How to detect over-etching

[buenos]

I suspect that some of my pre-production boards have over-etching problems.
About 1/3 of the boards fail with 1 or more interface, a different interface on each.
Previously a whole series of bare PCBs were scrapped as the PCB fab detected over-etching. This time it passed their inspection, but now some of the fully assembled boards have problems. We replaced BGAs, measured clocks and voltages, inspected small components for cracks and soldering, but it didn't help.

 

[kam1787]

To me over-etching is fairly obvious; jagged edges on the traces, traces that fade away, pitting

Maybe put a series of dots, of increasing diameter, in the middle and edge as a quick indicator.