It is often said that the use of nitrogen is mandatory in a solder process when soldering with lead free solder alloys. There is no doubt that nitrogen will assist in the solder process.
It is however in most cases very hard to proof that nitrogen is necessary to run the process successfully. Also the difference in effects in a 50-PPM atmosphere compared to 1000-PPM will be difficult to proof when we have to compare soldered products. Also we have the discussion which technology performs better. The closed tunnel is the best. It guarantees a reproducible process and gives a robust nitrogen level. But what is the difference in solder quality between an open tunnel and a nitrowave? Only the soldered product should be judged, keeping all other process settings at the same level.
The main reason to use a closed tunnel, instead of an open tunnel or a nitrogen hood or nitrowave, is that liquid solder is a very good getter for collecting oxygen.
This means that as soon as the liquid solder comes in contact with oxygen molecules, it will immediately form oxides. These oxides are covering the liquid surface as a "closed" layer. When we use an open tunnel or a hood or nitrowave, this layer will also be formed but with less thickness. Giving the flux an easier job to remove these oxides and so promote a more stable solder process. The best results, one gets when a closed tunnel is used, with a nitrogen supply with an oxygen level of less than 10-PPM. It has been found that during use of the (Sigma)machine this oxygen level becomes even lower, due to the fact that the liquid solder catches the remaining oxygen from the supplied gas, forming minor oxide particles which you will find in the tunnel as grey/black powdery dust.
Preheating under a nitrogen atmosphere will in theory prevent further oxidation of the heated metal parts. However since this preheating is at a relative low temperature during a relative short time, it will be hard to prove that the extra oxidation initiated by the preheating will have a measurable detrimental effect on the soldering process.
There are no specific data available on how much an open tunnel performs better than a hood or a nitrowave. The reason is that these results are very much depending on the PCB-layout. In many occasions one can get excellent solder results using no nitrogen at all in a standard wave soldering process. In other occasions there can be a clear difference in quality when nitrogen is used to support the solder process. In practical situations it has been proven by customers that the Sigma wave-soldering machine gave always a very reproducible solder quality. However due to the higher surface tension of the solder in a nitrogen atmosphere, sometimes solder bridging was found on components that did not show that bridging when soldered under air. This was clearly layout related.
There is no clearly measurable metallurgical effect when soldering under nitrogen, because the wetting mechanism is not different than with soldering under air. The only thing is that due to the absence of oxygen wetting is promoted, since the flux has more activity left. The other point to mention is that the joints will have a larger solder volume in wave soldering, due to the higher surface tension of the solder when soldering in a nitrogen environment.
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