Development of new reworkable epoxy resins for flip chip underfill applications

Underfill is a polymeric material used in the flip-chip devices that fills the gap between an IC chip and an organic board, and encapsulates the solder interconnects. This underfill material can dramatically enhance the reliability of the flip-chip devices as compared to non-underfilled devices. Current employed underfills are mainly silica filled epoxy-based materials that are not reworkable after curing, an obstacle in Flip-Chip on Board (FCOB) and Multi-chip Module (MCM) technology developments where unknown bad dies is still a concern. Reworkable underfill is the key material to address the non-reworkability of the FCOB packages. We have synthesized a series of thermally degradable epoxies that show good reworkability. However, all these epoxies have degradation temperature higher than 250°C. Besides, they are cycloaliphatic-only-based which have shown weakness in toughness and thus may not perform well for very high reliability applications. In the search for high performance epoxy resins for reworkable underfills, our approach is to incorporate aromatic moiety and thermally labile group that decomposes around solder reflow temperature. This paper reports part of our recent approach in the study and development of new epoxy resins that may offer better rework properties. Two new diepoxides containing tertiary ester, tertiary carbonate, and aromatic moieties were synthesized. These epoxy compounds exist as liquids at ambient temperature. The curing properties of these two epoxides and thermal properties of cured resins of these epoxy compounds were characterized with DSC, TGA and TMA