Bypass caps under BGA: Should I isolate vias from planes?
The ESL for an 0201 capacitor looks like this:



Source: https://ds.murata.co.jp/simsurfing/mlcc.html?lcid=en-us#

The inductance for a 5mil via is about 1.546nH


Source: http://www.saturnpcb.com/pcb_toolkit/

So the ESL of the capacitor is being reduced significantly by the vias, however, with a BGA part we can't put the capacitor next to the part so, a capacitor is better than nothing at all. If we look at the circuit we would like to use the one on the right below, but since the BGA will get in the way we place vias on the opposite side of the board and deal with the inductance. Bigger vias have lower inductance. If there is a significant worry for noise and a way to parallel the vias, then paralleling the vias could minimize inductance.







simulate this circuit &ndash; Schematic created using CircuitLab