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HaneSOM: A coin-sized 500MHz / 128MB ARM Cortex-A5 Linux computer
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You solder them onto the edges.
https://d3s5r33r268y59.cloudfront.net/datasheets/23894/2021-06-09-13-30-09/Hane-SOM-Full_SoM_pinout.pdf
There's the pinout. This is a common technique for electronics. See Sparkfun for a good tutorial: https://learn.sparkfun.com/tutorials/how-to-solder-castellated-mounting-holes/all