What I'm seeing is an occasional pattern of tear drop shaped features in a sea of nice shiny solder. The little islands don't really have any relief.....it's more of a surface finish feature than a 3-dimensional feature. The wetting angles are good, and there's nothing wrong with the profile (according to the paste specs.) It's not prevalent (a few components per board, and not every time we run them), and it's not obvious (I can only see it at 10X and above), so I haven't worried about it too much. Still, I'd like to know what it is and address it.
My hunch is that it's flux related, and that I'm occasionally not getting enough of it with reduced, homeplated apertures (it doesn't seem to show up on boards using 1:1 aps. in the stencil, although I just recently started looking at that). Before I go off crankin' up a DOE like a headless chicken, what do you all think?
Dr. Ning Cheng Lee from Indium Corp. of America has written papers on this phenomenon.Perhaps they would be willing to assist you.
I am pretty sure I know exactly what you are talking about. We see this as well.Does it kinda resemble a disturbed solder joint? Years ago we had Indium do an analysis on this. Its not flux related. It has something to do with component/pad metalization.
Again, this sounds like what we have seen in the past but to be sure I would recommend contacting Indium.They have an excellent applications team.
I've used ipa (actually "70/30 rubbing" alcohol) to clean these for a better view (this is a no-clean), and found the above described anomaly to be intact. So, I assumed it was something in the solder itself. Well, today I used some 100% ethyl acohol and ended up with nice glossy fillets, no anomaly present where there had been one before. The feature is clearly not only flux related, but is part of the residue and not visibly affecting the joint at all.
I discovered this on a board that had been misprinted and wiped, probably with 70/30 IPA, apparently not washed (BAD operator....go to your rug!), and reprocessed and subsquently scrapped for numerous loose solder fines. Coincidence? I wouldn't bet on it, and I'll sure be looking for it again.