Wirebonding is the weakest area of device packaging of power IGBT modules. Accelerated thermal fatigue testing cracks to form and propagate in the aluminum wirebond at the foot area. This study examined the relationship of the wirebond reliability and the aluminum wire grain structure, which can be affected by post-wirebond heat treatment. A series of wirebonded IGBTs were annealed at a temperature range from 280 deg C to 400 deg C for up to 60 minutes. Wirebond shear strengths versus temperature cycles were examined. Cross-sectional SEM was used to examine both aluminum grain size development by annealing and crack initiation and propagation in the wirebonds after temperature cycling. It was found that aluminum grain size was increased by post-wirebond annealing. With temperature cycling, the wirebond shear strengths of the as-wirebonded samples decrease rapidly,and lifted wirebonds were present after 1500 temperature cycles. The lifted wirebonds typically break within the aluminum wire near the wire/metalization interface. The shear strength of the wirebonds with post-wirebond annealing ashowed no significant change even after 5000 temperature cycles, and there were no signs of significant deterioration of the wirebonds either. The wirebond crack initiation and growth rates were depressed substantially by larger aluminum grains. Annealing of the aluminum wire after wirebonding provided increased aluminum grain size resulting in improved reliability of the wirebonds of high power modules.
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