Soldering filler materials are available in many different alloys for differing applications. In electronics assembly, the eutectic alloy of 63% tin and 37% lead (or 60/40, which is almost identical in melting point) has been the alloy of choice. Other alloys are used for plumbing, mechanical assembly, and other applications. Some examples of soft-solder are tin-lead for general purposes, tin-zinc for joining aluminium, lead-silver for strength at higher than room temperature, cadmium-silver for strength at high temperatures, zinc-aluminium for aluminium and corrosion resistance, and tin-silver and tin-bismuth for electronics.
A eutectic formulation has advantages when applied to soldering: the liquidus and solidus temperatures are the same, so there is no plastic phase, and it has the lowest possible melting point. Having the lowest possible melting point minimizes heat stress on electronic components during soldering. And, having no plastic phase allows for quicker wetting as the solder heats up, and quicker setup as the solder cools. A non-eutectic formulation must remain still as the temperature drops through the liquidus and solidus temperatures. Any movement during the plastic phase may result in cracks, resulting in an unreliable joint.
Common solder formulations based on tin and lead are listed below. The fraction represent percentage of tin first, then lead, totaling 100%:
63/37: melts at 183 °C (361 °F) (eutectic: the only mixture that melts at a point, instead of over a range)
60/40: melts between 183–190 °C (361–374 °F)
50/50: melts between 183–215 °C (361–419 °F)
For attachment of electronic components to a PCB, proper selection and use of flux helps prevent oxidation during soldering, which is essential for good wetting and heat transfer. The soldering iron tip must be clean and pre-tinned with solder to ensure rapid heat transfer. Components which dissipate large amounts of heat during operation are sometimes elevated above the PCB to avoid PCB overheating. After inserting a through-hole mounted component, the excess lead is cut off, leaving a length of about the radius of the pad. Plastic or metal mounting clips or holders may be used with large devices to aid heat dissipation and reduce joint stresses.