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The image below shows the manual soldering of a stripped wire. The iron (right) heats the solder wire (top) to join the wire to a surface or another wire; in this case the stripped wire is being tinned. The smoke seen in the image is a typical byproduct of most soldering processes.
At its most basic, soldering consists of a solder alloy and a heat source, commonly a soldering iron or soldering gun, to melt and flow the solder into place. Soldering machines are more advanced pieces of equipment that provide additional features beyond manual soldering techniques. Solder is typically constructed using alloys with melting points between 180° and 190° C. It is important to note that, while solder is used to create a strong metal joint, it does not actually fuse with the solid metals to be joined. Because solder alloys need to wet the surface of metal parts before joining them, the parts must be heated above the melting point of the solder. A solder's composition may include flux, an additive to improve flow. Because heating a metal causes rapid oxidation, flux is also used to clean the oxide layer from the metal surface to provide a clean surface for soldering; this process is shown in the image below. Common fluxes include ammonium chloride, zinc chloride, rosin, and hydrochloric acid.
Solder AlloyA solder's melting point, toxicity, and uses are almost solely determined by its alloy metals. All solders formerly contained lead, but recent concerns about toxicity and lead poisoning have encouraged more widespread use of lead-free solders. Alloys are specified as a chemical "formula" of sorts, with the percentage of each element represented as a subscript. For example, a tin/lead solder containing 63% tin and 37% lead is referred to as Sn63Pb37. Lead SoldersTin/lead (or Sn/Pb) alloys are very common, versatile solders with a wide range of uses. Like most solders, Sn/Pb is manufactured with different elemental concentrations dependent on the intended application. A few common concentrations, melting points, and uses are listed in the table below.
Due to increasing restrictions on products containing lead, the use of tin/lead solders and lead solders in general is steadily decreasing. Sn/Pb solders have generally disappeared from plumbing applications in favor of silver alloys, but remain in use in electrical and electronics manufacturing, gas lines, and brass soldering. Lead/zinc (Pb/Zn) solders are less expensive than traditional Sn/Pb solders due to the relatively higher cost of tin. Some lead/zinc alloys, such as Sn30Pb50Zn20, are widely used for economical joining of metals, including aluminum and cast iron. This composition has also been used for repairing galvanized surfaces. In general, zinc is added to solder alloys to lower the melting point and reduce costs. Lead-free SoldersLead-free solders have become much more common due to new legislation and tax benefits regarding lead-free products. The Waste Electrical and Electronic Equipment (WEEE) and Restriction of Hazardous Substances (RoHS) directives — both passed by the European Union (EU) in 2006 — have effectively prohibited intentional use of lead solders in European-made consumer electronics. Lead-free solders typically use some combination of indium (In), tin (Sn), or aluminum (Al). Interestingly, cadmium-zinc (Cd-Zn) solder, while considered a lead-free alloy, is not RoHS compliant due to the directive's ban on cadmium as well as lead. Other than Cd-Zn, most lead-free solders are not considered toxic. The graph below provides a helpful visual comparison of the melting points of various tin-based lead-free solders, many of which are discussed in detail below.
Pure indium solder is commonly used in electronics manufacturing. Indium alloys are very useful for soldering surface mount (SMT) components and parts with gold, ceramic, quartz, or glass base materials. It features a low melting point of around 157° C (314.6° F). Indium solders are most suitable for low temperature applications and can maintain seals in cryogenic environments. Tin/antimony (Sn/Sb) is a high-strength alloy extensively used in the plumbing industry. It is also used in electronics applications for pin soldering and die attachment. Tin/antimony solders create strong bonds with good thermal fatigue strength even in high temperature environments. Sn/Sb alloys melt at around 235° C (455° F) and are also used in air conditioning, refrigeration, stained glass, and radiator applications. Tin/Silver (Sn/Ag) solders represent a common group of alloys often used for wave and reflow soldering. Generally speaking, silver is added to alloys to improve mechanical strength, although it is usually restricted to less than 3% of the total alloy composition to reduce the risk of poor ductility and cracking. Common compositions include Sn95.8Ag3.5Cu0.7 and Sn96.5Ag3.5, which have relatively high melting points of 217° C and 221° C, respectively. Zinc/Aluminum (Zn/Al) solder has a very high melting point of 382° C (719.6° F) and is particularly useful for soldering aluminum. Zinc/aluminum has a composition favorable for good wetting. Cadmium/Zinc (Cd/Zn) alloys are medium-temperature solders used to join most metals, especially aluminum and copper. Cadmium/zinc solders form strong, corrosion-resistant joints and are suitable for high-vibration and high stress applications. While Cd/Zn alloys are available in several different compositions, most share a melting point of around 265° C (509° F). Form Factor
Solder paste consists of solder powder mixed with a thick flux material and is "printed" onto a PCB using a stencil. The flux serves as a temporary adhesive to hold components onto the board until the paste is heated; after heating, a stronger physical bond is formed. Pastes are typically made of tin/lead alloys. Solder wire is available in a range of thicknesses and configurations. Wire may or may not contain flux. StandardsSolder alloys and their uses are governed by a wide range of standards. The standards bodies below are linked with their relevant standards. The Aerospace Material Standards (AMS) are published by SAE International, formerly known as the Society of Automotive Engineers. This set of over 6,400 technical documents includes technical recommendations applying to missiles, airframes, ground-control equipment, propellers, and propulsion systems. ASTM International — formerly known as the American Society for Testing and Materials — is one of the oldest continuously operating international standards organizations. ASTM maintains over 12,000 standards, including a wide variety pertaining to solder, including:
The International Organization for Standardization, or ISO, is a well-known international standards body. Its standard ISO 9453 is a major standard covering a wide variety of soft solder alloy compositions. MIL-SPEC standards are United States defense standards ensuring interoperability, quality, commonality, and general compatibility of military products. MIL-S-12204 is a well-known tin/lead solder standard. Other standards pertaining to solder alloys can be found here. ReferencesEPE Magazine - Basic Soldering Guide Image credits:Kester | Purdue University | Integrated Publishing | The Minerals, Metals and Materials Society | MBO |
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Solder is a metal alloy used to join metals together. The term "solder" represents a group of filler metals used as consumables when joining two pieces of metal together - a process known as "soldering." Soldering has been a very common metalworking technique throughout human history and remains a permanent process in applications as diverse as jewelry making, 
Solder is available as a number of form factors, including paste, powder, wire, and preformed. Selecting between these solder types requires an analysis of the application and general needs. Preform solder is the most specific (and limiting) type and consists of a pre-made shape designed for a specialized application. Preform solders are often stamped and may include integral flux.
