Forum: Reliability & Maintainability Questions and Answers
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 –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?
Posted by: Ben Walters (bwalters@etec.com ) Organization:Reliability Engineering Manager Date posted: Tue Jan 12 20:15:37 US/Eastern 1999 Subject: Aluminum Electrolytic Capacitors Message: Can anyone provide info concerning the use of aluminum electrolytic capacitors in a controlled, benign environment? From past employers in the defense/aerospace industry, we always avoided these capacitors due to possible outgassing, creating a fire/explosion situation if a spark occurred. This was over 10 years ago, though. Has the technology of these capacitors changed and are they safer to use? Appreciate any help you can provide. Please e-mail to me as well as respond to this forum. Thanks again.
Posted by: Tom Donnelly (tquality@gte.net ) Organization:Consultant Date posted: Sun Jan 3 1:59:31 US/Eastern 1999 Subject: Screening solder workmanship Message: I am currently working with space related hardware. The issues I am having are with solder joint reliability and possible sources for specific failure rate information as it relates to the thermal stress testing of PWB's. What is the difference in solder joint failure rates when the electronic equipment is exposed to -45F versus -30F? and what failure mechanism's did I miss by only exposing the equipment to -30F. I have the same question for taking the equipment to +60F instead of +85F. What is the differnce in failure rates for the solder connections and what failure mechanism's did I miss. From my many years in Reliability I believe that the difference is insignificant, but my customer's would like me to quantify it....I appreciate any thoughts you could contribute to solving this problem...... T.Donnelly tquality@gte.net
Posted by: Ananda Perera (Ananda.Perera@alliedsignal.com ) Organization:AlliedSignal Aerospace Canada Date posted: Wed Dec 23 12:52:53 US/Eastern 1998 Subject: Kolmogorov-Smirnov Goodness-of-Fit Test Message: In all the books and publications that I have seen, Kolmogorov-Smirnov (K-S) Goodness-of-Fit Test Critical Values are given in the Significance Levels of 20%, 15%, 10%, 5%, 1%. Are there any publications that give K-S Critical Values at Significance Levels greater than 20% Please reply through my E-Mail as well Ananda Fax: 905-608-6178
Posted by: Bob Moore (bob.moore@bae.co.uk ) Organization:British Aerospace Date posted: Wed Dec 23 5:24:48 US/Eastern 1998 Subject: HALT for potted modules Message: Does anyone have any experience of conducting Highly Accelerated Life Testing on potted modules or is this a complete waste of time? Any comments gratefully received. Please copy any reply to my email address as well as this forum as web access is infrequent. Bob Moore
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