Filling VIA in pad - After
[Editor's note: This post is the second 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.]
Here we are again with our board that has giant via holes in the middle of several BGA land pads. (If you haven't yet, read the original post here)
Take a look at the call-out in the picture now. You can see what the via holes look like once filled with high-temperature solder plugs. Now, when we apply the solder paste, those plugs will act as the substrate. The solder balls will bond properly and no solder will wick down and drip out the other side of the board.
For a second run, we would probably recommend re-spinning the board to have the vias plugged, but for a prototype, dealing with it like this will deliver a perfectly reliable board.
There are a variety of processes available at board fab houses to properly deal with vias in pad. You can have the vias capped with solder mask on either the top or bottom. We don't like this option. If the vias are capped on top, the BGA may not get enough of a mechanical connection. If the vias are capped with soldermask on the bottom, expansion of trapped gasses can pop the cap, causing the solder paste to run out as though there wasn't a cap at all.
The board house can plug vias with high-temperature solder just like we did. Some will fill vias with a conductive epoxy or insert a small copper peg. These are all decent options. The best option is to do one of these three and then plate over with copper. If that is done, an assembly house can't even tell that the vias are there, so the process and reliability will be unaffected by the vias.
Happy BGAs R good