Effect of load sequence interaction for low ∆Tj's on the reliability of bonded aluminum wires in IGBTs

This paper focuses on the effects of load sequence on the reliability of insulated gate bipolar transistors (IGBTs). Precisely, the effect of junction temperature swing ($∆T_j$) is investigated in low ranges. A series of power cycling tests have been done; first, two tests with a single $∆T_j$ conditions are performed in order to serve as test reference. Then, combined power cycling tests with two-level of $∆T_j$ are conducted sequentially. The crack propagation for both combined tests specimens is analysed. Results show that a sequencing in $∆T_j$ of the high-low stress level leads to crack retardation and then a crack acceleration after a number of cycles. In addition, the crack propagation of the low-high stress level follows the same evolution of the high stress crack propagation. These results were correlated to the microstructure parameters using Electron Backscatter Diffraction (EBSD) technique with a focus on the effect of residual stress. Local misorientation, grain size and grain boundaries evolution at the sites of crack propagation confirmed the phenomenon observed for the crack propagation for both combined tests.