Non Solder Mask Defined pads on BGAs
The industry generally recommends NSMD (non solder mask defined) pads on BGAs and so do we. There are some exceptions though. When you get into really small ball pitch, you probably need solder mask defined pads to help prevent bridging. See this post here.
This photo does a good job of showing NSMD pads up close and personal. The registration could be a bit better, but this isn't too bad. There's a nice small gap around the pad with no mask up on the copper.
You'll also notice that there's a good bit of solder mask between the pad and the escape via. That's also good practice. And, of course, no vias in these pads.
You can put vias in the pads, but if you do, you really want them filled and plated over. Never, never, never put an open via in a BGA pad. There are plenty of options at the board house for filling and plating these days.
I hope NSMD doesn't mean "Never Send Money to Duane"