Solder is a fusible alloy or metal applied in the molten state to join pieces of metal. Solder must melt more easily than the metal to which it is applied. A solder is used to hold parts together, make a liquid-tight or gas-tight joint, seal joints against corrosion, or provide a reliable electrical connection.

Soldering differs from brazing in that temperatures below 800°F (427°C) are used. The most commonly used solders are alloys of lead and tin, although antimony, bismuth, cadmium, or silver sometimes is added to improve strength or obtain a desired melting range. Materials that are readily solderable include gold, silver, copper, tin, lead, and brass. Materials that are moderately difficult to solder include iron and nickel. Some materials, such as tungsten, must be plated with a solderable metal before they can be soldered.

Solder Alloys

Many alloys are produced for soldering. A 50%-lead, 50%-tin alloy is the most popular alloy for general purposes. A low-cost alloy of 90$ lead and 10% tin is used in dip pots. A 97%-lead, 2.5%-silver alloy or a 95%-tin, 5%-antimony alloy is used for high strength at high temperatures.

Metal Surfaces

The binding power of a solder largely depends on adhesion, so its wetting ability and penetration power are very important qualities. To accept solder readily, metal surfaces must be free of grease, dirt, and oxide films. Grease should be removed by using a solvent, and dirt and oxide films by using emery cloth or steel wool. If large areas are to be joined, they should be precoated with solder or pure tin.

Photo by Nicolas Raymond
Photo by Nicolas Raymond


Cleaned parts usually are coated with a flux before heat is applied to the surfaces to be joined. The flux protects the metal surfaces from oxidation while being heated, dissolves any oxides on the metal surfaces, and lowers the surface tension of the solder so that it flows freely into the joint. Rosin is a commonly used flux for making electrical connections. Rosin with added chemical agents sometimes is packaged as the core of a solder in wire form. Where more chemically active fluxes are needed, zinc chloride and ammonium chloride are used, either as a wire core or as a separate paste.

Soldering Methods

There are several methods of heating the parts to be joined by soldering. An electrically heated soldering iron commonly is used for heating light parts and thin sheet-metal joints. The hot soldering iron is tinned and then held flat against the metal surfaces to be joined. Solder in wire form is then fed by hand into the joint. A gas torch can be used to heat larger surfaces for soldering. A soldering pot filled with molten solder can be used for precoating and for soldering assembled joints. An electric furnace provides a fast and easily controlled method of heating the metal pieces.

Joint Strength

The mechanical load on the solder in a joint should be as low as possible. Lock seams should be used in joining sheet metal, and wires should be hooked, clamped, or clipped whenever possible. A good method is to fasten the parts so that they will stay in position and hold the required load before the solder is applied to them. Where the solder must be depended on to carry some load, lap joints should be used. Butt joints should not be used. History. Soldering was first practiced in the Middle East and Egypt some time between 4000 and 3000 B.C. Copper-gold alloys and lead-tin alloys were used. By 2000 B.C., soldering had become a part of the jewelry technology.

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