Unused pins connected to ground. Could they cause short?

[VVest.1]

I just had my first STM32 pcb done and I've been following the reference designs and checked many times everything that there is no shorts etc. However when I power the circuitry 3.3V side draws >200mA and the STM32L4S5ZIT6 does heat up when looked through thermal camera. I did connect all unused pins directly to ground. Could this be the culprint that immediately when powered it is like shorted? It makes no sense that these pins would be output high default when power on. I'm not even able to program the chip. And yes the chip is not 180 degrees positioned wrong.

AN4555 reference design says for some reason PC3 pin (29) to connect to 1u capacitor. I used this pin as ADC to sense battery voltage. I'm not finding any explanation why this pin was connected to capacitor. Could this be the culprint?

[VVest.1]

I already soldered the 3.3V regulator back so it is the other one heating here (lower right corner) due to the huge power the UC is draining. Like you can see the middle of the UC heats up and it heats up quickly. Sorry for the crappy image, my thermal camera just didn't want to communicate with pc so took a picture of the screen..

[Tomas SIRUCEK]

Unused pins should never be connected directly to power rail, because microcontroller can be destroyed, when activating some of these pins in output mode of opposite logic level. This can happen also in embedded bootloader, which usually boots, when the main memory is empty.

From AN4555: To increase EMC performance and avoid extra power consumption, the unused features of the device should be disabled and disconnected from the clock tree. The unused clock source should be disabled and the unused I/Os should not be left floating. The unused I/O pins should be configured as analog input by software; they should also be connected to a fixed logic level 0 or 1 by an external or internal pull-up or pull-down or configured as output mode using software.

Best regards,

ST MCU Support Team

[VVest.1]

I redesigned the pcb, left the unused pins floating and now it works. So the issue was that the empty MCU pulls pins high when powered. That in my opinion is wrong and shouldn't happen. Anyhow, I've learned my lessons and from my part this case is closed.