High performance underfills for low-cost flip-chip applications

To develop the no-flow underfill materials suitable for no-flow underfill processing for flip-chip solder joint interconnects, we studied several catalysts for epoxy/anhydride base no-flow underfill formulations. These catalysts include metal acetylacetonates and imidazolium salts and were found to be potential candidates for no-flow underfill applications based on the curing profile of the formulations, glass transition temperature (Tg), and coefficient of thermal expansion (CTE) of the cured formulations. The added amount of catalyst and hardener has separate effects on curing reaction peak temperature of the prepared formulations, Tg, and CTE of the cured formulations. The added concentration of catalyst mainly affects the curing reaction peak temperature, but has little effect on the Tg and CTE of the cured formulations. The added concentration of hardener mainly affects the Tg and CTE of the cured formulations, but has little effect on the curing reaction peak temperature of these formulations. Water molecule complexed with metal ions in metal acetylacetonates can be released by heating which consequently shifts the curing reaction peak to lower temperatures by several decades of degrees. The curing profile and the correlation between the curing reaction peak temperature as latent catalysts for epoxy/anhydride resins and their decomposition temperature suggests that the decomposition fragments are most likely the active species responsible for initiating polymerization in epoxy/anhydride resin systems.