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[Moved]: Why / How does a large ground plane help with PCB heat dissipation ?

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It seems to be accepted that a large ground plane/copper pour helps with PCB heat dissipation.

I understand the idea that more mass will help dissipate/distribute heat…

…but why the ground plane? It seems more intuitive that the power rails would be where most of the heat was being produced?

Can anyone explain the relationship between the ground plane size and the generation/dissipation of heat on a PCB?

Thank you!
 

The heat is not dissipated by the power rails, they just carry the power (thus the wires carrying power in your house don't get hot).
The heat is produced by the various components that dissipate power due to their operating current.
A copper plane has a low thermal resistance which helps spread the power dissipated by the various components to minimize hot spots and allow it to be more readily radiated or conducted to the PCB mounts.

There are programs that can help with calculating this effect related to ground plane size and thickness but they are not simple. Here's one discussion that mentions this.
 

Replace the term GROUND plane with copper plane... it is any copper plane will HELP dissipate heat. There is more to it and thermal management can get very interesting... Also because of the limited thickness of PCB copper planes the distance the heat can spread from a heat source is limited, the higher the copper weight the further the heat will travel though. Without digging out my info I believe it is about ~40mm for 1oz copper.
But as copper is a far better heat conductor than FR4 the planes help dissipate heat through the Z axis...
Then you get onto thermal vias.

IPC-7093 is a good read and a search for PCB thermal management should provide hours of happy reading material.
 

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