Lead-tin alloys are binary metallic systems where lead and tin are combined to create materials with properties superior to either pure metal, most famously used as solder. The defining characteristic of this alloy family is the presence of a eutectic composition. A eutectic is a specific mixture that melts and solidifies at a single, lower temperature than any other mixture of the two components. For lead-tin, the eutectic point occurs at approximately 63% tin and 37% lead (by weight), with a melting temperature of 183°C (361°F). This is known as 63/37 solder.
Alloys with tin content above or below the eutectic point have a melting range (a solidus and liquidus temperature) rather than a single melting point. For example, common 60/40 solder melts between 183°C and 190°C. This property is harnessed in soldering; the alloy remains in a pasty, semi-solid state within this range, allowing the worker to adjust components before full solidification. The addition of tin drastically improves the wetting ability, strength, and electrical conductivity of the solder joint compared to pure lead.
Beyond the ubiquitous 60/40 and 63/37 solders, other compositions exist. High-lead alloys (e.g., 95%Pb-5%Sn) have higher melting points and are used for high-temperature applications like radiator repair. High-tin alloys (e.g., 90%Sn-10%Pb) offer better strength. The versatility in melting behavior and joint properties cemented lead-tin alloys as the cornerstone of joining technology for most of the 20th century across electronics, plumbing, and sheet metal work.
