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Since FR4 is epoxy is a thermal insulator or rather the conductance is <<10% of copper, you are basically only using 1 side of copper and as I recall forced air can improve this up to 5x with 1m/s air velocity over the surface depending on geometry of obstructions, lack of eddy currents etc. Vias will improve this but also have low conductance so hundreds of holes per sq.in. to utilize both sides. Whereas solid aluminum or MCPCB is far better. http://www.daycounter.com/Calculators/Heat-Sink-Temperature-Calculator.phtml
e.g. using 1oz copper 43 deg C/Watt per square inch on both sides or
with FR4 substrate ~80 deg C/(W*sqin)