High strain rate behavior of Sn3.8Ag0.7Cu solder alloys and its influence on the fracture location within solder joints

Significant work has been done on the characterization of SnAgCu solder alloys at low strain rates (10^-6 to 10^-2s^-1), and as a result, the behavior of solder over these strain rate regimes is well understood. On the other hand, there is a lack of accurate and consistent data for solder at high strain rates. In this paper, we will present data obtained using a servo-hydraulic mechanical tester and split-Hopkinson bar for the Sn3.8wt%Ag0.7wt%Cu solder alloy over strain rates spanning 0.001 to 500s ^-1. It is shown that the saturation stress correlates well with strain rate over nine decades on a log-log plot. It is also shown that a fit using Anand model based on low strain rate regime (4×10^-6 to 2×10^-4s^-1) data captures the high strain rate results to a reasonable accuracy. It is commonly observed that in low strain rate failure, as in thermo-mechanical fatigue, failure tends to occur through the bulk of the solder. However in high strain rate failures, as those seen in drop tests, fractures occur through the intermetallic layer. We present finite element simulations of ball shear and ball pull tests using the above high strain rate data. It is demonstrated how the shift in failure mode from the bulk solder to intermetallic compound may be explained based on the high strain rate behavior of the SnAgCu solder alloy.