Soldering Gaseous fluxing

Freedman, G, Atmospheric Pressure Gaseous Flux-Assisted Laser Reflow Soldering, US. Patent No. 5,227,604, July 1993. 

FIG. 56 Spreading studies earried out at 250 C of Sn, Sn-0.7Cu, Sn 3.5Ag, and Sn-3.5Ag-0.7Cu solders on 0.05-pm Au over 4.5-jun Ni deposited on a Si wafer. All alloys are given in weight percent. A gaseous flux mixture of formic acid and nitrogen was used. (From Ref. 173.) The initial sphere diameters were 0.889 mm and were deoxidized with the ROSA process. (From Ref. 174.)... 

Toward certain metals, molten NH4C1 is more reactive than gaseous FIC1. The acidic properties of NFI4CI are responsible for its use as a soldering flux (known as sal ammoniac) to remove oxide coatings. 

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