Current-induced degradation of isotropically conductive adhesives

The application scope of conductive adhesives increases continuously, requiring new and improved properties. In general, isotropic conductivity is achieved by loading a resistive polymer (mainly epoxy resins) with 70-80 wt% metal filler particles. During cure, resin shrinkage lowers contact resistance between neighbouring particles, giving a conductive 3D network. However, at high currents, local current density at neighbouring particle contact spots may be a limiting factor. Due to the filler content percolation effect, electrons flow through very small contact spot areas, so local current density cannot be calculated from applied current and adhesive bond geometry. This may lead to electromigration, resulting in Ag atom transport within the filler particles and then by diffusion through the polymer. This effect occurs even if self-heating is neglected. In this study, early-stage resistance degradation for bisphenol-A type and cycloaliphalic resins, loaded with Ag flakes and porous Ag powder respectively, is examined using adhesive stripe samples on FR-4. Resistance is measured by a four-point technique. Samples are held at constant temperature in an inert liquid, and a pulsed DC-current is applied. Tests with temperatures from 80-130/spl deg/C and 2-33 A/mm/sup 2/ current densities are performed. A linear increase or decrease in resistance with time is observed for flake and porous Ag filled adhesives, respectively, due to current application, whereby a sintering process for the porous Ag must be taken into account. Current density and temperature dependences for various parameters are discussed; a degradation model is proposed.