While we don't like it, with some designs, if done properly, there can be a number of compelling reasons for putting vias directly in the SMT pads for BGA and QFN packaged components.
- It can make routing easier with big BGAs
- It can allow really close placement of bypass capacitors
- It helps with thermal management
- It can help with grounding on high-frequency parts.
The reason we don't like it is that when left open, those via holes act like little capillary straws and suck solder off of the pad. If not properly prepared, that can happen and leave the part without enough solder or can leave solder bumps on the back side of the board. It can even suck the solder bumps off of the BGA. It can make assembly impossible.
Ideally, any designer needing to put vias in pads would have them filled with an appropriate material and capped with copper at the board fab house. When that is done, we can't even tell that the holes are there. However, in the prototype world you don't always have the option of doing things the ideal way.
Open vias in pads are not an industry accepted practice. If you do need vias in your pads, our first recommendation would be to re-design the boards so the vias are in between the pads, plug or cap the vias at the board house or to use microvias that don't go all the way through the board. If you can't do any of that, it may still be possible to get you a reliable board. The first step is to call us and discuss your design. We'll want to take a look at the gerbers and the parts and the boards, if you have them already. We may very well tell you no. We may tell you that we can give it a try without any guarantees.
Sipping solder through a straw...