Forum: Reliability & Maintainability Questions and Answers

Topic: Reliability & Maintainability Questions and Answers

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

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Original Message:

Posted by: Peter Kirchhofer (peter.kirchhofer@jaquet.com )
Organization:JAQUET Ltd
Date posted: Thu Jan 14 5:47:11 US/Eastern 1999
Subject: potted coil temperature cycling stress
Message:
Temperature cycle tests with electromagnetic speed sensors, working after the variable reluctance principle, between 25C and +180C let about 20% of the sensors fail within the first 1 to 20 days: 1. the sensor coil resistance (4200 turns of enamelled,bakeable Cu-wire, 0.04 mm, temperature class H for applications up to +180C) drops from a nominal 830 Ohm to 2 ... 790 Ohm, either suddenly or by and by. 2. the sensor coil connecting wires ( 0.04 mm) suddenly break, mostly directly on the soldering joint to an intermediate printed circuit board to the sensor cable. 3. sensors which were opened after the temperature cycle test often (but not always) showed a strange, more or less pronounced deformation of the coil winding along the coil axis. Not all of the failed sensors showed this deformation and not all senors with deformed coils failed during the test. Sensor assembly: the sensor coil with iron core, the TORLON (Polyamidimide) centering piece, a permanent magnet and one end of the TEFLON (PTFE or FEP) sensor cable are moulded in a silicone potting material in a cylindrical metal housing which is closed on the front side. Has anyone experience with respect to the reliability of similar wirewound items at similar temperature levels, knows anybody about research for the causes of such failures?

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