This paper, for the first time, investigates the reliability of wire-bonded low-voltage discrete power trench n-MOSFETs that have been subjected to repetitive unclamped inductive switching (RUIS). Automotive MOSFETs driving inductive loads may be subjected to RUIS; hence, there is a need to characterize the failure mechanisms in such applications. The failure mechanisms of repetitively avalanched wire-bonded MOSFETs are shown to be wire-bond lift-off and source metal degradation/fatigue due to thermomechanical stress cycling. Temperature excursions from avalanche pulses cause thermomechanical stresses on the wire-bond/source-metal interface as a result of differences in thermal expansion coefficients between silicon and aluminum. Trench MOSFETs exhibited an average of 10% increase in on-state resistance due to source metal fatigue after 100 million cycles of repetitive avalanche. The number of cycles to failure is investigated as a function of the avalanched induced temperature changes and is shown to follow the Coffin-Manson law. These results are important for designers of automotive systems since they are capable of predicting the long-term reliability of wire-bonded discrete power semiconductor components.
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