The term “via” probably comes from the Latin word meaning “road.” Therefore, open via would essentially mean “open road.” Open roads here in Oregon sometimes run through open range ranch land. That means that cows have as much right to be on the road as do cars. If you don’t want cows on your PCBs where the BGAs go, don’t leave open vias in the BGA footprint.
Don’t do this:
Here’s what happens:
Your assembled board will likely be missing connection between some of the BGA balls and the board. That’s almost as bad as having a cow step on it.
If you are putting the via in the BGA pad, your only choice is to have the vias filled and plated over at the board fabricator. (Read this post). If you’re putting the vias between the pads, you have two options. You can put solder mask dams on the short trace between the pad and the open via. This will prevent solder paste from migrating. I’ve modified the image below to show what that would look like.
The other, and better, option is to cap the vias with solder mask. This gives a bit of extra protection in case any of the solder mask dams are too thin or chip off. Just make sure you cap these things on the solder side. If you cap them on the back, solder can still spread on the trace and partway into the via. That still puts the electrical and mechanical connection at risk.
We don’t want cows on PCBs but we have built PCBs that go on cows
The IoC: Internet of Cows