Pete,

  I would suggest that paste apertures should be part of a PCB CAD system though such is seldom, if ever, the case. Most CAD system do not support paste apertures well.

 Yes, there are a lot of paste aperture modifications made to paste gerbers and that "Hand Tweaking" is a source for errors and inconsistencies.

 As an example here is a link for something I worked up for parts reduction patterns, it is something I would like to see implemented in a paste aperture conversion code. I would also like to see the same kind of thing done for home plate aperture designs though that requires some knowledge of the components sitting at the aperture locations. This patter provides good support at thermal pad corners yet reduces allows for better outgassing than window pane patterns.

See: www.kondner.com/files/Stencil_5_Dot_Math.pdf

 Dot Diameter = Sqrt(Rectangle) / 2

Bob K.
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Pete
Sent: Wednesday, December 14, 2011 9:13 AM
To: [log in to unmask]
Subject: Re: [TN] TI Paste Recommendations

William, there's been some great information on QFNs posted in this thread, they are different than most SMT and it's good to know as much as you can about the assembly issues.  

But from  a CAD library standpoint, do you really need to define the paste footprint?  Do you build home plate apertures into your 0603 footprints?  There's a very good chance that manufacturing engineering is making quite a few modifications to your paste mask gerber file anyway.  Check with them, they might prefer you just give them a gerber file that matches the pad shapes, and they will apply their expertise from there.

Now, via placement in and around the QFN, sodlermask apertures - those you need to be concerned with in your design.  The Desginers Council Listserver had a discussion about some of this in the October archive.

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