Whether soldering tiny electrical circuits in a computer, or copper water pipes in your plumbing, you need to use soldering paste, sometimes called flux. Without it, your electrical connections may come apart or your pipes may leak.
Soldering paste serves three purposes. It helps clean the copper as it heats, it helps the solder flow more evenly and it helps the solder adhere to the copper.
Paste flux can come in several types of containers, including a flat can, a tube or a small bottle. It contains cleaning chemicals in a resin base.
Tinning flux has a small amount of solder mixed into it, which puts a small amount into the joint before heating. Personal preference and craftsmanship determine whether this makes soldering easier.
To apply flux to copper pipes, use a small brush, often provided with the flux when you buy it. For electronic circuits, dip the soldering iron tip, the solder itself and/or the wire directly into the can of flux.
Flux Core Solder
Some solder, called resin core or flux core, comes on a spool-like wire and has a center core filled with soldering paste. Most electronics technicians prefer flux core solder.