Sn-Zn-AlPb-free solder - An inherent barrier solder for Cu contact

Flip chip solder bumps were produced on Cu contact applying Sn-9Zn-xAl Pb-free solder by dipping method. The solder bumps were tested under 85 degreesC/85% RH (relative humidity) or at 150 degreesC for 1000 hours to explore the shear strength and the interfacial interaction behavior. Experimental results revealed that Al and Zn, while not Sn, diffuse to the Cu/solder interface during the extended period test. A thin layer of Al4.2Cu3.2Zn0.7 compound, characterized by XRD, was formed at the interface of the as-produced solder bump. This compound, which resulted from the gathering of Al at the interface, provides a barrier to Sn diffusion toward Cu substrate and, thus, no Cu-Sn compound was detected. This is the first time to find a Sn-containing solder which, in contact with Cu, does not form Cu-Sn intermetallic compound during heat treatment and, thus, the Sn-Zn-Al solder is termed an inherent barrier solder.