Thank you all for taking time to answer my questions! You are a big help....

The reason I stated an 18mil drill for the vias was that I was trying to
make it easier for the fabricator to drill for .093 thick boards, but since
we allow the vias to be +.003-.013 (plate closed) they can use a smaller
drill anyway. Sorry for the confusion.

No one argued with the guideline that trace to drill should never be less
than 10mil.

So my final question is regarding the reduced pad size for inner layers. Can
someone point me in the right direction here? It just seems like for
multilayer boards you are processing all the inner layers first, then
laminating, then processing the outer layers. My own (faulty) logic says
that each succeeding step adds more and more error, so it seems like the
pads on the inner layers would need to be LARGER than the outside layers to
make sure they all get hit. Or at least the same size. Why can inner layers
be smaller?

onward thru the fog...  (and thanks again for all your help)

Jack

                -----Original Message-----
                From:   Olson, Jack

                We typically remove unused pads from inner layers. (please
don't change the subject to warn me against removing inner-layer pads,
that's not the issue I need help with right now)
                I am being asked to use the extra routing space that has
been freed up by the pads being removed. I have heard of this before but
never tried it, and frankly I'm a little worried about it.

                Somewhere I heard that the gap between traces to the drilled
hole should be something like 11-15mils, but that hardly makes sense from
what we are already doing. A 13mil via hole in a 25mil pad is probably
drilled with an 18mil drill bit, right? If we were already using a 4mil
clearance to the 25mil pads, then removing the pad still only leaves 7mils
to the edge of the drilled hole right? (sorry for all the numbers in one
sentence)

                So how much extra routing space is there? I say virtually
none. I can't remember where I heard that drill to route should be
11-15mils, has that become tighter these days?

                Anyway, that brings up another question. Maybe we can use
smaller pads on inner layers? That's yet another thing I've never tried....
Is there a rule of thumb for that or am I just asking for trouble?

                Any responses will be greatly appreciated

                Jack

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