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A solder is a fusible metal alloy, with a melting point or melting range below 450 °C (840 °F), which is melted to join metallic surfaces, especially in the fields of electronics and plumbing, in a process called soldering.

The word solder comes from the Middle English word soudur, via Old French soldure and soulder, from the Latin solidare, meaning '‘to make solid’'.

Desoldering a contact and a wire attached with solder.

In electronics, solders are usually 63% tin and 37% lead by mass in order to produce a near-eutectic mixture (lowest melting point — below 190 °C [374 °F]). These are commonly designated Sn63/Pb37. The eutectic ratio of 63/37 corresponds closely to a Sn₃Pb intermetallic compound.

In plumbing, a higher proportion of lead was used. This had the advantage of making the alloy solidify more slowly, so that it could be wiped over the joint to ensure watertightness. Although lead water pipes were displaced by copper when it was realized that the lead was poisonous, 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. Since even small amounts of lead have been found detrimental to health, lead in plumbing solder was replaced by copper or antimony, with silver often added, and the proportion of tin was increased.

As used for brazing, is generally a copper/zinc or copper/silver alloy, and melts at higher temperatures. Eutectic solder has the lowest melting point for solders, which is 360 °F or 183°C.

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 used in these alloys. These solders also come in a variety of hardnesses, known as 'enamelling', 'hard', 'medium' and 'easy'. Enamelling solder has a high melting point, close to that of the material itself, to prevent the joint desoldering during firing in the enamelling 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 soldering a new joint. Easy solder is also often used for repair work for the same reason. Flux or rouge is also used to prevent joints desoldering.

A tube of multicore electronics solder used for manual soldering - the flux is contained in five cores within the solder itself

Solder often comes pre-mixed with, or is used with, flux, a reducing agent designed to help remove impurities (specifically oxidised metals) from the points of contact to improve the electrical connection. For convenience, solder is often manufactured as a hollow tube and filled with flux. Most cold solder is soft enough to be rolled and packaged as a coil making for a convenient and compact solder/flux package. The two principal types of flux are acid flux, used for metal mending, and rosin flux, used in electronics, where the corrosiveness of the vapours that arise when acid flux is heated could damage components. Due to concerns over atmospheric pollution and hazardous waste disposal, the electronics industry has been gradually shifting from rosin flux to water-soluble flux, which can be removed with deionised water and detergent, instead of hydrocarbon solvents.

Since solder can occasionally splash (due to the superheated flux inside or from contact with water in the cleaning sponge), it is recommended that safety goggles be worn when soldering. Though small solder splashes on skin are painful, they usually do not cause lasting harm. For large scale work additional protective clothing may be needed.

A coil of lead-free solder wire

According to the European Union Waste Electrical and Electronic Equipment Directive (WEEE) and Restriction of Hazardous Substances Directive (RoHS), lead had to be eliminated from electronic systems by July 1, 2006, leading to much interest in lead-free solders. These contain tin, copper, silver, and sometimes bismuth, indium, zinc, antimony, and other metals in varying amounts. The lead-free replacements for conventional Sn60/Pb40 solder have higher melting points, requiring re-engineering of most components and materials used in electronic assemblies. Lead-free solder joints may produce mechanically weaker joints depending on service and manufacture conditions, which may lead to a decrease in reliability using such solders. "Tin Whiskers" are another problem with many lead-free solders, where slender crystals of tin slowly grow out of the solder joint. These whiskers can bridge a short circuit years after a device's manufacture.

• SnAgCu solders are used by two thirds of Japanese manufacturers for reflow and wave soldering, and by about ¾ companies for hand soldering.
  • SnAg3.0Cu0.5, tin with 3% silver and 0.5% copper, has a melting point of 217-220 °C and is predominantly used in Japan. It is the JEITA recommended alloy for wave and reflow soldering, with alternatives SnCu for wave and SnAg and SnZnBi for reflow soldering.
  • SnAg3.5Cu0.7 is another commonly used alloy, with melting point of 217-218 °C.
  • SnAg3.5Cu0.9, with melting point of 217 °C, is determined by NIST to be truly eutectic.
  • SnAg3.8Cu0.7, with melting point 217-218 °C, is preferred by the European IDEALS consortium for reflow soldering.
  • SnAg3.8Cu0.7Sb0.25 is preferred by the European IDEALS consortium for wave soldering.
  • SnAg3.9Cu0.6, with melting point 217-223 °C, is recommended by the US NEMI consortium for reflow soldering.
• SnCu0.7, with melting point of 227 °C, is a cheap alternative for wave soldering, recommended by the US NEMI consortium.
• SnZn9, with melting point of 199 °C, is a cheaper alloy but is prone to corrosion and oxidation.
• SnZn8Bi3, with melting point of 191-198 °C, is also prone to corrosion and oxidation due to its zinc content.
• SnSb5, tin with 5% of antimony, is the US plumbing industry standard. Its melting point is 232-240 °C. It displays good resistance to thermal fatigue and good shear strength.
• SnAg2.5Cu0.8Sb0.5 melts at 217-225 °C and is patented by AIM alliance.
• SnIn8.0Ag3.5Bi0.5 melts at 197-208 °C and is patented by Matsushita/Panasonic.
• SnBi57Ag1 melts at 137-139 °C and is patented by Motorola.
• SnBi58 melts at 138 °C.
• SnIn52 melts at 118 °C and is suitable for the cases where low-temperature soldering is needed.

Different elements serve different roles in the solder alloy:

• Silver provides mechanical strength, but has worse ductility than lead. In absence of lead, it improves resistance to fatigue from thermal cycles.
• Copper lowers the melting point, 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.
• Bismuth significantly lowers the melting point and improves wettability. In presence of lead and tin, bismuth forms crystals of Sn16Pb32Bi52 with melting point of only 95 °C, which diffuses along the grain boundaries and may cause a joint failure at relatively low temperatures. A lead-contaminated high-power part can therefore desolder under load when soldered with a bismuth-containing solder.
• Indium lowers the melting point and improves ductility. In presence of lead it forms a ternary compound that undergoes phase change at 114 °C.
• 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 ones.
• Antimony is added to increase strength without affecting wettability.

• Soldering gun
• Soldering iron
• Solderability
• Welding
• Soldering

• Basic soldering guide
• Solder and flux selection guide
• Physical Properties Table: Specialty Alloys and Solders
• Lead-free solder alloys
• Common Solder alloys and their melting ranges
• Database for Solder Properties with Emphasis on New Lead-free Solders Release 4 (MS Word Document format also available)
• Phase Diagrams of different types of solder alloys