Using planes as additional capacitor - useless, good or bad?

[eTobey]

I am designing a powerrail, with 2 more powerrails after that (with a AP66300). I was thinking about additionally using big planes of Vin/Vout with oposing GND plane to act as an additional capacitor.

Is this a good idea, or a bad one? Or wouldnt it matter at all?

[ataradov]

It does not matter. Calculate the resulting capacitance, it is negligible. It is a good idea to use power planes in general, and possible additional capacitance is not going to make things worse. But don't rely on it to do anything particular.

[moffy]

The planes can provide electrostatic shielding, and low impedance but as ataradov said "Calculate the resulting capacitance, it is negligible"

[Someone]

"negligible" depends on what you are looking at, but planes only start appearing as a significant factor up above a few hundred MHz:
https://resources.system-analysis.cadence.com/blog/msa2021-power-plane-resonance-analysis-in-your-pcb

[Marco]

Do any of the hobbyiist friendly PCB manufacturers offer planar capacitor layers yet or is it only available for boutique and ridiculously high volume users?

[DavidAlfa]

Will only provide few picofarads, maybe 1nF at most.

[Siwastaja]

As others have said, doesn't matter. Use one value of MLCC, first priority being smallest possible package, second priority largest possible capacitance available in that package, sprinkled over the bus as close as the consumers as possible. Additionally, use lossy (e.g. electrolytic) bulk capacitance near power input. Power plane can be full contiguous plane, or segmented fills with different power rails on the same layer, or just bunch of wide traces.

[SiliconWizard]

And if you're looking to save a few cents (and PCB area) on bypass capacitors with the thought that large planes will do it, think again. I don't think that was the idea here, but just saying, I have seen ultra-cheap designs trying to get rid of any part possible even if it's just 1 cent.

Apart from the low equivalent capacitance, the inductance and how it is distributed over the board would make this completely unpractical.

[Marco]

Unless you pay for Faradflex or similar.