Support and heat
Support the whole part of the board you are working on. Use preheat for mass, keep the nozzle local and short, and stay below the coverlay limits.
Flux and cleaning
Use a flux that suits the alloy and clean it off; residue on flex is harder to remove and can affect coverlay adhesion.
Inspect after rework
Check the joint, the coverlay edge and the bend area under magnification. If the flex will be bent in service, a bend test on a sample is a better indicator than a visual check. When the same joint keeps failing at the coverlay edge, the coverlay definition rather than the operator is usually the cause — a conversation to have with the flexible pcb manufacturer before the next build.
