Jack,

I like to think at the inner layer pads as drill targets, or plated hole keep-out
areas.
If designed well, (and assuming a tangent annular ring requirement), the innerlayer
pads represent the area on the innerlayer where the plated hole is allowed to be.  Let
the router rout traces as close as it wants to these innerlayer pads, taking them
out... or leaving them in... would make no difference.  Why?   Because it is the plated
hole barrel you are trying to stay away from... not the innerlayer pad. The hole will
still be there even when the pad is gone.

If there really is a routing advantage in your design thru removing non-functional
pads..... I would recommend you first reconsider your pad stack design.

George Franck
Raytheon Falls Church Va

"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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