A new approach in the development of no-flow underfill materials for both eutectic and lead-free solders with high melting temperature

In recent years, no-flow underfill technology has been drawing more attention due to its potential advantages over conventional underfill technology and several no-flow underfill materials have been developed and reported. However, most of these materials are not suitable for lead-free solder applications that usually have higher melting temperatures than the eutectic solder Due to the increasing environmental concern, the demand for friendly lead-free solders has become an apparent trend. This paper demonstrated the study on two new formulas of no-flow underfill developed for lead-free solders with melting point around 220 degreesC. As compared to the G25, a no-flow underfill material developed in our research group, which uses a solid metal chelate curing catalyst to match the reflow profile of eutectic solder (melting point 187 degreesC), these novel formulas employ a liquid curing catalyst thus provides ease in preparation of the no-flow underfill materials and more choices in combining fluxing agents. In this study, the curing kinetics, glass transition temperature (Tg), thermal expansion coefficient (TCE), storage modulus (E') and loss modulus (E ") of these materials were studied with differential scanning calorimetry (DSC), thermo-mechanical analysis (TMA), and dynamic-mechanical analysis (DMA), respectively. The pot-life in terms of viscosity of these materials was characterized with a stress rheometer. The adhesive strength of the material on the surface of silicon chip was studied with a die-shear instrument. The influences of fluxing agents on the materials curing kinetics were studied with DSC. The materials compatibility to the solder penetration and wetting on copper clad during solder reflow was investigated with both eutectic and no-lead solders on copper laminated FR-4 organic boards and heated in a reflow oven.