Effects of prestrain, rate of prestrain, and temperature on the stress-relaxation behavior of eutectic Sn-3.5Ag solder joints

Abstract

Stress-relaxation studies on eutectic Sn-Ag solder (Sn-3.5Ag in wt.%) joints were carried out at various temperatures after imposing different amounts and rates of simple shear strain. Stress-relaxation parameters were evaluated by subjecting geometrically realistic solder joints with a nominal joint thickness of ∼100 µm and a 1 mm × 1 mm solder-joint area. The peak shear stress during preloading and residual shear stress resulting from stress relaxation were higher at the low-temperature extremes than those at high-temperature extremes. Also, those values increased with increasing simple shear strain and the rate of simple shear strain imposed prior to the stress-relaxation events. The relaxation stress is insensitive to simple shear strain at 150°C, but at lower temperatures, a faster rate of simple shear strain causes a higher relaxed-stress value. The resulting deformation structures observed from the solder-joint side surfaces were also strongly affected by these parameters. At high temperature, grain-boundary sliding effects were commonly observed. At low temperature, intense shear bands dominated, and no grain-boundary sliding effects were observed.