Influence of the Interfacial Reaction Layer on Reliability of CSP Joints Using Sn-8Zn-3Bi Solder and Ni/Au Plating

In this study, we used a solder paste of Sn-8Zn-3Bi. The structure and specification of the CSP used in this study are shown in Fig. 1. The composition of the solder ball of the CSP is Sn-3.0Ag-0.5Cu. The CSPs were reflow-soldered on the Cu pad of the FR-4 PCB using Sn-8Zn-3Bi solder paste in air. The surface finish on the Cu pad was Ni-P(5 mm)/ Au(0.05 mm), Ni-P(5 mm)/Au(0.5 mm), or heat-resistant preflux (Fig. 2). The Ni-P layer was formed by electroless plating and the Au layer was formed by substitution plating. Figure 3 schematically illustrates thermal process for reflowsoldering. The reflow profile was composed of preheat at 403423 K for 60 s, followed by 20 s at a temperature above