Solder Paste (Flux)

Soldering flux is a chemical substance used before and during the soldering process of electronic components. Soldering flux can be used for both manual and automated soldering processes. Soldering flux is mainly used to prepare the metal surfaces before soldering by cleaning and removing any oxides and impurities. Oxides are chemical compounds that are formed when metals are exposed to air. Oxides prevent the formation of a perfect solder joint. The presence of metal oxides on circuit boards may result in electrical conductivity and inefficient flow of electricity across the electrical board or circuit board. Other reasons for using solder flux include:

Prevent reoxidation of the metal surface during the soldering process
Help in reducing the surface tension and the viscosity of the molten solder. This helps to improve wettability.
The flux allows the molten solder to form a permanent electrical and mechanical joint.