Liquid flux is a chemical agent that serves several purposes: it cleans a metal surface, creates a barrier that prevents oxidation and allows solder to flow more freely over a work area.In order to create a sturdy solder joint, the joining copper parts should be sanded, properly cleaned and free of dirt and dust that may interfere with the joint. The problem with copper is that it oxidizes quickly, and this can lead to weak solder joints.
Clean the surfaces that will be joined together.
Apply the flux with a small brush. Some types of liquid flux come with a small brush inside the cap. When you apply the solder, it will flow freely over the flux, so limit the area in which you apply the flux.
Using the soldering gun, heat the copper and apply the solder to the surfaces you want to join. You will see the solder flowing into the area where you applied the flux.
Continue applying solder until you have a solid, satisfactory joint. The liquid flux will make a sizzling sound as the solution evaporates in the heat.
Allow the joint to cool.