I worked for a contract manufacturer building extremely sensitive sensors for a large customer on a build-to-print order. These sensors had a finite life, and they were replaceable in the field. We had received several production orders and were well into delivering them when we started to see field returns coming back. The reported defect was "erratic behavior in service."
In service, the sensors were mostly in stable environments with no inputs. The output, therefore, was supposed to be constant. When a sensor’s output was not constant, it was flagged as "erratic" in the field, removed from the system, and returned to us for evaluation and repair. A spare was then installed.
When we received a field return, it was inspected for visual damage and then put into a storage oven. Sensors were always stored at operating temperature so they could be tested more quickly. There was a long stabilization period if stored at room temperature. Engineering would review the reason for the return, look at the field history, and then prepare a test plan to confirm the reported condition (or not).
As time went by, we received more field returns. A pattern emerged -- we were able to confirm the failure in very few cases. Mostly, the sensors worked as specified during the evaluation testing, so they were then routed to receive a full final acceptance test and were reshipped. There were a few confirmed failures that did not have an obvious cause, so we did the simplest repair, replacing the external electronics, and retested to confirm that it was fixed.
Since we kept receiving sensors back, we looked more carefully at their field history. We discovered that in every case the failures occurred in sensors that were installed in "Gen 2" systems. By and large, they became erratic weeks or months after being put into service, but they never failed in "original design" systems.
The Gen 2 change was to be both a cost reduction and a reliability improvement -- point-to-point wiring was replaced with cable harnesses, which eliminated many hours of hand wiring and soldering, as well as eliminating human errors in wire-routing. But how would that cause sensors to fail? A good number of engineers -- both at the customer and at our plant -- were trying to figure this out.
A great example of sticking to the problem and following the trail until total problem resolution is achieved. All I can say is I wish more engineers and companies practiced this kind of dogged determination to get to the truth. Kudos to you and your design team.
David, I agree with Beth. It took a lot of attention to detail to find the change you mentioned. It is interesting to note that the environment was such that even a "small" material change could cause a failure. Good 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.
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.
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?"
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 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.
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.
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