Energy-based micromechanics analysis on fatigue crack propagation behavior in Sn-Ag eutectic solder

Sn-Ag-eutectic-based solders are replacing Sn-Pb eutectic solders in the electronics industry. The current paper extends the recently developed approach based on phase transformation theory, micromechanics, and fracture mechanics to treat fatigue crack nucleation and propagation for steels and alloys to predict fatigue crack propagation in solder alloys. To verify the proposed method, fatigue experiments were conducted on Sn-3.5Ag solder alloys. Finite element analysis is performed to predict the stress intensity factor range ΔK and the required energy U to increase the crack by a unit area. Unified creep-plasticity theory and a cohesive zone model are incorporated to predict the creep and hysteresis effects on fatigue crack propagation in solder and the interfacial behavior between the solder alloy and the intermetallic layer, respectively. With U determined numerically, the predicted fatigue crack propagation rate using phase transformation theory is compared with experimental data for Sn-3.5Ag and Sn-37Pb eutectic solders. Reasonable agreement between theoretical predictions and experimental results is obtained.