Fortunately, I rarely risk anything if the board fails. And if it does I always try to understand what caused the failure. I never considered flux compatibilit

Fortunately, I rarely risk anything if the board fails. And if it does I always try to understand what caused the failure. I never considered flux compatibility between my solder rosin core and the flux paste and never considered measuring salt residues on the board. I almost exclusively use the same Kester sn64pb37 rosin core solder and MG Chemicals #8342 RA rosin flux paste. Both must be cleaned promptly. Now, I don't bother cleaning if I don't intend to use the board after I test it (like when I do prototyping) but anything else gets a thorough wash. I use various PCB cleaners, typically ethanol (not IPA!) based and then rinse boards in electronic water. I never saw any oxidation on any of the traces unless there was some external contamination.