Study of additive-epoxy interaction of thermally reworkable underfills

Underfill is the material used in a flip-chip device to dramatically enhanc e its reliability as compared to a nonunderfilled device. Current underfill s are mainly epoxy-based materials that are not reworkable after curing, wh ich is an obstacle in flip-chip technology developments, where unknown bad die is a concern. Reworkable underfill is the key to address the nonreworka bility of the flip-chip devices. The ultimate goal of this study is to deve lop epoxy-based thermally reworkable underfills. Our previous work showed t hat when incorporated into epoxy matrix, special additives could provide th e epoxy formulation with die-removal capability around solder reflow temper ature. The additive-epoxy interactions were studied and the results show th at the additives do not adversely affect the epoxy properties. Moreover, wh en 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 additi ves studied, Additive1 and Additive2 may be used in reworkable underfills t hat can be reworked around solder reflow temperature, Additive3 cannot be u sed 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. (C) 2001 John Wiley & Sons, Inc.