Most of the basic rules of thumb for printed circuit board layout are aimed at high-density, high-frequency boards. While keeping trace angles under 45 degrees and minimizing board size are good things to keep in mind, there are higher priorities when it comes to high power board design.
Spread Out
There will always be components that generate more heat than others on your board. The best thing you can do for heat dissipation is to get those components as far away from each other as possible. On an LED board, all the LEDs will be generating heat and should be as evenly dispersed as possible. On a power conversion board the culprits may not be so obvious. The main switching component in a power supply is often at risk for overheating and any components that are in line with the main current flow should be distanced at least somewhat from the main switcher. These components should already be selected and rated to high wattages, so they should be fairly obvious. Typically, a designer needs to keep an eye on thermals for the main IC, any external FETs, and any high-current inductors. These are often very close together in the schematic, but should be given room to breathe on the PCB. Components tend to be rated to a maximum current or power dissipation given a minimum copper area beneath them, so check the datasheet if you are worried your parts are too close. If you are creating a double sided board, keep in mind that the distance should be regarded 3-dimensionally. Putting the IC and FET back to back on a double layer board isn’t doing you any favors.
Go Deep and Wide
Copper has a very low resistance, but it isn’t perfect. If large amounts of current are moving around on your board, you need to calculate trace resistance both to manage any thermal effects and predict any voltage drops from one end of a trace to the other. There are numerous online calculators that can help you out, or you can do the math yourself. A 1oz copper trace has a height of 1.4mils, which means you can see a huge performance jump just by moving to 2oz copper. This upgrade is far from free, but may make the difference between a functional board and a molten mess in extreme cases. Once you have gone as deep as you can, go wide. Special consideration needs to be given to internal traces in a 4+ layer board, as they cannot dissipate heat into the air. They should generally be twice as wide as an external trace carrying the same current. Copper pours are great for distributing heat evenly through a board, but exposed copper and external heat sinks are key for actually removing that heat from a board. If you use pours to carry large amounts of current, watch closely for any choke points that may limit your thermal efficiency. Current choke points tend to show up where a pour shrinks to fit between two mounting holes or around the edge of a component. Current can only flow throughout your board as well as it can through the narrowest point.
Punch Air Holes
Many high power chips, especially those with an internal FET, have a large pad on the bottom that aids in heat dissipation. At a minimum, these pads should be in contact with an equally large and exposed copper pad on the PCB. If you have room, thermal vias are a great way to draw heat away from the chip. Thermal vias are smaller than normal current-conducting vias, using an 8mil to 10mil diameter drill bit. The vias are arranged in a grid such that their annular rings are touching and no soldermask is between them. Soldermask should also be removed on the back side of the via array to give the copper direct access to air. The amount of heat that can be dissipated into the air is directly related to how much surface area is available for dissipation, so thermal vias are literally piping the heat to the outside of the board rather than dumping heat into a copper plane isolated by soldermask.
With these three design priorities in mind, you can make sure your build stays high-functioning and stable when under high-power operating conditions.