| Howdy Richard, | Welcome to the club! I now dub thee a full fledged member of the "Spotty Gold Finger and Bleeding Ulcer Society"...or SGFBUS for short... (GRIN) | I know it's not funny to be in that position, but there's so many different places to pick-up those pesky spots that you gotta just sit back and grin once in a while about it or you'll go coo-coo... | I think that most of us have been in this business long enough to have had it happen to us at least a few times. | Just when you think you figured out where it was coming from and been watching the area like a hawk over the past week, all the while you're patting yourself on the back for being such a genius, Final QA comes to you and says; "You need to come take a look at some boards we have in final QA, they've got spots on the gold fingers..." It's about then you grab your ol' trusty bottle of Maalox and take a healthy swig... | Like I said, it could be happening anywhere, but I'll give you one of the more obscure places that I found where it was happening... | I worked at a memory company where we used DEK printers. We had all kinds of different boards that had gold fingers, but when I was working there, only a couple of assemblies would seem to have the problem. They were some new Level-2 Pipeline burst Cache modules that we started building. | I went through the same drill as you, dinking with the profiles, inspecting the fabs before they hit the floor, marking certain boards and following them thru the process, inspecting them at each step to see if I could learn where it was happening. | I think it's one of Murphy's laws that the harder you look for something, the less likely you are to actually see it...at least it would seem that way to me sometimes. Because when I tried to find it, I couldn't. But as soon as I turned my back, here they were again. | I finally figured out where it was coming from, and why it was happening only on the L-2 Cache modules. It was happening in the screen printer. The culprit that was causing it was the automatic stencil cleaner in the printer. | Dek has the same kind of "butt-wiper" toilet paper roll cleaner that a lot of other printers have, and what was happening was that whenever the stencil cleaner ran out of solvent, or the roll of paper would jam up when it tried to index to a clean area of paper during a cleaning cycle, would cause the solder paste to just be smeared on the bottom of the stencil instead of being removed. | The pattern image for the level-2 Cache stencil was positioned differently on the foil than all of our other modules because of the panel shape and size, and the way we going to run it in the rest of the line. It had the gold fingers pointing towards your belly if you were standing in front of the printer with the board inside the printer. | So what was happening that when the paste got smeared on the bottom of the stencil because of what I talked about earlier, the first few boards would get a few tiny solder particles on the fingers that were transferring from underside of the stencil...and once they hit reflow, that little, teeny, tiny solder particle liquifies and spreads itself out on the fingers, sticking out like a sore thumb when you look at it. | So after I figured out what was happening, I rotated the image in the stencil 90-degrees and did the same for the pick and place programs. | VIOLA'!! No more solder spots! | That's just ONE place that it can happen...and I know there's a bunch more places, and probably a bunch as well that I don't have a clue about. | Maybe this will help you, if it doesn't, I got some extra coupons that I cut out of the Sunday paper this weekend for some Maalox...it's 39-cents off for the quart sized bottles... | I'll send ya a few if ya want...(GRIN) | -Steve Gregory- We had a similar problem. But whenever the solder came from the screen printer the solder would wet to the gold fingers, with the help of residual flux. But the problem is balls, that I assume are not wetted to the gold fingers but instead are just stuck on them. We had a case where we had Tin/lead plated vias plugged both top and bottom with solder mask, well what happened was pressure would build in the vias and it would blow out with little balls, and sometimes big ones. Look at the vias and see if you notice any balls in them. Just another place to check. Mike C
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