Adhesion performance and thermo-mechanical property of epoxy-based underfill

The adhesion and thermo-mechanical properties of epoxy underfills depend on the epoxy resin, the hardener, and the catalyst. In this study, three different epoxy resins, ERL4221 (cycloaliphatic type), EPON862 (bisphenol F type), and EPON 8281 (bisphenol A type), were cured with 4-methylhexahydrophthalic anhydride (MHHPA) as the hardener using different catalysts: cobalt acetylacetonate (CAA), imidazole derivatives, and tertiary amines. The flow behavior of the epoxy systems was studied with a rheometer. The curing profiles were recorded using a differential scanning calorimeter (DSC), revealing varying catalytic effect for the different catalysts. The curing peak temperature increased in the following order: tertiary amine < imidazole derivatives < cobalt acetylacetonate. The bulk properties of the systems were studied through a thermo-mechanical analyzer (TMA). Epoxy resins cured with different catalysts showed different glass transition temperatures (T-g) and coefficients of thermal expansion (CTE). Among them, the CAA catalyzed systems showed the highest T-g and low CTE. Due to the cycloaliphatic structure of the ERL4221 resin, the cured ERL4221 systems showed the highest T-g compared to the other systems with the same catalysts. EPON8281 systems generally had lower moisture absorption than the other epoxy systems. The surface tension of the underfills was measured. The adhesion strength was evaluated by a die shear test with SiO2 and Si3N4 (SiN) passivated silicon dies as substrates. Although there was no significant difference among the surface tension and contact angles of the underfill materials on the SiO2 or SIN, there were significant differences in the adhesion. For the ERL4221+MHHPA systems, the CAA catalyzed system exhibited excellent adhesion with Sig and SiN passivation. However, poor adhesion resulted with the catalysis by the imidazole derivatives and the tertiary amines. The bisphenol A and bisphenol F type epoxy resins showed excellent adhesion. Although the CAA catalyzed systems had higher T-g compared to the other systems, their adhesion was worse after aging in a high humidity environment at high temperatures.