Solder is a metal alloy with low melting point. It is used to join two metal surfaces by alloying (in the molten state) with their surfaces. Solder is used with flux, a substance that keeps the surfaces to be soldered clean during the soldering process. There are many types of solder and flux; the solder and flux must be suited to the metals to be soldered together and to each other. For example, using fluxes other than rosin on electronic circuit boards will lead to corrosion that destroys the boards.

Alloys with melting points above 1000°F are also used to join metals and are sometimes referred to as solders, or hard solders. Actually that process is brazing, not soldering, and this entry is not about such materials.

Copper, tin, gold, silver, platinum and palladium are easily soldered, nickel and brass a bit less so. Cadmium, lead and bronze are still harder. Finally, zinc, steel, and stainless steel are very difficult to solder. Aluminum cannot be soldered with tin-lead alloys, but can be with a barium-aluminum alloy at around 750°F, while continuously scrubbing the surfaces with a stainless steel brush to remove oxide. Only certain aluminum alloys can be “soldered.”

Solder for electronics

The composition of tin-lead solder is sometimes shown in the form “Sn60”. “Sn” is the chemist’s symbol for the element tin. This designation means the solder is 60% tin and the rest lead. When the alloy designation contains a slash, for example, “60/40”, the first number is the percent tin and the second the percent lead.

An important tin/lead solder is 63/37, called “eutectic,” meaning it goes directly from solid to liquid without a pasty stage. This alloy melts at 361°F (183°C), the lowest melting point of any tin-lead alloy. Other eutectic solders include

The advantage of a eutectic solder is that it flows very easily. Surface mount components are usually soldered with a eutectic solder. A slightly different alloy, 60/40, provides a thicker coating on wires.

Almost all the solder sold for hobbyist use in electronics is in the form of wire with cores of rosin flux. Some guidelines for hand soldering of electronics:

Health hazards

Fumes from fluxes and lead oxide. Good ventilation is required.


Clifford L. Barber.
Solder. Its Fundamentals and Usage. 2nd Ed.
Chicago: Kester Solder Company, 1961.

Kester's website,, is a good source of current information, including a webography on the development of lead-free solders.

National Aeronautics and Space Administration. Office of Technology Utilization.
Soldering Electrical Connections. A Handbook. 4th edition.
NASA SP-5002.
Washington, D.C.: U. S. Gov't Printing Office, 1967.

Health problems associated with rosin fluxes.

Solder for plumbing

Since 1988 in the United States, solder used to join copper water pipes is no longer permitted to contain lead. It is more than 90% tin with the rest antimony.

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