Microstructural evolution of copper thick films observed by environmental scanning electron microscopy

Abstract

A generic copper thick film ink system was formulated to be representative of the inks currently used in industry. The microstructural evolution of two of the copper-based inks based on the generic composition was investigated using an environmental scanning electron microscope (ESEM) equipped with a hot stage. The first ink, consisting of copper and organic constituents (binder and solvent), oxidized heavily during firing while the second ink, consisting of copper, glass, and organic constituents, oxidized only slightly. The sequence of events observed in firing the ink comprised of copper, glass, and organics is as follows: The burnout process reached completion by about 320°C. Copper oxidation was observed to occur between 500° and 600°C in nitrogen. The glass was observed to soften over the same temperature range. By 700°C, the glass had softened and coated the copper particles. Prior to glass softening, oxidation occurred regardless of the presence of the glass. However, once the glass had softened, copper oxidation was retarded. Cu2O was found to be sufficiently soluble in the glass to allow it to be removed from the surfaces of the copper particles. Thus, the presence of glass facilitated the formation of conductive paths by two mechanisms. It inhibited oxidation and it dissolved the oxide that formed. Both processes allowed effective sintering.