A new underfill was developed for a flip chip application with a large VLSI die. The new underfill is resistant to hydrolysis, exhibits a significantly lower moisture saturation level than widely used carboxylic anhydride cured underfills, demonstrates superb penetration capability between such a large die and an organic substrate without void formation, and delivers excellent adhesion characteristics and reliability performance during temperature cycling test and PCT (pressure cooker test).
The new underfill utilizes amine catalyzed ring opening polymerization of epoxides, forms ether linkages during cure rather than ester linkages which anhydride cured underfills form, and strongly resists to hydrolysis. The underfill is less sensitive to moisture contamination and provides improved floor life as well as storage life in contrast to anhydride cured underfills as well. Unique low stress performance and penetration capability of the underfill are attributed to the use of proprietary silicone modified epoxy resin as well as highly loaded filler of optimized size, shape and size distribution in reference to the gap between a die and an organic substrate.
An optimum cure schedule and a desirable viscosity range have also been identified for the new underfill to minimize filler segregation. Proper preheating of a die‐substrate has effectively reduced void formation while facilitating the removal of volatile from an organic substrate.