Why is Solder Flux Used in Soldering?
The main function or use of flux in any Printed Circuit Board Assembly or rework is to clean and remove any oxide and impurities from the board. These oxides deposited on the board may not permit good solder joints. If the solder joint is poor, there will be poor electrical conduction or poor flow of electricity through the circuit.
Solder flux helps to de-oxidize metals (copper tracks on the PCB and leads of electronic components) and helps better soldering and wetting. The activators in the flux promote wetting of the molten solder to the surface mount lands and component terminations or leads by removing oxides and other surface contaminants.