Some weeks ago we had some results on Vishay hermetic resistors, ending with the recommendation of FriedLogic to watch the DUT for extended periods of time, not just some oven cycle. My last result had been a 4 ppm drift after an oven cycle with a time constant of about 45 minutes.

Meanwhile i improved my resistor measurement and observed the HCZ500R00 again.

After heating it to 37 °C it showed that 4 ppm drift again. But then it drifted back with a time constant of about 3 days. After that i did some oven cycles, but it became clear that the results are meaningless, if the resistor shows relaxation at time constants of 3 days. So i started measuring the step response after an oven temperature step 40 °C => 30 °C. Since the oven isn't perfect, the temperatur step was in fact from 38,6 to 29,6 °C, about 9 °C.

The log shows four distinct reactions at different time constants and the effective TC becomes zero twice after the step. On the long term TC is positive and about 0.46 ppm/K.

1) First reaction is a negative resistance change of about 2 ppm after 5 minutes, so that would be a TC of -2 ppm / -9 °C = 0,22 ppm/K

About 11 minutes after the step TC = 0.

2) Next is a positive resistance change of about +1 ppm after 22 minutes. If you stop then, TC = 1 ppm / -9 °C = -0,11 ppm/K

About 40 minutes after the step TC = 0 again.

3) Then we have a resistance change of about -5.2 ppm after 6 hours. This is a TC of -5.2 ppm / -9°C = 0,58 ppm/K

4) About 1.1 ppm of those -5.2 ppm get lost after about 4.5 days. So the long term TC is about -4.1 ppm / -9 °C = 0,46 ppm/K

At the end i left the measurement running some more days logging the linear drift of my bridge, which is about 0,11 ppm/day. Standard deviation of the filtered measurement during that drift was 0,0106 ppm of resistance. In the "slow" diagram residual temperature variations of the oven and the bridge linear drift have been subtracted.

I think the complicated step response mirrors the complicated construction of the Vishay hermetic resistor (foil, glue, carrier...). Andreas already explained that the glue with its creeping feature may constitute a long term memory. For me it is obvious now that all measured TC curves for such resistors are more or less meaningless.

Next i will probably try a similar measurement on a UPW50 thin film resistor. Hope its response will be more straightforward.

Regards, Dieter

Edit: I meant UPF50 (thin film resistor) instead of UPW50 (wirewound).