PCB Assembly Services - Screaming Circuits: Super Small Via In Pad


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?"

Beagleboard U6 viasLogically, 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 0.3 mm in diameter.

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

Our recommended method (se more detail here and here) is to plug the via with copper or epoxy and have it plated over at the board fab house. Next, we'd recommend via caps on the component side. FInally, capping the back side with soldermask, 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?


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In my experience, via in pad on large (ground/heatsinking) pads are rarely a problem, as long as you don't go nuts with huge vias (and have a large section of open copper on the bottom...). Even if some solder vanishes down the hole, I've had more trouble with _too much_ solderpaste than too little.

Of course, on smaller (.5mm pitch BGA, anyone?), putting open vias in the pad is a horrible idea.

At the end of the day, I think keeping the via hole to pad size ratio down will lead to entirely satisfactory results. Something like less than 5-10% open via area in the pad; if it manages to suck down that much solder into a comparatively tiny hole, you're using better solder paste than me!

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