Try having the FAB house repress PCBS using a controlled (slower) cool down.
This will commonly flatten even the worst warp for pre heat operations (
stenciling and pick-and-place). Reflow or wave will most likely cause the
warp to return so be careful with post operation cool down ( the warp
reappears during ramp-up of temp for adhesion process, so do not try to
unwarp assembled PWB).

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