Lead-free thick film resistors: an explorative investigation

Pastes prepared with seven lead-free glass particles were screen-printed and fired on 96%-Al₂O₃ substrates. The fired films were studied in terms of sintering, devitrification, crack propagation and bleeding. The most promising glass compositions were selected for the preparation of RuO₂-based thick film resistors (TFRs), on pre-fired PtAu- or PdAg-based terminations; electrical properties of the resistors (sheet resistance vs. RuO₂ volume fraction, temperature coefficient of resistance and size effects) were investigated. The study evidenced a myriad of complex phenomena, including devitrification, relevant bleeding of the glass on alumina substrates, anomalous distribution of conductive grains in the glassy matrix, phase separations. Contrarily to what happens in traditional systems, it is difficult to get a wide resistance range simply by changing the fraction of the conductive phase. These results emphasize the criticality in the choice of the glass composition for the preparation of lead-free thick film resistors.