I'm would like to try intrusive reflow (pin in paste, paste in hole, etc.) on a connector using WS-609 solder paste (halide free, water-washable). I'm concerned about entrapped flux in the barrel of the solder joints. If there were to be entrapment, will the flux remain active? Could there be any long-term reliability issues if it were to remain active? Changing to no-clean is not an option.
I have not done pin in paste in production quantites, but speculation is always a blast so;
OA fluxes are active at room temperature, they grow 'fur' with humidity if not washed off, great stuff for soldering, nasty stuff for reliability (if not removed).
However, we use OA fluxes in wave solder and flux 'could' be entrapped in thru hole solder joints. But we've been doing that for decades with very little concern about entrapped flux in solder joints (under components is another matter).
So why be too concerned about entrapped flux with intrusive reflow? Be sure to get hot enough, long enough to let the surface tension of solder force flux out, perhaps do some x-ray examination to look for voids.
Make sure your washer is up to snuff, do a few and get to testing them; for inonics, SIR, temperature, humidity, functional and so on, whatever will satisfy the requirements.