Development of no-flow underfill materials for lead-free solder bumped flip-chip applications

The authors develop an epoxy-based no-flow underfill material for lead-free bumped flip-chip applications. Many epoxy resin/HMPA/metal acetylacetonate material systems have been screened in terms of their curing kinetics. Some potential base formulations whose curing peak temperatures are higher than 200°C are selected for further development towards fluxing no-flow underfill materials for lead-free solders. The proper fluxing agents are then developed for these potential base formulations. The effects of fluxing agents studied on the curing kinetics and cured material properties of the potential base formulations are studied in detail. Fluxing capability of the developed no-flow formulations is evaluated using the wetting test of lead-free solder ball on copper board. The instrumentation involved includes differential scanning calorimetry (DSC), thermo-mechanical analyzer (TMA), dynamic-mechanical analyzer (DMA), thermo-gravimetric analyzer (TGA), and a rheometer. DSC is used to study the curing kinetics of the prepared formulations and glass transition temperature (DSC Tg) of the cured formulations. TMA is used to investigate the heat distortion temperature (TMA Tg) and coefficient of thermal expansion (CTE). DMA is used to measure the storage modulus (E\´) and loss modulus (E") within the temperature range from 25°C to 250°C. TGA is used to investigate the outgassing during curing and thermal stability of the cured material systems. The rheometer is used to investigate the viscosity change versus temperature for the interested formulations