Development of Electrical Conduction in Silver-filled Epoxy Adhesives

Abstract

Although percolation theory predicts that insulators containing ca. 20% or higher volume loadings of dispersed metallic particles should be electrically conductive, some commercial silver-filled epoxies fail to be so unless subjected to thermal cure. It was found that this is due to an insulating coating on the silver particles, presumably added to promote dispersibility. This coating consists of a mixture of fatty acid salts of copper, and is removed under the action of heat, primarily through decarboxylation. When the silver powder is heated alone or as a dispersion in an inert matrix (silicone oil), this occurs above 200°C, rendering the specimens conductive. The onset of electrical conduction can be brought down to much lower temperatures by addition of small amounts of solubilizers, such as long chain organic compounds containing numerous ether linkages. Such materials, when added to commercial conductive epoxies at very low concentrations, increased the conductivity by a factor of two to three. Shrinkage of the matrix during heating did not have any significant effect on ultimate resistance or on the initiation of electrical conduction.