Paul Rako@ edn.com discuss about thermal design in electronics and how to design your board to dissipate it effectively.
If you have high-powered LEDS, or a power supply, or are trying to control larger motors, you have to get a lot of heat out of your circuit boards. The classic way to dissipate heat is to bolt your power transistor to an aluminum heat sink. That is a slow, messy, and expensive proposition, especially if you need thermal grease between the transistor and heat sink.
It’s always a good idea to make thermal management inherent in your PCB design. Experience has shown you can get about 2W of heat out of a 3×5” copper area on a conventional FR4 PCB.
Thermal design: Get the heat out of the electronics – [Link]