Using Package on Package (POP)
Back in August of 2009, we at Screaming Circuits assembled our first package on package chip set. We did a number of test components first to tune the process and then built up some Beagleboards . We've done a few more since then. It's not yet a high-demand item, but it is getting more popular.
At the ESC (Embedded Systems Conference) last week, we had a number of folks stop by our booth and ask about how to use the part. The OMAP processor from Ti that comes in a POP form factor is a great high-performance part, but I think a lot of designers are still intimidated by it. Really, though, there's nothing special about designing with POP.
It's a 0.4mm pitch BGA and that gives some challenges with escape routing and PCB masking, but those are standard BGA-type issues. For escape routing, go to www.beagleboard.org , download the beagleboard reference manual. They have their version of the escape routing in the book. And, check out this post  for some advice on the BGA footprint.
That's pretty much all there is to it. The memory chip just plops on top of it, so as a designer, you don't have to worry about that. Just do a good 0.4mm pitch BGA layout and your POP will come out just fine.
Does Kellogg make Raspberry flavored Package on Package Tarts?