Introduction
Most soldering processes in electronics are done in a normal air atmosphere. The prevention of oxidation of the joint areas during the process, which is necessary for good soldering, is in that case fully provided by and depending on the function of the flux. If an inert gas is used in the process at that point where soldering actually takes place it can support the flux and increase so the process window.
How inert gas can support the flux function
The flux support function of the inert gas is most important at the point where the solder (wave) separates from the joints. If the flux is not able to keep the oxygen away from that process, oxides will be formed over the liquid solder. These oxides will affect the correct solder drainage, so that spikes, flags or bridging caused by this oxide film can be formed. The use of an inert gas that will displace most of the air (oxygen) especially in this separation area will prevent such effects and so assist the process. If the use of an inert gas will be beneficial for a specific process depends on the layout of the boards and the type of flux that is used. If that combination provides a good process quality, one can often refrain from the use of an inert gas. The saving on dross with the use of inert gas seldom equals the costs for the gas itself. So for dross savings alone the use of inert gas is for that reason not advised.
The use of nitrogen in comparison with other gasses
What inert gas should be used for displacing the air in the solder drainage/separation area is not that important. Nitrogen is commonly used, mainly for financial reasons. This gas is mostly easily available at reasonable costs, certainly in comparison with noble gases such as helium, argon etc. In principle every gas, that is not directly dangerous for the environment and will be able to displace the oxygen on that spot, can be used for this process. The use of nitrogen is obvious also for the reason that it is already the main part of the air. The use of hydrogen is not recommended. Due to the relatively low process temperatures it will not reduce oxides. Apart from that, at a certain mixture level with oxygen it can become very explosive. What should be recognized is that no combustible or dangerous gases should be used for inert gas soldering. It is important to keep in mind that the gas should be really inert.