Reliability studies on high current power modules with parallel mosfets

In the current range up to 300 amperes, MOSFETs have proved to be the most acceptable and cost effective devices in power modules for power and auto sectors; beyond 600 amps, IGBT are the preferred devices. To extend the usable range, MOSFETs in parallel are becoming more and more acceptable, provided the design, manufacturability and long term reliability have been fully evaluated for built in current stresses, thermal stresses under steady and transient current loads and ruggedness under the harsh load environments. A half H bridge module with two MOSFETs in parallel (per arm of the bridge) with a combined current rating of 220 amps have been evaluated for various manufacturing approaches like heavy Al wirebonding, TAB contacts and Au bump approach. The studies have been done under current and thermal stressing with a view to identify possible failure modes. Studies under SEM/SAM indicate sensitivity to damages, if there is improper current distribution along the die as well as substrate; these local variation in current distributions are improper locations of the die contacts (in wire bonding approach), inherent potential variations arising out of current crowding, variations in the effective conductance / inductance of the source and drain circuits. The studies have resulted in the optimization of a power module design addressing critical second order reliability considerations. This work is significant, as the industry is looking for extension of MOSFET usage in higher power needs and at the same time would like to identify and eliminate all long term failure modes.