The evolution of microstructure and microhardness of Sn-Ag and Sn-Cu solders during high temperature aging

The changes in microstructure and microhardness of Sn-0.5%Ag, Sn-1.0%Ag, and Sn-0.7%Cu Pb-free solders were investigated during high temperature aging at 200°C for 2 h. As-solidified microstructures, revealed by cross-polarized light microscopy, consist of relatively large grains of β-Sn phase with twinned microstructure in both Sn-Ag and Sn-Cu solders. The bright-field light microscopy displays Sn-dendrite cellular structure in Sn-Ag, and a mixture of Sn-dendrite cells and the eutectic microstructure in Sn-Cu. Upon aging at 200°C, 2 h, Sn grains become smaller compared to the as-solidified ones. In addition, the microhardness of Sn-Ag solders surprisingly increases after the aging, while that of Sn-Cu solder decreases. Detailed inspection of the redistribution and coarsening of intermetallic particles in each system further explains this unusual response of mechanical properties during high temperature aging. In this study, it is demonstrated that the microhardness of Sn-Ag and Sn-Cu solders is better correlated with the characteristics of their intermetallics, such as particle size, density and distribution, rather than Sn grain size.