Jack,

Why not create the part in its nominal size and then add a keep-out zone (or
some such thing like that) to the part to represent the "don't place closer
than..." area? Only a suggestion.

Mary Deichmann
Design Engineer
3M Austin Center
6801 RiverPlace Blvd.
Austin, Texas  78726-9000






"Olson, Jack" <[log in to unmask]> on 2000/07/13 04:40:29 PM

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Subject:  [DC] CAD library philosophy?



Ok you guys, I need a quick survey here....

I seem to have gotten myself into a big raging discussion here about whether
library footprints should be created (and placed) using the nominal dimensions
or the maximum dimensions.
Most companies have some kind of placement standard that says something like
"don't place discretes closer than 30mils to each other" and similar guidelines
for different types of parts. I would think the 30 mils is for the pick-n-place
machine to have a little room to work, save some space in case the part needs to
be reworked manually, and take into account any variations in component size.

For example, if I make an 0603 footprint, I make it assuming the part is 60mils
by 30mils, which is the nominal size. If a part comes in at 61x31 we have
already allowed a 30mil gap to the next part, so it shouldn't matter. We use
this 60x30 rectangle for DRC checking, back and forth to mech department, etc.

Now some are saying we should be using the MAXIMUM size, which I don't think
buys too much and creates other headaches (different vendors list different
tolerances, etc) so I would like to know how YOUR library is created.
NOMINAL or MAXIMUM?