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What is Soldering?

By S. Mithra
Updated May 17, 2024
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Soldering joins two pieces of metal, such as electrical wires, by melting them together with another metal to form a strong bond. Many people use this technique in their field, from electrical engineering and plumbing to jewelry and crafts. In a delicate procedure, a special material, called solder, flows over two pre-heated pieces and attaches them through a process similar to welding or brazing.

The process of soldering is tricky and intimidating in practice, but easy to understand in theory. Basic supplies include a soldering iron, which is a prong of metal that heats to a specific temperature through electricity, like a regular iron. The solder, or wire, is often an alloy of aluminum and lead, and needs a lower melting point than the metal that is being joined. Finally, a person performing this technique needs a cleaning resin called flux that ensures the joining pieces are incredibly clean. Flux removes all the oxides on the surface of the metal that would interfere with the molecular bonding, allowing the solder to flow into the joint smoothly.

The first step in soldering is cleaning the surfaces, initially with sandpaper or steel wool, and then by melting flux onto the parts. Sometimes, flux is part of the alloy of the wire, in an easy to use mixture. Then, the pieces are both heated above the melting point of the solder (but below their own melting point) with the iron. When touched to the joint, this precise heating causes the wire to "flow" to the place of highest temperature and makes a chemical bond. The material shouldn't drip or blob, but spread smoothly, coating the entire joint. When it cools, it produces a sturdy, even connection.

Various metals can be soldered together, such as gold and sterling silver in jewelry, brass in watches and clocks, copper in water pipes, or iron in leaded glass stained windows. All these metals have different melting points, and therefore use different solder. Some "soft" wire, with a low melting point, is perfect for wiring a circuit board. Other "hard" solder, such as for making a bracelet, needs a torch rather than a soldering iron to get a hot enough temperature. Electrical engineers and hobbyists alike can benefit from learning the art and science of this process.

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Discussion Comments

By anon321802 — On Feb 24, 2013

How would I remove any coating from metal costume jewelry -- the gold tone that is on costume pieces.

By elizabeth23 — On Jan 31, 2011

I always sort of wanted to learn to use a soldering iron so that I could try to make metal jewelry. Up until now, though, it still sort of frightens me. I'm pretty much as useless with power tools as it is possible to be, including a near complete inability to so much as get a drill to work. Oh well, I hope someday I can figure it out.

By anon65473 — On Feb 13, 2010

Soldering is a mechanical process. Wet solder behaves just like water while a liquid and can be wiped off of a wire if it is held in a liquid state. The copper never melts. At the atomic level, you still have an amalgam of copper and solder. There is no new alloy formed. When solder is cool and solid, it is just like placing 2 straws in a block of ice.

Heat-activate Flux is a bit of a chemical process in that it removes oxide, but a chemical process being involved does not make this a chemical bond.

Cadmium welding and brazing (for examples), where all the metals involved actually melt and blend together both seem like examples where a chemical bond is happening, but only cadmium welding really is. Brazing happens between similar materials and no new alloy is formed (yes; if a new alloy is formed, it's a type of weld, now brazing).

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