Electromigration of Sn-9Zn solder in contact with Cu

Pb-free solder has become an important issue in electronic industry. Sn-9Zn solder with a relatively low eutectic temperature of 198°C has attracted attentions as an alternative for Sn-37Pb solder. Recent studies have pointed out the importance of electromigration in advanced packaging solder joint like flip chip bonding. This present study investigated the electromigration behavior between Cu and Sn-9Zn solder under a current density of 650 A/cm² for up to 216 hours. The thickness of the solder specimen was 100 μm while that of the Cu plating was 20 μm. The surface of the Cu plating on both sides of the solder specimen was further protected with an Au layer deposited with sputtering deposition. The experimental results indicated that Cu-Zn intermetallic compound was formed at the interface between Cu and cathodic side of the Sn9Zn solder, and within the bulk solder near the interface. This compound was identified to be Cu5Zn8 with EDS. These compounds were found to exist as far as 60 μm away from the interface. Consumption of Cu was also observed for the Cu plating on the cathodic side. Similar occurrences were observed on the anodic side but with less extent of compound formation and Cu consumption.