Ruthenium resistor glazes for thick film circuits

The rapid development of thick film micro-circuits has created a need for preparations that will provide resistor films on a variety of substrates. Until recently the majority of theresistive preparations available required a firing profile with a peak temperature around 750°C, necessary to complete complex chemical changes within the glaze resistor. This not only imposed the need for very close control of the firing cycle and furnace atmosphere to ensure reproducible values, but limited the choice of substrate to materials, such as 95¿98% alumina ceramics, capable of withstanding this temperature. A prerequisite to attaining consistent values is the surface condition of the substrate. Relatively low-cost materials like mica and glass, whichinherently possess flat, highly finished surfaces, are excluded, since they either exfoliate (dehydrate) or deform at temperatures exceeding 650°C. To overcome these limitations and take advantage of the mechanism of conduction through ruthenium dioxide a series of glaze resistor preparations has been developed. The objective was an ink based on a chemically-inert system that would be far less dependent on firing conditions than those hitherto available. To be viable thesystem has to be capable of producing a wide range of resistivities; possess acceptably low temperature coefficients; contain the spread of resistance values within narrow limits and be economically competitive. The use of valency control and the techniques adopted to achieve these requirements are described.