Creep Effect on the Crack Growth Rate of Wire Bonds in IGBT Power Modules

In this article, a comprehensive physics–based study is carried out on creep phenomena occurring in heavy aluminum wire bonds of insulated gate bipolar transistor (IGBT) power modules during power cycling tests conducted under different heating times between 1 and 32 s. Postanalysis confirmed that wire bond lift-off was the root cause of failure. The study provides an efficient crack growth model to explain the effect of the heating duration on the rate of on-state voltage increase across the tested IGBTs. The developed model is based on a nonlinear fracture mechanics parameter known as modified or creep J-integral (J*). As in Paris’ law, the proposed model provides a simple relationship between the crack propagation rate and the cyclic value of the modified J-integral (ΔJ*). ΔJ* is calculated using temperature field mapping between a 3-D electrothermal model and a 2-D thermomechanical model using finite element (FE) tools. The model predictions are compared with crack growth rates estimated under different heating times through measurements of the on-state voltage rate. The relation between the on-state voltage and the crack length is obtained with an FE electrothermal model. The satisfying correlation between the fracture mechanics model and the experimental results confirms the creep phenomena is responsible for the dependency of the crack growth rate with heating duration, and provides a physical way to estimate the wire-bond degradation and lifetime under complex mission profiles.