This procedure covers the techniques for identifying various coatings so that the appropriate coating removal method can be selected.
Minimum Skill Level - Advanced
Recommended for technicians with soldering and component rework skills and exposure to most repair/rework procedures, but lacking extensive experience.
Conformance Level - High
This procedure most closely duplicates the physical characteristics of the original, and most probably complies with all the functional, environmental and serviceability factors.
Scraper, Curved Tip
Hardened stainless steel tip for scraping solder mask and removing defects.
Additional Items and Supplies
General purpose cleaner for removing contamination.
Thermal Parting Tool
Thermal removal tool using controlled heat through specially shaped tips.
Nonabrasive, low-linting wipes for cleanup.
The coating must first be identified to determine the appropriate coating removal procedure. During original manufacture, the specific coating is usually known. Consequently, the coating removal methods can usually be specified based on the known coatings being used.
When coating identification is unavailable, simple observation and testing will help identify the coating characteristics to specify the proper removal procedure.
Note: The generic or commercial identification of the coating material is not necessary to accomplish coating removal.
Hardness Penetration test in a non-critical area to determine relative hardness. The harder the coating, the more suitable it will be for abrasive techniques. The softer and gummier the coatings, the more suitable for the brushing removal procedures.
Transparency Obviously, transparent coatings are usually more suitable for removal than the opaque type. Removal methods used with opaque coatings must be far more controllable, less sensitive to damaging the covered components and printed board surfaces, and are usually slower.
Solubility Test the coating for solubility characteristics in a non-critical area with trichloroethane, xylene, or other solvents with low toxicity and mild activity.
Caution: Printed board assemblies should not be immersed in harsh solvents.
Thermal Removal Use a thermal parting device with controlled heating and without a cutting edge to determine whether the coating can be thermally removed. Start with a low temperature, approx. 100 degrees C (210 degrees F), and increase the temperature until the coating is removed. If the coating flows or gums up, you are too hot, or the coating is unsuitable for thermal removal.
Caution: Do not exceed the maximum component storage temperature or other limitations.
Stripability Carefully slit the coating with a sharp blade in a non-critical area and try to peel it back from the surface to determine if this method is feasible. Due to the adhesion required of coating materials, strippable techniques without a chemical aid is usually very limited.
Thickness Determine if the coating is thick or thin by visual means. Thin coatings show sharp component outlines and no fillets, while thick coatings reduce sharp component outlines and show generous fillets at points of a component or lead intersection with the printed board. Thick coatings usually require two-step removal methods to prevent surface damage to the board. First, reduce the thick coating to a thin one and then use pure abrasion methods to reach the board's surface.
The specified coating to be removed may have one or more of these characteristics, and consequently, the removal method selected should consider the composite characteristics.
See Table 1 for Conformal Coating Identification.
See Table 2 for Conformal Coating Removal Methods.
See Table 3 for Conformal Coating Characteristics.
Table 1 Conformal Coating Identification
1. Does the coating feel soft, rubbery, or spongy?
2. Does the coating have a noticeable reaction to heat?
3. Is there a reaction to alcohol?
4. Is the coating thick, and does it have a dull surface?
5. Does the coating have a noticeable reaction to heat?
6. Does the reaction form white powder?
Table 2 Conformal Coating Removal Methods
2.3.2 Solvent Method
2.3.3 Peeling Method
2.3.4 Thermal Method
2.3.5 Grinding Scraping Method
2.3.6 Micro Blasting Method
The preferred order for applying individual removal methods to specific coatings is numerically indicated. These removal methods are listed in ascending order.