Conformal coating is typically applied as air-dry or vacuum deposited onto the PWB at ambient temperature. CTE is in the range of 55-65 ppm/ºC for acrylic and epoxy based types, and 35 ppm/ºC for the Parylene type. The differential CTE causes strain in the conformal coating bonding onto the stronger PWB and thus induces stress on the conformal coating and stress concentration at sharp interfaces such as around components and solder joints.
In simple terms, the stress induced on the conformal coating increases with:
- increase in differential CTE
- compressive stress increases with temperature excursion below ambient
- tensile stress increases with temperature excursions above ambient.
Compressive stess, tensile stress, and stress concentration also increase with higher modulus of conformal coating. Sometimes thousands of psi in stress are induced. This stress may cause cracks, from micro-and-hairline to large cracks, or even delamination of the conformal coating.*