You failed to mention if you bothered to profile the boards thru wave
with a mole. This should have been your first step as the owner of the
wave soldering process. I'm not sure what solder you were using, but
the industry standard (Sn63/37) is eutetic and once the surfaces reach
361F - the leads reflow. There is no "plastic state" for this
material.
Most likely, the QFP's reflowed over the wave. Any post reflow touch
done by the operators at the end of the SMT line on these QFP's where
leads were "tweeked" to better place them on the pads was undone. The
leads pop back to the position of least stresses as the board crosses
and exits the wave.
Note: I find most of my secondary reflow occurs a few inches after the
board exits the wave. This is the point where the board has soaked
enough heat from the solder pot and the top surface of the board now
exceeds reflow temp of the solder.
SOLUTIONS: COOLER POT TEMPS, FASTER SOLDERING SPEEDS, PLUGGED VIA'S -
SO NO THERMAL TRANSFER OCCURS AT THE QFP LEADS.
RAV
----------
From: SteveZeva
To: [log in to unmask]
Subject: Re: [TN] Secondary reflow during wave
Date: Friday, March 20, 1998 6:24PM
Jim,
In the past I have had some problems with the solder joints on
QFP's
when
the temps start getting that high. It was really baffling at the time
that
we
first noticed something was wrong. What was occuring was that these
boards
were failing when they went into ICT. Up until that point everything
looked
fine, and we looked at them too...we had a pretty decent SPC program
going
at
the time. We looked closely at the joints that ICT caught (they always
were
on
QFP's), and there indeed were open joints.
But what was weird with these opens, was that the fillet was
completely
and cleanly separated from the pad. It was still attached to the foot of
the
lead, and the shape of the fillet indicated that there was good wetting
taking
place during initial reflow on both the lead and the pad...you could
tell
that
by the angles of the fillet that had formed on the sides, heel, and toe.
So then we started looking VERY, VERY closely after SMT reflow and
no
problems were seen, and we did the same after wave solder, again no
problems.
But test was still getting boards that were failing for the exactly the
same
thing. I went back and double-checked my reflow profile, and dragged a
mole
behind the board while it was going through wave. I had the
thermocouples
at
one of the QFP's that was failing. I saw just what you've been seeing,
temps
up there pretty high...not liquidous yet, but not too far away from
it...maybe
between 165-175.
When the boards came out of the wave, I looked at the joints again,
they
looked great...about that time my ulcers started getting a little
bothersome.
In the meantime, test was still failing boards for the same thing...they
even
failed one that I had hooked the mole to and looked at after it came out
of
wave. I knew that was one of the boards I looked at because I had marked
the
edge of the board with a "sharpie"...my ulcers were bleeding big time
now.
I was down in test drinking some Malox after looking at one of the
latest
failures, and I noticed something that the test tech was doing. After he
would
place a board in the fixture, he would take a sheet of conductive rubber
and
drape it over the assembly, and then start the test. (The sheet of
rubber
is a
common thing I've seen used with a single-sided fixture to help vacuum
the
board down to the pins...) But sometimes he wouldn't get contact on all
the
pins, and then I saw him pushing all over on top of the board to help
make
pin
contact...THAT WAS IT! He was flexing the board and that's what was
causing
the fillets to separate from the pads! But I still had a problem, we
discovered by tracking each failure that it was only happening on a few
locations...there were other QFP's on the board that didn't have one
single
failure...so what's up with that? I was wondering...
Turns out that the QFP's that had been failing were the ones that
had
via's right at the ends of each pad...there may have been a little bit
of
trace between the via and the pad, but not much. Heat transfer was
causing
the
joint to start to get in the plastic range, and somehow weaken the bond
at
the
lead and pad interface...not enough to be able to see anything, but
enough
to
cause it to easily separate if there was any stress put on it at all.
What
we
wound-up doing to fix things was to use temporary mask to cover the
via's
until they could redo the gerbers to tent them on the bottom...which by
the
way, is the best side to tent vias on, don't tent the topside!!
Sorry this was so long...I can go
on
sometimes, can't I?
-Steve
Gregory-
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