Effect of copper over-pad metallization on reliability of aluminum wire bonds

Abstract

Recently, there have been moves to replace Si power devices with SiC ones, which will require new materials with higher thermal conductivity for power module assemblies owing to their higher operating temperatures. Al wire bond interconnections suffer from a mismatch in the coefficient of thermal expansion (CTE) between the SiC chip and the wire material, leading to a degradation of the power module reliability owing to the higher junction temperature (Tj) operation caused by the current density increase. Various studies have been conducted to improve the reliability of such interconnections. In this study, Al wires were bonded on a thick Cu over-pad metallization (OPM) layer that was formed on an Al pad of a SiC Schottky barrier diode, and active power cycling tests were performed while changing Tjmax to investigate the contribution of such a thick Cu-OPM layer on the reliability of Al wire bonds. Compared with the case of Al wire bonds on an Al pad, in active power cycling tests, the lifetime of Al wire bonds on a 25 μm-thick Cu-OPM layer was 1.3-times longer at ΔTj = 75 °C, and 1.5-times longer at ΔTj = 100 °C. The results of Joule heating analysis and elastic thermal-stress analysis showed that the lifetime prolongation effect of Al wire bonds on Cu-OPM was not derived from the heat dissipation effect of the thick Cu-OPM layer, but from a stress-reduction effect obtained by keeping the Al wire bonds away from the SiC diode chip, with which the CTE mismatch is large.