Hi Grant! Just wanted to thank you and the rest of the Technet community
for your assistance. We have also run a number of DOE investigations to
understand the interaction of O2 levels and solder joint formation. Both
the internal and external data sources put the O2 level in the primary
reflow zone in the range of 100-300 ppm - pretty neat how everyone is
arriving at similar values (physics is a wonderful thing). The nitrogen
purity level is a primary driver when using on-site gas generation in
comparison to the use of cryogenic N2. The posed question/survey was a
sanity check to get one final review of our overall review of N2 options.
Thanks again for everyone's responses.

Dave



                      <[log in to unmask]
                      m>                       To:       <[log in to unmask]>, <[log in to unmask]>
                                               cc:
                      03/11/2004 02:13         Subject:  RE: [TN] Nitrogen Purity Question/Survey
                      PM







Hi Dave,

At a previous employer, one the engineers did an excellent DOE to
determine the actual threshold of Nitrogen level necessary to
significantly reduce BGA voids.  The number came to 250 ppm of O2 in the
oven as the "bare minimum" to make it worth it.  Our process then became
one of actually measuring the Nitrogen level at change-over and
waiting/verify that the O2 was below 250 ppm.  We would actually, get
down around 25-50 but we would have to turn off the supplemental cooling
fans on the exit conveyor because the air would reflect off an exiting
board and shoot into the oven.  Otherwise the O2 level would shoot up to
150 ppm every time a board would exit.  I did find that if the fans were
sharply angled away from the oven opening, we could keep it from
contaminating the nitrogen levels.  Worth considering since large dense
boards would generally be too hot to handle right out of the oven, even
with the supplemental cooling fans.

Although many other engineers would use Nitrogen to improve the
appearance of the solder joints, the only reason for dropping O2 levels
to 250 ppm was to control BGA voids.  If voids are already under
control, or you can switch solder paste formulations to control voids,
then the low O2 ppms would not be necessary.

Ryan Grant

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Dave Hillman
Sent: Tuesday, March 09, 2004 11:00 AM
To: [log in to unmask]
Subject: [TN] Nitrogen Purity Question/Survey


Hi folks! Question for those folks using nitrogen for an inert
atmosphere
in their reflow processes or as a blanket gas on their wave solder
processes. Cryogenic nitrogen is typically 99.998% purity (in terms of
O2
content). On-site nitrogen generation equipment can deliver N2 gas
ranging
from 99.5% to 99.99% purity (again, in terms of O2 content). What gas
purity level are you using in your processes and do you have a rationale
for selecting that purity level?  The reason for the question is that I
am
involved in an exercise of comparing on-site nitrogen generation versus
cryogenic nitrogen supplies.

Thanks in advance for you assistance.

Dave Hillman
Rockwell Collins
[log in to unmask]

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