Electronics Assembly Knowledge, Vision & Wisdom
Will Nitrogen Reduce Wave Solder Defects?
Will Nitrogen Reduce Wave Solder Defects?
We plan to install a nitrogen environment around the wave soldering tank. Is this likely to improve our defective parts-per-million level? Phil Zarrow and Jim Hall, The Assembly Brothers, discuss this question.
Board Talk

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Board Talk is presented by ITM Consulting

Phil Zarrow
Phil Zarrow, ITM Consulting
With over 35 years experience in PCB assembly, Phil is one of the leading experts in SMT process failure analysis. He has vast experience in SMT equipment, materials and processes.

Jim Hall
Jim Hall, ITM Consulting
A Lean Six-Sigma Master Blackbelt, Jim has a wealth of knowledge in soldering, thermal technology, equipment and process basics. He is a pioneer in the science of reflow.

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Welcome to Board Talk. This is Jim Hall and Phil Zarrow, the Assembly Brothers pick and place. What is today's dilemma, Jim?

Improving DPM for wave soldering using nitrogen. This comes from A.S.

We are wave soldering complex board with 3000-plus components using OSP finish and SAC305 alloy. Our current defective parts-per-million level for soldering is 400 to 500 and for solder voids is around 1000.

To improve our DPM, we plan to install a nitrogen environment around the wave soldering tank to reduce oxidation. Is this likely to improve our DPM level?

With regard to voids, that's another question. Let me say this about nitrogen before we dive into it. Anybody who has listening to this program knows we're not big fans of using nitrogen for reflow. However, wave soldering is a totally different process. With the addition of flux, addition of solder, it's a totally different beast.

We have found both from our personal experiences as well as studies that were done by Soltec and Electrovert, that nitrogen really does help in the wave soldering process with through hole components by improving the surface tension and by reducing the oxidation, it helps the solder go in the right places. 

So things like less propensity towards bridging, reduced solder balls, reduced open and skips, you can see some improvement. And the other advantage of inerting the wave is you're only inerting the wave areas.

So you can do that out of a gas bottle. However, with voids, well, this kind of leaves a void in the problem here.

I'm not totally convinced that I understand exactly what they're describing.  What's the difference between DPM level for soldering and solder voids. But if they're experiencing solder voids in wave soldering that waves the flag of inadequate preheat.

Voids are typically caused by solvent in the flux being encaptured in the wave and causing bubbles and voids. The answer is to make sure you're profiling your preheat correctly, putting thermocouples on top of the board on a land adjacent to a heavy through-hole component and making sure that the top surface of the board is reaching the appropriate temperature for the flux.

Typical numbers are for alcohol-based flux, it's 100 degrees. For water based VOC-free fluxes it's 110 to 120. But in any case check what your manufacturer recommends because I suspect that's the principle cause of voiding.

I don't know that nitrogen is going to help that very much. But I agree that it should help the other defects, particularly with insufficient solder and insufficient hole fill. The question is raised, if you are not preheating adequately, in addition to not boiling off the solvents, you may not be fully activating the surface.  

I agree that nitrogen is a good idea for reduction of dross, particularly with the high tin content lead-free alloys.

No magic bullets, nitrogen can help but is not a magic bullet. 

Remember, whether you're trying to get rid of voids in wave soldering or you are avoiding wave soldering altogether, don't solder like my brother.

And don't solder like my brother.  
Reader Comment

We have found (and tested) that the HOT nitrogen injection on to the soldering bath get a real soldering defects reduction (e.g shortcut) and the dross quantity is the same as the "nitrogen tunnel system" dross quantity. Obviously, the costs are completely different, you decide.

Pierleonardo Zoppellaro, Carel Industries s.p.a.
Reader Comment

Will Nitrogen Reduce Wave Solder Defects? The short answer is yes, but to better understand how nitrogen will reduce defects in the wave soldering process you must understand how nitrogen works in the soldering process.

Nitrogen is an inert gas which displaces oxygen in the soldering process, and there are different levels of nitrogen purity which has an effect on the soldering process. As the purity level of the nitrogen decreases the impurities found in the nitrogen gas increase, which has a direct effect on the wave soldering process.

For example a purity level of 5.0 (99.999%) has a typical impurity level is <10 ppm, and a purity level of 3.0 (99.9%) has an impurity level of < 1000 ppm.

In a nitrogen wave process with reduced oxygen levels, oxidation is not occurring at the board level and this has a direct effect on solder spread, wetting force and surface tension. A study on solder spread was conducted in different atmospheric environments and one major finding was that solder starts to flow at lower temperatures if the rest oxygen level is low.

For example Sn63/Pb37 has a melt point 183 degrees C, and with a rest oxygen level of 10 ppm solder started to spread at 205 degrees C, versus a rest O2 level of 1000 ppm solder started to spread at 270 degrees C. SAC 305 has a melt point 221 degrees C and with a rest O2 level of 10 ppm solder started to spread at 230 degrees C, versus a rest O2 level of 1000 ppm solder started to spread at 240 degrees C.

Wave soldering in a nitrogen atmosphere is accomplished by one of two methods, nitrogen inertion over the soldering area with a rest O2 level of approximately 1000 ppm or a complete tunnel system encompassing the fluxing area, preheat and soldering area with a significantly lower rest O2 level of 20 ppm to 500 ppm. Wave soldering in an inerted and oxide free atmosphere versus ambient air provides many advantages when you consider all the relevant parameter, PCB layout, temperature profile, solderability of the materials use, and the defect rate. The reduction of typical defects such as incompletes, barrel fill, insufficient wetting, bridging and solder balls are due primarily to improved wetting characteristics, changing surface tension of the liquid solder in a nitrogen atmosphere.

Lower defect rates and overall improvement in quality are reflected in the reduction of PCB repair work. Studies have shown that nitrogen inertion over the soldering area versus ambient air will reduce PCB rework by 40%, and a full tunnel nitrogen inertion versus ambient air will reduce PCB rework by 75%.

Studies have also shown that there are considerable differences in the defect rates between both methods of wave inertion. Due to the lower rest oxygen levels defect rates in a full tunnel nitrogen wave system are significantly lower versus inertion just over the soldering area.

Gus Mavrou, SEHO North America, USA
Reader Comment

A slight clarification on one benefit of nitrogen. It increases wetting forces (a good thing) while also increasing surface tension (not so good) unless your wave is specifically designed for nitrogen and is set up properly.

Don't let customers get fooled into thinking that just plumbing nitrogen in or around a wave will make it inert. Years of trials and tribulations (as well as patents) went into insuring that when and where nitrogen was input into a system yielded an inert soldering environment. My 2 cents.

Ray Chartrand, CharTrain Consulting, USA
Reader Comment

Thank you for bringing this issue to the Board Talk. Phil, you should see more benefits of nitrogen in reflow. I am sure you will be surprised. In wave soldering, using our system (ALIX Inertwave) that is already installed in more than 800 machines around the world, you can reduce dross formation in more than 80%. Flux consumption the reduction is near 40%. We use heated gas with our HT system (TM). The heated gas improve yield (better soaking and heat transfer). The savings are fantastic comparing with air processes. But be careful, it is not a matter of just injecting nitrogen. The goal is atmosphere CONTROL. Improved wettability (barrel filling), better surface tension (less shorts and bridges) and with less flux, less dirtiness on the PC board and in your machine. Cleaning time (dross on solder pot) reduced in over 80%. Quality gain and savings all together.

Luiz Felipe Rodrigues, Air Liquide, Brazil
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