Thermal Sensitivity of UBM Adhesion in WLCSP

Developments in WLCSP proved polyimide to be a good alternative to benzocyclobutene as a passivation material and dielectric interlayer. An interface adhesion failure between PI and Ni-V under bump metallurgy was reported after significant exposure to high temperature. This paper aims to provide a better understanding of thermal sensitivity of UBM/passivation adhesion through determination of activation energy. Four devices with different structures were exposed to different temperature settings and exposure times to simulate thermal degradation of UBM adhesion. Results showed that bump shear strength degrade exponentially with exposure to temperature. The time-to-failure equation was determined to follow the Arrhenius model.