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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