Till

@caesar1111 alright. One last shot... could you post a picture of the boards especially the STM32 controller? This behaves so erratic that I almost suspect counterfeit stm32 hardware that is...

here one of my boards for comparison.  What I see in a quick comparison: - no visible rev marking on your STM (mine has a "B") - way too...

@swanepoeljan thanks! I had no chance to identify this 6pin DC/DC converter without nowing that only "BN" was the identfier. ;) I will google a bit more around this weekend.

Not sure, if this is a real indicator as ST has multiple packaging and test facilities so there should not be much difference in the controller, if they are genuine....

found our guy (via aliexpress "BNOG" search...): https://datasheet.octopart.com/AOZ1282CI-Alpha-%26-Omega-Semiconductor-datasheet-67314984.pdf  It runs the PWM on 450kHz +/-90kHz as we measured. That range might explain, why different countermeasures work on different boards....

If I find time this weekend I will heat one of my boards up and check if it fails more rapidly (not heating the OLED!). Secondly, I will place a...

> >The only board which runs almost flawlessly is the one where the 6R8 inductor is soldered in upside down > > This is interesting and worth testing out. Maybe...

Found some time to at least follow the supply voltage traces.  The buck converter is at the end of the complete chain. C3 has likely 220nF but it looks...

I would habe piggy packed 0806 but had no 10uF available. You can bet that the output cap is same as in the schematic above. Adding 100nF for the 100ns...

@kablek there are not too many connections or components for the 3V3 so tracing should be job of 10 minutes, I hope. But all information is welcome, of course. :)...