Lead is the traditional partner for tin in many solder alloys.
Traditional soft solder commonly used an alloy of tin and lead, often near a 60:40 or 63:37 composition. These alloys melt at relatively low temperatures and flow readily, making them useful for joining metal surfaces in plumbing and electronics.
The 63:37 tin–lead composition is close to a eutectic alloy, meaning it changes from solid to liquid over a particularly narrow temperature range. This behavior helps create neat, reliable joints. Modern electronics and plumbing increasingly use lead-free solders because lead is toxic and can accumulate in people and the environment.
Solder should not be confused with welding. Soldering joins surfaces using a filler metal that melts below the workpieces’ melting points, while welding generally melts and fuses the base materials themselves.