MICROSTRUCTURAL EVOLUTION OF COPPER THICK-FILMS OBSERVED BY ENVIRONMENTAL SCANNING ELECTRON-MICROSCOPY

A generic copper thick film ink system was formulated to be representa tive of the inks currently used in industry, The microstructural evolu tion 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 a nd 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 follo ws: The burnout process reached completion by about 320 degrees C. Cop per oxidation was observed to occur between 500 degrees and 600 degree s C in nitrogen, The glass was observed to soften over the same temper ature range, By 700 degrees C, the glass had softened and coated the c opper particles, Prior to glass softening, oxidation occurred regardle ss of the presence of the glass, However, once the glass had softened, copper oxidation was retarded, Cu2O was found to be sufficiently solu ble in the glass to allow it to be removed from the surfaces of the co pper particles. Thus, the presence of glass facilitated the formation of conductive paths by two mechanisms, It inhibited oxidation and it d issolved the oxide that formed, Both processes allowed effective sinte ring.