Silver-polyaniline-epoxy electrical conductive adhesives - a percolation threshold analysis

Electrical conductive adhesives (ECA) find extensive applications in electronic manufacturing and packaging industries. Compared to the soldering technology, adhesives joining offer numerous advantages including mild processing conditions, fewer processing steps (reducing process cost), and especially, the fine pitch capability due to the availability of small size conductive fillers. ECAs function based on the conductive fillers, typically 1-10μm in size, supported in a polymeric matrix. To be conductive, the loading of fillers needs to be above its percolation threshold - the minimum fraction of fillers needed to establish the conducting network. In this paper, we report a systematic study of percolation thresholds of silver-epoxy (Ag-epoxy) and silver-polyaniline-epoxy (Ag-PANI-epoxy) system. The polyaniline (PANI) has a moderate conductivity in between the silver and epoxy; it was used to dope the epoxy matrix to minimize two problems in the ECAs: (1) localization of charge carriers because of the aggregation of the silver fillers, and (2) the interfacial polarization arising from the intrinsic difference in polarities and surface energies of fillers and epoxy. Two different methods were used to prepare the Ag- PANI -epoxy adhesives. The first method was to add silver fillers into liquid epoxy resin, add PANI, and then the amine hardener. The second method was to add PANI into liquid epoxy resin, add silver fillers, and then the amine hardener. Our experiments showed that silver fillers were well dispersed in cured adhesives prepared in the first method while silver fillers formed aggregates in cured adhesives prepared in second method. We also propose a mechanism of conduction based on surface properties and interactions between fillers and the epoxy matrix. Our results and analysis may help to explain the reduced percolation threshold and enhanced conductivity.