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It's possible that the Chinese CM and/or the designer could have been asked to make changes that we never documented.
Although the new CM might also have build issues. This is what we need someone to help us determine.
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Yes, I've seen this in practice, although some time ago.
In one case, the PCB files at the CM didn't match the schematic and PCB files at the design house. The designer made
some "last-minute" changes shortly before he left the company. It was a 5-person start-up, so forget about version control.
It wasn't just connectivity (tracks), there were also problems with PCB hole sizes for through-hole components.
Another case: a problem with PCB footprints. The PCB footprint for a high pin count, fine-pitch component was slightly off
- the pitch was too small. The CM could sometimes make the component connect to the pads, just barely, with careful manual touch-up,
but not always. The CM informed the client company there was a problem, but somehow the message didn't sink in. You could just detect
the pin pitch discrepancy with the naked eye. Several revisions of the PCB went through the CM, with the same bad footprint.
Until one day, a particularly handsome and dashing engineer (ahem), new to the company, took it upon himself
to audit the board's PCB library, and clean up all the problems in one go. Rumor has it that he particularly enjoyed standing over
some snooty PCB layout specialists, demanding that they justify every dimension of every footprint ... NOW.
Auditing the design and build files you have, against what the old CM was supplying you, is a good place to start.