SRC Forum - Message Replies
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
Posted by: Peter Kirchhofer
Date posted: Thu Jan 14 5:47:11 US/Eastern 1999
Subject: potted coil temperature cycling stress
Temperature cycle tests with electromagnetic speed sensors, working after the variable reluctance principle, between –25°C and +180°C 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, Ø 0.04 mm, temperature class H for applications up to +180°C) drops from a nominal 830 Ohm to 2 ... 790 Ohm, either suddenly or by and by.
2. the sensor coil connecting wires (Ø 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?
Subject: Coil Temperature Cycling
Reply Posted by: Bruce Dudley
Organization: Reliability Analysis Center
Date Posted: Tue Feb 16 14:03:53 US/Eastern 1999
Oil Drilling Companies experience environmental conditions for down-hole electronics such as you have described. These companies do extensive high stress (temperature, vibration, shock and etc) testing to find and fix problem areas. One design change that I noticed, in an audit of an oil company's reliability, was the use of conneting posts to solder the wires from the coil sensors rather than a solder pad or hole. This allows some relaxation of the tension of the connecting wire. Potting was also used to reduce vibration fatigue.