Solder and Soldering Iron

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Solder and Soldering Iron


The soldering principle

The soldering technique

Brazing and welding


Soldering is the process by which two pieces of base metal are joined to each other by means of a filler alloy, which generally have a melting point of below 840°F(450°C). The tool used to make this type of joint is called a soldering iron, and the alloy from which the connection is made is called a solder. The resulting connection, or joint, is not as strong as the base metal, but still has enough strength, conductivity, and other desirable features to satisfy its uses. Soldering can be used for making either a mechanical or an electrical connection. An example of the former case is the situation in which a plumber uses plumbers solder to connect two pieces of pipe with each other. An example of the latter case is the situation in which a worker connects an electrical wire to a printed board.

The technique of soldering has been known to human artisans for many centuries. Some metal work recovered from the remains of ancient Egypt and Mesopotamia, for example, contains evidence of primitive forms of soldering. As workers became more familiar with the properties of metals in the late Middle Ages, soldering became a routine technique in metal work of various kinds.


The vast majority of solders are alloys that contain tin, lead, and, sometimes, one or more other metals. For example, the well-known general solder known as plumbers solder consists of 50% lead and 50% tin. A solder used to join surfaces that contain silver is made of 62% tin, 36% lead, and 2% silver. In addition, a solder that melts at unusually low temperatures can be made from 13% tin, 27% lead, 10% cadmium, and 50% bismuth. The most widely used solders for making electrical connections consist of 60 to 63% tin and 37 to 40% lead.

Solder alloys are available in many forms, such as wire, bar, foil, rings, spheres, and paste. The specific kind of solder selected depends on the kind of junction to be formed. Foil solder, for example, may be called when the junction to be formed has a particular shape that can be stamped or cut out prior to the actual soldering process.

The soldering principle

The solder alloy used to join too pieces of metal, the parent metals, has a melting point less than that of either parent metal. When it is placed between the two parents, it slowly changes from a liquid to a solid. The soldering iron is used to melt the solder and it is then allowed to cool.

While the process of solidification is taking place, the solder alloy begins to form a new alloy with each of the parent metals. When the solder finally cools, therefore, the joint consists of five segments: parent metal #1; a new alloy of parent metal #1 and the solder alloy; the solder alloy itself; a new alloy of parent metal #2 and the sold alloy; and parent metal #2.

The primary function to the soldered junction, of course, is to provide a connection between the two parent metals. However, the junction is not a permanent one. In fact, an important characteristic of the soldered connection is that it can be broken apart with relative ease.

The soldering technique

The first step in making a soldered connection is to heat the solder alloy until it melts. In the most primitive form of soldering irons, this step can be accomplished simply by heating a metal cylinder. Then, the cylinder is used to melt the alloy, which is attached to the parent metals. However, most soldering irons are now heated by an electrical current that is designed to apply exactly the right amount of solder in precisely the correct position between the two parent metals.

The joining of two parent metals is usually more difficult than might be suggested by the foregoing description because most metals oxidize when exposed to air. That means that the faces (that is, the metal oxides that cover their surfaces) of the two parent metals must be cleaned before soldering can begin. In addition, care must be taken that the surfaces do not re-oxidize at the high temperature used in making the solder. The most common way of accomplishing this goal is to use an acidic flux in addition to the solder itself. An acidic flux is a material that can be mixed with the solder, but that melts at a temperature less than the solders melting point. As soldering begins, therefore, the flux insures that any new oxide formed on the parent metals will be removed.


Acidic Having the qualities of an acid, one of which is that it will react with and neutralize metallic oxides.

Alloy A mixture of two or more metals with properties distinct from the metals of which it is made.

Flux A low melting point material used in soldering and other processes that helps keep surfaces clean and aids in their joining with each other.

Parent metal One of the two metals that is joined to each other during soldering, brazing, or welding.

Brazing and welding

Brazing and welding have sometimes been described as specialized forms of soldering. These two techniques also involve the joining of two metals with each other, but each differs from soldering in some important ways. Probably the single most important difference is the temperature range at which each takes place. While most forms of soldering occur at temperatures in the range from 356°F (180°C) to 590°F (310°C), brazing usually takes place in the range from 1,022°F (550°C) to 2,012°F (1,100°C), and welding in the range from 1,832°F (1,000°C) to 6,332°F (3,500°C).

The first step in both brazing and welding is to clean the two surfaces to be joined. In brazing, a filler is then inserted into the gap between the two surfaces and heat is added, either at the same time or immediately after the filler has been put into place. The filler then fuses to form a strong bond between each of the two surfaces. The filler used in brazing is similar to solder and performs the same function, but it melts at a higher temperature than does solder.

During the welding process, a thin stick of filler is added to the gap between the two surfaces to be joined the same time, a hot flame is applied to the gap. The filler melts, as do the surfaces of both metals being joined to each other. In this case, the two metal surfaces are actually joined together and not just to the filler itself, as is the case with soldering and brazing.

Most alloys used for brazing contain copper and zinc, often with one or more other metals. The term brazing itself, in fact, derives from the fact that copper and zinc are also the major components of the alloy known as brass.

See also Metal production.



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David E. Newton