STM32 Primer2 break down, even if it is workarounded with ceramic capacitor and Transil diode.

I inspected broken "STM32 Primer2". The C23 and C56 are reworked. The C23 is ceramic capacitor with Transil diode. I guess the diode is SM2T3V3A from marking. The C56 is also ceramic capacitor.
Picture: The C23 with Transil diode. The Top-Left corner of the picture is zoomed view of C23.

Picture: The C56.

My Primer2 PCB has all workaround reworks described in Known problems. But "Primer2" was broken.

I wonder the cause of breaking down Primer2. When the L6928D shutting down, the followings are happen.

The low side MOS transistor in L6928D turns off.
The inductor current still remain and rush into L6928D body didoe.

If the body didoe has not enough current rating, the L6928D will be breakdown.
To resolve this problem, try insert free wheel diode GND(anode) to LX(cathode).

Go related blog entry in Japanese.