Robert Kollman of TI has a great article on Power Supply Layout Considerations (http://focus.ti.com/lit/ml/slup230/slup230.pdf) that covers heat transfer through the board and its traces, (as well as lots of useful info on parasitic inductances / capacitances, ground design, and voltage losses through traces).

We’ll summarize the highlights and conclusions, but first some basic structure on how these types of heat transfer problems are thought about and solved:

In most cases, there is a temperature that will break something: either the pcb board material (FR4) will break down or a device will become permanently damaged or shut down. In the case of our voltage regulator, it shuts down at 125 degrees C.

The next question to ask is “What will the ambient temperature be in which the device operates?” The best heatsink in the world won’t help if it needs to operate in the desert and components fail at desert temperatures (or automotive: -40 to 125 deg C).

The final operating temperature will be:

T_device = T_ambient + Q*R, where Q is the heat generated and R is the total thermal resistance between the device and the outside air.

Rearranged, (Td-Tamb)/R = Q, which is analogous to V/R = I, where voltage is T and current is heat.