Most of the via-in-pad writing I do concerns BGAs and QFNs. I do cover other parts from time to time, but the subject seems to come up most often with those packages. It is an important subject with passives too though. If you need to make your board smaller, putting vias in the pads of all of your passives may seem like a viable option to gain a lot of space. If you fill and plate over the vias, then, yes. It’s a good plan. If you leave the vias open, then no. It’s not.
Here are some via-in-pad guidelines:
| Part type | open via | Solder mask capped via |
6mil or smaller open via |
filled and plated over |
| BGA and LGA land pads | Bad | Bad | Bad | Good |
| QFN, TO-(power part) thermal pads | Bad | Acceptable | Maybe | Good |
| QFN signal pads | Bad | Bad | Bad | Good |
| Passive pads | Bad | Bad | Bad | Good |
You can probably see a somewhat common theme in the table above.
All of these pictures show bad stuff. These are from the “don’t ever do this” camp. Open vias on passive parts can lead to tombstoning, poor mechanical connections, solder blobs on the back side of the board and crooked parts. Open vias on BGAs can also lead to the solder ball being sucked off of the the BGA. Bummer dude.
If you do use solder mask capped vias in a thermal pad, most manufacturers recommend the via cap be about 100 microns bigger then the via. This prior post here shows a decent example of using solder mask caps in the center thermal pad of a QFN (the rules from QFPs and DFNs are the same as for QFNs). And, I’m calling it a thermal pad in the center of the QFN, but the rules still apply of the pad is just for grounding and not for cooling.
Duane Benson
Where are we going? Planet ten
When are we leaving? Real soon
I don’t understand how you can ever have signal vias under a grounded thermal pad? Can you trust the soldermask to insulate vias from the thermal pad?
Charles – I whole heartedly agree with you. Signal vias under a thermal pad like in the QFN pic I posted are bad, bad, bad. They can ruin your whole day.