I've had the "final" layout for this project ready for a while. I've been fiddling with it so that it "looks nice" as well as being electrically correct. All component outlines are 0.2mm lines. Text is 1mm high and 0.15mm lines. All component holes are 0.6, 0.8 or 1.4mm. Hi-current traces are 1.5mm wide. Normal traces are 0.3mm wide, except the 5V traces which are 0.5mm wide. That should leave enough headroom for expected currents. I am not including the EEPROM that is required for the complete Pi HAT specification, I am just using the same outline and dimensions.

I am doing the layout with KiCad, which is a great program, but all of the symbols and footprints came from different places, plus I had to draw some of my own. Next time I will create library specifically for the project and copy all the devices I use into there and normalise the line thicknesses, text, and drill holes as I do it.

The board has not changed significantly since the first draft, so it's ready to send to dirtypcbs.com for manufacturing. Unfortunately the HAT boards are slightly bigger than the smallest (cheapest) size of 5x5cm. However, the next size up, 10x10cm is still a good price. I could add some smaller boards to fill up the remaining space, but I'm not going to. After submission it will take about 8 weeks to receive the boards, which is fine as I will be travelling. That means I also won't be working on the software either.