High-temperature storage and thermal cycling studies of thick film and wirewound resistors

Abstract

The operating ambient temperature for underhood automotive and aerospace applications is increasing. This work was undertaken to evaluate the suitability of thick film and wirewound resistors for distributed aircraft control systems in a 200/spl deg/C-225/spl deg/C operating environment. High temperature stability testing of power wirewound and thick film resistors is reported. Dale power wirewound 1 /spl Omega/, 100 /spl Omega/, and 10 k/spl Omega/ resistors with power ratings of 5 W and 25 W were tested. The TCR of the 100 /spl Omega/, and 10 k/spl Omega/ resistors was very small, however, the 1 /spl Omega/ resistor varied by 5% over the temperature range from 25/spl deg/C to 300/spl deg/C. Stability with long term storage (10000 h) at 300/spl deg/C was measured for the wirewound resistors unpowered and powered at 20% of rated power. With the exception of the 10 k/spl Omega//25W resistor, the change in resistance was less than 4%. Wirewound resistors were also thermal cycled 1000 times over a temperature range from -55/spl deg/C to 225/spl deg/C with only one failure due to a broken internal connection. Three 900 Series thick film resistor pastes from Heraeus-Cermalloy were studied: 100 /spl Omega//sq., 1 k/spl Omega//sq., 10 k/spl Omega//sq. The temperature coefficient of resistance (TCR) was measured from 27/spl deg/C to 500/spl deg/C in 50/spl deg/C increments. The change in resistance was </spl plusmn/6% up to 300/spl deg/C. A 2 /spl times/ 2 matrix of variables was included in the 300/spl deg/C storage test: untrimmed resistors, resistors trimmed up 50% in value, unpowered, and powered at 1/8 W. Palladium/Silver was the initial termination choice for these 300/spl deg/C studies, but silver migration under electrical bias lead to electrical shorts between conductor traces on the substrates with powered resistors. Gold terminated thick film resistors were used for powered storage testing at 300/spl deg/C. The change in resistance after 10000 h at 300/spl deg/C was < 3% for all test combinations.