Effect of water condensation on electrochemical migration in case of FR4 and polyimide substrates

Abstract

The effect of water condensation on electrochemical migration was investigated by water drop and thermal humidity bias tests. The investigations were carried out on immersion silver surface finish on glass fiber epoxy (FR4) and polyimide (PI) substrate. During the thermal humidity bias tests, water condensation was monitored by an in situ and real time optical inspection system. Simultaneously, electrical measurements were also done in order to validate the visual results. The water drop tests (without condensation effect) have not showed any dendrite growth time difference between the substrate materials. However, in the case of the thermal humidity bias tests (with condensation effect), it was observed that the water condensation intensity is significantly related to the electrochemical migration failure mechanism. In the case of FR4 substrate the mean time to failure was observed to be lower than in case of PI which effect was caused by the different water condensation intensity which depends on the surface roughness and the thermal diffusivity of the substrate materials. Therefore, the water condensation effect has to be considered in the classical electrochemical migration model.