Bulk soldering: Conductive polymer composites filled with copper particles and solder

The electrical conductivity of plastics can be improved by metal filler particles, but high metal loadings (several ten volume percent) are generally needed to realize percolating pathways that can conduct electricity. We explore the improvement of the electrical conductivity of polystyrene by simultaneously adding two different metal fillers: copper particles, and a lead/tin solder alloy. The essential idea is that during mixing, the solder can bind together the particles via numerous menisci and the resulting copper-solder aggregates can then form a percolating network - hence the term "bulk soldering". We show that this approach can realize high conductivities at total metal loadings that are far lower than if the copper particles were used as the sole conducive filler. SEM confirms that the microstructure consists of copper particles bonded by the solder, and that such aggregates can percolate throughout the sample volume. Indeed at a total metal loading of 20 vol%, the mixing process can create an all-metal scaffold that remains free-standing even when continuous phase polystyrene is removed. We examine the effect of composition on the microstructure and conductivity, and show that there is an optimal solder:copper volume ratio of roughly 0.6 which gives the highest conductivity. High conductivity is achieved only if a flux is added during mixing. The flux removes the copper oxide layer on the copper particles, and hence ensure wetting of copper by solder. Finally we comment on the similarities and differences between these polystyrene/solde/copper mixtures versus other liquid/fluid/particle mixtures in which capillarity induces aggregation of particles.