Designing Somu, a tiny security key

Somu is the same technology as Solo, but in a tiny form factor that fits in your USB port. You can use Somu as a U2F or FIDO2 authenticator, which allows you to log into websites and other services, . The name is inspired by Tomu, which was the first open source key using this form factor.

The team at SoloKeys and I are planning to Crowdfund Somu at CrowdSupply. If you’re interested, you can sign up to get an announcement when it’s available.

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The PCB is a 1 mm thick 2-layer board, with Z-axis milling. The milling makes the short tabs on the sides so the PCB can "slide fit" into the case.

The end of the circuit board is fully edge plated and makes 2 capacitive touch sensors, which act as the buttons. Right now there’s no use case for 2 buttons, so they will be used as one button. Maybe there will be a case in the future.

The case will be a relatively stiff rubber material so it will slightly flex around the PCB tabs and still be fine to plug into a USB port. Circuit components are on the bottom.

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Judging from my rubber hardness sampler, Shore 90A *feels* about right. I also used this to help pick the case hardness for Solo.

I printed the circuit board and case. Circuit fits in case well. Circuit+case fit in USB-A port well. Next is to order real PCBs.

An interesting challenge with the PCBs is that the board is nearly one layer since you can only make very few vias without interfering with the USB traces.

And the components make a very tight fit. 1 MCU, 9 jelly beans, 1 RGB LED, 1 linear regulator. Plus two capacitive buttons, USB traces, and 4 test points. I was doubtful this would fit on a 10x11mm space on a "1.5" layer board but it does!

Since the components are all the same as on previous Solo models, and the routing is also largely the same, the same firmware can be used for all models. This keeps things simple.

I ordered the PCBs from PCBWay for about $250 since they support edge plating and Z-axis milling for their prototype service. Communicating the requirements was pretty straight forward, and I got the PCBs back in about 2 weeks.

I hand soldered all 20 PCBs that I ordered. It took some patience, but it’s for sending samples to reviewers while we run our Crowd Supply campaign. If you’re interested in reviewing (or anything else), let me know and I can send you a sample.

Just like Solo, Somu is open source and easy to develop for. It’s also a secure option to use as a security key! If your interested, check out Crowd Supply, or sign up to our announcement list.

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