An Evaluation of the Electrical Stability of Copper Filled Isotropic Conductive Adhesives in High Moisture Environments

Abstract

Isotropic Conductive Adhesives (ICAs) consisting of SAM (Self-Assembled Monolayer) coated copper (Cu) powder dispersed in a two-part epoxy adhesive (hereafter referred to as Cu-ICAs) were prepared, stencil printed as tracks on to glass substrates and cured. These test samples were stored within four different environments: a room temperature ambient environment; deionized water at room temperature; $85^{\circ}\mathrm{C}$ and 85% relative humidity (RH); and $85^{\circ}\mathrm{C}$ and 1% RH. Samples of a commercial silver (Ag) filled ICA (Ag-ICA), also based on a two-part epoxy resin system, were prepared for comparison. Changes in the electrical conductivity of the stored ICA samples were evaluated using four point probe resistance measurements made by contacting either the top surface (for both ICAs) or lower surface (Cu-ICAs only) of the tracks. The as-prepared Cu-ICAs and Ag-ICAs displayed similar electrical conductivity and showed good electrical stability during long-term storage in ambient conditions. During storage in deionised water the conductivity of the commercial Ag-ICA samples was also stable, whereas the conductivity of the Cu-ICAs measured through the lower surface contact decreased by around 25% after 280 days. Dry high temperature storage $(85^{\circ}\mathrm{C}$ with 1% RH) accelerated the electrical aging of the Cu-ICAs and the addition of 85% RH considerably accelerated the degradation rate. It was typically seen that there was an apparent greater decline in the conductivity of the specimens when measured through the exposed top surface of the tracks compared to the enclosed lower surface and this was linked to morphological changes seen in the structure of the Cu particles at the sample surface.