Curing Circuit Board Soldermask and Legend Simultaneously

[Johnny Bollocks]

Are there any circuit board manufacturers in the house?!?!?!?!?!?

Does anyone have any objective evidence showing studies of cross contamination while curing SM and Legend at the same time, in the same oven.

[AndyN]

Johnny!

Are we talking about screen printed resist and legend? It's been a while since I was in the biz, and we used both screened resist and film. I'm thinking with inks there would be some problems with bleed and hence definition of the legend as a result. What's your question based on?

I'm fairly sure there are board makers represented on the Cove, tho'

[Johnny Bollocks]

Thanks for responding.
The question basically asks if soldermask and legend ink are cured at the same time in the same oven. I have been trying to find some kind of evidence for or against, but am having a hard time. I know that we have been doing it for years without incident, but I don't trust that that would be enough objective evidence for a Nadcrap auditor.

[bobdoering]

I has been decades since I was in circuit boards, but my guess is that a capability study (perhaps from history) that shows no issues of solder adherence that can be attributed to curing legend. You might also do a worst case, place a thin haze of legend ink on the pads and see if the solder burns off the residue and provides good adhesion to the pads. That may be the evidence that it would not be an issue. Just some blue sky ideas.

What is the solvent for the legend?

[Pistonbroke]

I can't cite any specific studies, but having spent the best part of a decade working in all aspects of PCB manufacture, and assembly, I can tell you that I have personally seen issues in this area.

1) where ident (legend) has bled onto the copper pads prior to final finishing (gold, HASL etc) and prevented adequate adherence of the final solderable coating

2) Where ident has bled onto the pads where it is applied post finishing, causing a non solderable surface on the top of the final coating (though this does tend to break up on HASL).

3) where a photoimageable resist has not been developed off adequately, and has had a 'quick cure', with the intent of the full cure taking place post development of the ident.

4) Where ident being cured in an oven has seemingly deposited some sort of residue on a different batch of boards upon which solder mask is being cured simultaneously preventing decent adherence of the final solderable coating. Though this may have been related to issues with the different temperatures the ident and the solder mask SHOULD have been cured at.
This last was visually evident by varied discoloration (oxidisation) of the copper, rather than a consistent color, and was resolved by the application of a sligtly pronged micro etch prior to surface finishing.

In reality, there should be no reason why, assuming the solder mask and the ident are of a similar nature (i.e. both two pack, or both photo-imageable) and have similar cure characteristics, that they should not be cured side by side.

Another reason for this issue I have seen is that bad prints or coatings have been washed off by the photomech or screen printing guy, to a visually acceptable level, yet have left an invisible (or at least very hard to see) residue on the suface of the copper - this is normally be evidenced by traces of solder mask down the via (through) holes.

Sorry I cant cite anything but my own experience, but I hope that this helps in any case.