Fracture behavior of Sn-3.5Ag-0.7Cu and pure Sn solders as a function of applied strain rate

The demand for environmentally benign Pb-free solders is increasing, and the push toward smaller portable electronics will make it more likely for solder interconnects to encounter mechanical shock through dropping or mishandling. Thus, fundamental understanding of the relationship between solder microstructure and mechanical shock resistance is essential for developing reliable numerical models of mechanical shock behavior. In this paper we report on the strain rate-dependent mechanical behavior of pure Sn and Sn-3.5Ag-0.7Cu solders, measured from tensile tests conducted in the strain rate range from 10 ^-3 s ^-1 to 30 s ^-1. Local strain and strain rate distributions were measured by digital image correlation. Finally, the strain rate dependence of fracture mechanisms is discussed. For a given strain rate, water-quenched tin-silver-copper (SAC) had the greatest ultimate tensile strength (UTS), followed by furnace-cooled SAC, then pure Sn. Furnace-cooled SAC had lower ductility than water-quenched SAC, due to large Ag ₃Sn needles that nucleated elongated voids which easily coalesced.