We are using "XC7Z020-2CLG484I" in our design. Please provide detail about "Failure rate / MTBF" for the same.
The system is Ground Benign(GB) and ambient temperature is 25 degree C.
UG116 has the reliability information and details.
For such a benign environment you state, the device will only (eventually) fail from intrinsic reliability concerns many (many) more than 25 years in service.
The hard fail rate from our reliability program is 11 FIT, which is greater than 10,000 years (per ug116).
For actual returned failure data, please contact your local Xilinx sales office.