Study of a controlled thermally degradable epoxy resin system for electronic packaging

In flip-chip technology, the reworkable underfill material development has been one of the keys to the recovery of highly integrated and expensive board assembly designs by replacing defected chips. This paper reports the synthesis, formulation and characterizations of two new diepoxides, one contains secondary and the other contains tertiary ester linkages that are thermally degradable below 300°C. The secondary and the tertiary ester diepoxides were synthesized in three and two steps, respectively. Both compounds were characterized with NMR and FT-IR spectroscopies, and formulated into underfill materials with an anhydride as hardener and an imidazole as catalyst. A dual-epoxy system was also formulated containing the tertiary ester diepoxide and a conventional aliphatic diepoxide, ERL-4221E, with the same hardener and catalyst. The curing kinetics of the formulas was studied with differential scanning calorimetry (DSC). Thermal properties of cured samples were characterized with DSC, thermogravimetric analysis (TGA) and Thermomechanical analysis (TMA). The dual-epoxy system showed a viscosity of 18.7, and 0.87 Poise at 25°C and 100°C, respectively. The cured secondary, tertiary and dual-epoxy formulas showed decomposition temperatures around 265°C, 190°C and 220°C, glass transition temperatures (Tg) 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 Tg, respectively. The shear strength of the cured dual-epoxy system decreased quickly upon being aged at 230°C. The reworkability test showed that the removal from the board of a chip underfilled with this material was quite easy, and the residue on the board could be thoroughly removed with a mechanical brush without obvious damage of the solder mask. In summary, the synthesized tertiary epoxide can be used as a reworkable underfill for flip-chip application