Take a look at the closeup of one of our Beagleboards here on the right. That's what package on package (POP) looks like up close. The bottom chip is a Texas Instruments OMAP processor, in BGA form with 0.4mm pitch solder balls. It has a land pattern on its top for the top layer, which is a Micron memory chip in 0.5mm pitch BGA form.
A few years back, we built a small handful of Beagleboards ourselves, just to showcase our POP capabilities. It's hard to believe that we did that back in 2009. The Beagleboard has undergone a few iterations and spawned the Beaglebone since then, but 0.4 mm pitch is still pretty small.
Small, but not really all that uncommon anymore. "Smaller" is on the way. In fact, though it's a special process, we've even built a few 0.3mm pitch BGAs.
If you're joining the fun and starting to use on of the Ti 0.4mm pitch BGAs, you might want to take a look at what we learned from the Beagleboard folks about the land pattern.
The sandwich needs pickles.