Talk to xilinx. They test the cells before shipping, although it is
possible a fault might have made it out undetected, in which case they
will be VERY interested in knowing what the fault is as well as how to
detect it. It is extremely rare for a cell to go bad later, but again
there is a possibility. Generally speaking, if the FPGA does have a
hard failure, it is usually going to occur on an I/O that got
overstressed (often by ESD). What are the symptoms that lead you to
believe it is a hardware fault?
--Ray Andraka, P.E.
President, the Andraka Consulting Group, Inc.
401/884-7930 Fax 401/884-7950
"They that give up essential liberty to obtain a little
temporary safety deserve neither liberty nor safety."
-Benjamin Franklin, 1759