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I am using 0402 nominally 10k\$\Omega\$ NTC thermistors that are soldered onto a flex PCB which is then adhered to the point of interest in an Alumina based thermal adhesive.

The thermistors have been working properly through temperature cycling from room temp to 85C many times over the past year. The points of interest are cooled by forced air when in use.

Suddenly, several of the thermistors have begun failing by reading a nominal resistance between 13k\$\Omega\$ and 17k\$\Omega\$ leading to obviously wrong temperature measurements across their use range. I removed the failed ones to look for visible cracking due to thermal shock, but I don't see anything.

After going through many avenues as to figure out what is causing this problem, I am still stumped.

I know all parts have a mean time before failure, but I am nowhere near that, and clearly something is expediting their failure time. There is no shock nor vibe in the system so what else causes thermistors to fail once installed and in use over time? Corrosion? Leakage paths in the thermal adhesive?

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  • \$\begingroup\$ What power is being dissipated? (I know this is an unlikely cause or the temperature read would be higher than 85°C.) \$\endgroup\$ – Transistor Jun 15 '17 at 22:49
  • \$\begingroup\$ 3.3V through 10k\$\Omega\$ thermistor and 10k\$\Omega\$ resistor (halfway point is input to ADC), so nominally 1/4mW. \$\endgroup\$ – Nino Jun 15 '17 at 22:52
  • \$\begingroup\$ You measured the resistance after removing them from the flex? \$\endgroup\$ – The Photon Jun 16 '17 at 0:07
  • \$\begingroup\$ Yes, resistance was also nominally high even after removal of the excitation voltage (3.3V). And then after removal from the circuit it would measure high as well. It sustained permanent damage somehow. \$\endgroup\$ – Nino Jun 16 '17 at 0:39
  • \$\begingroup\$ I'm troubleshooting an A/C which stopped turning on automatically. Do thermistors commonly fail? \$\endgroup\$ – Tom Russell Aug 25 at 23:53
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Three possibilities come to mind:

It could be that the thermal epoxy has attacked the exterior potting of the thermistor.

It could be that the thermal compound has a different coefficient of expansion than the thermistor leading to mechanical stress on the thermistor.

It could be that the exterior potting material on the thermistor is defective leading to moisture ingress.

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  • \$\begingroup\$ Thank you for your response! Follow up to your three points: 1) Maybe I am confused on the "stackup" up a thermistor. I thought they were pretty much ceramic all the way to the surface? 2) This also crossed my mind. The curious thing about this is that the failures are concentrated in the cooler part of the system. If this were the case I would expect to see a concentration of failures in the warmer part that sees more thermal shock. 3)This would mean corrosion right? Is there a way to test for this? \$\endgroup\$ – Nino Jun 16 '17 at 0:46
  • \$\begingroup\$ There is usually a hermetic coating on the thermistor. Contact an applications engineer at the manufacturer for more details. For point 3, this would mean some type of internal corrosion. Cross sections examined with an SEM is the normal test method. \$\endgroup\$ – Glenn W9IQ Jun 16 '17 at 1:01

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