Microarchitectures of STM32F4 devices?

Can't help much, but i am curious what package you are using. (Other things I am curious about, but I don't want to try your patience - is it a battery operated device, is it a consumer product, what do you suspect might have caused the damage.... sorry, but I find the thought of dead/killed processors in a product a bit unsettling. Sympathies.)

> We'd like to find out what killed some processors on our boards.

Exceeding the maximal ratings of the device.

Most probably through inadequate power supply / ground and external connections causing potential differences.

> A microarchitecture picture would probably help us to figure out which pin(s) need more protection.

I think ST published the general protection circuitry of GPIO inputs.

This ist most probably the weak point in your design.

You are doing motor control, or drive other large inductivities ?

Shorts are normally a sign that over-voltage was applied. Been there, done that ...

Most of the boards are fine. There are just a couple that are shorted. No inductive loads are driven.

Not consumer product, but a part in a biggish vehicle. Can't tell much more due to NDA.

And yes, battery operated, except the battery is 24 Volt. :D

Circumstances of the damage are unknown.

You've probably seen this already, if so, others might find it interesting/relevant (AN2689 Application note - Protection of automotive electronics from electrical hazards, guidelines for design and component selection) https://www.st.com/resource/en/application_note/cd00181783-protection-of-automotive-electronics-from-electrical-hazards-guidelines-for-design-and-component-selection-stmicroelectronics.pdf

The thing is trying to find out which pin(s) (still) have inadequate protection.

 The chips were studied applying power to the board with current limit, and using thermal camera to see how the processors warm up. It could be seen that they started to get heated at one edge. The question is, what's there.

BTW, I think the ESD protection in our board mostly uses ESDA6-devices, that seem to be mentioned in the linked document as well.

The 24V power lines are a different thing.