VIA-IN-PAD

WHAT IS A VIA-IN-PAD?

A via-in-pad design, as the name indicates, is a printed circuit board design with the vias directly on the BGA pads. The main benefit of a via-in-pad design, also called VIP design, is that you reduce the area needed for the vias, making it easier to manufacture miniaturized PCBs and dramatically minimizing the amount of board area you need for signal routing. With via holes connected directly to layers beneath the component, you can have signal routing without escaping the device footprint perimeter.
 
IS IT CONSIDERED BAD PRACTICE TO PUT VIAS ON BGA PADS?

Is this a good practice or a bad practice? Why isn’t everybody doing it? In fact, many people are. It is becoming a common practice to put vias on BGA pads. Why don’t all designers do it? The main reason is that if you put a via in pad, you have to fill it — either with copper or a copper-covered non-conductive material. Non-conductive fill is most popular and more price competitive.  If you do not, the solder will flow away from the pad and you will not get a functional electrical connection.
Filling the vias is an extra step, and some designers may not want to incur the cost and lost time required to do it. Putting vias in pad also affects the drill diameter you will need. Nevertheless, there are many good reasons to opt for a via-in-pad design, which is why many PCB users do call for them, despite the slightly added cost and time commitment.
Next, what are the advantages and disadvantages of VIP vs. traditional via placement?
 
TRADITIONAL VIAS VS. VIP TYPES ADVANTAGES AND DISADVANTAGES

As mentioned, in a traditional via layout, you can simply apply solder mask to prevent the solder from drawing into the barrel of the via and creating electrical connection problems. But when you have via-in-pad, this will not work. You must completely fill the vias so there will be no air entrapment with resulting outgassing in the assembly phase. You also need a flat planar surface in order to attach fine-pitch BGAs as well as components effectively.
How can you fill these in-pad vias? After mechanically drilling and plating your in-pad vias, you must fill them with epoxy. Alternatively, you can laser-ablate your vias and fill them with copper. Which you choose to do will depend on your specific application and needs when it comes to your printed circuit boards as well as the size of the via. The main issue when deciding your process will be pad diameter. You need to make sure the pad size is large enough for the via diameter while still being able to accommodate manufacturing tolerances and meet the minimum IPC Class 2 or 3 annular ring requirement.
The VIP advantage is that once you effectively place the vias in-pad, you will enjoy some incredible space savings, and this can not only increase your efficiency but may also be required for certain modern applications. If you have a revolutionary application that necessitates space flexibility, VIP can be the ideal choice and may, in fact, be the only choice.