Recent advances in the development of no-flow underfill encapsulants-a practical approach towards the actual manufacturing application

No-flow underfill technology has been proven to have potential advantages over the conventional underfill technology, and a no-flow underfill material (called G25) has been developed and reported in our prior papers. In this paper, two improved no-flow underfill materials were studied. Compared to the G25 no-flow underfill material, these two materials could be fully post-cured at the temperature below 170°C. These two materials also exhibited lower CTE, lower moisture absorption, better adhesion, and more fluxing stability. In this study, differential scanning calorimetry (DSC) was used to study the curing kinetics and glass transition temperature (DSC Tg) of the two materials. Thermo-mechanical analyzer (TMA) was used to investigate the heat distortion temperature (TMA Tg) and the coefficient of thermal expansion (CTE). Dynamic-mechanical analyzer (DMA) was used to measure the storage modulus (E\´) and loss modulus (E") at the temperature range from 25°C to 250°C and then estimate the cross-linking density (ρ) of the cured material systems. A rheometer was used to investigate the material viscosity. Die shear machine was used to investigate the adhesive strength between the cured underfill material and polyimide passivation layer. SMT reflow oven, quartz chips and copper laminated FR4 substrates were used to in-situ test the processability of the two materials. Scanning electron microscopy (SEM) was used to observe the cross-section of the reflowed solder interconnects integrity and the filler interaction during the formation of interconnects. A potential approach towards the production application of no-flow underfill material was then proposed