Strain rate effect and Johnson-Cook models of lead-free solder alloys

Drop/impact causes high strain rate deformation in solder joints of microelectronics package. It is important to understand mechanical behavior of solder joints under high strain rate for reliability design of products. In this paper mechanical behaviors of two lead-free solder alloys, Sn3.5Ag and Sn3.0Ag0.5Cu, were investigated by quasi-static tests and the split Hopkinson tension/pressure bar testing technique under high strain rate (600~2200s^-1). The experimental results show that the two materials are sensitive to strain rate and their dynamic flow stresses are much greater than their static flow stresses. The higher the strain rate, the greater their tensile strength but less their fracture strains. Based on the experimental data, constitutive models in the Johnson-Cook form for the two lead-free alloys were derived. The models were then used to simulate the testing process by incorporating it into ABAQUS. The good agreement between the numerical simulations and the experiments indicates that the presented Johnson-Cook models are suitable and reliable to describe dynamic behavior of the two solder alloys under high strain rates. Finally, the proposed constitutive models were used to compute peeling stress and plastic strain of solder joints under drop/impact loadings, and the results were compared with that by linear elastic model and tri-linear elastic-plastic model to show the necessity of using a strain rate dependent material model in simulation of drop/impact of solder joints.