Operating IGBTs above rated junction temperature limits: Impacts to reliability and electrical performance

A fixture was designed and built to test power cycling reliability at maximum junction temperatures of 150°C and 175°C. A 1200V, 150A IGBT six pack module was mounted on a single cold plate and three sets of cold plates were connected to DC power supplies controlled by LabView. The IGBTs on each cold plate were cycled to give a junction-to-case temperature rise of 100°C with a maximum junction temperature of 150°C. Every 2000 elapsed power cycles, the cold plates were removed from the test setup and the junction-to-case thermal resistance and collector-emitter saturation voltage was measured. Power cycle tests were run until all IGBT modules exceed the failure criteria (5% change in collector-emitter saturation voltage or 20% change in junction-case thermal resistance). The power cycle test was repeated on a new set of IGBTs with the same temperature rise of 100°C, but at 175°C maximum. Weibull analyses of the power cycle test data resulted in shape parameters of 8.6 and 8.8 for the 150°C and 175°C maximum junction temperature data, respectively, that is indicative of rapid wear out. Inspections of the failed modules indicated wire lift-off. The B5 life at 175°C was found to be 36% lower than the B5 life at 150°C. The power cycling test result at 150°C; however, was over two times higher than the life specified by the manufacturer. High temperature reverse bias testing at 175°C was also performed to stress silicon die and packaging materials under a DC bias in order to accelerate migration of ionic contaminants and silicon die impurities. Die temperatures of 175°C caused no degradation of the key electrical static parameter in 1400 hours, which exceeded industry standard test of 1000 hours.