We have been experiencing intermittent problems with the solder masking bubbling up from the bottom side of the board, after wave soldering. Our board fabricator claims that none of his other customers are experiencing this problem, and that it is probably due to excessive preheat temperature. I have put in a call to the manufacturer of the solder mask they use, in order to get a spec sheet on the recommended temperature range. (All copper is bare prior to solder masking - so having tin plating under the solder mask is not the culprit.) I have also put in a call to the wave solder machine manufacturer for help, but haven't heard back yet.
We have a two stage preheater; Zone 1 is set for 769 F and Zone 2 is at 360 F (measured at the thermocouples near ceramic glass top that protects quartz-heater tubes). With temperature dots we measured the board top surface, after passing through the preheater, to be between 210 - 220 F. The solder pot temperature is at 500 F.
Question: Do these preheater settings seem so far off as to cause intermittent solder mask problems to the bottom side of boards?
We use a fixture with adjustable rails and fingers during the wave soldering process. It has been noted that a majority of the problems with bubbling/peeling solder mask occurs near the edges, of the bottom side, of the board where the fingers and rails grip the board. One thought was that leftover flux residue on the fingers/rails might be reacting with the solder mask and causing an adverse reaction during the heating process. Of course, when we last used a brand new fixture, we still noticed the problem (even in areas not touched by the fixture).Thus, we are bringing in the rep for the flux manufacturer for suggestions/solutions, as well. We drain/clean the flux pot once a week, but I am not sure if we maintain the proper flux density, upon refilling.
As you can see, we are trying to tackle this problem from all angles. Unfortunately, we are not experts in wave soldering.
Any assistance would be greatly appreciated.