I failed to maintain a Single Source of Truth for this project—there's this HaD.io project, the git repo, and a twitter thread. The twitter thread has newer details than the other two, so here's are the highlights!

After the thread started getting attention, I went ahead and just cut the flex connector off, thereby locking in the code that was on the ATtiny84A at the time. It did nothing when I put it in a USB-C cable connected to a wall wart, I have not tried plugging it directly into a computer at this time. But I did jury-rig a power-only setup using a micro-USB breakout and a USB-C breakout. Red and blue worked, but then something crackled and damaged the red and green LEDs. I know, right? What a surprise, pushing the exposed components against long parallel rows of conductive fingers caused problems! So I soldered a new LED on and applied two layers of conformal coating. That worked great for the duration of one tweet, then red stopped working. The conformal coating that I have is not strong enough to resist the scraping motions of the USB contacts, apparently. But I was left with a very pleasant blue-and-green doodad, which was fine by me. That was a good stopping place and I haven't had the chance to go back to it since then.

I've since measured the OSH Park PCB to be closer to 0.2 mm (8 mil), thicker than I imagined. This makes it a bit harder to do what I want to do next, which is a """proper""" double-sided port made of two flex PCBs sandwiched around a microcontroller. If the current setup is 0.7mm, putting another 0.2mm board on top will be way too thick. Two possible things to try:

1. an even shorter microcontroller. Do you know of one? The ATtiny20 in WLCSP-12 is at most 0.53 mm tall, are there any other options out there?
2. cut a hole in the second flex PCB so the microcontroller can poke through but the other components are shielded. This needs a diagram to illustrate what I mean, I think, but I can't make one at the moment.