Relevance of primary creep in thermo-mechanical cycling for life-time prediction in Sn-based solders

The time and temperature dependant creep deformation of solder alloys has to be studied to describe material characteristics and failure behaviour in order to use it for lifetime evaluation by FE-simulations. It is often found in the literature that material behaviour of eutectic SnPb and lead-free such as SnAgCu solders is described considering only secondary creep. To improve the material model, which is already used to predict damage and lifetime of the solders joints, it is now important to investigate primary creep. As this paper will show, primary creep may not be neglected as it is clearly seen in the performed characterisation tests. In the present paper, we will present a possibility to test and to measure both primary and secondary creep of solders under cyclic loading conditions. Combined creep laws using primary and secondary creep for SnPb and SnAgCu will be introduced. In addition it will be shown how this creep law can be implemented in FEM software like Ansys® in order to perform thermo-mechanical simulations of a thermal cycling test. As an example, the behaviour of a flipchip under thermo-mechanical fatigue load was simulated. The calculation was performed with or without primary creep, and the two results were compared to see the relevance of primary creep: we found up to 24% increase in accumulated creep strain using a model combining primary and secondary creep.