It is also suggested to characterize the maximum board flex or strain made during construction, assembly, test, and use to ensure excessive board stress does not contribute to potential BLR issues.
The way to measure this is through the use of a strain gauge. Xilinx suggests keeping the stress on the board within + or – 500 µstrain. Dye and pry analysis can also be used in conjunction to strain measurement to ensure proper device contact to the pads exists after final assembly and test as further evidence of a reliable process.