I have seen in the last couple of months more boards with a white hazing under the solder mask. I ran a few tests on a board to determine the cause of the white hazing underneath the solder mask. First I confirmed, or at least I think I confirmed, that the hazing was indeed coming from underneath the solder mask. I tried to clean off the surface with various solvents to see if it was by chance on the outside of the solder mask. I was unable to remove the hazing, leading me to the conclusion that it was indeed underneath. I baked one board for 1 hour at 250 F and 95% of the hazing disappeared. I heated anouther board with a heat gun and was able to eliminate the hazing. This leads me to the conclusion that whatever is causing the hazing is volatile. The hazing pattern had somewhat of a regular pattern that appeared to me to be left from a roller coater. My guess is that this was left from the pre-clean process prior to the solder mask application. The hazing possibly occurred due to poor rinsing/drying of the board prior to the solder mask being applied. This could be mineral deposits from hard water, or some chemical residues that were poorly rinsed prior to the drying process. I only see this problem on the bottom of boards which makes me think of flux, but I've seen it with water soluable and no-clean (Alpha K183 and Kester 951). I don't see it where masking has been applied and then removed. Normally if the solder mask hasen't been cured properly I can remove it with some solvent like methynol. We use de-ionized water for cleaning but no chemicals. I also can run different boards one after the other with the same flux and see it on one type of board and not anouther. I've tried cleaning with both inline and batch cleaners and see no difference. What bothers me the most is that I'm seeing this problem a lot more often than in previous years. Has anyone else seen this problem with through hole and SMT boards they have been running? Am I missing something we are doing? Randy Holland
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