Studies of latent catalyst systems for pot-life enhancement of underfills

Underfill encapsulant is the material used in flip-chip devices that fills the gap between the IC chip and the organic board, and encapsulates the solder interconnects. This underfill material can dramatically enhance the reliability of flip-chip devices as compared to nonunderfilled devices. Current underfill encapsulants generally consist of epoxy resin, anhydride hardener, catalyst, silica filler, and other additives to enhance the adhesion, flow, etc. Underfill properties that are mainly determined by the catalyst include pot-life, cure speed, and cure temperature. Long pot-life and fast cure at relatively low temperature (~150°C) are desired, such that it requires the catalyst to be latent at room temperature, but able to catalyze the epoxy curing efficiently at desired temperature. Currently, the pot-life of commercial underfills is normally less than 1 day. The underfills must be put in the freezer at -40°C for storage and in dry ice for shipping. The goal of this work was to test various catalyst systems that have the potential to enhance the pot-life of the underfill without adversely affecting its curing. The viscosities of the underfill with various catalysts were measured periodically by using a stress-controlled rheometer in order to establish their viscosity-time relationships. The curing of the underfills was studied using DSC. The pot-life and curing data of the underfill mixed with each of these catalysts are presented in this paper