Syntheses and characterizations of a controlled thermally degradable epoxy resin system for electronic packaging

Abstract

In flip-chip technology, reworkable underfill development has been key to the recovery of highly integrated board assembly designs by replacing defective chips. This paper reports the synthesis, formulation and characterization of two new diepoxides containing secondary and tertiary ester linkages that are thermally degradable below 300/spl deg/C. The secondary and tertiary ester diepoxides were synthesized in three and two steps, respectively. The compounds were characterized via NMR and FTIR spectroscopy, and formulated into underfills with anhydride hardener and imidazole catalyst. A dual-epoxy system was also formulated using tertiary ester diepoxide and an aliphatic diepoxide, ERG-4221E, with the same hardener and catalyst. The curing kinetics were studied by DSC. Thermal properties of cured samples were studied by DSC, TGA and thermomechanical analysis (TMA). The dual-epoxy system showed a viscosity of 18.7, and 0.87 Poise at 25/spl deg/C and 190/spl deg/C, respectively. The cured secondary, tertiary and dual-epoxy formulas showed decomposition temperatures around 265/spl deg/C, 190/spl deg/C and 220/spl deg/C, glass transition temperatures (T/sub g/) around 120-140/spl deg/C, 110-157/spl deg/C and 140-157/spl deg/C, and CTE of 70 ppm//spl deg/C, 72 ppm//spl deg/C and 64 ppm//spl deg/C below their T/sub g/, respectively. The shear strength of the cured dual-epoxy system decreased quickly upon being aged at 230/spl deg/C. Reworkability tests showed that removal from the board of a chip underfilled with this material was easy and the board residue could be removed with a mechanical brush without obvious damage to the solder mask. In summary, the synthesized tertiary epoxide can be used as a reworkable underfill for flip-chip applications.