Electrical conduction models for isotropically conductive adhesive joints

Abstract

An electrical conduction model for silver filled isotropically conductive adhesives (ICA) was developed. The model combines the microscopic resistance of the bulk silver particles and the contact between silver flakes with the macroscale resistor network calculation by percolation theory. The resistivities of the composites were calculated by resistor network simulations considering both contact effects and particle size effects. Three different types of film typically exist on the silver surface: residual organic films; tarnish films; and a thin epoxy layer. The contact resistance between silver flakes can be due to a constriction resistance, to the tunneling resistance through insulating films, or to the resistance of more conductive layers. The constriction resistance is produced by the restriction of the current flow by small contact spots and is controlled by the actual contact spot area (metallic contact), which is dependent on the contact force between flakes. The tunneling resistance is caused by the very thin layer which may reside on the silver flakes between the metallic contact spots, and is dependent on a barrier film thickness and potential. Oxide and sulfide tarnish films are typically degenerate semiconductors. Two- and three-dimensional (2-D and 3-D) computer simulations were performed to predict the effects of particle sizes, shapes, and distribution on the percolation conduction thresholds and cluster sizes. The model predicts that the percolation threshold decreases with broad particle size distributions and high aspect ratio particles. The effective resistivity of the adhesive depends on the thickness dimension of the adhesive pad geometry, with very thin layers resulting in high percolation thresholds and high resistivities. Resistivity does not change with the pad thicknesses greater than a certain thickness level. Silver flake orientation on the surface increases the resistivity of the conductive adhesive pads, but in the same magnitude range. The resistivities of the materials are controlled by silver flake sizes and the nature of the contacts.