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Conformal Coating Discoloration

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Conformal Coating Discoloration | 29 August, 2011

Greetings...I have a customer that has an assembly that is conformal coated with Humiseal 1A33. This assembly has some components (resistors and FET's) that heated up during operation and has discolored the conformal coating around their location. (Turned the conformal coating a brownish color).

Per IPC-A-610, discoloration of conformal coating is a Defect Class 1,2,3. However, this discoloration is NOT a result of the conformal coating process or the conformal coating itself but rather from the heat given off by the components on the board.

Does this discoloration pose a reliability issue for the customer? (Note: the coating is still in tact...just discolored).

Thanks for your help.


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Conformal Coating Discoloration | 29 August, 2011

Rich Here's what Humiseal says: Thermal Properties The coating must be able to withstand localized thermal 'hot spots' generated by components. High temperatures can cause short-term changes in coating characteristics, such as softening and lowered electrical resistance. Long-term effects of high temperature operation include discoloration, brittleness, cracking, and shrinkage and could result in lower dielectric strength, along with possible reduction in moisture and insulation resistance.

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Conformal Coating Discoloration | 30 August, 2011

Guess the better question is why are the resistors and FETs heating to the degree that they discolored the coating? If the coating is discolored, I'd be concerned with the possibility of cracking as dave mentioned. Cracks defeat the purpose of the coating.

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Conformal Coating Discoloration | 2 September, 2011


That is a standard reaction with Humiseal 1A33, nothing to worry about. Find a board that was coated over year ago and had been heated up at all and it will be an amber color. It does not violate the IPC for discoloration. We have coated boards that went into under-street sensors in Arizona. They came out with a beautiful tan!!!!

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