Outline
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.
Micro Drill System
Versatile power tool for milling, drilling, grinding, cutting and sanding circuit boards.
Mixing Picks
Unique mixing sticks have a paddle shape on one and sharp pick on the opposite end.
Scraper
Hardened stainless steel tip for scraping solder mask and removing defects.
Thermal Parting Tool
Thermal removal tool using controlled heat through specially shaped tips.
Wipes
Nonabrasive, low-linting wipes for cleanup.
Procedure
To determine the appropriate coating removal procedure the coating must first be identified. During original manufacture the specific coating is usually known. Consequently, the coating removal methods can usually be specified and based on the known coatings being used.
When identification of the coating is not available, simple observation and testing will help identify the coating characteristics so that the proper removal procedure can be specified.
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 to abrasive techniques. The softer and gummier the coatings the more suitable to the brushing removal procedures.
Caution
Abrasion operations can generate electrostatic charges.
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 and 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 not suitable for thermal removal.
Caution
Do not exceed the maximum component storage temperature or other limitation.
Stripability Carefully slit the coating with a sharp blade in a non-critical area and try to peel 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 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 down to a thin one and then use pure abrasion methods to reach the surface of the board.
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
Test
Yes
No
1. Does the coating feel soft, rubbery or spongy?
2
3
2. Does the coating have a noticeable reaction to heat?
Polyurethane
4
3. Is there a reaction to alcohol?
Acrylic
5
4. Is the coating thick and does it have a dull surface?
Silicone Thick
Silicone Thin
5. Does the coating have a noticeable reaction to heat?
6
Paraxylyene
6. Does the reaction form white powder?
Epoxy
Polyurethane
Table 2 Conformal Coating Removal Methods
Conformal Coating
Removal Method
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
Paraxylyene
1
2
3
Epoxy
1
2
3
Acrylic
1
2
3
4
Polyurethane
3
1
2
4
Silicone, Thin
1
2
3
4
Silicone, Thick
1
2
Note
The preferred order for applying individual removal methods to specific coatings is numerically indicated. These removal methods are listed in ascending order.
