particular attention to the areas under and behind control boxes.
(2) Inspect control box units for corrosion and contaminants. Pay particular attention to switches, dials, knobs, electrical connectors, control box and instrument panel mating surfaces, and attaching fasteners.
(3) Remove corrosion products from metal surfaces using an abrasive nylon mat, conforming to A-A-58054, Type I, or an appropriate cleaning and polishing pad.
(4) Clean the affected area with an approved solvent or Dry Cleaning Solvent, MIL-PRF-680, Type II, and a clean cloth or cheesecloth.
NOTE: Local air pollution regulations may restrict the use of this solvent.
Comply with all local air pollution regulations.
WARNING: Dry Cleaning Solvent, MIL-PRF-680, is 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.
(5) Following solvent cleaning, perform a final wipe using a clean cloth or cheesecloth and Isopropyl Alcohol, TT-I-735.
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(6) Repaint or touch-up existing paint as required in accordance with the OEM’s maintenance instructions manual or the procedures of Chapter 5, paragraphs 503 and 504.
(7) Apply a thin film of Water-Displacing Corrosion Preventive Compound, MIL-PRF-81309, Type II, on exterior metal surfaces and wipe off any excess with a clean cloth or cheesecloth. Do not get preservative on the acrylic plastic faceplates of instruments.
(8) Clean and preserve toggle, rotary, and push-button switches in accordance with subparagraph 603k.
(9) Clean control box faceplates with a solution of 1 ounce nonionic liquid detergent, conforming to MIL-D-16791, Type I, and 1 gallon of clean water. Wipe faceplates with a clean, soft flannel cloth or cheesecloth and the nonionic liquid detergent solution. Dry and polish the glass faceplate with a clean, soft flannel cloth or cheesecloth.
(10) Clean and preserve cockpit and control box electrical connectors in accordance with subparagraphs 603k(2) through (5).
(11) Hardware (panel fasteners, clamps, etc.) is generally replaced if found corroded.
FIGURE 6-4. TERMINAL BOARD INSTALLATION
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h. Terminal Boards, Junction Boxes, Relay Boxes, and Circuit Breaker Panels.
Remove covers and access panels, as required. Figure 6-4 shows a typical terminal board installation. Treat corrosion and preserve as follows:
WARNING: Ensure that all electrical power is disconnected from the aircraft and all electrical systems in the aircraft are deactivated. Disconnect all batteries.
(1) External Surfaces:
(a) Inspect, clean, and treat in accordance with subparagraph 602k.
(b) If the application of conversion coating, primer, and paint cannot be accomplished, temporarily preserve the external surfaces with a thin film of Water-Displacing Corrosion Preventive Compound, conforming to MIL-DTL-85054.
(2) Internal Surfaces and Components:
(a) Remove corrosion with an abrasive nylon mat, conforming to A-A-58054, Type I.
(b) Clean the affected area by wiping or brushing with Isopropyl Alcohol, TT-I-735. While the surface is still wet, perform a final wipe with a clean cloth or cheesecloth and allow the surface to air dry.
(c) Clean and preserve electrical connectors in accordance with subparagraphs 603k(2) through (5).
(d) Preserve internal surfaces of terminal boards, junction boxes, relay boxes, and circuit breaker panels by applying a thin film of Water-Displacing Corrosion Preventive Compound conforming to MIL-PRF-81309, Type III. Avoid applying preservative to relays and circuit breaker contact points.
i. Metallic Equipment Covers and Housings. Avionics equipment cases, covers, housings, and associated hardware can also be exposed to harsh environmental elements. Inspect, clean, treat, and preserve in accordance with subparagraph 602b, except for the following:
(1) Hardware (fasteners, clamps, etc.) is generally replaced if found corroded.
(2) Hinges and latches on equipment covers should be preserved and lubricated in accordance with subparagraphs 602k(6) and (7).
j. Nonmetallic Covers and Housings. In some cases, avionics equipment, support equipment, and general test equipment use nonmetallic high-impact plastic, fiberglass, Kevlar, or graphite/carbon epoxy covers and housings. These should be inspected and cleaned, and the paint touched-up as follows:
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(1) Inspect nonmetallic covers and housings for structural damage (cracks and delaminations), corrosion around metallic hardware, hinges, latches, and damaged paint.
(2) Repair any structural damage per the OEM’s maintenance instructions.
(3) Inspect hardware, hinges, and other metal attachments for signs of corrosion.
Remove any corrosion with 320-grit abrasive cloth, conforming to A-A-1048, or an abrasive nylon mat, conforming to A-A-58054, Type I.
(4) Clean nonmetallic covers and housings with a solution of 9 parts fresh water and 1 part Aircraft Cleaning Compound, conforming to MIL-PRF-85570, Type II. Apply with a brush or clean cloth.
(5) Rinse with fresh water and wipe dry with a cleaning cloth.
(6) Touch-up paint finish in accordance with Chapter 5, paragraph 504.
(7) Lubricate and preserve hinges and latches per subparagraph 602k.
(8) Preserve unpainted metallic hardware, hinges, and latches in accordance with Chapter 4, paragraph 407.
k. Equipment Hinges and Latches. Inspect hinges and latches on equipment boxes, doors, and covers for corrosion and proper operation. Treat as follows:
(1) Clean hinges and latches with a cleaning cloth dampened with an approved solvent.
(2) Remove corrosion with 320-grit abrasive cloth, conforming to A-A-1048, or an abrasive nylon mat, conforming to A-A-58054, Type I.
(3) Wipe residue with a clean cloth dampened with Isopropyl Alcohol, TT-I-735, and allow to air dry.
(4) Clean and treat bare aluminum and magnesium hinges and latches in accordance with Chapter 4, subparagraph 405m, and Chapter 5, paragraph 503.
WARNING: Chemical conversion material is 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.
CAUTION: Exercise care when using chemical conversion materials near electronic hardware. The solutions can cause corrosion of delicate electronic devices not suitably protected.
(5) Touch up paint finish in accordance with Chapter 5, paragraph 504.
(6) Lubricate hinges and rotating latches as follows: