Richi,
From what I read in your post, I'm assuming that you're assembling these
boards, not the manufacturer of the bare fab.
You said that you've built these boards before without any problems huh?
Well, then go back to the way you were building them before!!! (GRIN) Just
joshin' ya! Lemme ask ya' this, did you just get a new stencil for this board
recently? If you did, is the image oriented the same way as it was before? The
reason I'm asking, is because something like this has happened to me before.
I worked for a large memory company and we were having some of the same
problems. All of the boards we built were in panels, and when we started
building a newly designed cache module, we started seeing solder spots on the
fingers. Same deal as you, we built thousands and thousands of SIMM's with
gold fingers before without having this issue, and couldn't figure out where
the solder spots were coming from. After what seemed like forever, I finally
found out where the solder was coming from.
The new cache module was in a 22-up panel, and because of the size and
shape of the panel it was oriented differently than the rest of our panels
when running in our SMT assembly line. We used DEK-265 screen printers with
the automatic "toilet-paper roll" style of underside stencil cleaner...most of
ya'll have seen em' before haven't ya'?
Because of the new orientation, when the board was sitting in the
printer, the fingers were running in a north/south orientation instead of a
east/west orientation like all our other panels did. "What in the heck does
that got to do with anything ya' bonehead?" you maybe asking yourself about
now. What was happening was that the operators would sometimes let the solvent
tank for the stencil cleaner go dry, or sometimes the paper in the cleaner
system would jam and not index correctly when the printer went through one of
it's pre-programed cleaning cycles. In either case, all it was doing was sorta
dragging and smearing the paste that was around the stencil apertures on the
bottomside, right to where the gold fingers were. So the next few boards that
would get printed after the printer went thru a cleaning cycle would also get
a little of the paste on the gold fingers....not enough to notice before
reflow, but after it had a chance to melt and spread it was noticable for sure
then.
I hope
this might help...
-Steve
Gregory-
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