Electrically conductive adhesives (ECAs) have recently become a critical technological area in component development behind solar cell packaging for die attachment, solderless interconnects, and heat dissipation. The standard example of an ECA employs the use of conductive fillers within a polymeric matrix or host to render the final composite conductive. Electrical conductivity of an ECA is governed by percolation theory, wherein the necessary fillers that host electrons transfer, via physical connection or tunneling, must reach some critical volume fraction to accommodate probable conductive pathways that would be large enough to be considered isotropic [1,2]. Many fillers exist for use in this role, but commonly silver is chosen for its high electrical and thermal conductivities [3]. However, silver (especially micro- or nano-structured) remains an expensive commodity, and typical volume fraction loadings in ECA can approach >30%. This is necessary as the theoretical critical volume fraction required for monodisperse spheres in a randomly oriented isotropic system is ~16% [4]. Such excessive filler loading not only invalidates economic feasibility, but also deteriorates mechanical properties inherent for the host polymer.
To mitigate the critical percolation threshold (pc) for volume fraction loading of a filler, combative methods are articulated herein. One approach is to use low-dimensional, high-aspect ratio fillers, such as graphene and carbon nanotubes (CNTs), which have been shown to lower pc [5,6]. Typically, such fillers are more expensive than silver; however, given the low-loading implied to achieve a percolated network, this approach could improve the economic feasibility as an added filler for reducing total filler loading required [7]. In this work, commercial CNTs are employed as a high-aspect ratio filler for the reduction of silver filler loading in an ECA system. Graphene nanoplatelets are also synthesized and used to demonstrate a route for creating tailored high-aspect ratio, low-dimensional fillers which are effective at generating a percolated network at relatively low loading. Utilizing a pre-percolated CNT system, a hybrid silver/CNT system was then generated to achieve enhanced conductivity at lower total loading over pure silver systems, which exhibited a conductivity of 54 S/cm at 12 vol.% loading with a CNT loading of only 8 wt.%.
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