Acceleration of temperature humidity bias (THB) testing on IGBT modules by high bias levels

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 hrs tests at 85 degree Celsius and 85 percent relative humidity (85/85) are used to predict up to 25 years of operation. With regard to the respective standards asking for limited self-heating, the bias was commonly limited to 80 V. Nevertheless, recent THB tests on 1.7 kV IGBT modules have shown that higher bias is a more severe test condition. Failure analysis confirmed corrosion of the Al chip-metallization as well as Cu-and Ag-dendrites as the relevant failure mechanisms. In order to determine the acceleration due to bias, 1.2 kV IGBT-modules were tested in THB at 65 percent and 90 percent of their nominal voltage V_nom, respectively. A characteristic degradation consisting of three phases has been identified. The 2^nd phase seems to be determined by Al corrosion and a factor of about two has been estimated for the acceleration between the aforementioned test-voltages. Within the 3^rd phase, the devices stabilized probably due to localized self-heating. Thus, this degradation mechanism is kind of self-limiting, but the higher leakage increases the risk of thermal runaway, i.e. catastrophic breakdown especially when biased close to V_nom.