Development of CSP using Au ball bumps as external connection terminals

We developed a new-structure, fine-pitch chip size package (CSP) with Au ball bumps used as external connection terminals. In the manufacturing process, a substrate consisting of a base material made of a Cu alloy (0.3Cr-0.25Sn-0.2Zn-Cu) and spot Ag plating applied to one side was used as a temporary interposer. After the molding process, only the substrate was dissolved selectively, and an interposer-less structure was realized. We founded that we can directly connect Au ball bumps to tip balls of Au wires exposed on the back surface of a package by applying etching dissolution to a substrate. Thus, terminals can now be arranged in as fine a pitch as a wire bonding pitch. We verified that each bond between a Au ball bump and a Au wire has sufficient strength, and the strength did not decrease after 1000 cycles of -55°C/+125°C. We attempted mounting on board using the Au-Au bonding method with an insulating adhesive. No void occurred in the connection areas, and the electrodes on the substrate were properly deformed and connected. After 1400 cycles of -55°C/+125°C, electric resistance of interconnect portion did not decrease, and thus, a high degree of mounting reliability was confirmed.