Effects of shrinkage on conductivity of isotropic conductive adhesives

Our previous study proved that intimate contact between conductive fillers caused by resin cure shrinkage, rather than lubricant removal, was the main mechanism for establishment of conductivity of isotropic conductive adhesives (ICAs). The purpose of the present study was to investigate the changes in properties, especially in dimension (cure shrinkage), of an ICA during cure and to correlate them with establishment of conductivity. An ICA was cured nonisothermally by a temperature increase from 30 to 250 degrees C; and its heat flow, storage modulus, dimension change, and electrical conductivity were studied with a differential scanning calorimeter (DSC), rheometer, thermomechanical analyzer (TMA), and electrical multimeter, respectively. It was found that all of these properties changed dramatically over the same small range of temperature. Changes in these four properties of this ICA with time in the course of an isothermal cure were also investigated. It was found that all of the properties showed significant changes within the same period of time. In addition, the conductivities of three different ICA formulations which were filled with a blank Ag powder and whose resins had different cure shrinkages were measured and compared. From this study, it was concluded that (a) conductive adhesives achieved high conductivity only when enough cure shrinkage was achieved, and (b) ICAs with higher cure shrinkage showed higher conductivity. (C) 2000 Elsevier Science Ltd. All rights reserved.