Rheological characterization of a novel isotropic conductive adhesive – Epoxy filled with metal-coated polymer spheres

Novel isotropic conductive adhesives (ICAs) filled with metal-coated polymer spheres have been introduced in order to improve the mechanical reliability compared to conventional silver-filled ICAs. The topic of this study is the rheology of an epoxy filled with monodisperse polymer spheres with or without Ag coating. In trials with uncoated spheres, the viscosity increases with increasing sphere fraction, while the sphere diameter (6 vs. 30 μm) only has a minor effect. With 45 vol% of spheres (giving an ICA with adequate electrical properties), the Ag coating on the spheres has a large effect on the rheology of the adhesive paste. Compared to the epoxy with uncoated spheres, the epoxy with Ag-coated spheres exhibits higher viscosity, higher storage and loss moduli, and a higher ratio of storage modulus to loss modulus (about 10 times). Furthermore, the viscosity of the epoxy with coated spheres increases with time in oscillatory measurements with low to intermediate frequencies. With hardener added to the epoxy containing Ag-coated spheres, the viscosity increase at room temperature is small within the first three hours. In curing trials, the viscosity development is different for unfilled and sphere-filled epoxies. The rheological properties of the ICA with 45 vol% Ag-coated spheres were found to be suitable for stencil/screen printing and dispensing processes.