Placement of decoupling capacitors is highly critical and must be placed as close as possible to the BGA pad. Otherwise, a higher inductance path than necessary is created. Ideally, the designer wants to maintain the shortest possible distance between the two. The PCB designer makes certain of low ac impedance targeted at reducing noise and storing energy. That is accomplished by having decoupling capacitors between the device’s power and ground pins. Several capacitors with different values are used as the way to attain low impedance over a wide frequency range.
The first figure below shows 0201 and 0402 sized capacitors placed inside a 1mm pitch BGA with vias in pad.
The second figure below shows 0402 capacitors placed inside a 0.8mm pitch BGA with dog-bone fan-out. The technique of via plugging avoids solder starvation.
The third figure shows another way of placing the 0402 capacitor inside a 0.8mm pitch BGA. Most of the decoupling is further apart from the pin. But it offers better results during assembly and easier rework.