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I realize that, in principle, conformal coating should be inert and not affect a circuit's behavior, but are there any real-world performance changes to look out for when using conformal coating?

I have two boards that pass IRIG signals (low-speed digital, 5 V) that are nominally identical except that one is conformal coated. The coated board has sloppier IRIG edges, but I suspect this is because of variations in the performance of the ICs in the chain (an optocoupler, a transistor, and an inverter), rather than the conformal coating.

Has anyone experienced problems that they have directly tied to conformal coating?

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    \$\begingroup\$ conformal coating has a dielectric constant and a thermal resistance \$\endgroup\$ – PlasmaHH Aug 16 '18 at 13:15
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    \$\begingroup\$ There may be minor performance differences due to conformal coat at relatively fast edge rates, but I have never personally seen anything significant at rates up to 5Gb/s. At low speed, it should have a negligible effect provided the electronics is well within the thermal limits. \$\endgroup\$ – Peter Smith Aug 16 '18 at 14:06
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I once designed a Battery Management System for an electric car prototype. There were a lot of peripheral PCBs that could be exposed to a mild to moderate environmental hazard.

Long story short: The CAN Bus didn't have any problem with the conformal coating. Heat management was tighter to prevent any thermal runaway (i.e. we lowered the max temperature rating and ensure that the circuit was not over the temperature rating).

The only Problem I had with conformal coated boards: you can't easily do field repair on them and you need to be extra careful with your connectors. You don't want to jam them in conformal coating or overexpose them. Enclosed connectors like Molex Microfit are efficient at that. You can even plug in a dummy connection to protect the inner part of the connector while you put your conformal coating.

Bear in mind that different coating may yield different results. Ours wasn't a particularely thick one. I saw once black conformal that was almost rubber like at the end and almost 1/8 of an inch thick...it was heavy duty to say the least.

Finally, when I apply conformal (don't know if you do it yourself), each layer is crisscrossed at 90 degrees. 3 to 4 coating did the trick most of the time. however, like I said previously, my conformal coating is on the thin and light side.

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A friend of mine worked for a company that makes solenoids and solenoid driver boards.
He designed a custom board that worked fine until it was conformal coated. The board would misfire in some way. I can't remember the details, being a lot of years ago. I suspected that there was a critical change in parasitic capacitance somewhere in the control circuitry. I suggested he try a ground plane and that fixed the problem.
The dielectric constant of the coating would be several times higher than air.

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