Evolution of Electrical Conductivity in Silver-Loaded Electrically Conductive Adhesives Composed of an Amine-Cured Epoxy-Based Binder

Abstract

To investigate interfacial phenomena related to the evolution of electrical conductivity in electrically conductive adhesives containing silver fillers, the curing processes of model adhesives composed of an amine-cured epoxy-based binder were examined using several analytical methods. Shrinkage of the adhesives during curing was not a predominant factor in determining the electrical resistivity. Rearrangement of the fillers occurred at the onset of gelation. Afterwards, the electrical conductivity evolved in the adhesives through the following two steps: development of conduction paths during gelation of the epoxy-based binder, and microstructural evolution between the silver fillers. The use of adipic acid as a surfactant enhanced the first of these steps during gelation. The latter process occurred concurrently with cross-linking of binder molecules to increase the elastic modulus. In this step, the amine molecules probably reacted with silver fillers to induce necking between the fillers. Because interfacial chemical phenomena probably influence the electrical conductivity evolution, the conventional hypothesis, which is based on the mechanical contact concept for inter-filler contacts in electrically conductive adhesives, needs to be expanded.