Simplified explanation:
If you store a metal in air it will oxidise slowly.
If you heat a metal in air it will oxidise rapidly.

If you heat a metal in flux it will be de-oxidised.
If you heat a flux it will be dissipated.
IF the flux is dissipated before it has de-oxidised you need nitrogen.

In addition at the flux/component/air interface, the flux will be wicking
and deoxidising just ahead of the spreading wetting solder, but meeting an
increasing oxide barrier.
In nitrogen the oxide barrier does not increase so wetting is faster. There
are also some surface tension effects and less flux degradation in nitrogen
which amplify this.

So the effect of nitrogen is basically to decrease the oxide removal burden
on the flux and to increase wetting speed. A bit like increasing the flux
activity in other words.

However if your flux is already  virtually 100 % efficient (as determined by
say first time pass rates) clearly the incremental benefit of nitrogen is
going to be small to say the least, and the cost hard to justify.

If your flux is a very low residue type intended say to leave as little
residue as if it had been cleaned, then you need nitrogen. How many ppm
oxygen can you stand? Dunno, but your supply has suggested 180 ppm. What you
should do is "profile" your process with an oxygen meter, to determine how
many ppm oxygen you can go to before your rework reaches an undesirable
level. Clearly - unless your reflow oven has perfect containment - this has
to be less than the ppm in the nitrogen entering the system. At 5000ppm
against a guideline requirement of <200 ppm this is not the case.

Then you need to understand why you are using a low residue material
requiring nitrogen, and do some sums to work out the cost benefits of your
process compared to alternate methods of achieving same results and ....

good luck! Hope this helps.


Best regards

Mike Fenner
Applications Engineer, European Operations
Indium Corporation
 T: + 44 1908 580 400
M: + 44 7810 526 317
 F: + 44 1908 580 411
 E: [log in to unmask]
W: www.indium.com
Leadfree: http://Pb-Free.com



-----Original Message-----
From: TechNet [mailto:[log in to unmask]]On Behalf Of Shoda, Steve
Sent: Friday, August 31, 2001 4:51 PM
To: [log in to unmask]
Subject: [TN] Nitrogen Purity for SMT IR Reflow


Our facility is considering changing from stored liquid nitrogen at 99.9999%
purity to onsite generated at 99.5% purity (5000ppm impure).  Our reflow
manufacturer recommends a maximum of 180ppm oxygen in the nitrogen
environment.   What problems could occur if the 180ppm level is breached?
What maximum impurity levels have been observed without any adverse effects?

Thanks in advance for your inputs.

Steve Shoda
BAE SYSTEMS Controls
Fort Wayne, IN

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