Lead-free low-firing thick-film resistors based on bismuth glasses and ruthenium oxide

Recent developments in thick-film technology have seen the widespread introduction of lead- free alternative to traditional conductive, dielectric and overglaze compositions. Resistors, however, have lagged behind this trend, as developing balanced and well-behaved resistive compositions from scratch is significantly more involved than other thick-film materials. Bismuth- containing glasses based on the bismuth-zinc-boron-silicon oxide system, which have been studied for over 20 years, constitute a promising alternative to the usual lead borosilicate materials, with which they are quite similar. This paper presents, after a historical perspective of thick-film resistor technologies, a short overview of bismuth glasses and recent developments in applying them to lead-free thick-film resistors, with a focus on low-firing piezoresistive compositions. Recent work, both outside and in our laboratory, suggests that successful development of these novel lead-free materials is possible, and that they can be applied to the manufacture of thick-film piezoresistive sensors on steel substrates. Moreover, as firing temperature below 600°C are achievable, application on glass and some aluminium alloys is also possible.