Anisotropic effect when using isotropic conductive adhesives

Conductive adhesives are an enabling technology for a growing number of electronic packaging applications. In the last few years anisotropic conductive adhesives have gained increased popularity, between another applications, in flip chip packaging. In general anisotropic conductive adhesives are prepared with complicated technology by dispensing electrically conductive particles inside an adhesive matrix. The conductivity of these materials is limited to the Z-axis (between the chip pads and the substrate) with electrical isolation provided in the XY-plane (between the adjoining chip pads). When using standard formulations of isotropic conductive adhesive such diversity between the directions of current's conduct is impossible. In this paper we describe the results of investigations, which show that by modification of the surface tension of the silver filled isotropic conductive adhesives the anisotropic effect is possible. Adhesives with filler content far before percolation threshold have been used for experiments. In proper level of additives influencing the surface tension, filler type and content as well as curing temperature and pressure, the agglomeration of silver particles is observed. As the result of the polymer matrix shrinkage, the clusters of filler's particles are trapped between tighten, gold coated pads, while areas which have to be electrically isolated are filled with non-homogenous material, in which the conductive clusters are isolated by polymer base material.