The Effects of Filler Volume Fraction and Film Thickness on Resistivity of Conductive Adhesives

Conductive adhesives are widely used in electronic packaging applications such as die attachment and solderless interconnections, component repair, display interconnections, and heat dissipation. This research intends to provide an understanding of the effects of film thickness as functions of filler volume fraction, conductive filler size, shape, as well as uncured base adhesive viscosity on the electrical conduction behavior of such adhesives. Epoxy-based adhesives were filled with particles of different size, shape, and types, including Ni powder, flakes, and filaments, Ag powder, and Cu powder. The filaments were 20 µm in diameter, and 160 or 260 µm in length. Acid solutions including HCL, H₃PO₄, and HF were used to etch and remove the surface oxide layers. The plane resistance of filled adhesive films was measured using the four-point method. In general, the addition of micron-sized metal fillers to the conductive mixture resulted in distinct thickness thresholds for transition from three-dimensional conductivity to one-dimensional conductivity with considerable increases in plane resistivity when the film thicknesses were smaller than these threshold values. The relationships between the conductive film resistivities and the filler volume fractions were derived mathematically, based on the experimental data. The effects of surface treatment of filler particles, the type, size, shape of fillers, and the uncured epoxy viscosity were also included in this formulation.