Constitutive models of creep for lead-free solders

Abstract

The constitutive modeling of creep has been extensively studied due to the important of the creep failure mode in solder joints. However, there are very few studies that considered room temperature aging contributions in their creep modeling studies. This study investigated constitutive modeling of creep of solders by taking into account the possible contribution room temperature aging. Lead-free solder (Sn–4.0Ag–0.5Cu) was found to have a higher creep resistance than Sn–Pb solder at the same stress level and testing temperature. The higher creep resistance was contributed by the second phase intermetallic compounds, Ag3Sn and Cu6Sn5. The precipitation of these intermetallic compounds can significantly block the movement of dislocations and increase the creep resistance of the material. Constitutive models of creep for both lead-free and Sn–Pb eutectic solders were constructed based on the experimental data. The activation energy for SAC405 is much higher than that of Sn–Pb, which also indicates that SAC405 possesses higher creep resistance. The constitutive models can be used in finite element analysis of actual electronic packages to predict solder joint failure. The creep mechanisms of both lead-free and Sn–Pb eutectic solders were also extensively discussed in this dissertation. Dislocation gliding and climb is believed to be the major failure mode at high stresses, while lattice diffusion and grain boundary diffusion is believed to be the major failure mode at low stress levels. Grain boundary sliding is believed to contribute to creep deformation at both high-stresses and low-stresses. For eutectic Sn–Pb, superplastic deformation is a major the creep mechanism at low-stresses and high-temperatures.