Welding is a process that combines materials together at a high heat to melt and form a weldment. Welding is done on metals, thermoplastics, and even wood and is responsible for the creation of many different projects.
Flux welding is a semi-automatic welding process that comes into play when working with thicker materials.
Keep reading this guide to learn more about the purpose and function of flux welding.
Flux consists of a mixture of chemicals and other materials that protect molten metal in the weld pool from contamination. The use of flux in welding began in the 1950s and has since become the preferred alternative over stick welding.
The main reason why flux is so important in welding is that it helps prevent the metal from rusting and oxidizing. Flux also helps the flow of the filler rod which prevents the metals from becoming porous.
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Flux consists of four major components.
Short answer: Flux removes oxidation, aids in heat transfer, cleans and prepares the joint to accept the solder, and promotes even solder flow.
http://en.wikipedia.org/wiki/Flux_(metallurgy)
Many various fluxes contain metal halides which are metals combined with halogens. Halogens are a group in the periodic table consisting of five chemically related elements: fluorine (F), chlorine (Cl), bromine (Br), iodine (I), and astatine (At). These halides are the activators. Because flux has a low melting point it will liquefy before solder solidifies. Metal halides often promote corrosion which will help aid in the dissolution of the oxide allowing the contaminate to flow away from the joint. Then the solder will flow into the joint forming a strong bond that actually fuses with the metals involved. Which is why metals like lead and tin are used to solder metals like copper because they form a bond with the metal that creates a thin layer of alloy metals. I do not believe there are any "products" from this reaction. There was a saying in Chemistry I learned once "like dissolves like". It takes corrosion to remove it. However, heavy corrosion will not remove from the metal by just using solder flux which is very mild and not acidic as flux used in copper tube welding.
I wasn't able to research "voids" in the solder. In my experience that is due to soldering with extremely high temperatures. Lead melting point is about 621 degrees Fahrenheit. If your iron is too hot it can super heat the lead and cause it to "explode" or pop off the joint. Perhaps this is a cause of voids. Also, if the material being soldered is very dirty it could cause contaminates to be trapped under the solder that the solvents in the flux are unable to clean away. Which, as mentioned above, can cause spattering and boiling off of solder particles which could cause the "voids".