I’ve written a bit about soldermask defined (SMD) vs. non soldermask defined (NSMD) pads for BGAs.

Quick summary: 0.5 mm pitch or wider spacing, go with NSMD pads. 0.4mm pitch seem to need SMD pads to prevent bridging (unless the pads are staggered. Then NSMD is fine)

But what about the LGA (land grid array)? It’s different due to not having the solder balls. Does that make  a difference? According to Freescale and a few other manufacturers, in most cases, you should treat an LGA just like a BGA and use NSMD pads. However, if you need extra strength holding the pad on to the PCB, you may want to consider using SMD pads. As always, consult the data sheet for your specific part for the final word.

Duane Benson
Checkers anyone?

http://blog.screamingcircuits.com/