Influence of Flux on Wetting Behavior of Lead-Free Solder Balls during the Infrared-Reflow Process:

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In reality, the chemistry of flux interactions at oxide surfaces is complicated and involves acid-base, oxidation-reduction, and coordination-type and adsorption-type reactions. Most fluxes react as Bronsted-Lowry acids with metallic oxides to form their respective salts and water, and the salts serve as surfactants that promote solder wetting.


A "Bronsted-Lowry acid" is an acid defined by the fact that it donates a proton in a chemical reaction. A surfactant is something that reduces surface tension. Why surface tension exists and how it is reduced are beyond the scope of this post.
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Bitt Faulk