Controlling the Conduction Mechanisms in Isotropic Conductive Adhesives with Silver-Coated Polymer Spheres

Abstract

Replacing the silver flakes in conventional isotropic conductive adhesives (ICAs) with micron sized polymer spheres coated with tens to hundreds nanometer of silver (AgPS) drastically reduces the silver content required to obtain good electrical and thermal conductivity. In addition to cost-benefits, this reduction in amount of precious metal can improve the mechanical properties of the ICA by mitigation of thermomechanical mismatch, as the polymer cores can be customized with material properties similar to those of the adhesive matrix. It has previously been shown that the silver thin films form metallic connections in the contact zones between particles during adhesive curing. This contact structure reduces the contact resistance between filler particles and is thus of vital importance for the good conductive properties relative to the low silver content shown by these ICAs. In the present work, the mechanisms behind the formation of the metallic contacts were further investigated by examining the impact of various curing conditions on ICA electrical conductivity, and relating these measurements to the structure of metallic contacts in cross-sectional SEM images. In addition, the conductivity of these ICAs has been shown to improve during storage at 85°C and 85 % relative humidity. This has been attributed to grain growth and improved contact between particles at these conditions. Here, these mechanisms were elucidated by separating the contributions from heat and moisture, and it was discovered that massive grain growth and large improvement of conductivity only occurred in samples stored at elevated humidity.