Kathy,
Check IPC-D-325A, Doc Requirements for PCBs, Assemblies, and Support Drawings, Figure 5-1: "Typical Multilayer Master Drawing, Sheet 1 of 5", on page
32.
For those without this reference spec, the drawing shows the thickness
requirement as specified "over laminate"; which is to say, if you measured
an edge of the PCB without any copper foil, plate up, or mask. Now this of
course can cause some confusion, but it is the only way to be precise
about the laminate stackup, proper. You have to make sure that if you have
a critical thickness, say over gold fingers, that you call that out in a
detail or a note, and take into account the plating. Also, if you have
soldermask out to the edges, and the board needs to slide into rails, you
need to make sure that the nominal thickness of the stackup is nudged down
by the expected soldermask thickness. Last, you need to train inspectors
that the "thickness" shown, is not the thickness they may get when the
copper and soldermask is added to the upper tolerance maximum ....
So, there is a specification for the multilayer as laminated, with a
tolerance on the stackup. Then you need to add copper and soldermask, plus
the upper tolerance limit; total that and call out a "Maximum overall",
which would be the most any inspector should get when they drop a caliper
down anywhere on the board. If there is a specific maximum thickness, at
the edges or over gold fingers, then that should be the call-out for
maximum, and the nominal laminated multilayer thickness should be
back-calculated to not exceed that maximum.
Actually "overall thickness" is not such a clear thing as one would think;
there are a couple of ways it can be misunderstood.
Valerie
Dear Technetters:
I am looking for an IPC guideline for measuring the overall thickness of a
bare PCB. Several independent customer specs I have read list the
measurement to be taken over the metal surface (metal to metal or mask to
mask), and that is certainly what I am familiar with. But I have been
challenged. I have searched and been unable to find a method or
requirement
in my IPC A-600, 6012, and Test Methods. Also, I wonder if I could get
your
inputs on how you all would measure bare board thickness.
Thank you,
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