What is soldering flux? Flux is a chemical cleaning agent used before and during the soldering process of electronic components onto circuit boards. ... The flux also protects the metal surfaces from re-oxidation during soldering and helps the soldering process by altering the surface tension of the molten solder.
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What is soldering paste?
Solder paste is a mixture of minute solder spheres held within a specialized form of solder flux. As the name indicates it has the texture of a paste, hence the name. The fact that it is a paste means that it can be easily applied to the board during PCB assembly. The solder particles are a mixture of solder. Traditionally this used to be tin and lead, but legislation has been introduced around the world, to only use lead-free solder. These may be made from a variety of mixtures. One is 99.7% tin and 0.3% copper, whereas other mixtures include other metals including tin.
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So...what is the difference between soldering flux and soldering paste?
Solder paste contains solder particles and flux. It helps ‘tin’ a surface with a coating of solder, resistant to oxidation.
Solder Flux is just ‘flux’, a chemical to clean the surface, ready to accept solder. Solder has to be added to make the joint or tin surface.
Hope this will clear some doubts...