Lifetime modeling of solder joints based on accelerated mechanical testing and Finite Element Analysis

Solder fatigue is among the predominant failure modes observed in power electronic modules. Under service conditions power electronic parts are exposed to repeated temperature swings originating from resistance heating. In consequence of a mismatch of the coefficients of thermal expansion, thermomechanical stresses are generated at material interconnects. Nevertheless, lifetimes of up to 30 years are requested for high reliability applications. Therefore, there is a demand for accelerated testing methods. However, due to strain rate dependence of inelastic deformations theoretical lifetime modeling is necessary to compare the results of accelerated test methods with usual service conditions. The present study reports on a mechanical testing method operating at the ultrasonic frequency of 20 kHz. During testing samples are exposed to repeated bending deformations until the solder joint finally breaks. The number of cycles to crack initiation is determined for different temperatures ranging from room temperature to 175 °C. Thereafter, an FEM computer simulation of the fatigue experiment is performed, where the visco-plastic Anand model serves as material model of the solder. The time to crack initiation in the solder is evaluated with a model of damage accumulation, which combines the Coffin-Manson model with a multiaxial version of the Goodman relation. It is demonstrated that this model can be applied to the solder alloys PbSnAg, Sn3.5Ag and SnSbAg.