A Creep Model for Solder Alloys

Demand for long-term reliability of electronic packaging has lead to a large number of studies on viscoplastic behavior of solder alloys. Various creep models for solder alloys have been proposed. They range from purely empirical to mechanism based models where dislocation motion and diffusion processes are taken into account. In this study, most commonly used creep models are compared with the test data and implemented in ABAQUS to compare their performance in cycling loading. Finally, a new creep model is proposed that combines best features of many models. It is also shown that, while two creep models may describe the same material stress-strain rate curves equally well, they may yield very different results when utilized for cycling loading. One interesting observation of this study is that the stress exponent, n, also depends on the grain size. INTRODUCTION Solder alloys are extensively used in electronic packaging because of their favorable wetting properties and low melting temperature. However, most electronic packaging solder alloys operate at thermally-activated condition at room temperature, due to their low melting point. Several thermally activated processes, including dislocation glide, dislocation climb, grain boundary diffusion, and lattice diffusion produce and accumulate a large amount of creep deformation which leads to various types of failure in electronic packaging soldered joints.