I'm aware that you can turn down the volume to reduce power consumption and the heat generated, but if they're claiming the amplifier is a 20W per channel amp, then I expect there is a way to get 20W of output power continuously.
Anyway, in looking at that continuous power rating again, I noticed something:
Continuous Power Dissipation (TA = +70°C)
44-Pin Thin QFN (derate 27mW/°C above +70°C, single-layer board) ...................................................2162mW
This bit:
(TA = +70°C)
I think it's a safe assumption that the ambient temperature isn't going to rise to 150F in my intended usage.
Also, there's this:
?JA, Single-Layer Board................................................37°C/W
So, if I assume 5W of power disspation, which I don't actually know is a correct assumption if I am driving two 4 ohm loads, then I can assume a temperature rise of 185C over ambient for which I think 30C (86F) is a reasonable assumption.
So that would give me a total temperature of 215C, which is too high.
Of course this also assumes I am running the amplifier at maximum power with something approaching a square wave at full volume. But it's best to consider the worst case.
If I used a multi-layer board, which I am considering, then the temperature rise would be 27C/W, so at 5W now my total temperature rise above ambient is 135C, ,making the total temperature at 30C ambient... 165C. This is slightly above the allowed junction temperature of 150°C.
So, assuming that 5W power dissipation is for two channels at once, and again, I am unsure if it is, then I think for the typical case of playing music in stereo, the power dissipation will probably be more like 2.5W, and my temperature rise will be 68C, making my maximum temperature 98C. And 2.5W of power dissipation would require around 5 in^2 of four layer PCB. Which seems, roughly, to be the the size of the Adafruit PCB. Though it appears to be only two layers.