I don't think it's a matter of flexibility but rather rigidity. One of the products I work on has a BGA with over 1400 balls at a very fine pitch (smt done at c

I don't think it's a matter of flexibility but rather rigidity. One of the products I work on has a BGA with over 1400 balls at a very fine pitch (smt done at contract manufacturer with serious machines, of course). The really high ball-count devices always go on thick multilayer PCB's (ours has 32 layers). Mechanical flexing certainly can break connections but that requires a significant force that "should never" been seen in the field. BTW, one way to troubleshoot for already existing poor connections on a BGA (other than using boundary scan) is to blast it with cold spray or push down on it while running a test. If it stops working or suddenly starts working, you know you have a problem. The cold-spray/heat-gun/press-down troubleshooting techniques won't hurt good connections. These things can be a pain in the ass, but a properly soldered BGA on an appropriate board with appropriate heat sinks and fixturing is very reliable-- they're everywhere.