# Acceptable conformal coating expansion ratio for PCB with delicate SMT parts?

I have heard that conformal coatings can put stress on SMD parts because of expansion/shrinking while curing or because of temperature differences. How much of a problem is this?

The only value relating to this which I found in the datasheet for the coating I'm looking at was "Cured film Coating: Coefficient of Expansion: 90ppm" which I guess is about expansion while curing because it has the wrong dimension for a thermal expansion coefficient. I assume this means that for a 20cm board there would be expansion of $90\cdot10^{-6}*200mm=0.018mm$ which should be negligible. Is this right?

• How do I use inline Latex? – Nobody Dec 30 '17 at 20:45
• We don't have LaTeX on this webiste but we have something similar to it, called Mathjax. You have to use the $equations here$ syntax. – KingDuken Dec 30 '17 at 20:51
• @KingDuken Thanks, that's what I meant. But seems it was already fixed for me. :) – Nobody Dec 30 '17 at 21:04
• I haven't heard of many problems with conformal coating. Usually it is at least slightly flexible and fairly thin. I have heard of problems with potting. The larger and more fragile the component, the bigger the problem. Leaded electrolytic caps, and large inductors seem to be the most at risk. Epoxy is the least flexible potting material. Consider silicone or polyurethane potting if you are worried about breakage. – mkeith Dec 30 '17 at 21:06
• @mkeith Quite possibly I (or the random internet person who is the source of that piece of information in my head) mixed up potting and conformal coating. In that case the silicone coating I'm considering will surely be fine. – Nobody Dec 30 '17 at 21:09