At 12:20 PM 10/21/2005, Brooks,Bill wrote:
We had a part mis-defined with a PAD that was a 'toplayer' pad (not a
'multilayer pad') that had a drill hole defined in it... not on purpose, but
by accident.
Now DRC did not pick up any shorts with planes or traces running under this
pad [...] Has anyone ever run across this before?
I do recall there being error conditions with single-layer pads with
holes. It appears that the programmers designed single-layer pads to
not have holes, an easy mistake to make. If you wanted to put a hole
in a single-layer pad (for example, for microvias-in-pad), you would
place an extra pad, multilayer, with the hole. Because the hole does
exist on other layers, it is an error to define the pad as being Top
or Bottom. But an easy one to make; it could be argued that the
programmers should have made it impossible to create this condition,
using an error dialog ("which do you want, a hole or a single-layer
pad with no hole"), and also a default design rule that can't be
turned off (there are a few of these), to deal with imports
containing this little beastie.
Or, if you really do want a single-layer pad with a hole and that's
that, as long as the hole being in the center of the pad is okay,
then you would use a multilayer pad with a padstack, and defined the
pads on the other layers to be small. I'd still recommend making them
larger than the hole, just because I get nervous about DRC's ability
to correctly measure clearances around such an animal. Whenever one
strays from common practice, there is a risk of running into a rare bug.
In particular, holes are generally drilled 3 mils oversize or so
(diametric), and then plated back, so this reduces clearance to hole
metal by 1.5 mils as against the nominal hole size, which is what
Protel would know about. I've argued in the past that DRC should
factor for this, but I don't think it ever has. (There would be a
design rule specifying drill oversize; it would be simple for this to
be a default 1.5 mils radial, and then this correction would always
be applied to holes for DRC purposes.)
I'd think it possible to set up a design rule or rules to detect any
top/bottom layer pads with holes. It would take a little
experimenting, which I can't do right now.... Such a rule could then
become part of a standard design rule set.
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