Filling via in pad - Before
[Editor's note: This post is the first of a three-part post about a rather difficult and undersireable via in pad PCB. This board violates industry-wide recommended practices. The right thing to do in a case like this is to redesign the board without vias in the pads or have them plugged at the board house. We did get this one to work, but it wasn't a sure thing. Some designs require vias in the pads. If you have one of those, get the vias plugged and plated over at the board house. If you can't do that, you can ask us to try what we did here. It may work. It may not. There are no guarantees in a situation like this and in may incur additional labor charges. Call first for situations like this.]
This is a leaded board with a HASL finish. The footprint is for a 9 x 13 ball BGA. In this case, the vias just about completely fill the individual BGA land pads. Without treatment of some sort, this BGA would be almost impossible to mount reliably.
This board also gives a good representation of why many users of BGAs prefer ENIG (Electroless Nickle, Immersion Gold) or immersion silver finishes over HASL (Hot Air Solder Leveling). The ENIG and silver finishes are almost completely flat, while the HASL can leave an uneven or bulgy surface.
Tomorrow or the next day, I should have an "after" photo showing the same pad with the vias filled with a high-temperature solder.