Vias in BGA pads
In past posts, we've talked quite a bit about vias in the heat-slug or flag pad of a QFN, listing techniques and hints. We haven't said as much about vias in BGA pads though.
The BGA pad via is an even more difficult situation than the QFN because there is less room. In general, our advice regarding via in BGA pad is: "don't do it. Don't do it. Don't do it." Oh, and did I say: "don't do it"? However, there are some applications that simply require it so just saying no, in this case may not be all that helpful. If you don't absolutely need to put the vias in your BGA pads, don't, but if you don't have a choice, here are some guidelines:
- Best: Have your board fab house plug the via and then plate copper over it. They can plug with metal or a thermally and electrically conductive epoxy before the final plating steps. This will give you all of the benefits of via in pad without causing problems in assembly. It works for prototypes and for production. This is industry best-practice.
- Okay, but still a bit of a challenge: Use a micro-via that only goes through one layer of the board. This can still cause some problems and you should call us before sending in an order. The solder can wick down into the via chamber so we may need to adjust the stencil to ensure enough solder stays on the pad.
- Not too horrible, but not good either: Cap the underside of the board with solder mask. This will usually stop the solder from completely wicking out, but sometimes the cap can pop open and sometimes the void is big enough to still suck too much of the solder off of the pad. Sometimes, it can even suck down the solder ball off of the BGA leaving an open. Call us before you send in a board like this. We need to look at it to tell you if we can reliably assemble it.
- You should never do this: Open vias on the pad. This practice goes against industry standards. We have built some boards like this but there are no guarantees and, if we can do it, it will probably cost extra. The problem is that the via, through capillary action, sucks the solder off of the pad and may even suck the ball off of the BGA. Sometimes, no matter how much solder paste is applied, it still all gets sucked down to the other side of the board. Using really tiny vias or lead-free paste may help but our advice is to see above. If you don't have a choice, go ahead and give us a call, but we may have to say no or we may agree to give it a try without guarantees.
We want to be able to help in any situation that you need and sometimes the best and least expensive option may involve a re-layout with a more expensive board. Think of the cost of those expensive BGAs and the potential cost of having to re-build it.