Having designed circuit boards where some components have substantial power consumptions, I can also say from experience that the circuit board itself can spread heat fairly effectively.
Nearly all high speed digital PCB's have at least one copper 'plane' which spreads heat throughout the board fairly well. The heat flowing through, even a thin trace, only has to travel through a thin layer of fiberglass to get to the plane, and be spread effectively.
Using the PCB as a heatsink is very common practice, and many component datasheets discuss the amount of copper surface area that should be provided to adequately heatsink the component.
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