Sometimes, you can get by with vias in your pads. Sometimes, but not very often. I wrote about this a while back here. The thing is, I was talking about big pads — like QFN or QFP thermal pads and stuff like that. We never like to see it and it’s always a manufacturing risk at some level, but as described in the earlier post, sometimes you can just roll with it.
Pretty much never with a BGA pad, though. The picture above shows just about a worst-case scenario. Very big. Very bad. Relatively very big holes anyway. (This is for a 0.5 mm pitch Bluetooth module BGA.)
The vias in the image below worked okay with a QFP because they’re really small — practically closed up — and it was Pb-free solder.
We still wouldn’t want to see a via, even that small, in a BGA pad though. Process variations leave enough opportunity for a few of the vias to be open all the way through and even if one BGA ball gets sucked off the BGA, you’re out of luck. Even if it’s just partially sucked off and still connected, it’s much more susceptible to cracking and things like that. (By the way, we did find a way to build the board on the left and make it work. We won’t guarantee that we can make something like this work though.)
A lot of fabricators will epoxy fill vias these days. Even microvias. And, yes, you should even have your microvias filled and plated. Especially with small BGAs. It’s just not worth all the risks that come along with it.
We need little moles to fill those holes
Can fab house fill vias-in-pads with some sort of material (preferably conductive) to prevent dips or outright holes in vias that fail to plate through? How do you ask for this on a fab drawing?
Yes, and this is a very normal practice for me. I’m doing .5mm or less BGA devices, and always have via-in-pad microvias. I make sure it’s spec’d out in the fabrication drawing that the via-in-pad pad surfaces are “Plated flat to the surface”. That’s what my fabricator understands, and they know their processes. It’s worked extremely good for me.
You should ALWAYS contact your fabricator (or multiple vendors, if you need), and discuss what works best for the both of you. My practice is pretty simple, but your company’s standard might call for an extra effort. But, always discuss it with your fabricators to see what works best.
BTW, I have forgotten ONCE, and got boards that were not “plated flat” (little indentations), had them assembled, and they DID work. I still corrected it, and had new boards run, just in case.
Matt’s got the right advice – talk to your fab house. Filling and plating is getting to be pretty common practice these days and almost any fab house can do it. They each tend to have their own vocabulary with some of these techniques though, so a quick call or check on their website is the best bet to make sure you get what you need.
Here at Screaming Circuits, we’ve made a whole lot of “non-standard” things work, but it’s always a trade off between cost here vs cost there or expediency vs reliability. With vias in pads, the least expensive in the long-run and most relaiable way is definitely filling and plating flat.