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

Abstract

Recently, there has been an increasing interest in silver thin film coated polymer spheres as conductive fillers in isotropic conductive adhesives (ICAs). Such ICAs yield resistivities similar to conventional silver flake based ICAs while requiring only a fraction of the silver content. In this work, effects of the nanostructure of silver thin films on inter-particle contact resistance were investigated. The electrical resistivity of ICAs with similar particle content was shown to decrease with increasing coating thickness. Scanning electron micrographs of ion milled cross-sections revealed that the silver coatings formed continuous metallurgical connections at the contacts between the filler particles after adhesive curing at 150°C. The electrical resistivity decreased for all samples after environmental treatment for 3 weeks at 85°C/85% relative humidity. It was concluded that after the metallurgical connections formed, the bulk resistance of these ICAs were no longer dominated by the contact resistance, but by the geometry and nanostructure of the silver coatings. A figure of merit (FoM) was defined based on the ratio between bulk silver resistivity and the ICA resistivity, and this showed that although the resistivity was lowest in the ICAs containing the most silver, the volume of silver was more effectively used in the ICAs with intermediate silver contents. This was attributed to a size effect due to smaller grains in the thickest coating.

Investigation of thermal transport in polymer composites with percolating networks of silver thin films by the flash diffusivity method

“…36 The formation of metallic contacts helps explain why AgPS-based ICAs yield higher electrical conductivities relative to their silver content when compared with most conventional ICAs. 8 It is evident that the electrical conductivity (Fig. 2), thermal diffusivity (Fig.…”

“…In addition to lower intrinsic conductivities, the spherical shell geometry will induce a geometrical constriction resistance that would not be present in a conductive filler with a constant cross-sectional area, although this constriction resistance will decrease with increasing interparticle contact radii. 8,11 At $56 vol. %, the AgPS are above the loose packing limit of randomly dispersed spheres ($53.6 vol.…”

“…It has been shown that the silver films of AgPS penetrate the epoxy and form continuous metallic inter-particle contacts after curing of the ICAs. 8,9 Although electrons in theory can pass across very thin (<10 nm) non-conductive barriers by quantum tunneling, this current will be negligible compared to a metallic contact of the same size. 36 The formation of metallic contacts helps explain why AgPS-based ICAs yield higher electrical conductivities relative to their silver content when compared with most conventional ICAs.…”

“…6 In recent years, homogeneously sized polymer spheres coated with thin films of silver (AgPS) have been shown as a viable alternative to silver flakes and solid silver particles in ICAs. 7,8 AgPS-based ICAs are truly isotropic, and as the spherical silver films are much thinner than the diameter of the polymer cores, most of the AgPS will be polymer, and percolating networks can be formed at much lower silver contents than in conventional ICAs. AgPS-based ICAs have been shown to yield r > 1500 S/cm at less than 3 vol.…”

“…% silver, 7,8 and the mechanisms controlling the electrical properties of these conductive composites are starting to be clarified. [8][9][10][11] However, investigation of the thermal properties of AgPS-based ICAs have been sparse, and to our best knowledge, only one paper has been published on the subject to date. 7 The mechanisms controlling the thermal transport of these ICAs are still largely unexplored.…”