New model of crack propagation of aluminium wire bonds in IGBT power modules under low temperature variations

In this paper, a new model of lifetime prediction for wirebond degradation during power cycling in IGBT power module is proposed. This model is based on experimental crack propagation analyses and on a plastic strain empirical law. For the experiments, two power cycling tests in switching mode under high voltage were carried out respectively with temperature swing ∆Tj = 30°C and ∆Tj = 40°C and minimal temperature Tj,min = 55°C. The DUTs and test conditions have been chosen so that only degradations on chip top-side interconnections were observed. The on-state voltage (VCE) was measured during the tests as an indicator of the degradation in the wire and metallization and samples were removed at different aging stages. The removed samples were cross-sectioned in order to observe the crack propagation evolution with cycling. Concerning the plastic strain empirical law, we used results from literature that show the evolution of the plastic strain (∆εpl) with bond-wire contact crack growth. As results, this new lifetime prediction model based on a modified Paris's law for aluminium wire bonds fatigue of IGBTs shows a good fitting of the test results and is used to predict the lifetime. Finally, the lifetime of tested IGBT modules are estimated and used to verify the effectiveness of the proposed model.