If people use an inert gas, they use nitrogen for mass soldering. Inert gases get progressively more expensive as you move down the periodic table.
Many times no-clean flux is less active than other flux types. So, people use an inert gas to lower oxidation of solderable surfaces during preheat cyles, prior to soldering, widening the process window.
For background: * We have not talked about inerting a lot on SMTnet, but the are a few threads in the fine SMTnet Archives. * There's one white paper in the SMTnet Library. It give a contact at an inert gas supplier, where you could get more information.
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