From my practice there are simple measures to avoid failures.
Do not place these boards in the closed case, setup the fan to cool the bottom layer of PCB.
Do not allow the local overheat of any component like DC/DC or external power connector.
Turn on the overtemperature shutdown in the bitstream, and control the temperature in the mining software. It is easy.
Then the FPGA board will work for years.
Turn on overtemp shutdown in the bitstream
AFAIK theres no such magic button. If you want temp monitoring and thermal limits with an FPGA you have to include them in your design logic. Same as an ASIC. Otherwise you very much can exceed junction temps and damage the hardware if you have enough power going into the board to begin with.
This is also possible on some GPUs with poor drivers. A few of my old Titan Blacks from HPC work had power stages for memory that would overheat if the memory was trashed too much for too long, and the mosfet would slip down on the boards till they caused a short. Actually caught a entire server on fire that way once...
False. The better fpgas(like virtex 5/6, complete series 7 and newer xilinx) has pin terminals to an separated internal die temperature sensor transistor. A properly designed board should use it for temperature measurement and shutdown. It works even when FPGA isn't configured. An alternative is using jtag to read XADC (7 series and newer).