Many defence systems rely on electronic parts that are no longer produced by the original manufacturer. That reliance is based, in part, on the long life cycles of these systems.
A UK defence company was buying BGAs via a distributor. Their requirement was a SnPb finish (leaded) and the BGAs that were purchased were Pb free.
Removing the lead free balls and replacing them with SnPb was an accepted process, but one which affected reliability via multiple reflows.
Between us we managed to secure their allocation before the BGAs had balls placed on them, and they were subsequently finished on the Retronix BGA reballing laser machine. By adopting the Retronix laser process we improved the reliability compared to the old process.