Flux can be in many different forms. it can be in the solder, it can be in a soft form like butter, or it can be in a pen like a tide stain stick. it is used on the solder iron and on the thing you're soldering to make a good surface for the solder to stick to.
Basically it makes soldering easier, it can be done with out it.
It cleans the surface so the solder will stick
To prevent oxidation and allow for the alloy to flow better
Flux helps promote soldering. Soldering paste might contain flux and other items to help promote soldering. The paste helps hold the flux in place while beginning to solder.
In soldering, the function of a flux is to clean the metal surface such that the solder may adhere. Rosin was one early flux, and pastes loaded with HCl were also common, though the HCl left over caused corrosion. I don't know about ancient times however.
Flux is used to prevent oxidation while soldering wires and components in electronics.
The materials being soldered.
A: The pad is not clean it has tarnish or wrong solder with not enough soldering flux. Finally not enough heat.
Gd
Gd
Flux is used to keep metals from oxidizing. Solder will not bond well with copper-oxides which are formed when copper is exposed to high temperatures (ie a soldering iron) the flux prevents the oxidation. If you were to solder without flux the solder joint would be weak and possibly completely nonconductive.
A state of continuous movement or a medium used in metal soldering.
Soldering iron, solder, flux
One gallon of Kester 951 Flux weighs about 8.34 pounds. Flux is used for soldering purposes and is widely used in the making of electronics.
flux acts as a wetting agent in the soldering process, reducing the surface tension of the molten solder and causing it to better wet out the parts to be joined.