Effects of non-conductive film (NCF) resin formulation and bonding parameters on high-speed Cu pillar/Sn-Ag Micro-bump bonding

Abstract

As pitches of the bumps gets finer and finer as forty micro-meters, many problems occur such as flux residue, voids, and underfill overflow. As a solution for these problems, Non-conductive Film (NCF) for TSV chip stacking application is the effective solution. To interconnect chips on substrates using pre-applied NCF, thermo-compression bonding is the most common method, however conventional ramp-up bonding process takes about 300 seconds for next bonding process due to precise heating and cooling process. Isothermal bonding using hot bonding tool is alternative method for mass-production of pre-applied NCF. However, curing behavior of NCF should be considered because thermo-compression bonding time of isothermal bonding is too short to cure the NCFs. Liquid epoxy amount of NCF resin was optimized because liquid epoxy was related to adhesion of NCF at the room temperature before curing. Optimized NCF that contained 50 wt% liquid epoxy showed highest adhesion and appropriate elongation properties. Cu pillar/Sn-Ag micro-bumps using both conventional single chip packages and WLPs showed excellent daisy chain resistances of $12 \Omega$ , bump contact resistances of $3 \Omega$ , and equivalent reliabilities. 2-phenylimidazole was selected for curing agent of fast-cure NCF because curing on- set temperature was higher than film coating and NCF lamination temperature. Curing speed of imidazole-NCF was 67.5 times faster than that of DICY-NCF at bonding temperature. Conventional ramp-up bonding and isothermal bonding method were used to analyze the effect of bonding method. The heating rate of isothermal bonding was higher than that of conventional ramp-up bonding and final joint gap decreased enough to achieve the stable solder joints with imidazole-NCF. Solder joint gap of imidazole-NCF was maintained after physical contact of solder because degree-of-cure of imidazole-NCF reached to 90 % at solder melting temperature. Isothermal bonding parameters were also investigated in terms of the bonding pressure, bonding temperature, and bonding time.