You should strive for a level of less than 100ppm oxygen, but it depends
on your reflow oven. Not all ovens are created equally. If your oven was
truly designed to be a nitro oven, it should be able to maintain this
level within the reflow chamber. It is important that you measure the
oxygen level right at the point in the oven that reflow takes place. If
your oven does not or cannot get to 75-100ppm efficiently without your
going through millions of gallons of N, don't bother. The cost of the
nitrogen use is going to be far higher than the cost of touching up 2
more DPMO. Achieving 100ppm does not mean you will reap immense
reductions in solder defects; getting your suppliers to provide
solderable parts and managing the shelf life of those parts is a much
more beneficial pursuit. Focus your efforts on that.
I have seen factory operations where the oven's best performance was
around 300-400ppm oxygen, but they had 5-6 sigma yields, and the few
defects they had were usually tied to "incidents" rather than general
solder-type defects.

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Leland Woodall
Sent: Monday, May 21, 2007 3:21 PM
To: [log in to unmask]
Subject: [TN] Recommended Nitrogen Levels for Lead-free Reflow

Everyone,

I'd be interested to know what you recommend for nitrogen usage in a
lead- free reflow process utilizing fine pitch QFPs.

I've seen documentation where users were maintaining oxygen levels
everywhere from 25 to 2500 PPM.  I find that range to be quite wide.

What's your experience?

Thanks in advance,

Leland

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