nRF52832 exposed part of datasheet exposed die pad What is wrong with not connecting GND to this part?
Connecting the center pad to ground is mandatory as it's to be considered as the main ground connection of the IC.
What happens if I do not connect?
Mostly likely, the radio will be out of spec or may not even work.
Is there anything related to this 'NRF_FAULT_ID_SDK_ASSERT' error in that part?
Hard to say. There is some info in the ASSERT_INFO_T handler that will tell you why the assert happened.
In any case, without grounding the center pad, anything can happen. If have a design that doesn't have a grounded center pad, you have to fix it.
