Fatigue Reliability Evaluation for Sn–Zn–Bi and Sn–Zn Lead-Free Solder Joints

Abstract

The use of Sn-Zn-Bi and Sn-Zn solders is increasing because of their low cost and low melting point. Therefore, it is important to ensure the fatigue strength of Sn-Zn-Bi and Sn-Zn solder joints. In this study, the mechanical fatigue strength of Sn-Zn-Bi and Sn-Zn solder joints was evaluated by using chip scale package (CSP) specimens. The influence of surface treatment on the fatigue strength was investigated by using specimens with Ni/Au or pre-flux on a Cu-pad. These specimens were aged at 85, 125 and 150 C for 500 and 1000 h to investigate the influence of the appearance of intermetallic compounds at the joint interface. Through a series of isothermal mechanical shear fatigue tests and finite element method (FEM) analyses, it was found that the fatigue lives of Sn-Zn-Bi and Sn-Zn solder joints were greatly affected by the aging temperature. In particular, when Sn-Zn-Bi and Sn-Zn solder joints with Ni/Au plating of the Cu-pad were aged at a temperature above 125 C, the fatigue strength decreased remarkably in comparison with the specimens without Ni/Au plating.