On 15 Dec 2011 at 9:39, Pete wrote:

> Most CAD systems allow you to adjust the soldermask aperture over (or
> under) size at CAM output.  This lets you change based on the design. 
> Some designs force you to use a smaller oversize, meaning you need to
> pay more for a fabricator that can do that for you.  Other designs
> have more room, you can use a larger opening, and a less expensive
> fabricator.
> 
> Again, I wouldn't recomend adding soldermask apertures to the CAD
> library.  There's a reason the CAD systems allow you to define them on
> the fly.
> 
> IPC-7351 really is a great "best case scenario" document.  But we
> rarely get to work in a "best case" world.
> 
> And don't let your fabricator change your artwork!  They have to make
> some process adjustments, but the final result in copper should match
> the gerbers you gave them.  Give them an inch........

Real trouble begins with mixed boards containing "standard" stuff 
(unsually NSMD) and parts which need SMD (often BGAs with small 
pitch, µBGAs etc., depends often on fanout patterns and/or heat 
sinking.

Then it is absolutely necessary to talk with the board manufacturer 
AND the assembly house.

And because of alignment tolerances between copper and solder mask,  
you will need own SMD fiducials for the SMD components additionally 
to the "standard" NSMD fiducials for the "standard" NSMD stuff. 
Otherwise good luck to the P&P operator....

Best regards
Matthias Mansfeld




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