This repo has 3 open boards based on STM32 to make your own security key: https://github.com/solokeys/solo-hw https://github.com/solokeys/solo-hw It could be u

This repo has 3 open boards based on STM32 to make your own security key: https://github.com/solokeys/solo-hw https://github.com/solokeys/solo-hw It could be useful to take inspiration and build your own, with a working example in mind. From a "process" perspective, check out this project: https://www.crowdsupply.com/sutajio-kosagi/fomu https://www.crowdsupply.com/sutajio-kosagi/fomu If you look at the things you can buy, they have a EVT board, which is I think what you're asking for. Usually you first build a larger version of the board to validate the design (and that you can start using to build the firmware), and then you shrink it.