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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.
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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.