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.
- Xilinx appnote on QFN land pattern and via in pad capping
- An Amkor appnote on the same subject.
- If you're an SMTA member, you can check this paper out
We'll be watching this issue closely and pass on more hints and tips as we find them.
Via le pcb assembly