Deconstructing the myth of percolation in electrically conductive adhesives and its implications

Abstract

The modern emphasis on green technologies has caused the electronics industry to seek alternative solutions to lead-based interconnections. Electrically conductive adhesive (ECAs) composed of metallic fillers within a polymer matrix have received the majority of the interest in lead-free interconnect technology. However, ECAs are still unable to meet the demands of high performance consumer electronics. Previous research recognized a critical filler concentration where there is a dramatic increase in conductivity, followed by a plateau. Researchers have labeled this transition as evidence of a percolation, implying a continuous interconnected metallic network. Our work comprised of a series of "proof of concept" type experiments deconstructs the myth of percolation and emphasize the functional role of the polymer matrix. From a theoretical standpoint direct metal to metal contact is not feasible since silver particles coated with short chain acids are easily wet by the polymer matrix. Assembly conducted under low mechanical stresses is unable to displace the adsorbed surfactant to form metallic contact. Moreover, preparation of a high K epoxy (Dielectric Constant ~5.5), Co(III) acetylacetonates (Co(III) AcAcs) doped diglycidyl ether of bisphenol F had unstable conductivities orders of magnitude lower than the control samples; under similar applied DC. Dielectric constant has a minimal effect if metal to metal contact is the dominant charge transport mechanism. However, tunneling through materials with high dielectric constant impedes the tunneling efficiency. We clearly demonstrate that charge transport at the interface occurs via secondary conductivity pathways, dominated by thermally assisted tunneling mechanisms. The importance of these secondary conductivity mechanisms is highly dependent on the particle-thin film dielectric interaction. This revolutionary discovery provides a new approach for scientists and engineers to improve the performance of electrically conductive adhesives through the incorporation of electrically functional matrix materials.