Syntheses and characterizations of thermally degradable epoxy resins. III

摘要

In flip-chip technology, the development of reworkable underfill materials has been one of the keys to the recovery of highly integrated and expensive board assembly designs through the replacement of defective chips. This article reports the syntheses, formulations, and characterizations of two new diepoxides, one containing secondary ester linkages and the other containing tertiary ester linkages, that are thermally degradable below 300 degreesC. The secondary and tertiary ester diepoxides were synthesized in three and two steps, respectively. Both compounds were characterized with NMR and Fourier transform infrared spectroscopy and formulated into underfill materials with an anhydride as the hardener and an imidazole as the catalyst. A dual-epoxy system was also formulated containing the tertiary ester diepoxide and a conventional aliphatic diepoxide, 3,4-epoxy cyclohexyl methyl-3,4-epoxycyclohexyl carboxylate (ERL-4221E), with the same hardener and catalyst. The curing kinetics of the formulas were studied with differential scanning calorimetry (DSC). Thermal properties of cured samples were characterized with DSC, thermogravimetric analysis, and thermomechanical analysis. The dual-epoxy system showed a viscosity of 18.7 and 0.87 P at 25 and 100 degreesC, respectively. The cured secondary, tertiary, and dual-epoxy formulas showed decomposition temperatures around 265, 190, and 220 degreesC, glass-transition temperatures around 120-140, 110-157, and 140-157 degreesC, and coefficients of thermal expansion of 70, 72, and 64 ppm/degreesC below their glass-transition temperatures, respectively. The shear strength of the cured dual-epoxy system decreased quickly with aging at 230 degreesC. The reworkability test showed that the removal of a chip underfilled with this material from the board was quite easy, and the residue on the board could be thoroughly 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. (C) 2002 Wiley Periodicals, Inc. [References: 19]