Specialized Techniques for Escape Routing
When using fine pitch BGAs, vias on pad technique may be implemented. The vias are placed directly on the BGA pads and filled with a conductive material, usually silver, which can provide a fairly flat surface. In some instances, the only option is to use silver-filled vias. They are less expensive to fabricate compared to blind and buried vias, hence, in some cases, compromises must be made between cost and manufacturing time.
In a BGA pad design, using silver-filled vias, the need for dog-bone fan-out (see image below) is eliminated, and a small increase in density is realized by being able to route a few more signals on the outer surface. In addition, discrete decoupling capacitors can be placed directly over the vias on the bottom side, providing a low inductance path to the power system.