Last week in one of my Beagleboard posts, I was musing on the issues related to layout with a package-on-package form factor. I pondered a couple of options, but Occam's Razor held true. Gerald Coley, the designer of the BeagleBoard pointed out that the CAD software simply doesn't care. And that's perfectly logical.
If you're designing with POP like the Ti OMAP processor, just don't worry 
about it. There are no layout issues - except for all of the normal ones like escape routing, trace current capacity, crosstalk, components spacing, vias in pads, etc, etc. Just place the bottom part where it's supposed to be, like you would if it isn't POP.
The manufacturing folks, on the other hand, do care. But that's easy. Just put the POP top part at the same XYtheta location in the centroid (pick and place) file and make sure the build instructions are clear that it is a POP part. Simple.
Duane Benson
Curse you Red Baron!
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