Hi Jack,

We've been using vias with different sized pads for the internal layers for some time now...haven't heard
any complaints from the board vendor.
We started doing it for the same reason you want to do it -- get more routing space.  As far as totally removing
the pads, no, we leave at least 4-5 mil annular to let the DRC/Router keep away from the hole.  Hope this helps

Bill Brooks
Design Tech./Librarian
SLM Electronics
1901 Congressional
St. Louis, MO 63146
(314) 569-0141 Ext. 1660
E-mail: [log in to unmask]


-----Original Message-----
From:   Olson, Jack [SMTP:[log in to unmask]]
Sent:   Wednesday, April 26, 2000 12:18 PM
To:     [log in to unmask]
Subject:        [DC] 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