At 04:40 PM 7/13/00 -0500, Olson, Jack wrote:

>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. [...]

>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?

I've usually used nominal. It is simply easier.

There is something to be said for using MMC, though it is rather
complicated to actually realize this. To draw true MMC outline, not only
will there be the dimensional tolerances for the component dimensions to
consider, which, as noted, may vary from manufacturer to manufacturer, but
also placement tolerances as well, which may vary with the assembler.

So, in my opinion, the most practical idea is to draw the parts at nominal,
and allow sufficient clearance in the design rules to take care of
component and placement tolerances.

However, for visual identification of potential problems, it might make
sense to draw two outlines: one at nominal and one at MMC. If I were to do
this, I would put the nominal on the silkscreen and the MMC on a special
assembly layer, used just to check clearances.

I'll still stay with nominal alone and appropriate design rules. It's much
simpler.


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Abdulrahman Lomax
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