Epoxy-additive interaction studies of thermally reworkable underfills for flip-chip applications

Underfill encapsulant is the material used in flip-chip devices that fills the gap between the IC chip and the organic board and encapsulates the solder interconnects. This underfill material can dramatically enhance the reliability of flip-chip devices as compared to nonunderfilled devices. Current underfill encapsulants are mainly silica filled epoxy-based materials that are not reworkable after curing, which places an obstacle in flip chip technology developments, where unknown bad die is still a concern. Reworkable underfill encapsulant is the key to address the nonreworkability of the FCOB packages. The goal of this study is to develop epoxy-based thermally reworkable underfills. This paper presents work in development of additive-modified epoxies. Previous work showed that two special additives, when incorporated into the epoxy matrix, could provide the epoxy formulation with die-removal capability at around 200°C. The epoxy-additive interactions were studied and the results are presented in this paper. These include study of the interactions between the underfill composition and several additives before and after curing. The results showed that the additives did not interfere with epoxy curing and adversely affect the epoxy properties. However, when the additive decomposition temperature was reached, the decomposition of the additive caused a large CTE increase in the epoxy matrix