Conductivity mechanisms of isotropic conductive adhesives (ICAs)

Abstract

Isotropic conductive adhesives (ICA's) are usually composites of adhesive resins with conductive fillers (mainly silver flakes). The adhesive pastes before cure usually have low electrical conductivity. The conductive adhesives become highly conductive only after the adhesives are cured and solidified. The mechanisms of conductivity achievement in conductive adhesives were discussed. Experiments were carefully designed in order to determine the roles of adhesive shrinkage and silver (Ag) flake lubricant removal on adhesive conductivity achievement during cure. The conductivity establishment of the selected adhesive pastes and the cure shrinkage of the corresponding adhesive resins during cure were studied. Then conductivity developments of some metallic fillers and ICA pastes with external pressures were studied by using a specially designed test device. In addition, conductivity, resin cure shrinkage, and Ag flake lubricant behavior of an ICA which was cured at room temperature (25 C) were investigated. Based on the results, it was found that cure shrinkage of the resin, rather than lubricant removal, was the prerequisite for conductivity development of conductive adhesives. In addition, an explanation of how cure shrinkage could cause conductivity achievement of conductive adhesives during cure was proposed in this paper.

Contact Resistance and Metallurgical Connections Between Silver Coated Polymer Particles in Isotropic Conductive Adhesives

“…In conventional ICAs, the contact resistance is generally regarded as the dominating factor. 8,9 In addition to a concentration of conductive fillers above the percolation threshold, most ICAs require curing to lower the resistivity to an acceptable level. Curing leads to cross-linking and shrinkage in the epoxy matrix, which presses the particles together, decreasing the contact resistance.…”

“…Curing leads to cross-linking and shrinkage in the epoxy matrix, which presses the particles together, decreasing the contact resistance. 9 Constriction resistance and tunnelling resistance are the two main mechanisms behind the contact resistance, as described by Holm's classical contact theory. 10 If the contact areas are smaller than the cross-sectional areas of the particles, the flow of current will be narrowed, leading to a constriction resistance through the contacts.…”