Isotropically conductive adhesive filled with silver metalised polymer spheres

Isotropic conductive adhesives (ICAs) have a growing range of applications in electronics packaging and have recently emerged as an important material in photo-voltaic module interconnections, particularly for thin-film and other non-silicon technologies where soldering processes are often unsuitable due to the nature of the metallisation or the limited maximum temperature the assembly can be exposed to. ICAs typically comprise of a high volume fraction of solid metallic flakes, usually silver, in an adhesive matrix because of its highly conductive oxide however, this thesis will focus on adhesives containing a large volume fraction of silver coated/metalised mono-sized polymer spheres (Ag-MPS). Incorporating silver coated mono-sized polymer spheres is anticipated to deliver specific advantages such as a significant reduction in the required silver content, improvement of the overall mechanical properties and flexibility to tune the properties of the filler according to the application compared with conventional flake filled adhesives. In this research advancements in the understanding of Ag-MPS filled ICAs, both through theory and experiments, have been made. Analytical models to predict an individual Ag-MPS resistance and Ag-MPS filled ICA resistance have been developed. The experiments based on the flat punch nanoindentation technique have been conducted to determine individual Ag-MPS resistances. The theoretical and experimental studies establish Ag-MPS diameter, coating resistivity, coating thickness, contact radius, and contact geometry as the main contributors towards the resistance of an Ag-MPS filled ICAs. These studies showed that Ag-MPS resistance decreases with increasing coating thickness and contact radius but increases with increasing coating resistivity. The experiments have also been conducted to investigate the effect of Ag-MPS volume fraction, diameter, coating thickness, curing conditions and shrinkage (affecting contact radius) on ICA conductivity and comparisons are made with flake filled and commercial ICAs. The results showed that ICA conductivity increases with increasing volume fraction and coating thickness but decreases with diameter. More importantly the results showed that conductivities similar to those of flake filled ICAs, including those commercially available, can be obtained using 70% less silver. The results show that, Ag content can be reduced further to just 7% with use of larger 30μm Ag-MPS but with a lower resulting conductivity. Thus for applications where very high conductivity is not required larger Ag-MPS may offer even greater potential cost benefits, which is something flake filled ICAs cannot offer. This is a significant achievement which can allow tuning of ICA formulations according to the demands of the application, which is not possible with the use of silver flakes as there is only a limited range of silver flake volume fractions that will yield useful levels of conductivity.