Thickness-dependent conduction behavior of various particles for conductive adhesive applications

Abstract

In order to gain insight into the film thickness-dependent conduction behavior of adhesives containing conductive particles of different sizes, shapes, and types, the effect of adhesive film thickness was studied. Epon 830 resin was used as the base resin and 4-7 μm silver powder was used as the base particle for comparative purposes. Adhesive films of length 2.0 cm and width 0.7 cm were cast subsequent to mixing with conductive particles in the amount of 25% by volume. The conductive particles were 100% Ag or a 50% by weight mixture of Ag with Ni powder, flakes, or filaments. Silver-coated Ni flakes of size 20 μm were also mixed in 50% proportion with Ag powder to assess the effect of silver coating. Subsequent to cure, the resistance of the filled films was measured by the four-point method and resistivities were calculated based on these measurements. In general, the addition of flake and filament material (Ni) to the conductive mixture in epoxy films resulted in distinct thickness thresholds for transition from three-dimensional to two-dimensional conductivity with a considerable increase in resistivity when the film thicknesses were smaller than these threshold values. This behavior is attributed to the specific geometry of the flake and filament particles which, when added to a thin film resin, are likely to align flat in planar directions, thus reducing the number of three-dimensional conduction paths.