There is a property of lateral heat spreading, which depends upon the conductivity of the board relative to the conductivity of air.
Suppose you have a slug of, well, some device that's dissipating heat, a D2PAK's copper tab, say. That tab, being solid copper, can be reasonably assumed as a constant-temperature boundary condition, and a power source. That is, it's dissipating heat into the board, and all points of the board that touch it are held at the same temperature.
Within a short distance of the slug, there is not much board area, and little power dissipation. This area heats up, and power continues to spread, laterally.
As the heat continues to spread, the total area increases, and gradually, it cools down. This slows the spread of heat, so that 90% of the heat is dissipated within a given area, even if the board is infinite.
I once solved this equation, finding a solution in the form of the Bessel function. Namely, the zeroth order, where the value of the function corresponds to temperature (as a function of radius) and the first zero can be taken as the lateral spreading distance. (I'm probably remembering this very poorly, because a zero is certainly not part of an asymptotic function, which should be the correct result. Note that the "radius" is not the geometric value, but a dimensionless product taking into account board and air conductivities.)
Anyway, there is theoretical basis for the simple case (linear thermal conductivity), and you can run the diff eq if you like.
The practical result is this: PCB heat spreading radius is approximately as many inches as half the total ounces.
So, for a D2PAK on 2oz two-sided PCB (with vias and heat spreading on both sides), expect about a 1" radius, or ~6.3 in^2 (pi, then x2 sides). The corresponding convection constant, in mixed units, is about 150 K.in^2/W (what a horrible unit, I know
), so expect about 20 K/W, or a maximum dissipation of 5W at 100C temp rise (about the most you'd ever want to push, for a commercial grade product).
...I'm playing with a board right now that's a combined total of 14 ounces. I can dissipate 10W in the middle of it, at ~40C peak temp rise (that's the temp on top of the components). Downside: it's almost impossible to solder!
Tim