Jack Wrote,
>So does the chart indicate that I should use 75mil aperture in my CAD 
>system, or is my goal to take all factors into account and try to ensure 
>the END RESULT is 75 mils minimum?  
>
>Am I splitting hairs?	
Jack,
        The I believe the intention is stated somewhere in D-275. Your
database should reflect the desired results at nominal. Maybe you should
specify that on your Master drawing. It should be left up to the Fab house
to modify you artwork to take into account the tolerances of there
particular processes. But it should also be spelled out as such on the
drawing. If you compensate for a particular set of processes, and later
someone choses to fab with a different process the orig. intent would be
lost. For instance Additive process versus the subtractive process.
I know this leaves me open to the rath of those who don't want "anyone"
modifing their files, but you can't stop them from modifing them anyway. I
would rather the fab house know my intent and try to meet it, then for them
to guess or make assumptions about the intent.

I use this same theory on my soldermask swells. I make the soldermask the
same size as the pads/lands. It is then up to the fab house to swell them
the correct amount for the process and technology on a particular job. Who
is going to know there capabilities best, me or the person actually doing
the production CAM work. I specify "No exposed traces and no soldermask on
the pads". We put about 50 to 75 different designs into production each
year, and have had favorable results.


Regards
Fred Pescitelli
Phoenix Designs
1285 Turner Rd.
Lilburn, GA   30247
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770-923-3465

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