Revisit of wirebonding on immersion silver-finish board

In this article, we evaluated the bondability of aluminum and gold wires on multilayer FR-4 printed circuit board (PCB) with immersion silver finish by using an automatic wedge bonder equipped with ultrasonic transducers operating at a conventional 62 kHz and at a higher frequency of 138 kHz. Very good aluminum bondability was obtained at both bonding frequencies. However, the bonder with transducer at higher frequency provided a larger window for setting the bond power. Its bond points exhibit smaller deformation of the bonding wire, less heel cracking and less metal splash. Tarnishing of the silver finish was observed if the boards were left in the atmosphere for days. XPS analysis revealed an additional layer of sulfur compound on the silver surface. The adverse effect of this layer made wirebonding very difficult and proper board storage was important to maintain good bondability.