This invention relates to conductive pastes useful for producing conductor patterns adherent to substrates.
Conductive pastes which are applied to and fired on dielectric ceramic substrates (e.g. alumina substrates) usually comprise an inert liquid-vehicle having finely divided inorganic powders (e.g. metal powders and binder powders) and are applied to substrates using so-called "thick film" techniques. Upon firing the printed film of the pastes, the metal powders provides the functional (conductive) utility, while the inorganic binder (e.g. glass powder, Bi.sub.2 O.sub.3) bonds the metal powders to one another to the substrate. The most commonly used conductive pastes employ noble metals (e.g. Ag, Au, Pd, and their mixtures), especially Ag, as conductive media, since their inert characteristics permit firing at high temperature (e.g. 850.degree. C.) in air and these conductive pastes are used for making conductors.
Since the prices of expensive noble metals, especally Ag, has become higher recently, conductive pastes employing less expensive conductive powder in place of Ag powder has been proposed. For example, conductive pastes employing less expensive powders such as non-noble metal powder (e.g. Ni and Cu) and metallic compound powders (e.g. TiN and SnO.sub.2) have been developed and some of them are put on market. These conductive pastes, however, have some disadvantages. The conductive pastes employing non-noble metal powder (e.g. Ni, Cu) have often been confined to specialized uses due to their fired film having bad corrosion-resistant characteristics or have required the great practical inconvenience and expense of firing in non-oxidizing atmosphere, though they show low resistivities just after firing. The fired film of the conductive pastes employing conductive metallic compound powders (e.g. TiN, SnO.sub.2) have not shown low resistivities because the powders themselves do not show low resistivity. In addition to this, the conductive paste employing TiN powder have also required the practical inconvenience and expense of firing in non-oxidizing atmosphere.
Meanwhile, conductive pastes employing Ag-coated Al.sub.2 O.sub.3 powder as conductive media has been proposed. These pastes are economically excellent. But, because of bad wettability of Al.sub.2 O.sub.3 with Ag, Al.sub.2 O.sub.3 powder cannot be closely touched with Ag, so Ag-coating layer peels off when Ag-coated Al.sub.2 O powders are dispersed in a liquied vehicle. This makes resistivity of fired film larger. Additionally pastes employing powders of oxides such as Al.sub.2 O coated with Ag have a disadvantages that fired film cannot be soldered, as well as pastes employing TiN powders or SnO.sub.2 powders mentioned above.
Though some kinds of pastes have been proposed for saving Ag, as mentioned, these do not always meat conductivity corrosion-resistance, and solderability. Inexpensive pastes meeting these requirements are expected.