When near-eutectic Sn-Ag-Cu (SAC) alloys are used to make soldered ballgrid-array (BGA) assemblies, the grain size of the joints is very large. During thermomechanical cycling, the solder joint fatigue process is often initiated with recrystallization of the Sn grains, resulting in a smaller grain size in the deformed areas. Grain boundary sliding and increased grain boundary diffusion then results in intergranular crack nucleation and propagation along the recrystallized Sn grain boundaries. In this work, fatigue tests were used to study the initial stages of cyclic deformation damage in Sn-Ag-Cu alloy samples. To separate the solder properties from the constraints introduced by the substrate, the tests were done with free-standing solder specimens, instead of solder joints. The test samples were cast dog-bone specimens that have a cross-sectional diameter of 1 mm, which corresponds to a typical solder joint diameter in BGAs. Mechanical cycling was performed isothermally at several temperatures, from )25°C to 125°C. Typical test conditions were ± 1.5% strain and 15-minute hold at tensile peak stress to allow stress relaxation to take place. Optical microscopy, scanning electron microscopy, and electron backscattering diffraction were used to study the microstructures of the samples before and after fatigue testing in order to obtain insight into the nucleation and growth mechanisms of fatigue damage.

The deformation properties of near-eutectic Sn-Ag-Cu alloy were measured in temperatures ranging from Ϫ25 to 125°C, and down to strain rates of about 10 ϫ 10 Ϫ9 . 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.