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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