Welding and soldering

Last updated: February 27, 2009.

If you're building almost anything that uses metal, from a nuclear submarine to a laptop computer, one thing you'll need to be able to do is join metals together. Welding is a way of tightly bonding two metals by melting them where they meet, while soldering involves making a joint between components in an electric or electronic circuit. Both are highly effective, though they're very different and work in completely different ways—don't mix them up!

Photo: Arc welding uses electricity to generate intense heat that fuses metals together. Photo by Joshua C. Kinter courtesy US Navy.

What is welding?

You can't really join metals with adhesive—not with ordinary glue, anyway. But you can join them by melting them together in a process known as welding. The basic idea is simple: you apply a source of heat to melt the two metals so they fuse and form a secure joint. Usually (though not always) you add other materials as you apply the heat: a filler (an extra piece of metal, supplied from something called a welding rod, which seals up any gaps where the main metals meet) and a flux (a nonmetallic chemical that helps to stop the molten metals forming oxides and nitrides with gases in the air, which weakens the joint). As an alternative to using a flux, you can weld in an atmosphere from which the air has been removed (filled with other, nonreactive gases such as argon, for example).

Photo: Arc welding generates very bright light as well as intense heat; that's why welders need to wear protective masks. Photo by Lamel J. Hinton courtesy US Navy.

Most forms of welding involve joining metals with heat alone. But they differ in where the heat comes from. One common form of welding involves using an oxyacetylene gas torch, which makes an intense flame by burning acetylene (an energy-rich fuel made from a simple hydrocarbon molecule) in a rich supply of oxygen. Although convenient and portable, oxyacetylene torches are relatively expensive to use (because the fuel is supplied in gas cylinders). In factories, it's usually more convenient to weld with electrical power using a technique known as arc welding. Instead of a gas torch, you use a piece of metal called an electrode connected to a high-current power supply (hundreds of times higher than the ones that flow through appliances in your home). As you bring the electrode up to the joint you're welding, it creates a spark or arc that melts the metals together. Arc welding produces both bright visible sparks and discharges of ultraviolet light, both of which can lead to blindess; that's why you'll always see people arc welding behind wraparound protective visors. Other heat sources for precision welding include ultrasonics, lasers, and electron beams.

You can also weld materials by forcing them together through sheer pressure, with or without extra heat. This is known as pressure welding; used for many hundreds of years by blacksmiths and other artisans, it's one of the oldest metalworking techniques. The basic process involves heating metals in a forge and then hammering them together so they fuse.

Photo: One way to make arc welding safer: get a robot like this to do it for you! Photo by Idaho National Laboratory courtesy US Department of Energy (DOE).

What is soldering?

Soldering is quite similar to welding—but also quite different! In welding, you're trying to make a super-strong joint between two pieces of metal. Often a welded joint has to stand up to incredible stresses and strains—for example, if you weld parts of a car body or an airplane fuselage together. So the objective is to make a good mechanical connection. When you solder, the idea is usually to to make a good electrical connection using solder.

Solder looks a bit like an unwrapped paperclip, though it's much softer, and it typically comes in tubes and reels. It's an alloy of different metals that has a relatively low melting point. The solder I use, which is typical, is made of 99.25 percent tin and 0.75 percent copper, though other metals such as zinc, silver, and bismuth are also used. (Lead was once widely used in solders with tin, but has now been largely phased out for health and safety reasons). Solders sometimes also contain fluxes to prevent the formation of oxides.

Photo: Solder looks and feels like a long unwrapped paperclip, coiled up in a plastic dispenser tube like this one. You pull out a short length as you need it. This is lead-free solder made mostly from tin and copper.

Why do you need to solder? Electronic circuits are made of discrete components: tiny devices such as resistors, capacitors, transistors, and LEDs that do specific jobs. When you put them together in different ways, you can build all kinds of amazing electronic gadgets, from radios and televisions to calculators and computers. The components all have little metal legs—terminals that you use to connect them into the circuits. You could just wire these legs together with electrical cables, but the wires might drop off or wriggle free and the connections wouldn't be reliable, so anything you built this way wouldn't work very well. And that's where solder comes in: it makes a much more effective electrical connection.

If you want to make a good soldered joint, you don't solder straight away. First, you clean the components you want to join (for example, by scraping them with a knife to remove any surface oxides). Then you make a good mechanical connection between them (by wrapping the cable tightly round the component or whatever). Only then do you make a good electrical connection by melting some solder on top.

How does it work in practice? You melt the solder over a joint by applying a hot tool called a soldering iron (essentially a hot piece of metal with a pointed tip, with the heat generated inside it by an electrically powered heating element). It's very important to note that solder is not glue: it is not designed to make a mechanical connection. If you rely on solder alone to fasten two wires together, they'll probably break apart sooner or later. It's important to make a good mechanical connection and then solder on top. There are good and bad ways to solder, some of which make poor joints that don't conduct electricity properly. If you plan on doing your own electronic projects, the first thing to do is learn how to solder properly.

Photo: You solder by holding your hot soldering iron to the joint in your circuit where you want to make an electrical connection. Then, with your other hand, you apply the solder until it melts in a blob on top of the joint, usually with a puff of "smoke" (actually the metals in the solder turning into gas form). Photo by John Wagner courtesy US Navy, with annotations by Explain that Stuff.

What is brazing?

Brazing is a similar process to soldering except that the filler you use (equivalent to solder) works much more like an adhesive. When you solder, you're simply melting a low-temperature alloy on top of the terminals you're joining together to make a reliable electrical bridge between them. The terminals themselves don't actually melt and aren't usually changed in any way: the solder just sits on top. But when you braze, you work at a much higher temperature. The filler melts and seeps right into the surface of the metals you're joining, so it binds them together securely. Brazing is thus a bit like a cross between welding and soldering and it's mainly concerned with making a secure mechanical joint.