Steve-

I HATE those little buggers!  Let's face it, these things are BGA's
that the manufacturer saved a penny on by not mounting the balls.  They
are almost universally finished with that readily oxidized immersion
tin.  I still do a lot of PbSn processing, and its common to have
problems with wetting or voiding in the joints.  I don't think the lower
temperatures are doing a good job of getting rid of the oxide.

Anyway, if reflowing again seems to help, it's because the original
reflow had too little TAL (time above liquidus for wetting) and/or too
little fluxing action.  Re-fluxing and re-flowing often takes care of
it.

Good Luck!

Wayne Thayer

>>> [log in to unmask] 8/31/2006 2:58:00 pm >>>
Good Afternoon my fellow esteemed TechNet-ologists,

We're building a little double sided SMT board that has a National
Semi
6-pin LLP (Leadless Leadframe Package) device on it.
We call this little board a "Leaf" board because of it's shape:
http://stevezeva.homestead.com/files/LLP.jpg

The problem on this board is the LLP device. We've gotten quite a few
boards back from test stating that the board failed
test and that U4 needed to be re-reflowed.

This is not the first time I've heard this prognosis from a test
department to correct a problem with a board. Almost every time I've
looked
into this issue, I've not seen a problem with the solder joint, or a
problem with the reflow profile, or anything else that you would
naturally look at to try and explain why re-reflowing would correct
the
problem. Not to mention the fact that the parts that are
right next to the questionable device are fine with no problems
whatsoever. Yet when you re-reflow the part it will usually pass
test the second time around.

I've had this happen with QFP's, SOIC's, PLCC's, and now this LLP.
Obviously it must have something to do with the solder
joint, but what is it man?!?! I've scratched my head about this more
than a few times.

What does re-reflowing a perfectly good looking solder joint cause a
board to pass test where it didn't pass before?

-Steve Gregory-

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