What is solder

An overview of what is solder, the types to be used for electronics soldering and how it may be used for soldering in electronic construction.

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Although solder is widely used in the electronics industry and for connecting components and securing them in place, it is sometimes useful to ask what is solder to gain an understanding of what it is and how best to use it.

Solder is obviously a key element in any form of electronics construction where soldering is undertaken. It provides a mechanical and electrical joint that is essential to keep components in place once a circuit has been assembled. While the mechanical strength is important, it is also necessary to ensure that the soldered joint provides a good electrical connection is made between the two connections that require joining. This can only be achieved satisfactorily if the medium, i.e. the solder joining the two conducts electricity well.

A further requirement of the material making the joint is that it must be easy and convenient to use. Solder has the advantage, unlike other mixtures or pure metals that it melts at a relatively low temperature. This means that it is possible to use it in the construction of electronics equipment..

What is solder?

Traditional solder that used to be used for soldering in the electronics industry used to be a mixture of tin and lead. Different types of solder are used for different purposes. Solder is of course used in plumbing. The type of solder used for electronics usually contains 60% tin and 40% lead. This is often known as "60/40" solder. It is what is termed a "Eutectic Mixture". Although this is a chemical term, basically this type of mixture melts at a temperature well below that which might normally be expected.

In additional to the metal constituents flux is also required to ensure that good joints are made. Flux is an acidic mixture that helps remove oxides from the area of the joint and therefore helps the solder flow easily over the joint and form a good bond. The flux can be seen as a brown liquid as a solder is heated, and it sometimes gives off a pungent smoke that can act as a irritant. This is one reason why soldering should always be carried out in a well ventilated area.

For plumbers solder, a separate flux is used, but for electronics solder, it is contained within the solder itself. The solder used for electronics is normally in the form of a wire, and if it is carefully cut small cores of flux can be seen running through the solder. The amount contained is correct for making electronics joints and separate flux should never be used.

Solder sizes

Solder for electronic construction is sold in the form of a thin wire. When soldering, this makes it easy to handle and apply to the place where the soldered joint is to be made. Different sizes of solder are available and a choice will have to be made when buying any. For most applications for electronics soldering, this will make little difference. The thinner wire is much better for fine work. If it is used for normal soldering then the thin solder can be slightly less convenient because longer lengths are used making a normal joint.

The different solders that are produced are generally specified either in Standard Wire gauge (SWG) or America Wire gauge (AWG). For most printed circuit and general soldering work choose 20 to 22 SWG solder (19 - 21 AWG), although for larger joints a thicker solder, possibly 18 SWG (16 AWG) is better.

Lead free solder

With the drive to reduce the amount of lead being used for environmental and health reasons, lead free solder is now being widely used. A European Directive mandates that solder containing lead shall not be used commercially. This means that for any soldering to be undertaken by the hobbyist the traditional solder containing lead will not be available.

The traditional tin lead solder is being replaced by other types of lead free solder. A variety are appearing on the market. One type contains 99.3% tin, and 0.7% copper. This has a very similar melting point to tin lead solder and melts at around 227C. Another type is being sold and contains a small amount of silver. Although slightly more expensive than the tin copper solder it has a lower melting point at around 217C. This second type of solder is sometimes sold as "Lead free silver solder". However it should be remembered that it only has a small amount of silver, and there are other silver solders on the market. These solders are sold in reels or in dispensers in the same way that the traditional tin lead solder was sold.

In use these new lead free solders perform in a very similar fashion to that of the traditional solder. Although the melting point is slightly higher, this is not normally noticeable when used with ordinary soldering irons. There are also not believed to be any other differences in the method of use. It should therefore not present a problem to the home constructor or hobbyist.


When starting to build electronic circuits it is essential to choose a solder suitable for the job, and not a type of solder used for plumbing, etc. With the new directives on the use of lead free solder, these new forms of solder are now widely available, and should obviously be used if possible.

When soldering, every care should be taken to ensure that the hot soldering iron cannot cause injury or fire, and additionally it is necessary to ensure that soldering is undertaken in a well ventilated area so that the flux fumes are not inhaled. A general safety mindset should be used to ensure that no injury or health hazards are present, and in this way, circuits can be safely constructed and soldered. The end result can then be pride in the circuit that has been built.

Although solder is taken for granted in the electronics industry, it is often helpful to ask what is solder so that a greater understanding of its nature and properties can be gained so that it can be used correctly and better results obtained.

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