Development of No-Flow Underfill Materials for Lead-Free Solder Bumped Flip-Chip Applications

No-flow underfill process in flip-chip assembly has become a promising technology toward a smaller, faster and more cost-efficient packaging technology. The current available no-flow underfill materials are mainly designed for eutectic tin-lead (Sn/Pb) solders. With the advance of lead-free interconnection due to the environmental concerns, a new no-flow underfill chemistry needs to be developed for lead-free solder bumped flip-chip applications. Many epoxy resin/hexahydro-4-methyl phthalic anhydride (HMPA)/metal acetylacetonate material systems have been screened in terms of their curing behavior. Some potential base formulations with curing peak temperatures higher than 200 C (based on differential scanning calorimetry at a heating rate of 5 C/min) are selected for further study. The proper fluxing agents are developed and the effects of fluxing agents on the curing behavior and cured material properties of the potential base formulations are studied using differential scanning calorimetry (DSC), thermomechanical analyzer (TMA), dynamic-mechanical analyzer (DMA), thermogravimetric analyzer (TGA), and rheometer. Fluxing capability of the developed no-flow formulations is evaluated using the wetting test of lead-free solder balls on a copper board. The developed no-flow underfill formulations show sufficient fluxing capability and good potential for lead-free solder bumped flip-chip applications.