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
. At the retail level, the two most common alloys 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 ratio is notable in that it is a eutectic
mixture, which means:
- It has the lowest melting point (183 °C or 361.4 °F) of all the tin/lead alloys; and
- 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.