Current-induced degradation of isotropically conductive adhesives

The scope of applications for electrically conductive adhesives increases continuously requiring new and improved properties. In general, isotropical conductivity is achieved by loading a high resistive polymer (mainly epoxy resins) with 70...80 wt% of metal filler particles. During the cure the shrinkage of the resin lowers the contact resistance between neighbouring particles, resulting in a well conducting, three dimensional network. Nevertheless, for high current applications the actual local current density at the contact spots of neighbouring particles might be a limiting parameter. Due to the percolation effect of the filler particle content, electrons flow through very small contact spot areas, so the local current density cannot be calculated from the applied current and the adhesive bond geometry. This may lead to electromigration due to "electron wind" resulting in a material transport process of Ag atoms within the filler particles and subsequently by diffusion through the polymer. This effect will be present even if self-heating can be neglected. In this study the early-stage degradation of resistance for Bisophenol-A type and cycloaliphatic epoxy resin loaded with flake shaped and porous Ag powder, respectively, is examined. Each sample is made of a stripe of conductive adhesive on FR-4 measuring the resistance with 4-point technique. It is held at constant temperature in an inert liquid, and a pulsed DC-current is applied. Series with different temperatures (varying from 80 to 130 degrees C) and current densities (2...3 A/mm(2)) are performed. A linear increase or decrease of resistance with time for the flake and porous Ag filled adhesive, respectively, due to the application of current is observed, whereby a sintering process in the case of porous Ag has to be taken into account. The dependencies on current density and temperature for different parameters are discussed; a model for the degradation will be proposed.