Wire bondability of Au/Ni bond pads: effects of metallisation schemes and processing conditions

The wire bondability and surface characteristics were studied of Au/Ni bond pads on organic FR-4 PCBs with different gold plating schemes and gold thickness. Three different plating techniques were used to deposit gold layers, namely the electrolytic gold plating, immersion gold plating and immersion gold plating followed by electrolytic gold plating. Process windows for the individual metallisation schemes were established to optimize the wire bond force and bond power. Bond performance after wire bonding was evaluated based on the wire pull strength and wire deformation ratio. Various elemental and surface characterization techniques, such as SEM, optical profiler, XPS, nano-hardness test and surface energy analysis, were employed to characterize the bond pad surfaces, which were then correlated to the wire bond performance. The effects of plasma treatment using Ar and O₂ gases were also studied.