Investigation on electrical and microstructural properties of Thick Film Lead-Free resistor series under various firing conditions

The paper presents investigation of four lead free thick film resistor pastes, developed at ITME, denoted R-100, R-1k, R-10k and R-100k with sheet resistivities of 0.1, 1, 10 and 100 kΩ/□, respectively. The resistors were based on RuO 2 as the conductive phase. The aim of the work was to evaluate the influence of firing conditions of the resistive pastes on a sintering process. The pastes were screen printed onto alumina substrate with prefired AgPd lead-free terminations. They were fired at several temperatures from 750 to 950 °C for 10 min at peak temperature, as well as fired at the highest temperature for 6 h, in order to bring the sintering process into the equilibrium. The properties of the resistors, i.e , sheet resistivity and temperature coefficient of resistance (TCR), microstructure changes, glass crystallization upon firing, etc., were examined. Dried and fired resistor samples were evaluated by X-Ray diffraction analysis and by the scanning electron microscopy. The RuO 2 conductive phase maintained the same crystal structure regardless of the firing conditions. No devitrification was observed in lead-free resistors glasses. The lattice constants of RuO 2 were uniform after firing at temperatures over 800 °C. The resistors matched the desired resistivity and the TCR was the least temperature dependent at the firing temperatures around 850 °C .