I'd like to better understand what to expect when carrying current on multiple layers of a PCB. I've used trace width calculators to determine how wide a trace needs to be to carry a given amount of current while limiting temperature rise to a given delta. Let's say for example that I've calculated a trace width that limits the temperature rise on the top layer to 10 degrees. Let's say I also have a trace running in the inner layer below it that is intended to limit the temperature rise to 10 degrees. What is the total temperature rise I should expect? What happens if I have traces that are expected to limit temperature rise to 10 degrees on all 4 layers of a 4-layer board, or all 6 layers of a 6-layer board?
It's not an exact science unless you throw the board files into a 3d FEM thermal solver and accurately model the materials.
There are also multiple factors that will determine the temperature rise, such as what the temperature of the traces endpoints (how the heat spreads out horizontally) and what the ambient temperature is. It's also not so much as what the temperature is (simmilar to voltage) as the heat produced in the trace and the heat flow (simmilar to current in the thermal world).
If you have a trace that is 10 degrees in the layer above and the layer below then no heat flow will go between the traces but there will be more heat that will need to spread out into other areas of the board (similar to two voltage sources being the same voltage). In the end it matters how much power you are putting into the board and where it will go.