Fatigue Life Assessment for Lead-Free Solder Joints

The authors will indicate the basic reliability problems associated with use of lead-free solder joints. They investigated the thermal fatigue reliability of lead-free solder joints, and focused their attention on the formation of the intermetallic compound and its effect on the initiation and propagation of the fatigue cracks. An isothermal fatigue test method was used to improve the efficiency of fatigue study. Several lead-free solder alloys, Sn-Ag-Cu, Sn-Ag-Cu-Bi, Sn-Cu and Sn-Zn-Bi, were investigated.

It was found that there were two kinds of major failure mode in lead-free solder joints: the solder bulk fatigue mode, and the interface fatigue mode. It was found also that the mode shift of the fatigue crack was affected by not only the properties of the intermetallic layer, but also by the tensile strength of the solder material. In order to investigate the influence of plating treatment on the fatigue strength of Sn-Zn-Bi solder joint, specimens with Ni/Au or Cu plating treatment on Cu-pad were used. Through a series of isothermal mechanical shear fatigue tests and FEM (Finite Element Method) analysis, it has been found that the fatigue life of Sn-Zn-Bi solder joint was greatly affected by the environmental temperature and plating conditions.