Steve and all,

We run but two profiles on all our assemblies. We have nine
different board types/classes (SMT 1 through SMT 9 with obvious
variations of surface, through hole, and whatever else) and
run up to three reflow passes to effect soldering.

I've only seen our big bad BTU's sneeze once at an assembly
with so much thermal mass it lowered the final zone temperature
by 5 degrees and set off the alarm. No problem, we created one
more profile, based on the first two (air or nitrogen), by jacking
up the temp a few degrees in liquidous.

One profile is relatively classic and one uses the more modern
"tent" or straight up profile. The time in 183 does not exceed 100
seconds and between 205 C. does not exceed 45 seconds with a peak of
220. These are paste recommended profiles and work very well.

When we see voiding in X-Ray (done every new prototype), we randomly
X-Section. If evedince of retained flux in void pockets, we analyze
to determine content whether at the joint surfaces or centered. If
excessive, we analyze solder termination areas for contamination or
oxidation. If found we correct, though not often the case.

All this goes back to my very first findings with J-Leaded and Gull
wing device contamination and the voiding and "cracking" often found
in the early SMT days as transition was being made from tin/lead
plated/reflowed leads to those tin immersed. Excessive carboxcylic
acid entrappment often was found in the void pockets or oozing out
of cracks as the flux remained, no matter the profile or its length/temp.
It remained attempting to remove the oxidation or other contamination.

I am not trying to draw direct correlation with past findings. However,
it is reasonable to belive something similar happens in BGA's today -
though possibly on a different level as we find solder termination area
contamination less a factor with our improved supplier base.

Still, I have concerns in this area more for what's inside the void than
its size or location. No matter, I/we still hold up the 20% maximum
void size by volume as a primary criteria for concern even though we
have found a certain amount of voiding to be beneficial to solder joint
reliability.

I would also note, we have found little reliability difference between
solder pasted BGA reflowed parts and those reworked using not paste even
though we seldom practice this. Without paste, obviously, no voids.

Something to consider?

Earl Moon

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