considering both cpu and motherboard have temperature cutoff at which point power is shutdown around 100C, i dont think its possible for a non modified/non damaged motherboard to reach melting point, but even without security, without an actual short circuit, cpu themself dont really consume enough electricity to warm the whole area to the point of melting, it would get pretty darn hot for sure
around 150C youll start getting lots of smoke from the board but it wont be melting yet, just burning the first layer.
Actual melting point would vary a bit, but its usually in the 200-250C. but at that point, your cpu would be long dead.
The cases i know off with motherboard physically melting were from short circuits on power bus, and not really from the hardware itself heating up during usage.