Abstract Lead-free thick film resistive compositions suitable for hybrid microelectronics were prepared. The compositions were made with RuO 2 as the conducting phase and bismuthate glasses. This blend of bismuthate glasses constitutes a suitable choice for avoiding nega- tive effects such as devitrification, bleeding out of the glass on alumina substrates, anomalous distribution of conductive grains in the glassy matrix and phase sepa- ration observed in other systems. The morphology, microstructure and electrical properties have been studied. X-ray diffraction (XRD), electron scanning microscopy (SEM) and energy dispersion spectroscopy (EDS) show that a defect pyrochlore phase of bismuth titanate formed at about 700 °C in all the compositions studied. Transmission electron microscopy (TEM) analysis of the original RuO 2 powder shows that a sin- gle grain is made of many smaller grains of different crystalline orientations. The sheet resistance spans two decades by changing the RuO 2 fraction from about 14–52 wt%. The resistors exhibit good reproducibility and their temperature coefficient of resistance is in the range of ± 300 ppm/°C. 1 Introduction In response to the demand for lead-free processing, to reduce negative environmental effects resulting from disposal of electronic products and to conform to the relevant legislation, the electronics industry has been engaged for several years in the development of new lead-free materials. As a result, lead-free products are currently available for solders, packaging and thick-film conductive pastes for interconnections and contacts of components. However, the development of lead-free compositions for thick-film hybrid microelectronics is not a simple task, and only partially satisfactory com- positions [1–6] based on the usual conductive phases (ruthenium dioxide and perovskite ruthenate) have been obtained for lead-free thick film resistors (TFRs). In this paper, we report the development of lead- free RuO 2 -based TFR compositions in bismuthate glasses. The glasses were chosen so as to avoid the negative effects observed in other systems, such as devitrification, bleeding of the glass on alumina sub- strates, anomalous distribution of conductive grains in the glassy matrix and phase separations. The mor- phology, microstructure and electrical properties of the materials were determined. Resistors with low sheet resistance and with suitable electrical properties were produced from the developed compositions. 2 Experimental 2.1 Glasses, additives and resistor formulations Three resistor compositions denoted as JH024/3, JH024/4 and JH024/5 were the subject of this study. M. G. Busana M. Prudenziati (&) Department of Physics, University of Modena, via G. Campi, 213/A, 41100 Modena, Italy e-mail: Prudenziati.maria@unimo.it J. Hormadaly Zandman Center for thick film microelectronics and Department of Chemistry, Ben-Gurion University of the Negev, Beer Sheva, Israel e-mail: hormadj@bgumail.bgu.ac.il J Mater Sci: Mater Electron (2006) 17:951–962 DOI 10.1007/s10854-006-0036-x 123 Microstructure development and electrical properties of RuO 2 -based lead-free thick film resistors M. G. Busana M. Prudenziati J. Hormadaly Received: 30 March 2006 / Accepted: 12 June 2006 Ó Springer Science+Business Media, LLC 2006