When the original and Gen 2 system designs were compared, a material change stood out. PTFE (polytetrafluoroethylene) wire insulation was replaced by polyimide flex-circuit insulation. Comparing these materials, we noticed that polyimide absorbs a lot of moisture, while PTFE does not. It was beginning to make sense. In the hermetically sealed system, the flex circuits in the new wiring harness would release moisture when the system was brought up to operating temperature.
This moisture took some time to penetrate the epoxy potting over the back of the D-sub connector on our sensor, but it eventually did. This allowed some soluble salts in the epoxy filler to become conductive, effectively connecting adjacent circuits that were supposed to be isolated. As it turned out, a PWM (pulse-width modulation) heater circuit was adjacent to the sensitive pickup circuit, and once an electrical connection was made between them, lots of erratic output was seen.
As an interim fix, the completed systems were vacuum baked before sealing. We submitted a design change to replace the filled epoxy potting with a polyurethane compound rated for electrical applications. This solved the problem, and at least while I was with that company, there were no failures in sensors with the new potting.
This entry was submitted by A. David Boccuti, P.E. and edited by Rob Spiegel.
Dave Boccuti is a mechanical design engineer with more than 30 years of experience in a number of different industries, including consumer products, medical devices, and custom automation equipment.
Tell us your experience in solving a knotty engineering problem. Send stories to Rob Spiegel for Sherlock Ohms.
It is interesting naperlou, and something I am always trying to get across. Often times folks don't think twice about changing something material or mechanical and don't expect it to have an impact on the electronics. This story certainly proves otherwise...
Somebody needs to forward this to cochlear implant maker Advanced Bionics, which has been struggling to seal their implanted electronics for over a decade, causing losses of upwards a half-billion dollars.
It appears that on many occasions "epoxy" material is not suited for many kinds of electrical applications. I am aware of some antennas that don't work right when they are insulated with epoxy material, although one would think that they should. Moisture leaching out salts to short circuit a connector is a long way to go, though. It took good troubleshooting skills to find that problem.
The moisture absorption of polyamide is often overlooked in design. We manufacture a polyamide product that is used on average two years before discard. Consumers that were keeping the product over the two year mark complained of premature breakage of the product (non-safety related). Investigation showed that the PA absorbed enough moisture to push out the plasitcizer on the product making it brittle. As there was no other material available, we opted to put use by dates on the product to guide the consumer to when the products life was ending.
Thanks for your insightful comment Charles. In this case the change was made for the right reasons (reliability; cost was a secondary benefit) by the system engineering folks, but it had an impact on a sub-system (the sensor) - an unintended consequence. Lesson learned - evaluate everything that might be affected by a change, not just how it affects "your own stuff."
The interesting aspect of this is that the change was made, not just for cost reduction reasons, but for reliability purposes, as well. It makes me wonder if the original PTFE insulation had a problem, too. Was this a case of replacing something that wasn't working well with something that was even worse? Or was it a case of, "If it ain't broke, don't fix it?"
This story provides a good example of how a secondary effect (insulation change) caused a tertiary effect (shorted contacts). New engineers must keep these types of problems in mind when they look for the root cause of a defect. That cause isn't always obvious. Nice work.
Yes, Naperlou, this is a good example of attention to detail. Something as simple as wire insulation made difference between sensors that worked and sensors that failed. This is excellent Sherlock sleuthing.
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