I've just got back a Teensy 4.0 breakout board that I ordered from OSHPark and thought I'd post about it.

This board solders directly to the back of the Teensy 4.0 and uses some castellated (i.e. cut) holes to connect to the pads on the bottom. It brings these out to two additional columns of 0.1" header on the outside of the board.

This includes:
  • SD
  • USB Host
  • Pads 24-30 on the bottom
  • Five pins along the end: On/Off, Program, GND, 3.3V & VBatt

Looks like this:

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Front and back of the board:
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Top and bottom with breakout board attached:
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Close up of pads soldered to castellated pads:
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Bonus using right-angle headers:
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I've tested continuity on all the pins and it seems to work without shorts. Soldering the castellated connections was a lot easier than I expected.

I haven't soldered the headers on yet because I'm waiting on an order from digikey with some headers with longer pins on the "top" to make sure I can make solid connections through both boards. (Next time I'll order 0.8mm thick boards and shouldn't need special headers.)

I've posted the KiCad project here if anybody wants to take a look. It uses the SMD footprint from here.

If you are interested, you can order yourself up some from OSHPark here. $6.30 for a set of 3 delivered in the US.

A variation of this could include an SD card slot and/or USB host connector. I built it as a proof of concept for a larger board with a bunch more peripherals, I'll detail that in another post later...

Let me know what you think!