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I am working on a 6 layer job which placement of the given components in the rather limited area that I have to work with...is driving me a bit crazy.

I have no option but to have 2 different voltage levels on the same inner-layer (layer 5 in this case). I have never done this way before because I always considered it to be "safer" not to put different potentials on the same layer as a copper pour (but I have done this as normal tracks e.g. 10mil/15mil).

Consider the image below from my CAD software. As you can see I have created two pours almost next to each other with one being 28V and the other 12V (with a 200mil gap). To make it worse, I have to route 5V digital signals (with a max speed of 100KH - the green traces) on the layer above this.

Is it electrically safe to do such a thing? what kind of problems can arise in terms of electrical performance (e.g. creepage) and/or manufacturability?

enter image description here

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Creating large copper pours for different supplies is common practice even on two-layer boards. You appear to have a healthy gap between the pours and should not need to worry about creepage at these voltage levels.

If these are high-current areas, you may need to adjust for reduced thermal dissipation on internal layers.

And finally, running the 5V signals over these planes should be fine...if you also have a solid ground plane layer, which I'm not seeing in your image (perhaps you have hidden it for clarity). The ground return path should run as close to the signal paths as you can manage, and an unbroken ground plane is the easiest way to manage that.

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