Materials interaction between Pb-free Sn-9Zn solder and Au pad in advanced electronic packages

At present, the electronic packaging industry is actively searching for Pb-free solders due to environment concern of Pb-based solders. Recently, the European Union had proposed to ban the lead-bearing solders by the year of 2008. Moreover, several major Japanese electronic companies, such as Panasonic and Fujitsu, have decided to switch voluntarily to lead-free processes by the year 2002. Among many potential candidates, Sn9Zn solder is a very promising replacement. In fact, NEC-Panasonic and Toshiba have used this solder in their products. The objective of this study is to investigate the reactions between Sn9Zn solder balls with a common surface finish used in the industry. The Au/Ni surface finish is the most common and important one for solder pads in the industry now. In this study, two kinds of intermetallic compounds were formed at different temperature (240°C - AuZn₃, 250°C - AuSn₄). When the reaction between pure Au (50 μm) and Sn9Zn, with short reflow time (1 min) at 240°C, a lot of Au-Zn intermetallic compounds formed at the solder ball and a thin layer of Au-Sn intermetallic compounds formed at the interface. With the reaction time increasing, intermetallic compounds changed from Au-Zn to Au-Zn-Sn. Finally, the Au-Zn-Sn intermetallic compounds were formed completely. During the Sn9Zn reaction with pure Au (50 μm) at 250°C, a small amount of Au-Zn intermetallic compounds are also formed for a short time (1 min). Finally, they changed to Au-Zn-Sn intermetallic compounds completely.