Hi Carl!

Without some pictures, it's tough to make a call...pinholes and solderballs
in SMT joints are pretty hard to get, initially sounds like things didn't get
hot enough. BUT, I had a deal one time that had me baffled...

Q.C. inspectors called me to look at boards that they said had cold solder
joints...solderballs all over the fillets. These were SIMM memory modules and
were occuring on SOJ DRAM parts. Looking at them, it was really strange that
there would be one or two leads on the part that looked cold, but the other
leads looked fine.

From my understanding, convection doesn't work like that, if the temperature
was good for one pin on a part, it should be good for the pin next to it that
was only .050" away...

What it turned out to be was that the operator was trying to do a good job
keeping the stencil clean between prints (it was a MPM AP-20, no
auto-cleaning), and he was using a water moistened wipe to clean the bottom
of the stencil periodically between prints (we were using water soluble
paste). The wipe was so wet that water would remain in the apertures in the
stencil after he cleaned it, the water would alter the flux chemistry on the
subsequent print after the cleaning, it would look like crap after reflow...

Just a W.A.G. on my part...but you might look to see if there's any cleaning
fluids being left on the stencil after in-process cleaning...

-Steve Gregory-

<< Technetters,

 Need some help from all the soldering experts out there. We have a group of
 boards that show evidence of poor solder quality throughout the board. To
 the novice eye, it looks like the solder paste did not reflow completely.
 There appears to be small balls of unreflowed solder and small pin holes in
 some of the joints. Most of the quality group thinks that the profile was
 not hot enough. However, when the engineering group came to examine the
 joints, they stated, "Why No, these joints were just the opposite, they were
 liquidous too long!" Can I get some opinions on this? Any one out there with
 examples of solder joints that reflect these two scenarios would be
 appreciated. Please send them directly to me. We are having these joints
 cross sectioned, once the pictures come in I will forward to anyone
 interested. Thanks in advance of any help anyone can give.

 Carl Barnes
 Production Process Analyst >>

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