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That is, for example, filling the top layer of a 2-sided board with ground and the bottom with power rail.

Will this form a big decoupling cap across the board, or will it give me horrible crosstalk?

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    \$\begingroup\$ FYI: Landfill might not mean what you thought it means. link \$\endgroup\$ – The Photon Mar 27 '15 at 1:46
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It does act like a big capacitor but its not a large value unless the PCB is fairly thin. The main reason you fill is to create the lowest resistance and inductance distribution for your power and ground. That way they are more stable when the load changes.

I had a board where I forgot to fill so the power and ground were distributed in 10mil trace. I had some comparators powered off the rails. The moment the comparator switched state, the power rail bounced causing the other parts to change state. next rev I filled all available area with power on one side and ground on the other and it didn't move more than a mv or two.

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    \$\begingroup\$ But this still does not mean you can forget your bypass caps next to your devices. You still need local energy storage right at the parts that will be changing state. \$\endgroup\$ – hwengmgr Mar 27 '15 at 1:33
  • \$\begingroup\$ What if the board have 4 or 6 layers? \$\endgroup\$ – Maxthon Chan Mar 27 '15 at 1:53
  • \$\begingroup\$ Doesn't matter. It still lowers the resistance and inductance of the power distribution. And you still need bypass caps! \$\endgroup\$ – hwengmgr Mar 27 '15 at 2:01
  • \$\begingroup\$ On a 4-layer board, there is a common convention to use the two inner layers as (mostly) power and ground planes, and the outer layers as signals. The benefits being that there's then good shielding between signals on the two outside layers, and it's easier to debug (and fix) problems than if you had the power on the outside and signals on the inside. For 6-layer boards, the convention is signal-power-signal-signal-power-signal. \$\endgroup\$ – David Smith Mar 27 '15 at 13:21

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