Strain rate effects and material models of solders

Reliability of solder joints in electronic packages depends on themechanical properties of solder materials. Under drop impact loadings, thesolder joint undergoes high strain rate deformation. Therefore it isimportant to investigate the effect of strain rate on the mechanicalbehavior of the solder materials. In this paper, mechanical properties andstress-strain curves of one lead-containing solder, Sn37Pb, and twolead-free solders, Sn3.5Ag and Sn3.0Ag0.5Cu, were investigated at strainrates ranging from 600s^-1 to 2200s^-1 by the split Hopkinsonpressure and tensile bar techniques. Based on the experimental data of thequasi-static tensile tests and the split Hopkinson pressure bar tests,elastic-plastic material models independent of the strain rate andJohnson-Cook material models dependent of the strain rate of the threesolders were developed and employed to predict mechanical behaviors ofsolder joints in a board level electronic package under drop impactloadings. The results show that at high strain rates, the two lead-freesolders are more sensitive to the strain rate, and their tensile strengthsare about 1.5 times greater than that of the lead-containing solder, andtheir ductility is significantly greater than that of the lead-containingsolder. Under the drop impact, the solder joints experience a strain rate of1000s^-1, and the proposed material models in Johnson-Cook form areapplicable to predict more realistic stress and strain than theelastic-plastic models independent of strain rate.