Polyimide foil flip-chip direct bonding

Abstract

Recent developments in the field of neuroprosthetics have created a demand for cost-effective advanced bonding techniques to mount polymer-based, thin-film, multi-electrode arrays onto PCBs. Low thicknesses of polymer substrates present challenges for common packaging technologies. Here we implemented a cost-effective direct bonding process of polyimide thin-film electrode arrays using vapor phase soldering. Our polymer foil is composed of PI-2611 and contains embedded gold/platinum/chromium traces. Contact pads are platinum coated and no under bump metallization is required. Instead of using wire bonding techniques, the contact pads are flip-chip bonded directly onto the PCB using a lead free solder paste. Transfer of the foil is carried out by vacuum placement. Soldering is subsequently performed in a vapor phase soldering oven. In contrast to wire bonding on flexible substrate materials, vapor phase soldering results in enhanced contact yield of approximately 99%. The landing area of the thin-film electrode, containing the soldered contact pads, is then passivated with a low-shrinkage epoxy in a pin-transfer process. Using such a low-shrinkage material is paramount to achieve sufficient long-term stability of the solder connections and to stabilize the thin polyimide substrate.