Printed Board Type: R/F/W/C
Skill Level: Advanced
Conformance Level: High
Revision: D
Revision Date: Jul 7, 2000

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

ACCEPTABILITY REFERENCES
IPC-A-600 2.0	Externally Observable Characteristics
IPC-A-610 10.0	Laminate Conditions
PROCEDURE REFERENCE
1.0	Foreword
2.1	Handling Electronic Assemblies
2.2	Cleaning
2.5	Baking And Preheating
2.7	Epoxy Mixing and Handling
IPC7721 2.3.1	Coating Removal, Identification of Coating

TOOLS & MATERIALS
Brushes
Cleaner
Cutting Disks
Foam Swabs
Knife
Micro-Drill System
Thermal Parting Tool
Wipes
Wood Sticks

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.

1. Hardness
   Penetration test in a non-critical area to determine relative hardness. The harder the coating the more suitable to pure abrasive techniques. The softer and gummier the coatings the more suitable to the brushing removal procedures.

   
   CAUTION
   Abrasion operations can generate electrostatic charges.

2. 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.
   
   
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
   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° C (210° 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.

5. Strippability
   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 chemical aids is 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 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 specific 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.