Microstructure and electrical performance of eco-friendly thick film resistor compositions fired at different firing conditions

Perovskite ruthenates, viz. CaRuO3, SrRuO3, BaRuO3, etc. although exhibit many interesting physical properties, have been seldom used in thick film resistors. In the preliminary attempts, we have formulated lead free thick film resistor paste compositions using CaRuO3. Five different paste compositions by varying the ruthenate concentration were prepared. Thick film resistors were screen printed onto an alumina substrate. Different thick film firing conditions were adopted to fire the resistors. Although, for TFRs, the standard conventional firing cycle is 850 °C, our TFR compositions can be fired even at a peak temperature of 800 °C. The evolution of microstructure and electrical performance of these resistors were studied. The resistors fired in conventional thick film firing furnace (BTU make) were found to be morphologically more uniform than the resistors fired in normal tube furnace. However, the sheet resistance value ‘Rs’ is lower (54–1.36 kΩ/□) in case of resistors fired in tube furnace. On the other hand, resistors fired in thick film firing furnace exhibit higher range of sheet resistance (1.36–800 kΩ/□). The hot and cold TCR range is 180–560 ppm/°C in case of tube furnace fired resistors and 120–235 ppm/°C for BTU furnace, which is comparatively lower than those reported earlier (>600 ppm/°C) for other lead free resistors.