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

No-flow underfill process in flip-chip assembly is a promising technology for smaller, faster and more cost-efficient packaging technology. The 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 are selected for further study. The proper fluxing agents are developed and the effects of fluxing agents on the curing kinetics and cured material properties of the potential base formulations are studied using differential scanning calorimetry (DSC), a thermo-mechanical analyzer (TMA), a dynamic-mechanical analyzer (DMA), a thermogravimetric analyzer (TGA), and a 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 tests of lead-free solder balls on a copper board. The feasibility of developed no-flow underfill is demonstrated using a lead-free bumped flip-chip package, which shows 100% interconnection yield after the solder reflow