Understanding the impact response of lead-free solder at high strain rates

As resistance to drop impact is a standardized evaluation for consumer electronics, the dynamic behaviour of solder material is critical to ensure the prediction accuracy about damage and failure in solder joints using finite element analysis. In this paper, the dynamic constitutive behaviour of Sn-3.0Ag-0.5Cu (SAC305) lead-free solder is investigated at different strain rates from 733.8/s to 1758.1/s by using the split-Hopkinson pressure bar (SHPB) with the applied gas gun pressure increasing from 0.1 MPa to 0.5 MPa at room temperature. It is shown that the flow stress increases but the strain rate decreases with the increasing strain of a visco-plastic solder alloy. More importantly, it is found that the strain rate effect is pronounced and the peak strength is thus enhanced until the strain rate is up to 1172.9/s. Nevertheless, if the strain rate increases further, the peak strength deteriorates. Under such high strain rates, this interesting transition is speculated due to the temperature rise and the induced softening effect on material strength of the SAC305 solder with a relatively low melting point. This speculation is supported by the fact that compared with the solder samples under different strain rates, the microstructures of post-impact solder samples with degraded strengths are observed to be apparently different by using a scanning electron microscopy. A finer morphology of solder material results from the rapid cooling process after the transient temperature rise due to a high impact velocity. Lastly, the dynamic constitutive behaviour is further described in the framework of the strain rate dependent Johnson-Cook model and validated using finite element simulations.