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.
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