The Reduction of Outgas from Pre-Applied Underfill Materials by Optimizing the Combination of Base Resin and Flux Compound

This study investigated the relations between base resins and flux compounds applied to NCF (Non Conductive film) for thermal compression bonding with micro Cu pillar bumps. As the use of NCF expands, outgas during flip-chip bonding process is being recognized as a serious problem because it lowers both of the productivity and the package quality. The flux compounds which are normally used to get solder-ability are considered to be one of main sources of outgas [1], so its reduction is strongly expected. The selection of the base resin system is also an important item because the required amounts of flux compounds basically depend on resin types. So the combination of the base resin and the flux compound has to be determined on the detailed consideration. In this work, two step approach to optimize the combination of the base resin and the flux compound was performed to reduce outgas from NCF and to achieve good solder-ability at the same time. Firstly, several thermoset resins were evaluated to determine preferable candidates for the base resin. Next, screening of some flux compounds were conducted through the foundational evaluations on the flux-ability and weight loss at high temperature. Typically, most of the flux compounds can easily volatilize before reaching the melting point of solder, and it leads to the under-fill voids or the contamination on bonding tools. Therefore, the low weight loss at high temperature is an important parameter as well as the flux-ability when choosing the flux compounds. Finally, NCF was fabricated with the best combination of this study, and TCB (Thermal Compression Bonding) was demonstrated with inline 50um pitch test vehicles. The cross-section analysis of bonded samples showed sufficient wettability of solder (Sn-Ag) to Cu-OSP pads without any abnormality. In addition, HAST (High Accelerated Stress Test) has started by using test boards with 40um/40um line and space. So far, 100 hours has passed without any failure, and the test is ongoing up to 500 hours.