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 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.