Pete,

 I have found the paste aperture adjustment logic in PCB design software rather poor. Such tools will under and over size well but when it comes to QFN thermal processing or home plate designs for passives these tools are totally lacking. I use PADs and Altium here.

 I would suggest folks generate 1:1  paste to pad apertures from the CAD tools and then provide rules to the stencil processor. The rules defined need to be coordinated with the stencil fabricator capabilities and hopefully the data exchange can be done electronically.

 I would be curious what PCB design tools you are using and how you adjust paste aperture using that tool. I am sure there are new tricks and capabilities I am not aware of.

Thanks,
Bob K.

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Pete
Sent: Thursday, December 15, 2011 10:40 AM
To: [log in to unmask]
Subject: Re: [TN] BGA SMD vs NSMD

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........

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