Development of no-flow underfill for lead-free bumped flip-chip assemblies

The no-flow underfill process in flip-chip assembly is promising for a smaller, faster and more cost-efficient packaging technology. Currently available no-flow underfill materials are mainly designed for eutectic Sn-Pb solders. This paper presents a new no-flow underfill for lead-free solder bumped flip-chip assemblies. Many epoxy resin/HMPA/metal acetylacetonate material systems have been screened in terms of their curing kinetics. Some potential base formulations with curing peak temperatures higher than 200°C were selected for further study. The proper fluxing agents were developed and the effects of fluxing agents on the curing kinetics and cured material properties of the potential base formulations were studied by differential scanning calorimetry (DSC), thermo-mechanical analyzer (TMA), dynamic-mechanical analyzer (DMA), thermo-gravimetric analyzer (TGA), and rheometer. The results show that the addition of flux significantly reduced the curing temperature and limits the potential of many formulations for application in lead-free bumped flip chip assemblies. The fluxing capability of several no-flow formulations is evaluated using wetting testing of lead-free solder balls on a copper board. The feasibility of the developed no-flow underfill is demonstrated using a lead-free bumped flip-chip package, which shows 100% interconnection yield after solder reflow