Flux is added to solder paste to remove metal-surface oxides, prevent re-oxidation, reduce the surface tension of the solder, and improve wettability—thereby ensuring a successful soldering process, forming reliable solder joints, and conveniently transferring the solder to the soldering area. Its specific functions are as follows:

Remove oxides: All metal surfaces—including solder powder, component leads, and PCB pads—naturally form an oxide film in air (such as SnO₂ and CuO). This oxide film prevents molten solder from contacting the clean base metal, making it impossible to form a good metallurgical bond (i.e., a "cold solder joint"). The activator in the flux (usually an organic acid or halide) is activated when soldering heat reaches a certain temperature (e.g., 100°C–200°C), reacts chemically with the metal oxides, decomposes, dissolves, and removes them, exposing a clean, solderable metal surface.

Prevent re-oxidation: The soldering process requires heating, and high temperatures accelerate oxidation of the metal surface. Without protection throughout the heating process, a freshly cleaned surface will rapidly oxidize again, causing soldering failure. The film-forming agent in the flux (such as rosin or resin) melts upon heating and forms a liquid protective film that covers the metal surface and the molten solder, isolating them from air and effectively preventing re-oxidation at high temperatures.

Reduce surface tension: Molten solder, like a water droplet, has its own surface tension, which tends to make it shrink into a ball (non-wetting) rather than spread out (wetting) to cover the entire pad. Flux reduces the interfacial tension between the molten solder and the pad, allowing the molten solder to better "flow" and "wet" into every corner of the pad, forming full and reliable solder joints and producing a good self-alignment effect (for example, pulling BGA solder balls back into position).

Improve soldering quality: Flux also helps shape the solder joints, maintain their surface luster, and make them aesthetically pleasing. At the same time, it promotes energy transfer to the soldering area, improving soldering efficiency.

Flux provides the solder paste with adhesion, holding chips in place after component placement and preventing movement. At the same time, it enables the tin-powder solder within the paste to be transferred in paste form to the PCB soldering area, making smaller and denser package formats possible.