Electronic – 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?

Best Answer

Generally this material is applied to protect components from environmental damage.

The ratio of change Stress [N/m²] pressure to Strain displacement is called Young's Modulus and the resulting force is expected to be much lower than the breaking point for components when there is thermal expansion.

Stress = Modulus × CTE × ∆T