Numerical analysis of polymer cure kinetics in isotropic conductive adhesives

Abstract

The electrical and thermomechanical properties of isotropic conductive adhesive materials vary significantly during the material cure process as polymer crosslinking alters the molecular structure and volume of the material which, in turn, alters the percolation threshold causing step changes in the values of electrical and thermal conductivity within the material. It is therefore important to ensure that adhesive interconnects are sufficiently cured. As in-situ analysis of the cure state is highly complex, a number of numerical models have been developed to provide an estimate of cure. In this study, the accuracy of six cure kinetics models in determining the cure kinetics of commercially available isotropic conductive materials is assessed. Cure kinetics were determined experimentally using Differential Scanning Calorimetry (DSC) equipment. The numerical models were fitted to the experimental data using a particle swarm optimisation method, enabling the ultimate accuracy of the models to be determined. Results obtained show that the single-step autocatalytic model would appear to provide a more accurate estimate of cure kinetics process than the nth order type models or the more complex double step models. The overall accuracy of the models was found to be quite high with close agreement between experimental and numerical results. Further studies are required to consider the multi-component nature of underfill and encapsulant materials.