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N2 vs. Air Reflow Soldering

DaveJ

#2170

N2 vs. Air Reflow Soldering | 15 December, 2000

I need some advise from the reflow experts. In your experience in nitrogen versus air reflow, what would be the differences in profile parameters? For example, if I profiled a board in nitrogen and achieved (for a particular component) ramps rates under 2C/sec, a TAL (time above liquidous)of 61sec, and peak temp of 221C, what would I expect if I reflowed the same in air? What other profile changes would there be? You comments are appreciated.

DaveJ

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CAL

#2171

Re: N2 vs. Air Reflow Soldering | 15 December, 2000

Dave- Follow this link to our November empfasis. In the empfasis there is an article on N2. http://www.empf.org/html/November%202000%20empfasis.pdf

Cal

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ralph

#2172

Re: N2 vs. Air Reflow Soldering | 15 December, 2000

In my experiance, I have seen no difference in profiles between nitrogen and air reflows. What I understand is that the Nitrogen blanket allows for a greater solderability to gold plated pads. I also found that when dealing with fine pitch components I needed to use Nitrogen as well..again for solderability. Not once did I need to change my Oven profile in order to get better solder results. This is all based on having an accurate oven profile for the boards you are doing currently.

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DaveJ

#2173

Re: N2 vs. Air Reflow Soldering | 15 December, 2000

All, Thanks for your quick response. However, my primary concern is not the question of solderability advantages of nitrogen versus air. With all things (zone temps, track speeds, flow rates, etc.) being equal, what would be the profile differences of nitrogen versus air? For example, should I expect a hotter peak temp or longer TAL with one gas versus the other - or would it be the same?

Thanks again, DaveJ

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Hussman

#2174

Re: N2 vs. Air Reflow Soldering | 15 December, 2000

Dave,

It will all be about the same - changing atmospheres does not affect your heater output. You may experience some differences in the way your solder joints look (a little duller), but with some of the newer pastes out there, you can overcome it. I didn't even see a change with fine pitch like Ralph stated earlier, but that may be due to design guidelines, stencil design, oven type, paste type (well, you get it). If you're going to change atmospheres, I also suggest looking at some of the newer fluxes as well. Most new ones are designed to run in either nitro or air!

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Greenman

#2175

Re: N2 vs. Air Reflow Soldering | 20 December, 2000

Dave: In terms of heat transfer, air is 78% nitrogen, and the remainder is mostly (20.9%) oxygen (roughly the same molecular weight as nitrogen), with a bit of argon (0.9%), CO2, hydrogen, monosodium glutamate etc. So you would expect virtually no difference between nitrogen and air in terms of reflow profile. But as with most things in life, it ain't that simple...

The nitrogen is provided from a cryogenic (pressurized liquid )source - it's usually passed through a vaporizer (big bunch of parallel tubes), and (particularly at this time of year in, say, Minnesota) may well be coming into the reflow oven at 32�F or so (it liquefies at -196�C, so don't worry about it liquefying too much). If you have a well-designed forced convection reflow oven (most of them are now OK), this should not matter, as the PID controller/heater combo makes sure that the gas temperature is maintained no matter what. Watch the oven-balancing here, as you may find you need extra exhaust flow to keep the nitrogen from leaking out (it's not poisonous, it's an asphyxiant, and you need at least 19.5% oxygen in the air aroun the oven). By changing the oven balance (gas in/gas out relationships), you change the way heat flows in the oven, and it is usually changes here that give rise to actual profile differences between "nitrogen and air" that on paper, should not exist!!

IR is a different matter - IR relies on radiative heat transfer, and if you go from air-reflow in an IR oven, to suddenly purging with a cold gas, you will see a drastic change in the reflow profile, as the convection of the cold gas over the assembly chills it down.

There's a ot more to this than meets the eye.

I hope this helps.

Greenman

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Greenman

#2176

Re: N2 vs. Air Reflow Soldering | 20 December, 2000

Dave: In terms of heat transfer, air is 78% nitrogen, and the remainder is mostly (20.9%) oxygen (roughly the same molecular weight as nitrogen), with a bit of argon (0.9%), CO2, hydrogen, monosodium glutamate etc. So you would expect virtually no difference between nitrogen and air in terms of reflow profile. But as with most things in life, it ain't that simple...

The nitrogen is provided from a cryogenic (pressurized liquid )source - it's usually passed through a vaporizer (big bunch of parallel tubes), and (particularly at this time of year in, say, Minnesota) may well be coming into the reflow oven at 32�F or so (it liquefies at -196�C, so don't worry about it liquefying too much). If you have a well-designed forced convection reflow oven (most of them are now OK), this should not matter, as the PID controller/heater combo makes sure that the gas temperature is maintained no matter what. Watch the oven-balancing here, as you may find you need extra exhaust flow to keep the nitrogen from leaking out (it's not poisonous, it's an asphyxiant, and you need at least 19.5% oxygen in the air aroun the oven). By changing the oven balance (gas in/gas out relationships), you change the way heat flows in the oven, and it is usually changes here that give rise to actual profile differences between "nitrogen and air" that on paper, should not exist!!

IR is a different matter - IR relies on radiative heat transfer, and if you go from air-reflow in an IR oven, to suddenly purging with a cold gas, you will see a drastic change in the reflow profile, as the convection of the cold gas over the assembly chills it down.

There's a lot more to this than meets the eye.

I hope this helps.

Greenman

reply »

Greenman

#2177

Re: N2 vs. Air Reflow Soldering | 20 December, 2000

Dave: In terms of heat transfer, air is 78% nitrogen, and the remainder is mostly (20.9%) oxygen (roughly the same molecular weight as nitrogen), with a bit of argon (0.9%), CO2, hydrogen, monosodium glutamate etc. So you would expect virtually no difference between nitrogen and air in terms of reflow profile. But as with most things in life, it ain't that simple...

The nitrogen is provided from a cryogenic (pressurized liquid )source - it's usually passed through a vaporizer (big bunch of parallel tubes), and (particularly at this time of year in, say, Minnesota) may well be coming into the reflow oven at 32�F or so (it liquefies at -196�C, so don't worry about it liquefying too much). If you have a well-designed forced convection reflow oven (most of them are now OK), this should not matter, as the PID controller/heater combo makes sure that the gas temperature is maintained no matter what. Watch the oven-balancing here, as you may find you need extra exhaust flow to keep the nitrogen from leaking out (it's not poisonous, it's an asphyxiant, and you need at least 19.5% oxygen in the air aroun the oven). By changing the oven balance (gas in/gas out relationships), you change the way heat flows in the oven, and it is usually changes here that give rise to actual profile differences between "nitrogen and air" that on paper, should not exist!!

IR is a different matter - IR relies on radiative heat transfer, and if you go from air-reflow in an IR oven, to suddenly purging with a cold gas, you will see a drastic change in the reflow profile, as the convection of the cold gas over the assembly chills it down.

There's a lot more to this than meets the eye.

I hope this helps.

Greenman

reply »

Bentzen

#2178

Re: N2 vs. Air Reflow Soldering | 21 December, 2000

Check out my article about reflow soldering in nitrogen atmosphere on www.smtinfocus.com

Hopefully you will find some answers and more detailes on my web-site.

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Brad Kendall

#2179

Re: N2 vs. Air Reflow Soldering | 21 December, 2000

I used to switch my N2 on and off for different products and I had performed profiles on the products with and without the N2 on and there was not a difference in the profiles. So you should not have to adjust your profiles. I think everyone else covered the other technical aspects of this, I just wanted to give you my experience with this subject to help support what the others have already said.

Brad

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