Super Small Via-in-Pad

Via-in-pad is an old issue that still pops up now and then. Our standard answer hasn’t changed: No open vias in pads. But one of the questions we get related to the subject is: “What if we make the vias really small?”

Logically, that makes sense. In fact, in some cases, the via is so small that it’s essentially closed. If it’s so small that it really is closed, then it’s not an open via. But look close — if it’s closed with solder, that solder may melt during reflow leading to an open via.

The images here show some pretty small vias. I believe they’re 0.3mm in diameter.

In the first picture, on the left, it appears that the vias are open. They aren’t though. This board (an unstuffed Beagleboard) uses solder mask on the back side of the PCB to close off the vias, as shown in the image on the right.

Our recommended method (see more detail here and here) is to plug the via with copper or epoxy and have it plated over at the board fabricator. Next, we’d recommend via caps on the component side. Finally, capping the back side with solder mask, like this example can work, but it comes with the risk of voids. The via caps and also pop open, leading to an open via.

Duane Benson
No more open vias-in-pad, I mean it!
Anybody want a peanit?

http://blog.screamingcircuits.com/

About Duane

Duane is the Web Marketing Manager for Screaming Circuits, an EMS company based in Canby, Oregon. He blogs regularly on matters ranging from circuit board design and assembly to general industry observations.
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