Increasing the Reliability of Aluminum Wirebonds by Thermal Treatments: Analysis and Implementation

In this work, we studied the effect of a postbonding thermal treatment (TT) on thick aluminum wirebonds lifetime in insulated gate bipolar transistor (IGBT) power modules during power cycling (PC) at different temperature swings. A reliability improvement of up to 44% is demonstrated for the bonds located at the center of the chip and seeing the highest TT temperature. The physical phenomena occurring during the TTs and during PC are studied using microstructure analysis (Scanning Electron Microscopy techniques) and nanoindentation. Softening of the material results from recrystallization and/or recovery during TT, whereas during PC, the thermally treated material hardens due to strain accumulation and defect formation. Despite the hardening, our data suggest a decrease in crack growth rate at the second half of the testing time. We implemented an approach for applying TTs during typical operating conditions in a converter by controlling the OFF-state losses of a device in a multichip configuration. The stability and the limits of this approach are studied experimentally and described by an electrothermal model. This study allows us to conclude on a realistic process window for online TTs, by considering both material constraints and practical temperature control limits.