Low cost underbump metallisation for flip chip packages

Recent packaging roadmaps show that the flip chip technology will be an important technology for future packaging of electronic devices clue to ifs capability to provide high I/O density and superior electrical performance Since the dominant flip chip technology that is used extensively is relatively expensive, a number of alternative low cost processes are currently being investigated This work addresses such a process that is based on maskless deposition of electroless nickel/gold metallurgy as the underbump metallisation (UBM). in this process, aluminum terminal pads are sensitized by a zincate pretreatment, followed by electroless nickel and gold plating to form the UBM. In the present work a number of zincate treatment techniques were investigated and compared in order to establish the optimum UBM process. Lend free Sn-Ag solder balls were subsequently placed onto the UBM and reflowed to form the solder blimps. One of the accomplishments of the present work is the development of a relatively simple zincate process that is required for the formation of high quality nickel layer over the aluminum bond pad. In order to optimize the process parameters, scanning electron microscope (SEM) and energy dispersive x-ray (EDX) techniques were employed for morphological study and compositional analysis. In addition, sheer test was performed after solder reflow to evaluate the mechanical integrity of the solder bump-UBM structure. The fracture was found to occur within the solder region and the sheer strength was found to be at least comparable or higher than that reported in the literature.