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I think you are right.Placing the cap on the other side of the board
will add some 2mm if you can place the caps close to the vias.
The rule says "as close as possible". Like you said, in BGAs you simply
have no choice: the pins are underneath the IC, so going to the other
side of the board is the only option.
A pad used two vias and minimising the track length, is the usual
technique when putting the capacitors on the opposite side of the board,
with high-speed designs.
It is very important to have your decoupling cap connected to the
power or ground planes of the PCB than directly to the power pins of the
chip. This frequently lowers the overall impedance of the power traces
and improves the usefulness of the decoupling caps.
Great help to my new project.
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