3.2.1 — Flux: Selection and Application
Flux chemically limits oxidation during heating and improves solder wetting by reacting with and dissolving oxides that form on copper as it is heated. Without flux, the oxides produced during heating would prevent the molten solder from bonding to the base metal. However, excess or misapplied flux can create cleanliness and corrosion concerns — the correct amount applied to the correct surfaces is what produces a reliable joint.
Types of Flux for Copper Soldering
Paste Flux (Water-Soluble)
The standard flux for copper-to-copper soldering with lead-free solder. Applied by brush or flux brush to the cleaned tube end and fitting cup. Water-soluble flux residues can be removed with hot water after the joint cools; residue left in a water system can cause corrosion over time. Always clean flux residue from completed joints.
Self-Cleaning Flux
More aggressive than standard paste flux; contains activating agents that help dissolve heavier oxide films. Useful when copper surfaces cannot be fully cleaned mechanically. More corrosive than standard flux — residues must be removed promptly and thoroughly after soldering. Not recommended for refrigerant-system joints where residue can damage compressor oil.
Flux for Dissimilar Metals
Copper-to-brass and copper-to-bronze joints may require a flux specifically rated for the alloy being joined. Confirm flux compatibility with the fittings being used; some brass alloys — particularly high-zinc brasses — require a more active flux than standard copper paste flux to achieve reliable wetting.
Flux Application Procedure
Correct Flux Application
- Apply flux immediately after abrasive cleaning — do not allow the cleaned surface to sit exposed before fluxing
- Use a small brush (flux brush or acid brush) to apply a thin, even coat to the outside of the tube end; a thin coating is sufficient — excess flux creates more residue to clean and can contaminate the joint interior
- Apply flux to the inside of the fitting cup as well; the flux on both surfaces ensures full coverage once the joint is assembled
- Do not apply flux to the solder wire itself — the flux is on the joint surfaces, not the filler
- Assemble the joint immediately after fluxing; rotate the tube slightly in the fitting cup to distribute the flux evenly around the contact surfaces
- Wipe away any flux that squeezes out at the fitting face during assembly; excess flux at the joint face can cause runs and drips during heating
Flux does not clean copper — it protects a clean surface and assists wetting. Applying flux to uncleaned copper does not produce a reliable bond; the flux cannot dissolve heavy oxide layers or remove oil contamination. Mechanical cleaning with emery cloth and a fitting brush is always the first step.