Testing plan for flat flexible cable (FFC) connector
You don't have to test everything, only those parts that you want to make sure work.

For a large production run you usually make test jigs that carefully test everything.  Designing and getting the production test jigs made is usually a larger effort than for the thing you are testing.  However, for small volume as you have, you can trade off labor and some uncertainty with test jig creation.

In this case, a test jig for the cable sounds simple.  You use a ZIF socket for each end and have a microcontroller run a walking 1s and walking 0s pattern and the like.  That will be instantaneous in human terms.  This doesn't have to be on a fancy machined custom jig.  Test jigs can be built as one-off prototypes, as long a they are physically solid.

Can you test everything together as a single system?  Ask yourself what can still be wrong if you put known stimulus into the sensors on one end and see the correct values over the USB at the other end.  Obviously a lot has to be working for that all to be correct, although there could possibly still be a few things wrong.  For example, the unit may appear to operate correctly, but could be drawing more power than it should and something therefore getting warmer than it should, causing it eventually to fail over time or at higher temperature.  Of course drawing too much power is a failure in its own right.  In this example the solution could be as easy as using a bench supply with a current readout, and having the production tech verify the current is below some value.  However, keep in mind that anytime you are asking a human to do something, there will be a fraction of the time they get it wrong.

Think about it carefully.  Would a system test as described above excersize every line in the cable?  If so, then you need to do nothing further.  If not, you first have to ask yourself why that line is really there.  Then figure out a way to test it.

The test strategy needs to be part of the design from the beginning, and producing the test system, even if it is just documentation to the production techs, is something that needs to be considered in the schedule and budgeted for up front.  As I said before, usually the test fixture is a larger project than just the thing it is testing.  This is one example of where experience matters.  If you've gotten to the end and are just now thinking about testing, then you have already screwed up.  There simply may not be a way to fix the mess within all constraints of time, money, and test coverage.  Get someone who knows what they are doing next time.

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