Jack

I'm rather surprised no-one has mentioned that if you do not have inner pads and
particularly if you use coarse prepregs (e.g. 7628 cloth with 44% resin content), then
there is a much greater risk of  poor drilling due to insufficient resin flow into the
gap. Of course, this applies especially to ground planes with 35 um copper. I strongly
favour annular rings with the inner pad at least big enough to maintain a crown of
copper under the worst conditions of misregistration and drilling tolerances plus a 0,5
mm annular ring for voltages up to about 25 V. This moves the critical area inland from
the drill site, providing a better integrity and less risk of such nasties as CAFs. I
would apply the same rules for inner track layers. Another advantage is that, if you
have to manually tweak the inner layer routing, you can see exactly where the forbidden
zone is round a hole or via and you are less likely to violate it (some DRC systems on
ECAD software will not check the hole-barrel to conductor spacing unless there is a pad
there).

FWIW

Brian

"Olson, Jack" wrote:

> Sorry for the cross-post, but I couldn't decide if this was more of a
> fabrication issue or a design issue.
>
> 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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