An Experimental and Finite Element Study of Thermal Fatigue Fracture of PbSn Solder Joints

A solder joint specimen has been designed to determine the stress/strain hysteresis response and fracture behavior of 90 percent wtPb/10 percent wtSn solder alloy. The specimen consists of an Al₂O₃ beam and an Al 2024-T4 beam bonded together at the ends with solder. The specimen is subjected to thermal cycling to failure between 40°C to 140°C with a 10°C/min ramp rate and 10-minute hold times. Stress/strain hysteresis loops were experimentally determined as a function of thermal cycles. A method based on the stress relaxation data at hold times has been developed to determine the steady state creep parameters of the solder. A constitutive equation for the solder alloy based on elastic and creep deformation has been formulated and implemented in a finite element code, ABAQUS. Good agreement was obtained between the finite element model and the experimental results. In the thermal fatigue test, crack length versus number of thermal cycles was measured for two different shear strain ranges, and the fracture surface was examined with SEM. The SEM results show a combined transgranular and intergranular fracture. In addition, a significant amount of secondary cracks and voids were generated during thermal fatigue which led to material weakening. A thermal fatigue model based on the C* integral, the measured stress history, and creep properties was employed to model the fracture behavior.