Speaking of vias in pads, here’s a reason we’re not terribly fond of the technique of using solder mask to tent the bottom side of a via in a BGA pad.
As I mentioned in my last post, capping the bottom of the via with soldermask is sometimes an acceptable method for dealing with a via in pad. It’s not a desirable method, but it does sometimes work. However, with BGAs, the top-open via can still wick solder down to fill the space. At the very least, you will end up with a hefty void in the BGA solder ball. At worst, you will end up with an open, as illustrated here.
Best option: Fill and plate over the via.
Does cheap mass production = this perfect day?
Another problem we have had with solder mask tenting is incomplete cleaning of the via and subsequent inclusions in the solder joint causing defects. We first went to solder mask encroachment which helped but, fill and plate works best.