White spots on PCBA after Selective Solder

Hi!

We're noticing white spots on 2 PCBAs after the selective soldering & DI Wash processes.

----Facts---- 1) The marking is on all boards, irrespective of date code 2) It only happens on leads that have been selective soldered 3) The pattern of white spots is consistent with flux spraying (the spots are around the solder joints fanning out) 4) The solder mask is a black colored LPI mask in 1 assy & transparent green on the other 5) The problem doesn't happen when using wave solder & same selective solder flux 6) The boards have already gone through an SMT placement-oven-wash twice (top & bottom) by the time they reach selective solder 7) Selective solder performance is fantastic with other PCBAs 8) Both assys are lead-free/RoHS compliant products

----Our troubleshooting steps (despite which the problem persists)---- 1) Varied selective solder temperature, with & without top pre-heater 2) Changed the flux

----Current stop gap solution---- Cleaning with flux (surprisingly) and a subsequent DI wash process seems to make the white spots go away.

We're hunting for the cause and fix it so that we don't spend extra time trying to hand clean the spots.

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