Partial pre-applied underfill for assembly and reliability of Pb-free CSPs

A novel pre-applied underfill is presented. In this approach, a B-staged thermoset is applied to a chip-scale package (CSP) as a thin layer over the solder bump array. During reflow, the coating expands to fill the standoff gap between the PCB and package, providing reinforcement to the solder joints. The underfill material is stable at typical drying and burn-in conditions, allowing it to be applied early in the manufacturing process. The expanded underfill was found to improve drop impact resistance significantly relative to a non-underfilled CSP.