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

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°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°C and 190°C, respectively. The cured secondary, tertiary and dual-epoxy formulas showed decomposition temperatures around 265°C, 190°C and 220°C, glass transition temperatures (T_g) around 120-140°C, 110-157°C and 140-157°C, and CTE of 70 ppm/°C, 72 ppm/°C and 64 ppm/°C below their T_g, respectively. The shear strength of the cured dual-epoxy system decreased quickly upon being aged at 230°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