Evolution of Contact Resistance during the Bonding Process of NCA Flip-Chip Interconnections

Abstract

Non-conductive adhesive (NCA) flip-chip interconnects are emerging as an attractive alternative to lead or lead-free solder interconnects due to their environmental friendliness, lower processing temperatures, and extendability to fine-pitch applications. The electrical connectivity of an NCA interconnect relies solely on the pure mechanical contact between the integrated circuit (IC) bump and the substrate pad; the electrical conductivity of the contact depends on the mechanical contact pressure, which in turns depends to a large extent on the cure shrinkage characteristics of the NCA. Therefore, it is necessary to monitor the evolution of the electrical conductivity which could reflect the impact of cure- and thermal-induced stresses during the curing and cooling process, respectively. In this article, in situ measurement of the development of contact resistance during the bonding process of test chips was developed by using a mechanical tester combined with a four-wire resistance measurement system. A drop of resistance induced by the cure stress during the bonding process is clearly observed. With decreasing bonding temperature, the drop of contact resistance induced by cure shrinkage becomes larger, while the cooling-induced drop of resistance becomes smaller. The evolution of contact resistance agrees well with experimental observations of cure stress build-up. It is found that vitrification transformation during the curing of the adhesive could lead to a large cure stress and result in the reduction of the contact resistance. Furthermore, no obvious changes were observed when the applied load was removed at the end of bonding.