Hi Dave!
You aren't getting much of a response - all the N2 junkies out there
must be cryo-heads...
We currently use nitrogen in our wave solder, some times in our reflow
process, and in a cabinet for parts storage.
I did look into switching from cryogenic to on-site generation, and l
am guessing you came up against the same question and lack of info that
I did - how pure do you really need.
For the purposes of my analysis I decided that I needed 99.8% pure
nitrogen, but it wasn't based on serious science. At the time (~5 years
ago) the leap above 99.8% was very expensive, that may have changed, but
at that point there was no way a cost justification would have worked
for higher purity. At that point in time the cost difference between
99.5% and 99.8% was not much.
Would 99.8% have sufficed? I believe so. We are not running a tight
process window here, as some would with no-clean flux. Our wave solder
pot area is very open, so high purity nitrogen in that area is pretty
much wasted. With respect to the reflow oven we do not normally use it
- so not a big impact. We are running RMA flux, and we find that the
incidence of tombstoning is much higher when we run the oven with a
nitrogen atmosphere. We do run with N2 on a couple of boards where we
know we have parts with marginal solderability, the N2 will widen our
process window. Do not ask me why we are using marginally solderable
parts, that's a 35 minute rant encompassing things like deals with the
devil, and we won't get into that here.
As for the third use, the main requirement is that the supplied gas be
dry, so again the purity is not a big issue.
The final question - why am I talking about this in the negative?
Well, the cost justification looked good until the plant services folks
pointed out that the amount of compressed air required would mean they
would have to buy another (big) compressor, The installation, running,
and maintenance costs associated with that bumped my payback period well
above that magic 1 year. We were running low volumes at that point, so
our cost justification might look better now, but I don't have time to
revisit that at the moment.
Final point - the only thing that can easily go wrong with a cryogenic
set-up is that the tank runs dry. With the on-site generation you will
need excellent preventive maintenance, maybe a backup compressor, etc.
In fact, on occasion here when our compressor setup has packed it in we
have run a couple of critical processes on nitrogen for short periods.
Good luck, and can you let me know what you conclude?
regards,
Graham Collins
Process Engineer,
Northrop Grumman Canada Corporation
Halifax
(902) 873-2000 ext 6215
>>> [log in to unmask] 03/09/04 02:00PM >>>
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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