Abstract
In this paper, the authors have investigated mechanical fatigue strength of lead-free solder joints. The use of Sn-Zn-Bi and Sn-Zn solder is increasing because of the advantage of 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 has been studied, by using the CSP(Chip Size Package) specimen. In order to investigate the influence of plating treatment on the fatigue strength of the solder joint, the specimens with Ni/Au or Cu plating treatment on Cu-pad were prepared. And these specimens were aged by holding 200, 500, 1000 hours at 85, 125, 150℃, respectively to investigate the influence of the growth of the intermetallic compounds on the fatigue strength. Through a series of isothermal mechanical shear fatigue tests and FEM(Finite Element Method) analysis, it has been found that if the Sn-Zn-Bi and Sn-Zn solder with Ni/Au plating treatment were aged at the temperature above 125℃, the fatigue strength decreased remarkably in comparison with the specimens with Cu plating. By plating with the Ni/Au on the substrate side, the interface in the substrate side was strengthened. But, the interface in the chip side is weakened. It causes the interface fatigue crack between intermetallic compound layer and solder at the chip side, and the decrease of fatigue strength.
