Behavior of Nanosilica Filled Epoxies

Epoxy resins filled with silica are used in a wide array of applications. When used in microelectronic packaging, chiefly as an underfill encaplsulant, it is critical that such epoxy resins possess low viscosity as well as high fracture toughness. Traditionally, micron-size silica fillers are used but there is much interest in the use of nanometer size fillers as the feature size on silicon chips decreases. In this study, the rheological behavior of an epoxy resin containing nanosilica fillers was characterized in steady state shear using a Rheometerics ARES rheometer equipped with a Couette fixture. Two types of nanosilica particles were examined as potential fillers(22nm and 168nm in diameter) as well as mixtures of both. Interestingly, the unimodal formulations exhibited reduced viscosities larger than those predicted from Einstein's equation, thus suggesting significant interactions between particles. Note that shear rate studies did not reveal the presence of a yield stress nor structure formation. Bimodal mixtures of nanosilica were also explored as a possible means to reduce the viscosity for a given nanosilica content. Initial results look promising even though the nanosilica content is lower than what is traditionally used in these systems.