Modeling of the curing kinetics of no-flow underfill in flip-chip applications

No-flow underfill has greatly improved the production efficiency of flip-chip process. Due to its unique characteristics, including reaction latency, curing under solder reflow conditions and the desire for no post-cure, there is a need for a fundamental understanding of the curing process of no-flow underfill. Starting with a promising no-flow underfill formulation, this paper seeks to develop a systematic methodology to study and model the curing behavior of this underfill. A differential scanning calorimeter (DSC) is used to characterize the heat flow during curing under isothermal and temperature ramp conditions. A modified autocatalytic model is developed with temperature-dependent parameters. The degree of cure (DOC) is calculated; compared with DSC experiments, the model gives a good prediction of DOC under different curing conditions. The temperature of the printed wiring board (PWB) during solder reflow is measured using thermocouples and the evolution of DOC of the no-flow underfill during the reflow process is calculated. A stress rheometer is used to study the gelation of the underfill at different heating rates. Results show that at high curing temperature, the underfill gels at a lower DOC. Based on the kinetic model and the gelation study, the solder wetting behavior during the eutectic SnPb and lead-free SnAgCu reflow processes is predicted and confirmed by the solder wetting tests.