I come again to the collective Delphi Oracle!

I have the job of specifying to PCB fabricators what we want them to build,
even though I have no previous direct experience in doing this. I'm trying
to have a 12 layer board fabricated that is VME standard - 0.063+/-0.008"
thick. It contains 6 x 1 oz copper signal layers and 6 x 2 oz plane layers
in the stack-up 1-2-2-1-2-1-1-2-1-2-2-1. To maintain the 0.063" thickness,
I have called for 3.5 mil thick Core and Pre-preg in accordance with the
minimum requirements of MIL-PRF-55110 (I know this is being superceded by
IPC-A-6012, BTW).

The feedback I'm getting is that the dielectric spacing between adjacent 2
oz layers in particular is far too thin and should be 8 or 10 mils of
pre-preg. Even between 1 oz copper layers, the recommendation is for 5
mils. I can't seem to get a quick and clear explanation as to WHY the fab
house is recommending these thicknesses.

My question is: does the pre-preg thickness have to take into account the
bordering copper weights in addition to the minimum dielectric spacing?
i.e. for adjacent 2 oz copper planes, should my pre-preg thickness spec be
2.8+3.5+2.8 = 9.1 mils? What is the "formula"?

Sorry for such a basic question, and my TIA for the help I feel sure I'll
get.

Pete Duncan

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