Study of additive-epoxy interaction of thermally reworkable underfills

Underfill is the material used in a flip-chip device to dramatically enhance its reliability as compared to a nonunderfilled device. Current underfills are mainly epoxy-based materials that are not reworkable after curing, which is an obstacle in flip-chip technology developments, where unknown bad die is a concern. Reworkable underfill is the key to address the nonreworkability of the flip-chip devices. The ultimate goal of this study is to develop epoxy-based thermally reworkable underfills. Our previous work showed that when incorporated into epoxy matrix, special additives could provide the epoxy formulation with die-removal capability around solder reflow temperature. The additive-epoxy interactions were studied and the results show that the additives do not adversely affect the epoxy properties. Moreover, when the additive decomposition temperature is reached, the decomposition of the additive causes a microexplosion within the epoxy matrix. Subsequently, the adhesion of the epoxy matrix is greatly reduced. Among the four additives studied, Additivel and Additive2 may be used in reworkable underfills that can be reworked around solder reflow temperature, Additive3 cannot be used in underfill because it greatly reduces the shelf life of the underfill, and Additive4 may be used to develop reworkable underfill that withstands multiple reflows.