PCB trace size for pulsed current
Thermal TimeConstant of 1micron cube of copper is 9.6 nanoSeconds.
Thermal TimeConstant of 10micron cube of copper is 100X slower, or 960 nanoSeconds.
Thermal TimeConstant of 100micron cube of copper is another 100X slower, or 96 microseconds.
Thickness of standard CU foil (1 ounce/foot^2) is 35 microns, close enough to 31.6 microns so we can with confidence say the timeconstant of 35micron cube of CU foil is 9.6 microSeconds.

To unload heat through FR-4 is tough, what with that ThermalResistance being 200X higher than CU. So you only path is along the CU trace. And that Thermal Resistance is 70 degree Centrigrade per watt per square of foil. A trace 10 mils wide and 1 inch long has 100 squares, thus Rthermal is 70*100 = 7,000 degree Centigrade per watt.