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°C/85% RH (relative humidity) or at 150°C 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.
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