Effects of POSS-silanol addition on the whisker formation in Sn3.0Ag0.5Cu Pb-free solder

The whisker formation behaviors of Sn3.0Ag0.5Cu composite solder which reinforced by an organic-inorganic hybrid materials, silanol of polyhedral oligomeric silsesquioxane (POSS-silanol) was investigated. The thin solder films were obtained by reflow soldering and a thermal cycle from -40°C to +85°C was applied to accelerate the test process. Both surface and interface microstructures of the specimens were evaluated using scanning electron microscopy (SEM). The results revealed that the growth of whiskers was inhibited significantly in solders with the addition of POSS-silanol. Fewer whiskers with smaller size were observed on the Sn3.0Ag0.5Cu+POSS solder when compared with Sn3.0Ag0.5Cu solder. The main driving force for the whisker formation under such condition was considered to be the squeeze due to the deformation of the solder matrix which induced by the coefficient of thermal expansion (CTE) mismatch. The POSS addition alleviated the deformation which dominated by CET mismatch. Thus, the driving force of whisker formation was weaken through releasing of the stress concentration during the iterative of heating and cooling process.