Fix source and drain pins on AON7400A heater MOSFET.

[robets]

This PR fixes the reversed source and drain pins on the AON7400A heater MOSFET for #2.

I rotated the footprint to put the drain pins closest to the heater connector, and routed the gate around to the opposite corner. Routing could probably be improved, I'm somewhat of a novice on PCB layouts.

[henrikssn]

Thank you!

Could you please also add a few (say 2x2 or 3x3) vias under the AON7400A footprint and a small 4mm*4mm filled zone beneath it (on the bottom layer) for better heat dissipation?

[henrikssn]

Please use metric units for the drill files

[robets]

Hah, oops. I think I saw that the plot and footprint generation dialog had millimeters set as and assumed everything else would default to metric as well. Seems JLC handles mixed units though, it must have been set that way when I ordered my boards.

Should be fixed in next commit.

[robets]

I'm not sure if I've done this correctly for KiCad. The only way I could find to add the thermal vias and pad was to modify the footprint for the component. I copied the footprint to "DFN-8-1EP_3x3mm_P0.65mm_EP1.7x2.05mm_Thermal_Pad" to keep it optional.

Added a 4x4mm pad to the opposite side with 3x2 via grid.
Moved the nearby related pads to keep good continuity with the ground pour.

As a side note, the power handling increase going from the AO3400A to AON7400A is probably lost due to the SS54 diode on the power input being limited to 5A. Using another AON7400A could be suitable for reverse polarity protection instead of the diode. BOM cost increase would be fairly small since the extended part cost of the AON7400A is already done by using it once.

I'll open a separate PR for that if I can figure it out.

[henrikssn]

Thank you, looks great! Lets follow up in #4 on improving the reverse polarity protection.