Tin/Lead Solder

General Function of Solders
The function of solder is to join 2 or more metals at temperatures below their melting point. Solder provides a metal solvent action between the solder and metal(s) being joined. This "solution" of metal in the solder results in an intermediate alloy being formed. This provides metal and electrical continuity and results in low resistance connections. A solder joint also allows for torsional stress due to temperature changes without rupture of the joint. In un-soldered connections, the metals are mechanically attached (crimped), but still remain separate pieces of metal.

Tin/lead alloys are the most widely used of all solders. They offer an advantageous low melting range, making them ideal for joining most metals with no damage to heat-sensitive parts. When referring to tin/lead solders, the tin content is customarily given first. For example, 40/60 refers to 40 percent tin and 60 percent lead by weight. The melting temperature or range defines the usefulness of the alloy.

Low Tin Solders
Solders containing less than 5 percent tin are used for sealing pre-coated containers, coating and joining metals, and for applications where the service temperatures exceed 250°F. At those temperatures, the solder functions primarily as a seal, and strength is a function of design.

10/90, 15/85, and 20/80 Solders
These solders are used for sealing cellular automobile radiators, and is an old alloy used for filling seams and dents in automobile bodies. The high lead content offers good corrosion resistance.

60/40 and 63/37 Solders
These solders are used in the stained glass industry and also for electronic component such as computers and communications equipment where a minimum of heat can be used to make the connection. For the electronics industry, silver is added to tin/lead solders to reduce the dissolution of silver from silver alloy coatings. Silver may also be added to improve creep resistance.

70/30 Solder
This alloy is used for coating or pre-tinning before soldering.

Qualities of Tin/Lead Solder - Tin/Lead Phase Diagram
Pure metals have a distinct melting point. Most alloys (mixtures of metals) have a range over which they melt. Some alloys melt at a single point, like pure metals. These alloys are called "eutectics". The eutectic is also the lowest melting point alloy. The "Liquidus" is the temperature above which an alloy is completely liquid (Line A, B, C), and the "Solidus" is the temperature below which an alloy is completely solid (line A, D, E, C). The area between the Liquidus and Solidus is called the "Pasty Range". The Solidus and Liquidus are the same temperature for a Eutectic alloy. For a better look at the Tin/Lead Phase Diagram click the diagram below. You will need Adobe's Acrobat Reader.

Benefits of Tin/Lead Alloys

• Lowest cost joining method compared to welding, brazing, mechanical joining
• Very fast method, easy, low skill level required
• Versatile heat sources - soldering iron, torch, oven
• Minimal effect on metals jointed
• Easy to take joints apart
• Low melting point
• High electrical conductivity
• Relatively strong joint at and below room temperature