Effect of primary creep behavior on fatigue damage accumulation rates in accelerated thermal cycling of Sn3.0Ag0.5Cu Pb-free interconnects

Most studies on modeling cyclic thermo- mechanical fatigue of solder interconnects consider only the steady-state creep behavior of solder materials. This study explores the effect of including the primary creep behavior; since it may represent a significant portion of the total viscoplastic deformations in Pb-free Sn3.0Ag0.5Cu (SAC305) solder (G. Cuddalorepatta et al., 2007). The solder constitutive behavior is modeled using partitioned elastic, plastic, and creep models. The creep properties are obtained from traditional constant-stress tests on a modified Iosipescu shear specimen, at different temperatures and stress levels (G. Cuddalorepatta et al., 2007). Significant scatter is observed across different specimens since the large grains in SAC305 microstructure make the specimen statistically inhomogeneous. Suitable averaging schemes are developed to deal with this scatter. In this study, constant-deformation tests are first conducted to measure the stress relaxation behavior at different temperatures and deformation levels (G. Cuddalorepatta et al., 2007). Finite element models of the constant-deformation tests reveal that the average behavior can be predicted with the measured average creep properties. This prediction is improved if the primary creep is included in the constitutive model. This exercise verifies that creep behavior measured under constant stress can indeed be used to predict deformations for varying stress loads. Using these verified constitutive models, the cyclic thermo-mechanical response of solder is investigated under accelerated thermal cycling conditions. The goal is to identify the difference in the fatigue damage metrics with and without primary creep. A 3D viscoplastic finite element model of a selected BGA256 assembly is developed for this study. The model is subjected to an accelerated temperature cycle between -40degC and 125degC and the cyclic fatigue damage is assessed. These results are compared with previous studies where secondary creep alone was used to model the viscoplastic constitutive behavior solder. Comparisons show that the results are strongly sensitive to the inclusion or exclusion of primary creep behavior. These results suggest that it is very important to include primary creep when studying thermo-mechanical fatigue damage in Pb-free SAC solders. The results of the creep and stress relaxation tests and simulations suggest that the primary creep model can be improved to better capture the viscoplastic response.