Ductile-to-brittle transition strain rate

Solder joint failure mode depends on factors such as loading mode (shear vs. tensile), temperature, and strain rate. For any given loading mode and temperature, there is a transition from ductile failure to brittle failure as strain rate is increased. This ductile-to-brittle transition strain rate (DTBTSR) is one of the best indicators for robustness relative to impact loading. A higher value for DTBTSR is better. DTBTSR was measured for a wide range of lead free solder joints. The effects of pad metallization, solder alloy, reflow conditions, thermal aging, test temperature, and loading mode were characterized. DTBTSR was found to improve with 1) electroplated Ni/Au over other finishes (as long as the substrate supplier has a well controlled process), 2) less reflow time above liquidus for Cu pad metallization, 3) mild thermal aging, 4) higher test temperature, and 5) shear loading over tensile loading.