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

Jim
One of our favorite topics, the use of nitrogen. 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 1,000. 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?"

Phil
With regard to voids, that's another question. Let me say this about nitrogen before we dive into it. Anybody who has listening to Board Talk 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.

Jim
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 being applied.

Typical numbers are for alcohol-based flux, it's 100 degrees C. 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 with Phil that nitrogen is a good idea for reduction of dross, particularly with the high tin content lead-free alloys.

Phil
Well, there we go.  So no magic bullets, nitrogen can help but is not a magic bullet.  

Jim
Adjusting your preheat can be a magic pill. Just get a profile properly and get your preheat in line with your flux.

Phil
Who knows, maybe other people would be inspired by this to check the profile on their wave solder machine, particularly the preheat. And 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.

Jim
And don't solder like my brother.