By Richard Asmus, Updated Mar 24, 2022

Whether you’re soldering microelectronic circuits in a computer or joining copper water pipes in your plumbing, soldering paste—commonly known as flux—is essential. Without it, electrical connections can separate and plumbing joints may leak.

Purpose

Soldering paste serves three key functions:

• It cleans copper surfaces as they heat, removing oxidation.
• It promotes even solder flow across the joint.
• It improves adhesion, ensuring a robust mechanical bond.

Types of Flux

Paste Flux

Paste flux comes in flat cans, tubes, or small bottles. Its resin base contains cleaning agents that prepare the metal for soldering.

Tinning Flux

Tinning flux incorporates a small amount of solder. This pre‑adds a thin layer of solder to the joint, which can make the process smoother for experienced craftsmen.

Application Techniques

For copper plumbing, apply flux with a small brush—often supplied with the product. In electronics, dip the soldering iron tip, the solder, or the wire directly into the flux can before heating.

Flux Core Solder

Flux core (or resin core) solder is wound on a spool with a core filled with soldering paste. Many electronics technicians prefer it for its convenience and reliability.