Solder - Lead Solder

Lead Solder

Tin/lead solders, also called soft solders, are commercially available with tin concentrations between 5% and 70% by weight. The greater the tin concentration, the greater the solder’s tensile and shear strengths. Alloys commonly used for electrical soldering are 60/40 Tin/lead (Sn/Pb) which melts at 370 °F or 188 °C and 63/37 Sn/Pb used principally in electrical/electronic work. The 63/37 is a eutectic alloy, which:

1. has the lowest melting point (183 °C or 361.4 °F) of all the tin/lead alloys; and
2. the melting point is truly a point — not a range.

In plumbing, a higher proportion of lead was used, commonly 50/50. This had the advantage of making the alloy solidify more slowly, so that it could be wiped over the joint to ensure watertightness, the pipes being physically fitted together before soldering. Although lead water pipes were displaced by copper when the significance of lead poisoning began to be fully appreciated, lead solder was still used until the 1980s because it was thought that the amount of lead that could leach into water from the solder was negligible from a properly soldered joint. The electrochemical couple of copper and lead promotes corrosion of the lead and tin, however tin is protected by insoluble oxide. Since even small amounts of lead have been found detrimental to health, lead in plumbing solder was replaced by silver (food grade applications) or antimony, with copper often added, and the proportion of tin was increased (see Lead-free solder.)

The addition of tin—more expensive than lead—improves wetting properties of the alloy; lead itself has poor wetting characteristics. High-tin tin-lead alloys have limited use as the workability range can be provided by a cheaper high-lead alloy.

In electronics, components on printed circuit boards (PCBs) are connected to the printed circuit, and hence to other components, by soldered joints. For miniaturized PCB joints with surface mount components, solder paste has largely replaced solid solder.

Lead-tin solders readily dissolve gold plating and form brittle intermetallics.

Sn60Pb40 solder oxidizes on the surface, forming a complex 4-layer structure: tin(IV) oxide on the surface, below it a layer of tin(II) oxide with finely dispersed lead, followed by a layer of tin(II) oxide with finely dispersed tin and lead, and the solder alloy itself underneath.

Lead, and to some degree tin, as used in solder contains small but significant amounts of radioisotope impurities. Radioisotopes undergoing alpha decay are a concern due to their tendency to cause soft errors. Polonium-210 is especially problematic; lead-210 beta decays to bismuth-210 which then beta decays to polonium-210, an intense emitter of alpha particles. Uranium-238 and thorium-232 are other significant contaminants of alloys of lead.