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

http://blog.screamingcircuits.com/