Flux

Flux is a chemical paste or liquid applied to copper pipe and fitting surfaces before soldering (sweating) to prepare the metal for bonding. It serves two functions: cleaning the copper surface of oxidation and contamination that would prevent solder adhesion, and promoting the flow of molten solder evenly around the joint by reducing surface tension. Without flux, solder beads up on the copper surface instead of wicking into the gap — resulting in a porous, leaky joint.

Types of flux

• Paste flux (most common): a water-soluble paste applied with a brush to cleaned copper surfaces. Most residential soldering uses paste flux. Common brands: Oatey No. 95, LA-Co Regular Flux.
• Water-soluble (self-cleaning) flux: more aggressive, removes heavier oxidation without abrading — appropriate for older or moderately corroded pipe.
• Lead-free flux: required by NSF 61 (potable water standard) for any joint in the drinking water supply. Most modern paste flux products are lead-free; always confirm if soldering supply lines.

How to use flux when soldering copper

1. Cut the copper pipe cleanly with a pipe cutter and deburr the end.
2. Abrade the outside of the pipe end and the inside of the fitting socket with emery cloth or a fitting brush until bright and shiny.
3. Apply a thin, even coat of flux to both surfaces with a flux brush.
4. Assemble the joint and heat evenly with a propane or MAPP torch until the copper is hot enough that touching solder to the joint (not the flame) causes it to flow and wick around the joint.
5. Allow to cool, then wipe off excess flux residue with a damp cloth.

Why clean-up matters

Flux residue left on pipe is mildly acidic and will gradually corrode copper over time if not wiped off. Always clean residue from finished joints. Flushing the system with water after soldering also removes internal flux.