Circuit Board Repair Guide > Basic Circuit Board Repair Procedures > Coating Removal

Coating Removal, Identification of Coating

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This section of the circuit board repair guide covers the techniques for identifying various circuit board coatings so the appropriate coating removal procedure can be used.

IPC Acceptability References
IPC-A-600 2.0 Externally Observable Characteristics
IPC-A-610 10.0 Laminate Conditions
 
Related Procedure References
CTC 1.0 Foreword - Circuit Board Repair Guide
CTC 2.1 Handling Electronic Assemblies
CTC 2.2.1 How to Clean a Circuit Board
CTC 2.2.2 Cleaning Circuit Boards, Aqueous Batch Process
CTC 2.5 Baking and Preheating of Printed Circuit Boards
CTC 2.7 Epoxy Mixing and Handling
IPC 7721 2.3.1 Coating Removal, Identification of Coating

Tools and Materials
Brushes
Cleaner
Cutting Disks
Foam Swabs
Knife
Micro Drill System
Thermal Parting Tool
Wipes
Wood Sticks

2.3.1 Coating Removal, Identification of Coating

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

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 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 coating 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. Coating 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 coating 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
Coating Removal Method
2.3.2
Coating Removal Solvent Method
2.3.3
Coating Removal Peeling Method
2.3.4
Coating Removal Thermal Method
2.3.5
Coating Removal Grinding/Scraping
Method
2.3.6
Coating Removal 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 coating removal methods to specific coatings is numerically indicated. These coating removal methods are listed in ascending order.


Table 3   Conformal Coating Characteristics

Characteristics Conformal Coating Type
Epoxy Acrylic Polyurethane Silicone Paraxylylene
Hard X   X   X
Medium Hard   X X    
Soft     X X  
Heat Reaction X X X    
Surface Bond, Very Strong X     X X
Surface Bond, Strong   X   X  
Surface Bond, Medium     X X  
Surface Bond, Light       X  
Solvent Reaction   X      
Smooth Surface X X X X X
Lumpy Surface          
Nonporous Surface X X X   X
Glossy Surface X X X    
Semiglossy Surface X     X  
Dull Surface         X
Rubbery Surface       X  
Brittle X X      
Chips X X      
Peels/Flakes   X X   X
Stretches     X X  
Scratch, Dent, Bend, Tear     X X X




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