2.3.1 Coating Removal, Identification of Coating

Outline

This procedure covers the techniques for identifying various coatings so that the appropriate coating removal method can be selected.

Minimum Skill Level - Advanced

Conformance Level - High

Tools and Materials

Abrader Mild abrasive for removing oxides and contaminants.

Ball Mills, Abrasives, Cutting Tools Ball mills, abrasives and cutting tools for working on circuit boards.

Cleaner General purpose cleaner for removing contamination.

Cleaning Brushes Disposable brushes for solvent cleaning and application of coatings.

Coating Remover Pen For precise removal of silicone, acrylic and urethane coatings from circuit boards.

Foam Swab, Small Swabs for use with solvents and application of color agents and epoxies.

Knife with #16 Blades A must-have tool for precise cutting, scraping and trimming.

Micro Drill System Versatile power tool for milling, drilling, grinding, cutting and sanding.

Mixing Picks Disposable mixing sticks for mixing and applying adhesives.

Precision Tool Set Nine precision-crafted tools for detailed circuit board work.

Scraper, Curved Tip 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. The specific coating is usually known at the time of its original manufacture. Consequently, the removal methods can generally be specified based on these known coatings. 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.

1. 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 they will be for brushing removal procedures.
2. Transparency
   Transparent coatings are usually more suitable for removal than opaque coatings. Removal methods used with opaque coatings must be far more controllable, less sensitive to damaging the covered components and printed board surfaces, and usually slower.
3. 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.
4. Thermal Removal
   To determine whether the coating can be thermally removed, use a thermal parting device with controlled heating and without a cutting edge. Start with a low temperature, approximately 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.
5. 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 are usually very limited.
6. 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 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

Table 2: Conformal Coating Removal Methods

Note
The preferred order for applying individual removal methods to specific coatings is numerically indicated. These removal methods are listed in ascending order. 

Table 3: Conformal Coating Characteristics

Images

Coating Removal, Identification of Coating

This procedure outlines the techniques for identifying various coatings, enabling the selection of the appropriate coating removal method.

When identification of the coating is not available simple observation and testing will help identify the coating characteristics.