Composition
Lead based
Tin-lead (Sn-Pb) solders, also called soft solders, are sold with tin concentrations between 5% and 70% by weight. The higher the concentration of tin, the higher the tensile and shear strength of the solder. Lead mitigates the formation of tin beard "Beard (metallurgy)",[4] although the exact mechanism is unknown.[5] Today, many techniques are used to mitigate the problem, including changes to the annealing process (heating and cooling), the addition of elements such as copper and nickel, and the application of conformal coatings.[6] The alloys commonly used for electrical soldering are 60/40 Sn-Pb, which melts at 188 °C (370 °F),[7] and 63/37 Sn-Pb used mainly in electrical/electronic work. This last mixture is a eutectic alloy of these metals, which:
In the United States, since 1974, lead has been banned in solder and fluxes for plumbing applications intended for drinking water consumption, in accordance with the Safe Drinking Water Act.[8] Historically, a higher proportion of lead was used, commonly 50/50. This had the advantage of making the alloy more resistant to corrosion. This had the advantage of making the alloy solidify more slowly. Because the pipes were physically fitted together before being welded, the weld could be cleaned over the joint to ensure watertightness. Although lead water pipes were replaced by copper ones when the importance of lead poisoning began to be understood, lead solder continued to be used until the 1980s because it was thought that the amount of lead that could leach into the water from the solder was negligible in a properly soldered joint. The electrochemical couple of copper and lead promotes the corrosion of lead and tin. Tin, however, is protected by an insoluble oxide. Since even small amounts of lead have been found to be harmful to health as a potent neurotoxin,[9] lead in plumbing solders was replaced by silver (food applications) or antimony, to which copper was often added, and the proportion of tin was increased.
The addition of tin, more expensive than lead, improves the wettability of the alloy; Lead itself has poor wetting characteristics. High-tin tin-lead alloys have limited use, as the range of workability can be provided by a cheaper high-lead alloy.[10].
Lead-tin solders easily dissolve gold plating and form brittle intermetallics.[11] 60/40 Sn-Pb solder oxidizes on the surface, forming a complex 4-layer structure: tin(IV) oxide "Tin(IV) oxide") on the surface, below a layer of tin(II) oxide with finely dispersed lead, followed by a layer of tin(II) oxide. "Tin(II) oxide") with finely dispersed tin and lead, and the solder alloy itself underneath.[12].
Lead, and to some extent tin, as used in solders contains small but significant amounts of radioisotopic impurities. Radioisotopes that undergo alpha decay are of concern because of their tendency to cause soft errors. Polonium-210 is especially problematic; Polonium-210 beta decays to bismuth-210, which in turn beta decays to polonium-210, a strong emitter of alpha particles. Uranium-238 and thorium-232 are other important contaminants in lead alloys.[13][14].
Unleaded
The European Union Directive on Waste Electrical and Electronic Equipment and the Directive on the Restriction of Certain Hazardous Substances in Electrical and Electronic Equipment were adopted in early 2003 and entered into force on 1 July 2006, restricting the inclusion of lead in most consumer electronics sold in the EU, and having a broad effect on consumer electronics sold around the world. In the US, manufacturers can get tax benefits by reducing the use of lead solder. Commercial lead-free solders may contain tin, copper, silver, bismuth, indium ("Indium (element)"), zinc, antimony, and traces of other metals. Most lead-free substitutes for conventional 60/40 and 63/37 Sn-Pb solders have melting points between 50 and 200 °C higher,[15] although solders with much lower melting points also exist. Lead-free solders typically require about 2% flux by mass for adequate wettability.[16].
When lead-free solder is used in wave soldering, it may be advisable to use a slightly modified solder (e.g. titanium liners or impellers) to reduce maintenance cost due to the higher tin pickup of high-tin solder.
Lead-free solders are prohibited in critical applications, such as aerospace, military, and medical projects, because the joints are likely to fail due to metal fatigue under tension (such as that produced by thermal expansion and contraction). Although this is a property that conventional lead solders also have (like any metal), the point at which stress fatigue usually occurs in lead solders is well above the level of stresses that normally occur.
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 manual soldering. The widespread use of this popular family of lead-free solder alloys is based on the low melting point of the Sn-Ag-Cu ternary eutectic (217 °C; 423 °F), which is below the 22/78 Sn-Ag (wt%) eutectic of 221 °C (430 °F) and the 99.3/0 eutectic.[17] The eutectic behavior Sn-Ag-Cu ternary and its application for the assembly of electronic components was discovered (and patented) by a team of researchers from Ames Laboratory, Iowa State University and Sandia-Albuquerque National Laboratories.
Much of the recent research has focused on the addition of a fourth element to the Sn-Ag-Cu solder, in order to provide support for the reduced cooling rate of solder sphere reflow for the assembly of ball grid assemblies. Examples of these four-element compositions are 18/64/14/4 tin-silver-copper-zinc (Sn-Ag-Cu-Zn) (melting range 217-220 °C) and 18/64/16/2 tin-silver-copper-manganese (Sn-Ag-Cu-Mn; melting range 211-215 °C).
Tin-based solders easily dissolve gold, forming brittle intermetallic bonds; For Sn-Pb alloys, the critical concentration of gold to embrittle the bond is approximately 4%. Indium-rich solders (usually indium-lead) are more suitable for soldering thicker layers of gold, as the rate of dissolution of gold into indium is much slower. Tin-rich solders also easily dissolve silver; for welding silver metallizations or surfaces, alloys with added silver are suitable; Tin-free alloys are also an option, although their wettability is lower. If the soldering time is long enough to form intermetallics, the tin surface of a soldered-to-gold joint is very matte.[11].
Hard welding
Hard solders are used for soldering and melt at higher temperatures. The most common are alloys of copper with zinc or silver.
In goldsmithing or jewelry, special hard solders are used that pass the test. They contain a high proportion of the metal that is soldered and lead is not used in these alloys. These welds vary in hardness, which is designated as "enamel," "hard," "medium," and "easy." The enameling solder has a high melting point, close to that of the material itself, to prevent the joint from unsoldering during cooking in the enamelling process. The other types of welding are used in order of decreasing hardness during the manufacturing process of an item, to prevent a previously welded seam or joint from unwelding while other points are welded. For the same reason, easy welds are also often used for repairs. Flux is also used to prevent joints from unsoldering.
Silver solder is also used in manufacturing to join metal parts that cannot be soldered. The alloys used for these purposes contain a high proportion of silver (up to 40%), and may also contain cadmium.
Alloys
Different elements play different roles in the solder alloy:.
Impurities
Impurities typically enter the weld deposit by dissolving the metals present in the assemblies being welded. Dissolution of process equipment is not common, as materials are usually chosen to be insoluble in welding.[23].
Plate finishes against the accumulation of impurities in the wave soldering bath:.