Mechanical properties of near-eutectic Sn-Ag-Cu alloy over a wide range of temperatures and strain rates

The deformation properties of near-eutectic Sn-Ag-Cu alloy were measured in temperatures ranging from −25 to 125°C, and down to strain rates of about 10×10⁻⁹. Results have been combined into a stress versus strain rate master curve. The measurements were done with dog-bone specimens that have a 1-mm diameter, which corresponds to a typical solder joint diameter in ball grid arrays (BGAs). Effects of cooling rate were also studied, with cooling rates from 0.1 to 1 degrees/sec. The stress exponent of the fast-cooled samples was high, about 16. The activation energy was about 1 eV. The relatively high temperature dependence suggests that bulk diffusion is dominating. Optical microscopy, scanning electron microscopy (SEM) and electron backscattering diffraction (EBSD) were used to study the microstructures of the test samples. The slower cooled samples had large Ag3Sn plates, but the size of the plates was significantly reduced with the faster cooling rates. The yield strength increased with cooling rate, reflecting the larger amount of alloying elements remaining in the solution and smaller, dispersed precipitates. For comparison, experiments were also performed on binary AgSn and CuSn solders, pure Sn, and with two reduced silver content SAC alloys, Sn-2.5% Ag-0.7% Cu and Sn-3.0% Ag-0.7% Cu.