Cu wire bonding with Cu BSOB for SiP & stacked die application: Challenges & solutions

In this paper, key challenges for copper (Cu) BSOB wire bonding process and its solutions are discussed. Cu wire bonding is an alternative interconnection technology for gold (Au) wire bonding. Cu wire is attractive because of its cost, electrical and thermo-mechanical properties. BSOB is the short form of bond-stitch-on-ball. It is used on devices that require die-die wire bonding in multi chip modules (MCM), stacked die and system in package (SiP) applications. BSOB also widely used while performing reverse bonding. Replacing gold (Au) wire with Cu wire in the wire bonding process comes with technical challenges. In this study, the process and parameter requirements for a stable Cu BSOB process are identified. They are bonding concept and special software features for Cu wire bonding to have minimum bonding stress to the bond pad, Cu bump with flat top surface without any protruded tail of the Cu bump, oxidation control of the Cu bump and stable stitch bond parameters good enough to break the Cu oxide on the Cu bump at the same this should not be high to make the wire skid. The Cu BSOB process also applied in few case studies.