A solder paste is essentially powder metal solder suspended in a thick medium called flux. Flux is added to act as a temporary adhesive, holding the components until the soldering process melts the solder and makes a stronger physical connection. The paste is a gray material.
Flux medium is a very useful and critical factor, and has a large influence over the entire properties and performances of the solder paste. Fundamental roles of the flux medium are listed as follows:
Elimination of oxide film on substrates: The flux chemically melts and removes the oxide film formed on the surface of electrical components, substrates and solder.
Prevention of re-oxidation: The substrates, components and solder powder exposed to the heating environment in the reflowoven rapidly oxidize. Solids in the flux soften into a liquid state, effectively covering those soldering parts and prevent them from re-oxidation.
Reduction of surface tension of the solder to enhance wetting: The flux temporarily reduces the surface tension of the molten solder and helps increase the contact area (wetting) with substrate and components.