STM32 overheating when SMBUS device connected

[jdolins2]

I am using a STM32L4R9ZGJ6 that has the following issue: When it is powered up and a SMBUS slave device is connected, the CLK line over heats and destroys the processor. 

This is the device we are plugging into the board: 

This is the STM32 with the CLK and DATA lines shown on PB6 and PB7:

What could be happening that causes the processor to fail? Immediately upon plugin the STM32 gets red hot and burns. 

[TDK]

Do the processors share a ground? If not, common mode is probably killing you. Measure voltage between ground of one and ground of the other.

[jdolins2]

Separate grounds that are connected via this 0805 short:

[Tesla DeLorean]

>>What could be happening that causes the processor to fail?

You've created a conduction path through the MCU, and an excessive amount of current is flowing.

Like you have a board with significant power differentials, and the grounding between them is not working, or presents more resistance than the path it's found here.

Are these parts on the same board?

What voltages involved?

If you remove the STM32 from the circuit, what current flows between this pin, and ground or the supply?

Double check the netlists for the board, make sure you've not created multiple separate islands that should have been connected, and weren't or were miss-named.

[TDK]

This is within a board. I am talking about the ground between one board and the other.

[jdolins2]

Yes the grounds are connected between the boards

[jdolins2]

One is a main board for the product, the other is a battery management IC. They connect via a simple molex connector. The pin on the main board is pulled up to 3.3V with a 10k, the other board is a slave device tolerant to much higher voltage than we are using. I'm unable to prototype removing the STM at this time but I'll check on the other things, thank you