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Forum: Reliability & Maintainability Questions and Answers

Topic: Reliability & Maintainability Questions and Answers

Topic Posted by: Reliability & Maintainability Forum ( )
Organization: System Reliability Center
Date Posted: Mon Aug 31 12:47:36 US/Eastern 1998

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Posted by: Richard Golek
Date posted: Thu Sep 21 12:42:14 US/Eastern 2000
Subject: glass thermistors with axail leads
We have begun forming the leads on axial thermistors with glass bodies. 50 to 90X visual inspection reveals small cracks near the glass/lead interface on a small percentage of the parts. Our own test program has just begun, and the vendor has not been able to supply more data. I am looking for comments on what the long term humidity and resistance effects may be, paticularly in a tropical but benign environment. Thank you.


Subject: Thermistor Reliability
Reply Posted by: Bruce Dudley ( )
Organization: Reliability Analysis Center
Date Posted: Thu Sep 21 15:42:00 US/Eastern 2000
Any cracking of the glass package will result in a loss of hermeticity eventually. Small crack are usually propagated by thermal cycling and or vibration or shock. Given the statement that a benign condition is expected at the operating site, minor cracks should not be a problem as the temperature is controlled as well as vibration and humidity. Searching our failure mode information, I found that an open condition was the most prevalent failure condition followed by parameter change and overstress. Of these modes, only parameter change is significant for a moisture problem, so at 16% of the failure modes and non hermetic condition, you could have a long term problem for those devices that have lost hermeticity due to open package conditions.

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