Copper metallizations

#This invention relates to electronic compositions, and more particularly, to metallizations useful in producing conductors on dielectric substrates.

#Metallizations which are fired onto ceramic dielectric substrates to produce conductor patterns usually comprise finely divided noble metals and an inorganic binder, and usually are applied to the substrate as a dispersion of the inorganic powders in an inert liquid vehicle. Upon firing (usually in the range 700.degree.-1000.degree. C.), sintering occurs and the metallic component provides the functional (conductive) utility, while the binder (e.g., glass, Bi.sub.2 O.sub.3, etc.) bonds metal particles to the substrate and to one another.

#There is a need for conductor compositions in which noble metals are replaced by base metals such as copper, yet having good performance characteristics. Copper compositions are usually fired in an inert, nonoxidizing atmosphere (such as nitrogen) to prevent copper from reacting with oxygen in the air at elevated temperature. Some previous attempts to make copper compositions have used typical glass binders such as those having high bismuth oxide, cadmium oxide, or lead oxide contents. However, it has not been generally recognized that in the absence of air during firing at 700.degree.-1000.degree. C. copper metal can be oxidized to cuprous oxide, or further to cupric oxide, by reduction of some of the constituents of the glass binder. Bismuth oxide and cadmium oxide have been found to be particularly susceptible to reduction under such conditions, lead oxide being less susceptible. Accompanying the oxidation of copper metal to an oxide of copper is the reduction of bismuth oxide, cadmium oxide or lead oxide to the corresponding metal.

#The essential component in the copper compositions of this invention is a glass of Table 1. These glasses provide excellent fired conductor properties despite being fired in nonoxidizing atmospheres. The problems inherent in the use of glasses rich in Bi.sub.2 O.sub.3 and CdO are avoided, such as limited solder acceptance and failures in adhesion.

#PbO is an essential component of these glasses to provide low viscosity and low softening point. At least 40% PbO should be present based on the total weight of the glass. In excess of 70% PbO tends to be reduced by dissolved copper under some firing conditions. PbF.sub.2 may optionally be present in amounts up to 20%, PbF.sub.2 being a powerful flux in combination with PbO and being less reducible by copper metal than is PbO. The total weight of PbO and PbF.sub.2 in the glass is 50-70%. As PbO is increased, the beneficial effect of PbF.sub.2 is reduced. The beneficial effect of PbF.sub.2 is due to its higher anion to cation ratio than is the case in PbO. Preferred glasses comprise 45-55% PbO and 8-13% PbF.sub.2, for minimized tendency to reduce, yet provide a low softening point.

#SiO.sub.2 is 7-27% of the glass, preferably 7-20%. It serves to provide adequate viscosity to reduce spreading of the metallizing binder, and inhibits crystallization and consequent loss of strength.