Stress–Strain Characteristics of Tin-Based Solder Alloys for Drop-Impact Modeling

The stress–strain properties of eutectic Sn-Pb and lead-free solders at strain rates between 0.1 s−1 and 300 s−1 are required to support finite-element modeling of the solder joints during board-level mechanical shock and product-level drop-impact testing. However, there is very limited data in this range because this is beyond the limit of conventional mechanical testing and below the limit of the split Hopkinson pressure bar test method. In this paper, a specialized drop-weight test was developed and, together with a conventional mechanical tester, the true stress–strain properties of four solder alloys (63Sn-37Pb, Sn-1.0Ag-0.1Cu, Sn-3.5Ag, and Sn-3.0Ag-0.5Cu) were generated for strain rates in the range from 0.005 s−1 to 300 s−1. The sensitivity of the solders was found to be independent of strain level but to increase with increased strain rate. The Sn-3.5Ag and the Sn-3.0Ag-0.5Cu solders exhibited not only higher flow stress at relatively low strain rate but, compared to Sn-37Pb, both also exhibited higher rate sensitivity that contributes to the weakness of these two lead-free solder joints when subjected to drop impact loading.