Via-In-Pad - Let It Slide???
Sometimes, you can get by with vias in your pads. Sometimes, but not very often. I wrote about this a while back here. The thing is, I was talking about big pads - like QFN or QFP thermal pads and stuff like that. We never like to see it and it's always a manufacturing risk at some level, but as described in the earlier post, sometimes you can just roll with it.
Pretty much never with a BGA pad though. The pic on the left shows just about a worst-case scenario. Very big. Very bad. Relatively very big holes anyway. This is for a .5mm pitch Bluetooth module BGA.
The vias in the image on the right worked okay with a QFP because they're really small - practically closed up - and it was lead-free solder. We still wouldn't want to see a via, even that small, in a BGA pad though. Process variations leave enough opportunity for a few of the vias to be open all the way through and even if one BGA ball gets sucked off the BGA, you're out of luck. Even if it's just partially sucked off and still connected, it's much more susceptible to cracking and things like that. (By the way, we did find a way to build the board on the left and make it work. We won't guarantee that we can make something like this work though.)
A lot of fab houses will epoxy fill your vias these days. Even micro-vias. And, yes, you should even have your micro vias filled and plated. Especially with small BGAs. It's just not worth all the risks that come along with it.
We need little moles to fill those holes