錫-9wt％鋅銲料之電遷移研究

Abstract

Removing Pb from the electronics has became a global trend and the traditional SnPb solder will soon be replaced. The Sn-9Zn (in wt.%) solder alloy is one of the potential candidates because it has a melting point (198.5℃) that is closer to that of the conventional eutectic SnPb solder (183℃). The subject of this study is to investigate the microstructural evolution of the Sn-9Zn solder under current stressing with a current density of about 105 A/cm2. Two different cooling conditions, furnace and fan cooling, were used in the reflow of the solder, and by which coarse and fine Zn precipitates were formed in the solder, respectively. After current stressing, Sn extrusion was observed, suggesting that Sn is the dominant moving species under electromigration. In contrast, Zn appeared to be immobile. It was also found that the microstructure of the solder had a significant effect on the electromigration behavior. For the solder with coarse Zn precipitates, more Sn extrusion sites were observed, and they were located not only at the anode side but also within the solder. Coarse Zn precipitates appeared to block Sn migration, thus Sn migration was intercepted in front of the Zn precipitates. The Sn atoms accumulated there, which led to its extrusion. The blocking effect was found to depend strongly on the size and orientation of the Zn precipitates. Effects of the addition of Bi and Cu into the Sn-9Zn solder on its electromigration behavior were also investigated. The solders used are Sn-8Zn-3Bi and Sn-9Zn-1Cu. In the Sn-8Zn-3Bi solder, Bi migrated ahead of Sn along with the electron toward the anode side under current stressing. The Sn-9Zn-1Cu solder displays a microstructure different from that of the Sn-9Zn solder because Cu reacts with Zn to form the CuZn5 and Cu5Zn8 compounds in the solder. After current stressing, voids were formed in the solder matrix and mainly surrounded the Cu-Zn compounds.