Mr. Lomax wrote:
> But it does not make the gained space safe for routing, because we can
> assume that, if routing density is an issue, via pads have been already
> been reduced in size to the minimum annular ring necessary to avoid
> breakout, as Mr. Koitmaa notes. And thus it is possible for the hole wall
> outer edge to be at the same position as the pad would have been.
>

No no no   there is  huge misunderstanding here,   this has nothing to do
with flirting with either internal or external annular rings.
I can achieve this goal with annular  rings of  1000 mils and never see
break out on my  12 mil hole.  It would give me the clearance on other 988
mils back on
layers not connected  only.    My suggested feature  has nothing to do with
modifying the padstack.  the more I think about it      It is a DRC issue.

. One   might gain as much as a few mils of routing space.
>

You are correct but on many occasions   all I needed was a few mils.


Mike Reagan

----- Original Message -----
From: Abd ul-Rahman Lomax <[EMAIL PROTECTED]>
To: Protel EDA Forum <[EMAIL PROTECTED]>
Sent: Tuesday, September 17, 2002 5:49 PM
Subject: Re: [PEDA] Inner pad feature


> At 10:49 AM 9/17/2002 -0700, Mark Koitmaa wrote:
> >Maybe your fab house can do better, but the ones I've used need at least
> >5.5 mils around a via hole to prevent break out of the hole wall. This is
> >what we use as minimum annular ring. For a decent fab yield we need at
> >least 3 mils air gap to the nearest trace. Removing the unused via pad
> >during layout doesn't buy us any extra routing room so I don't see any
> >advantage in removing the unused via pad.
>
> It is quite clear that removing unused internal pads improves yield, the
> only question is *how much* it improves yield.
>
> But it does not make the gained space safe for routing, because we can
> assume that, if routing density is an issue, via pads have been already
> been reduced in size to the minimum annular ring necessary to avoid
> breakout, as Mr. Koitmaa notes. And thus it is possible for the hole wall
> outer edge to be at the same position as the pad would have been.
>
> Nevertheless, this only takes place at extreme drill wander in just the
> wrong direction, and it is possible that a few mils of space could be
> gained for routing by becoming dead-pad aware in DRC. Essentially, at some
> value for the variables, the gain in yield from dropping the dead pads
> could be balanced by the loss in yield due to extreme drill wander. One
> might gain as much as a few mils of routing space.
>
> To determine without experiment the exact values that one could attain
> would be difficult, I think, but the problem could be approached by a
> statistical analysis assuming that drill wander is random; the results
> might vary with the number of holes, i.e., what you might get away with if
> there are a few hundred holes might be unsupportable with a few thousand.
>
> In any case, DRC for dead pads is not a simple matter of dropping the pads
> and taking the holes at nominal value!!!
>
> (By the way, there is a similar issue with inner planes, which for many
> users actually have a few mils less clearance than might appear! -- since
> Protel uses the hole size to determine inner plane clearance. For normal
> conditions, the blowouts should be 3 mils larger diametrically to make the
> real clearances on the inner plane layers the same as on positive layers.
I
> don't think DRC is aware of this at all.)
>

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