nospam said:
They don't have to be big. They don't even have to be inside the board
outline to give the PCB manufacturer a very obvious check he got the
stackup right.
Your exposed copper doesn't have to be inside the board outline either if
you get boards supplied in a biscuit or get supplied the surrounding scrap.
I am a bit dubious about measuring thickness by looking at the edge of the
board. Have you ever done it?
I think you would have to grind or rub it down first because the routing
will burr and spread the copper. Then you need a decent measuring
microscope that you can fit a board into edge on. I suspect it would be
more practical to take a sample from outside the board which you can cut
down, clean up, and fit in a microscope.
I have not had the chance to do this yet. The instructor who taught
this has an inspection microscope just for this purpose that fits in
his toolbox. It looks like an 8 inch long loupe. I believe he said it
had a reticle so he could directly read off the thicknesses. I doubt
that there would be significant burring when the metal is embedded in
layers, but only a test would tell. A saw would cut the boards in a
motion across the thickness and could burr. The router cuts along the
length of the board and is not likely to burr. If it did, it would be
uneven and cause lumpiness in the viewable edge so it would be apparent.