Couldn't you take the BGA and the heatsink loose and perform this test? If
you could think of a manner in which to hold the BGA when trying to pull the
heat sink off then you wouldn't have to play with a soldered BGA. Word of
warning about the load cell of the torque tester, I had done some testing where
we were testing the sheer strength of SMT adhesives and used a torque
tester. We blew the load cell of the tester by over stressing the tester
beyond the max limits. It turned out to be an expensive sheer test.
Kathy