Study on B-stage properties of wafer level underfills

Abstract

The flip-chip on organic substrates has relied on the underfill to enhance the solder joint reliability. The invention of wafer level underfill technology has greatly improved the production efficiency of the flip-chip process. Nevertheless, because of the unique curing characteristics of the wafer level underfill, there is a need for a fundamental understanding of the underfill curing process. In this study, we have explored two underfill formulations and have investigated their curing properties and the gelation behavior. The B-stage feasibility of these two underfills was investigated based on modeling of the curing process and characterization of the material properties. It was found that the epoxy/anhydride formulation was not suitable for wafer level underfill application, due to its low degree of cure at gelation and its low Tg after the B-stage. The underfill formulation based on epoxy/anhydride system was optimized for good dicing property and curing latency in a reflow process. A successful wafer level underfill material and process were demonstrated.