Factors Affecting Conduction in Novel Isotropic Conductive Adhesives Filled with Silver Coated Polymer Spheres

Conventionally, ICAs comprise of a high volume fraction of solid silver flakes as a conductive filler in an adhesive matrix, however this paper will focus on newly developed ICAs containing a large volume fraction of metal-coated polymer spheres (MPS) as conductive filler. In our previous studies we have demonstrated the feasibility and benefits of using silver coated polymer spheres as a conductive filler in ICAs, considering their electrical, rheological and mechanical properties, and comparing them with silver flake filled adhesives. In addition, we have also reported the effect of sphere size and coating thickness on their electrical performance. This paper present results from a much more detailed experimental investigation of the effect of various factors on the electrical performance of ICAs filled with Ag coated MPS. These factors are the MPS diameter, the thickness and quality of silver coating arising from different plating processes, the adhesive curing conditions, and the type of adhesive matrix. The experiments have been conducted using Ag coated MPS with the same core material, but diameters of 10 mu m and 30 mu m. The results show that the diameter of the MPS has a major effect on the conductivity of the adhesive. The adhesives formulated with smaller (10 mu m) spheres have nearly twice the conductivity of those with 30 mu m spheres. For the same size MPS, adhesives containing spheres with a thicker Ag coating show lower resistivity. A significant effect of the adhesive matrix on the resistivity is also observed at filler volume fractions below 48%.