Method to Predict Performances of PCB Silicone Conformal Coating under Thermal Aging

Abstract

The stability of print circuit board (PCB) conformal coating is critical to guarantee the long-term performance of electronic components on PCB boards. Coating exposure to thermal shock or temperature cycles may initiate cracks, a common failure mechanism of conformal coatings. Different simplified approaches are compared to help identify desired mechanical profiles for coatings to be used in a harsh environment, focusing on silicone characterized by low rigidity and high deformability compared to alternative chemistries. Evaluation of the bi-material strip bending test method appears not to be effective in the conformal coating selection. The large difference between the coating’s elastic modulus of silicones compared to substrate modulus allows the use of a simplified formula to calculate the stress associated with the coefficient of thermal expansion (CTE) mismatch, the silicone accommodating displacement imposed by thermal changes. Both lateral tensile stress and local shear stress near the edge are estimated, with local shear stress decreasing quickly and moving apart from the edge with the stress relaxation preventing coating delamination. Predictions of simplified models agree with both results of grid-independent finite element analysis (FEA) models and observations of test pieces submitted to temperature cycles. This demonstrates the ability to use simplified models to predict coating’s performances under thermal aging and help in product selection depending on the working environment.