Development of conductive adhesives filled with low-melting-point alloy fillers

Conventional isotropic conductive adhesives (ICAs) are composed of a polymeric matrix with silver flakes. As an alternative to lead-bearing solder, ICAs offer a number of benefits, but limitations do exist for ICA technology. ICAs filled with silver flakes generally show higher initial contact resistance, unstable contact resistance, and inferior impact strength. In this study, a new class of isotropic conductive adhesives was developed by using two different fillers, a silver flake and a low melting point alloy filler, in the ICA formulations. After curing, the metallurgical connection between the silver particles and between the silver particles and the nickel (Ni) substrate was observed using scanning electron microscopy (SEM). Initial contact resistance and contact resistance shift during elevated temperature and humidity aging of the formulated ICAs on a non-noble metal (nickel) were investigated and compared to those of ICA filled only with the silver flake. It was found that (a) the low-melting-point alloy filler could wet the Ag flakes and nickel (Ni) substrate very well and formed metallurgical connections; and (b) this ICA showed especially low initial contact resistance and stable contact resistance during aging on nickel metal compared to the ICA filled only with silver flakes.