Lead-based Tin-
lead (Sn-Pb) 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. Lead mitigates the formation of
tin whiskers, Today, many techniques are used to mitigate the problem, including changes to the annealing process (heating and cooling), addition of elements like copper and nickel, and the application of
conformal coatings. Alloys commonly used for electrical soldering are 60/40 Sn-Pb, which melts at , and 63/37 Sn-Pb used principally in electrical/electronic work. The latter mixture is a
eutectic alloy of these metals, which: • has the lowest melting point () of all the tin-lead alloys; and • the melting point is truly a
point — not a range. In the United States, since 1974, lead is prohibited in solder and flux in plumbing applications for drinking water use, per the
Safe Drinking Water Act. Historically, a higher proportion of lead was used, commonly 50/50. This had the advantage of making the alloy solidify more slowly. With the pipes being physically fitted together before soldering, the solder could be wiped over the joint to ensure water tightness. 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. Tin, however, is protected by insoluble oxide. Since even small amounts of lead have been found detrimental to health as a potent
neurotoxin, 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. Lead-tin solders readily dissolve
gold plating and form brittle intermetallics. 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 troublesome;
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.
Lead-free The
European Union Waste Electrical and Electronic Equipment Directive and
Restriction of Hazardous Substances Directive were adopted in early 2003 and came into effect on July 1, 2006, restricting the inclusion of lead in most consumer electronics sold in the EU, and having a broad effect on consumer electronics sold worldwide. In the US, manufacturers may receive tax benefits by reducing the use of lead-based solder. Lead-free solders in commercial use may contain tin, copper, silver,
bismuth,
indium,
zinc,
antimony, and traces of other metals. Most lead-free replacements for conventional 60/40 and 63/37 Sn-Pb solder have melting points from 50 to 200 °C higher, though there are also solders with much lower melting points. Lead-free solder typically requires around 2% flux by mass for adequate wetting ability. When lead-free solder is used in
wave soldering, a slightly modified solder pot may be desirable (e.g.
titanium liners or impellers) to reduce maintenance cost due to increased tin-scavenging of high-tin solder.
Tin-silver-copper (Sn-Ag-Cu, or
SAC) solders are used by two-thirds of Japanese manufacturers for reflow and
wave soldering, and by about 75% of companies for hand soldering. The widespread use of this popular lead-free solder alloy family is based on the reduced melting point of the Sn-Ag-Cu ternary eutectic behavior (), which is below the 22/78 Sn-Ag (wt.%) eutectic of and the 99.3/0.7 Sn-Cu eutectic of .
Hard solder Hard solders are used for brazing, and melt at higher temperatures. Alloys of copper with either zinc or silver are the most common. In
silversmithing or
jewelry making, special hard solders are used that will pass
assay. They contain a high proportion of the metal being soldered and lead is not offen used in these alloys. These solders vary in hardness, designated as "enameling", "hard", "medium", "easy" and "repair".
Enameling solder has a high melting point, close to that of the material itself, to prevent the joint
desoldering during firing in the enameling process. The remaining solder types are used in decreasing order of hardness during the process of making an item, to prevent a previously soldered seam or joint desoldering while additional sites are soldered. Easy solder or repair solder are also often used for repair work for the same reason. Flux is also used to prevent joints from desoldering. Silver solder is also used in manufacturing to join metal parts that cannot be
welded. The alloys used for these purposes contain a high proportion of silver (up to 40%), and may also contain
cadmium.
Alloys Different elements serve different roles in the solder alloy: •
Antimony is added to increase strength without affecting wettability. Prevents tin pest. Should be avoided on zinc, cadmium, or galvanized metals as the resulting joint is brittle. •
Bismuth significantly lowers the melting point and improves wettability. In presence of sufficient lead and tin, bismuth forms crystals of with melting point of only 95 °C, which diffuses along the grain boundaries and may cause a joint failure at relatively low temperatures. A high-power part pre-tinned with an alloy of lead can therefore desolder under load when soldered with a bismuth-containing solder. Such joints are also prone to cracking. Alloys with more than 47% Bi expand upon cooling, which may be used to offset thermal expansion mismatch stresses. Retards growth of
tin whiskers. Relatively expensive, limited availability. •
Copper improves resistance to thermal cycle fatigue, and improves
wetting properties of the molten solder. It also slows down the rate of dissolution of copper from the board and part leads in the liquid solder. Copper in solders forms intermetallic compounds. Supersaturated (by about 1%) solution of copper in tin may be employed to inhibit dissolution of thin-film under-bump metallization of
BGA chips, e.g. as . Suboptimal amounts may be used to avoid patent issues. Fluidity reduction increase hole filling and mitigates bridging and icicles. •
Cobalt is used instead of nickel to avoid patent issues in improving fluidity. Does not stabilize intermetallic growths in solid alloy. •
Indium lowers the melting point and improves ductility. In presence of lead it forms a ternary compound that undergoes phase change at 114 °C. Very high cost (several times of silver), low availability. Easily oxidizes, which causes problems for repairs and reworks, especially when oxide-removing flux cannot be used, e.g. during GaAs die attachment. Indium alloys are used for cryogenic applications, and for soldering gold as gold dissolves in indium much less than in tin. Indium can also solder many nonmetals (e.g. glass, mica, alumina, magnesia, titania,
zirconia, porcelain, brick, concrete, and marble). Prone to diffusion into semiconductors and cause undesired doping. At elevated temperatures easily diffuses through metals. Low vapor pressure, suitable for use in vacuum systems. Forms brittle intermetallics with gold; indium-rich solders on thick gold are unreliable. Indium-based solders are prone to corrosion, especially in presence of
chloride ions. •
Lead is inexpensive and has suitable properties. Worse wetting than tin. Toxic, being phased out. Retards growth of tin whiskers, inhibits tin pest. Lowers solubility of copper and other metals in tin. •
Silver provides mechanical strength, but has worse ductility than lead. In absence of lead, it improves resistance to fatigue from thermal cycles. Using SnAg solders with HASL-SnPb-coated leads forms phase with melting point at 179 °C, which moves to the board-solder interface, solidifies last, and separates from the board. High ion mobility, tends to migrate and form short circuits at high humidity under DC bias. Promotes corrosion of solder pots, increases dross formation. •
Tin is the usual main structural metal of the alloy. It has good strength and wetting. On its own it is prone to
tin pest and growth of
tin whiskers. Readily dissolves silver, gold and to less but still significant extent many other metals, e.g. copper; this is a particular concern for tin-rich alloys with higher melting points and reflow temperatures. •
Zinc lowers the melting point and is low-cost. However, it is highly susceptible to corrosion and oxidation in air, therefore zinc-containing alloys are unsuitable for some purposes, e.g. wave soldering, and zinc-containing solder pastes have shorter shelf life than zinc-free. Can form brittle Cu-Zn intermetallic layers in contact with copper. Readily oxidizes which impairs wetting, requires a suitable flux. •
Germanium in tin-based lead-free solders influences formation of oxides; at below 0.002% it increases formation of oxides. Optimal concentration for suppressing oxidation is at 0.005%. Used in e.g. Sn100C alloy. Patented. •
Rare-earth elements, when added in small amounts, refine the matrix structure in tin-copper alloys by segregating impurities at the grain boundaries. However, excessive addition results in the formation of tin whiskers; it also results in spurious rare earth phases, which easily oxidize and deteriorate the solder properties. == Flux ==