Explanation for Superiority of Thermal Fatigue Characteristics of Sn-3.5Ag-0.5Bi-6In Solder

Abstract

  Recent increase in the working temperature of the electronics products caused larger thermal stress in the solder joints of the printed-circuit board. From the view point of consumer product safety, it is very important to prevent thermal fatigue crack of solder joints. In the paper, we have investigated the thermal fatigue property of the Sn-Ag-Bi-In solders after heat-cycle test from -40°C to 150°C. It was confirmed that Sn-3.5mass%Ag-0.5mass%Bi-6.0mass%In solder material is more reliable than conventional Sn-Ag-Cu solder. By the metallographic and mechanical analyses, it was also confirmed the phase transformation from γ (InSn₄) to Sn + γ at about 150°C affected the thermal fatigue properties by reducing the grain size of the solder. Thus, alloy composition should be optimized to control transformation temperature in accordance with applied conditions.