Study and modeling of the curing behavior of no-flow underfill

Starting with a promising no-flow underfill formulation, this paper seeks to develop a systematic methodology to study and model its curing behavior. 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 a reflow process is calculated and compared with experimental results. A rheometer and a Fourier-Transform Infrared (FTIR) spectroscope are used to study the relation of the underfill gelation with the reaction mechanism. Rapid heating rate favors the etherification of the epoxy over the esterification between the epoxy and the anhydride. This causes the early gelation of the underfill.