PCB Assembly Services - Screaming Circuits: No Open Vias in or near pads


No Open Vias in or near pads

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 PC boards where the BGAs go, don't leave open vias in your BGA footprint.

Don't do this:

Via near BGA pad closeup

Here's what happens:

BGA via near pad wicking

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 fab house. (Read this post). If you're putting the vias between the pads, you have two options. You can put soldermask dams on the short trace between the pad and the open via. This will prevent solder paste from migrating. I've modded the image below to show what that would look like.

Via near BGA pad damed closeup

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.

Via near BGA pad capped closeup

Duane Benson
We don't want cows on pc boards but we have built pc boards that go on cows
The IoC: Internet of Cows


Post a comment

If you have a TypeKey or TypePad account, please Sign In.

« A few Predictions for the Next Decade - The Eve of Disruption | Main | 150 Years of Predicting the Elements of Electronics »