When the first few generations of No-clean solder paste came into the 
market, they required an inert atmosphere.  Most of the new generations
of paste do not.  Normally a manufacturer supplied brochure or data sheets will
tell you if nitrogen is necessary.  For example:
Kester R244, R244C, and R244L do not require nitrogen, yet R251 and R215 do.
Indium NC-SMQ 90, 62, 65 and the 51 series can be ran in air, but not SMQ 71.
Alpha LR735 and LR725 can be ran in air, as can the ESP 575E.

                                  Hope this helps.
                                                       Steve A


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