Solder is a spool of wire that is used to join two metals. It is melted and applied to a metal joint by using an electric soldering iron or a propane/butane torch. Solder is typically composed of tin, lead and/or silver. The amount of each metal in the solder blend, or "alloy," affects how the solder behaves when it is melted. That behavior determines if a specific alloy of solder is appropriate for a task.
The term "alloy" refers to a "blend" of two or more metals. The most common solder alloy is a combination of tin and lead. Tin has a melting point of about 450 degrees F and lead has a melting point of about 620 degrees F. When 60 percent tin and 40 percent lead are combined in an alloy, the overall melting point of the solder is lowered to about 360 degrees F. The ratio of tin to lead affects the melting temperature, the strength when solid and how long the solder remains "liquid."
Flux is a chemical that is used with solder to "clean" the surfaces of the metal joint. That cleaning process is what helps the solder "stick" to metal surfaces. Flux is available as a separate jar of paste, or it can be included in the solder alloy, making it "flux-core" solder. Generally, flux doesn't affect the melting point but it does create a lot of the smoke and fumes that result from soldering.
The 60/40 alloy is one of the most common types of solder. The alloy contains 60 percent tin and 40 percent lead (tin is the first number in the label). The chemical symbols may also be included in the label such as "60Sn/40Pb" (tin is Sn and lead is Pb). The 60/40 solder has a melting point of approximately 360 degrees F and an electric soldering iron is used to melt and apply it. Flux-core 60/40 solder is a common choice for electronics work because it has a low melting point, it cools quickly and it "re-solidifies" quickly.
The 40/60 solder alloy contains 40 percent tin and 60 percent lead. It may also be labeled "40Sn/60Pb." Because of the increased amount of lead, the solder has a melting point of about 460 degrees F. One advantage of this alloy is that it takes longer to re-solidify. This means that the solder can be "adjusted" while it is in melted or liquid form. The 40/60 alloy is mainly used in automotive applications involving sheet metal, body work or radiator repair. A high-heat tool, such as a torch is typically used with 40/60 solder.