Temperature–humidity–bias testing on insulated-gate bipolartransistor modules – failure modes and acceleration due to high voltage

The temperature–humidity–bias (THB) test is the standard for accelerated stress testing with respect to corrosion and other humidity driven degradation mechanisms. Usually, 1000 h tests at 85°C and 85% relative humidity are used to predict up to 25 years of operation. The bias is usually limited to 80 V in order to fulfil the respective standards. Nevertheless, THB tests on 1700 V insulated-gate bipolar transistor (IGBT)-modules have shown that higher bias is a more severe test condition. The failure analysis confirmed Cu- and Ag-dendrites and corrosion of the aluminium (Al)-chip-metallisation as the relevant failure mechanisms. To determine the acceleration factor due to voltage, 1200 V IGBT modules were tested in THB at 780 V (65% of V CES) and 1080 V (90% of V CES). A characteristic degradation consisting of three phases has been identified. The second phase seems to be determined by Al corrosion and an acceleration factor of about two has been estimated from 780 to 1080 V. Within the third phase, the devices stabilised probably due to localised self-heating. Thus, this degradation mechanism is kind of self-limiting, but the higher leakage also increases the risk of thermal runaway especially when biased close to the rated collector–emitter voltage.