Conductive particle filled polymer such as silver epoxy is the most widely used thermal interface material in electronic packaging due to its low cost, ease of assembly, and reworkability. The electrical and thermal conductivities are significantly affected by the particles network, which has been verified by the theoretical model of percolation theory. The high filler loading of polymer composite results in high cost-performance ratio, poor processability and flow behaviour, which makes it less competitive compared with traditional solder.

In this dissertation, a simple and effective approach is introduced to fabricate a highly conductive silver paste with a low filler volume fraction. The idea comes from gelation in capillary suspension, where the formation of three-dimension...[ Read more ]

Conductive particle filled polymer such as silver epoxy is the most widely used thermal interface material in electronic packaging due to its low cost, ease of assembly, and reworkability. The electrical and thermal conductivities are significantly affected by the particles network, which has been verified by the theoretical model of percolation theory. The high filler loading of polymer composite results in high cost-performance ratio, poor processability and flow behaviour, which makes it less competitive compared with traditional solder.

In this dissertation, a simple and effective approach is introduced to fabricate a highly conductive silver paste with a low filler volume fraction. The idea comes from gelation in capillary suspension, where the formation of three-dimensional particles network is driven by capillary attraction. A particle-wetting ionic liquid, as a secondary fluid, is added to bridge platelet shaped silver flakes within an epoxy base. The secondary fluid preferentially wets the silver flakes over the epoxy base with a hydrophilic three phase wetting angle of 45° that facilitates the formation of a capillary network. This forms the basis for selection of secondary fluid for facilitating the capillary bridging network. With the appropriate volume fraction of ionic liquid, the ternary silver epoxy composite forms conductive pathways with a low sliver flake content. The hydrophilic ionic liquid in the capillary bridges creates an interfacial tension that draws particles together and form a three dimensional gel network. The capillary attraction forms a strong factual network with low filler loading, consistent with the jamming theory. This work delivers two major findings.

Firstly, a relationship between a silver flake network induced by capillary attraction and conductive performances is reported in an ionic liquid added ternary silver epoxy system. Gradually increasing the secondary fluid content leads to a microstructure evolution from highly dispersed particles, weak gel network, full pendular network, funicular network and ultimately to compact capillary aggregates network. The maximum electrical and thermal conductivities correspond to the formation of a full pendular network and a funicular network, respectively. The difference in ionic liquid volume fraction for maximum electrical conductivity and thermal conductivity is explained by the differing mechanism in electron and phonon conduction.

Secondly, a theoretical investigation is conducted on the capillary attraction between platelets. Anisotropy of silver flakes yields low dimensional filler networks, indicating a possible way to introduce anisotropic capillary attraction by filler shape anisotropy. A new theory has been developed to account for the formation of ionic liquid pendular bridge on platelets. The theory is based on the large face-to-face bridge configuration leading to strong adhesion between silver flakes. A collision model is put forward to describe the dynamic bridge formation process. The criterion governing bridge formation is that the Stokes numbers must be smaller than the critical ones. Below the critical Stokes number, no rebound occurs which caters for network formation. Lubrication force is found to be the major energy dissipative mechanism in the dynamic capillary bridge formation process.

In summary, we demonstrate that introducing secondary fluid to induce capillary attraction between conductive silver flakes is an effective way to improve the conductive performance of polymer composites. Experimental investigation of particle network structures, conductivities, and theoretical study of capillary attraction and bridge formation between platelets provide practical insights into network formation in colloidal suspension and polymer composites. Based on our current findings, future work will focus on capillary bridging of nanoparticles and the sintering of ternary composites.