First, make sure you are treating the real problem. The power wasted in
power divided by the thermal mass, minus the cooling (which is proportional to the temperature difference between the hot parts and the coolant). The temperature itself is the result of the temperature increase over time.
Since your problem is too high temperatures, there are several ways to combat it - and the best choice is application dependent.
- Reduce the current. Assuming the output power is fixed, you can do this by increasing the voltage.
- Reduce the resistance. You can do this with shorter tracks, thicker tracks, wider tracks, or more tracks (including inner layers if you are careful about cooling). Or you can add bus bars, external wires, etc. Removing the solder mask and adding solder helps a bit, but the resistivity of solder is much higher than that of copper.
- Increase the thermal mass. This will help if you have a short duty cycle for the active period, when you won't have enough time to reach high temperatures. Soldering over the tracks is an easy way to increase the thermal mass.
- Increase cooling. Fans, heat pipes, heat sinks, etc. are all good, if circumstances allow. You should probably also aim to spread the heat around the pcb - have plenty of vias (without thermal reliefs) from the internal tracks to external tracks or polygons. Good ground planes help to spread heat. And try to get strong thermal connections between the pcb and mounting frames.
- Reduce the time that the high power is running.
You might also find it practical to simply accept that the power board is going to get hot, and keep it physically separate from the control board (which is likely to have more sensitive components). That also lets you use thicker copper on the power board and fine dimensions on the control parts.