Creep Behavior of 95.8Sn–3.5Ag–0.7Cu Solder Joints, and a Modified Constitutive Model for the Joints

The creep behavior of 95.8Sn–3.5Ag–0.7Cu shear-lap solder joints was investigated at different shear stresses ranging from 2–26 MPa and test temperatures of 25, 75, and 125°C. The stress exponent can be clearly defined in the low-stress ( τ 15 MPa) ranges. The stress exponent is larger in the high-stress range, and decreases with increasing temperature in both low and high-stress ranges. The average modulus compensated shear stress transition point and the average activation energy were determined to be 1.08 × 10 −3 and 90.59 kJ/mol, respectively. A creep constitutive model with internal stress incorporated into the Garofalo hyperbolic sine law model was used to describe the creep behavior of 95.8Sn–3.5Ag–0.7Cu shear-lap solder joints. In this model, the relationship between creep strain rate and shear stress was determined by introducing internal stress that is a function of the shear stress in the low-stress range and a function of particle size and volume fraction of intermetallic particles in the high-stress range. The internal stress was calculated on the basis of the different creep mechanisms in the low and high-stress ranges. Results showed that the modified creep constitutive model was consistent with experimental data, which indicates that the model can be used to predict the creep behavior of 95.8Sn–3.5Ag–0.7Cu shear-lap solder joints.