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White Hazing under the solder mask of a through hole board

Dallas Electronics Inc.


White Hazing under the solder mask of a through hole board | 7 November, 2003

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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White Hazing under the solder mask of a through hole board | 8 November, 2003

We saw this quite a bit, I want to say 5 or 6 years ago, but it's probably more like 10 years ago. It's caused by poor solder mask crue. Talk to your board fabricator about solutions.

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White Hazing under the solder mask of a through hole board | 8 November, 2003


I have seen this problem before. It is caused by improper curing of solder mask by the fab manufacturer. When you run the board thru' wave solder, and then wash the board, you see the problem of white residue. However, when you apply heat, you can reduce them. Sometimes these are like white spots, often giving the impression of contamination. Good thing is apart from cosmetics, electrically it has no effect.

Problem is certainly not assembly but that of fab house. But getting them to admit is another issue. You may be able to obtain some studies about it from the manufacturer of the solder mask. One way to avoid this problem partially, is to bake the board yourself before loading, thus speeding up the curing action. The solder mask is epoxy based compound, and improper mixture or handling can cause those spots or blemishes.

If board house do not want to take responsiblity of the poor process, get another.

Good luck!!!

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