Screaming Circuits: Via in pad?


Via in pad?

In general, we would recommend against placing vias in any pads that will receive solder during the assembly process. A couple of undesirable events can happen depending on the method used during board fab.

If your vias are left open, solder will tend to wick down into the via hole. The larger the diameter, the worse the wicking problem can be. You might end up without enough solder left to secure the component. You might even see a solder bump on the bottom side of the board, which could interfere with other components or lead to shorts.

If your vias are capped or partially filled, the caps might pop off due to thermal expansion or out-gassing. Internal air bubble can migrate up, leading to voids in your solder joint.

In a perfect world, we'd like to never see a via in pad. However, the real-world is saying otherwise. Manufacturers of QFN parts are starting to recommend vias in the heat-slug pad for improved thermal conductivity. High frequency designs benefit from the shortest possible routing, which may indicate via in pad. Super fine pitch BGAs may not leave any other options.

This not very helpful, but somewhat humorous thread from 2004 on the SMTnet message forum illustrates common opinion on the process. If you need to place vias in the solder area, we would recommend that you use as small a diameter as the design will allow and follow the component manufacturer's guidelines for placement and via capping or filling.

References:

We'll be watching this issue closely and pass on more hints and tips as we find them.

Duane Benson
Via le pcb assembly

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Comments

Andy;
You still need to be concerned about the open vias. Much of your solder can end up on the bottom side of your board. This link has some guidelines for you:

http://blog.screamingcircuits.com/2007/04/soldermask_via_.html

Basically, treat it similarly to a QFN thermal pad. Check with the part manufacturer to see what they say about paste layer coverage too.

If your vias are so small as to almost be closed, that can work sometimes too, especially with lead-free solder.

Duane,
Great series on vias-in-pads, but there's one case I don't think you've covered - small vias in large pads. We encounter this with voltage regulators, where the IC manufacturer recommends vias to connect the thermal pad to the ground plane. We use 2-layer board for most of our designs so this requires through-hole vias in the pad. Unlike BGA’s, the pads on these regulators are big; 0.325” x 0 .430”. So even if we throw a dozen small vias on them, their area is tiny compared to the pad’s area. In this case, do we still need to concern ourselves with the issues you’ve described for the vias in small pads?

Andy Beck

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