Effect of the Curing properties and Viscosities of Non-Conductive Films (NCFs) On Ultra-Fine Pitch Cu-pillar/Sn-Ag Bump Joint Morphology and Reliability

In this study, flip chip assembly using NCFs was evaluated for ultra-fine pitch Cu-pillar/Sn-Ag bumps. The $4\ \text{mm} \times 4\ \text{mm}$ size Si chips had $32\ \mu \mathrm{m}$ height and $20\ \mu \mathrm{m}$ diameter Cu-pillar/Sn-Ag micro bumps, and the bump pitch was $30\ \mu \mathrm{m}$. And $12\ \text{mm} \times 12\ \text{mm}$ size printed circuit boards (PCBs) had $23\ \mu \mathrm{m}$ width Cu pads and organic solder-ability preservative (OSP) surface finishes. Solder joint morphology was evaluated and optimized by adjusting the curing properties such as curing onset, peak temperature and the viscosities of NCFs by changing curing agents and silica contents. And then, in order to understand changes of solder joint morphology during flip chip bonding process, solder joint was also evaluated depending on flip chip bonding times and temperatures. Based on the results, it was confirmed that solder joint morphology is determined at solder melting temperature of 221°C. And then, degree of cure and viscosity approximation was conducted to define the precise viscosity of NCFs at the solder melting temperature using measured degrees of cure at various bonding temperatures. As a result, concave shaped solder joint and NCFs trap were generated between 545,140 and 551,007 Pa·s of NCFs viscosity at solder melting temperature. Finally, high temperature and high humidity (85RH%/85°C) test, temperature cycling (T/C) test is performed to evaluate the hygroscopic and thermo-mechanical reliability performance depending on solder joint morphology. The reliability test results showed stable electrical resistance changes within 10% of average value.