Jack,

Typically the edge of hole to trace spacing on innerlayer is related much in
the same way as edge of hole to ground planes.  I have always found it
interesting how annular ring requirement on a  hole to pad ratio is
considerably less than the hole to ground plane clearance. I've been told
that it is related to keeping the conductive hole wall at a safe spacing to
the ground plane to prevent cross talk. Perhaps the same applies to hole to
circuit. However, on tighter hole to pad ratios, I suspect that there is an
inherent amount of random break out occurring inside the board that no one
sees.  Now if you tried to hold say a 5 mil annular ring and a 5 mil hole to
edge clearance. I would bet that the board would short out all over the
place. Make you wonder huh?

So, in the case of removing non functional pad then using that area to place
circuits closer to the hole, I wouldn't recommend it since I suspect the
hole just might hit an occasional trace or so..

Just my thoughts.

Ed Cosper


----- Original Message -----
From: "Olson, Jack" <[log in to unmask]>
To: <[log in to unmask]>
Sent: Wednesday, April 26, 2000 1:18 PM
Subject: [TN] PCB FAB: bending the rules?


> 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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