Influences of aging treatment on microstructure and hardness of Sn-(Ag, Bi, Zn) eutectic solder alloys

Abstract

The microstructural change and hardness, which strongly influence the life of soldered joints, were investigated by microstructure observations for three typical promising alloys as Pb free solder, Sn-3.5 mass% Ag, Sn-8.8 mass% Zn and Sn-57 mass% Bi alloys, and Sn-38.1 mass%Pb for comparison. The solders of Sn-Ag and Sn-Zn eutectic alloys and Sn-Pb eutectic alloy were found to age-soften during storage at 25-100°C (298-373 K). It seems that the coarsening of the microstructure and re-crystallization caused the age-softening of the Sn-Ag and Sn-Zn alloys. The hardness of the Sn-Bi eutectic alloy hardly changed during storage at 25-100°C (298-373 K) in spite of remarkable microstructural coarsening. It seems that the hardening effect caused by solid-solution hardening of Bi into Sn-rich solution and/or the precipitation hardening of Bi in the Sn-rich solution compensated the softening caused by the remarkable microstructural coarsening of the Sn-Bi alloy.