A reworkable epoxy resin for isotropically conductive adhesive

Abstract

Electrically conductive adhesive (ECA) is a promising alternative to the toxic eutectic tin-lead solder as an interconnect material. Typical ECAs use epoxy resin as their ma- trix, which has superior properties over other polymers, such as high adhesion, and low dielectric constant. However, once cured, it is not reworkable. In this study, a liquid diepoxide was designed and synthesized, and used in isotropically conductive adhesive (ICA) formulations. This diepoxide has a molecular structure able to thermally decompose at mild temperature that allows selective individual removal of the bad component without damaging the board and its surroundings. The characterizations including proton and carbon 13 nuclear magnetic resonance, infrared spectroscopy indicated the success of the synthesis. A dual-epoxy system containing this secondary diepoxide and an equivalent bisphenol-A diepoxide were formulated and cured with an an- hydride hardener and an imidazole catalyst. Thermal analyses, such as differential scanning calorimetry, thermo-gravimetry analysis (TGA), thermo-mechanical analysis (TMA) and dynamic mechanical analysis (DMA) were employed for the curing kinetics, thermal degradation behavior, glass transition temperature, coefficient of thermal expansion (CTE), and mechanical modulus, respectively. The dual-epoxy system showed two exothermal curing peaks at 140 C and 180 C, respectively. The thermoset of this dual-epoxy system has a decomposition temperature around 234 C, a glass transition temperature around 80 to 90 C, and CTEs of 74 ppm/ C and 225 ppm/ C below and above its Tg, respectively. The rework test on a surface mount component bonded to copper surface showed this ECA can be easily and quickly removed from the copper surface. The bulk resistance and contact resistance of ICAs were measured before and during an accelerated aging process in a temperature/humidity chamber (85 C/85% RH). The ECA showed good bulk resistivity and contact resistance comparable to its control and a commercial ECA on gold and copper surface finishes. Index Terms—Coefficient of thermal expansion (CTE), dynamic mechanical analysis (DMA), differential scanning calorimetry (DSC), electrically conductive adhesive (ECA), FT-IR, isotrop- ically conductive adhesive (ICA), nuclear magnetic resonance (NMR), thermo-gravimetry analysis (TGA), glass transmission temperature (Tg), thermo-mechanical analysis (TMA).