Polymer flip chip technology on flexible substrates-development and applications

Compared with classic solder flip chip technologies, the use of isotropic conductive adhesive for bumping and assembling results in a number of advantages such as a simple yet versatile process, lower temperatures and environmental friendliness. The polymer flip chip (PFC ^TM) technology requires suitable bond pads. The commonly used aluminum alloys result in unstable contacts for the conductive adhesives and hence there is an urgent need for an additional under-bump metallization for the PFC^TM technology. KSW Microtec has developed a technology which is based on electroless deposition of palladium. The metallization is executed as a back end process. The bond pads are deposited with a homogeneous and pin-hole free Pd thin film with a thickness of about 1 μm without significant reduction of the Al bond pad metallization. The deposited film surface does not require flash gold metallization because of the noble characteristics of palladium. This simplifies the process in comparison to the well known Ni/Au process. The contact resistance between conductive adhesives and palladium metallization are equal to or better than gold/adhesive interconnections. As an example of the versatility of the PFC_TM technology, products manufactured by KSW Microtec include chip on dycostrate, chip on flex with screen printed polymer tracks, and smart cards. Experiences with the set-up and qualification of PFC^TM technology at KSW Microtec are also discussed