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When manufacturing a small prototype batch of an embedded system at EMS I have detected different behavior between the units. All units work as expected, except that some units fail to operate same pump properly when it is controlled at a high speed (the drive displays some hick-ups and breaks when the speed increases). Approximately half of the batch functions flawlessly.

I have visually inspected the boards and measured values of critical passive components without noticing any issues. Next step will be throrough scope measurements.

While I cannot share the design I would like to ask expirienced members of the community:

When noticing a discrepancy within a batch of PCBA - what are some most common causes that go undetected by optical inspection?

NoobPointerException
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    It may be a design error instead of manufacturing error. They may be manufactured correctly. That's something you can't visually detect. – Justme Apr 03 '23 at 10:24
  • There's the ever popular "components with slightly different properties." You can see (on a visual inspection) when a component doesn't meet the expected specifications. Maybe you've got two batches of transistors that behave differently. They might meet the manufacturing specifications, but just marginally meet the requirements of your circuit. That can apply to diodes, transistors, ICs, and just about all other electronic components. – JRE Apr 03 '23 at 10:50
  • Can I ask what is "EMS"? Also: What number of devices did you get made? (Size of _n_ matters for different failure distributions.) – jonathanjo Apr 03 '23 at 10:55
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    EMS is Electronic Manufacturing Services. Batch size is 20. – NoobPointerException Apr 03 '23 at 11:17
  • Have you done any WCS to determine that you are ok even with max production spread? For 20 units, it's likely that all components come from the same reel, so there shouldn't be much spread from the start, but still. – winny Apr 03 '23 at 11:29
  • @winny ... what is "WCS"? – jonathanjo Apr 03 '23 at 11:50
  • Worst Case Scenario. – winny Apr 03 '23 at 11:54
  • I have considered this and verified that all important parts come from the same tape/reel. – NoobPointerException Apr 03 '23 at 15:34
  • The specific case is solved - I was wrong and one of the used ICs came from two different cut tapes. Turned out they worked differently despite being same part number. – NoobPointerException Apr 04 '23 at 14:18

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