Pulse stability of low ohmic thick-film resistors

Durability of low ohmic thick-film resistors on pulse load with micro-and milisecond duration are described in this paper. Standard thick-film resistive compositions with sheet resistance of 10 Ohms/sq (4311 and QS871 - DuPont; R400-10A - Heraus) and 3 Ohms/sq (QS870 - DuPont) were tested. Test resistors with resistance of 3 Ω were prepared on alumina substrates. Resistance changes were measured after 50 pulses at each voltage. Then the voltage was increased and the series of pulses were repeated until the resistance change exceeded 0.5%. The resistors made of 4311 paste exhibited the highest durability for long (20 ms) pulses, when the thermal interface between substrate and heatsink was filled by thermally conductive grease. The relative resistance change of +0.5% was observed at about 19 W/mm² and about 18 W/mm² for the samples with and without the overglaze, respectively. Short (microsecond) pulses had the least influence on QS871 paste, for which the resistance change of +0.5% was observed at electric field intensity of about 90 ± 10 V/mm. For 4311 paste the parameter reached maximum 50 ± 10 V/mm, whereas for R400-10 as well as QS870 pastes the value didn't exceed 40 ± 10 V/mm.