Solders are metal alloys with low melting points that are used to join metals together. They are mixed with flux, a reducing agent that removes impurities from the points of contact in order to improve electrical conductivity. Soldering applies heat to metal parts and causes the resulting joint to draw in the softened solder through capillary action. After the metal cools, the resulting joint is not as strong as the base material, but provides adequate strength, electrical conductivity, and water resistance for specific applications. Many soldering techniques use a hot-air soldering tool, an electric soldering iron or gun, an infrared (IR) lamp, or a brazing torch. Metal parts can also be soldered together by passing them over a bulk container of melted solder. Wave soldering and reflow soldering are methods for attaching components to printed circuit boards (PCBs).

Solders are based on metal alloys such as cadmium-zinc (Cd-Zn), lead-zinc (Pb-Zn), indium (In), tin-lead (Sn-Pb), tin-silver (Sn-Ag), tin-antimony (Sn-Sb), and zinc-aluminum (Zn-Al). Most electronics applications use solders that are 60% tin and 40% lead by weight in order to produce a near-eutectic mixture with a melting point below 374° F. Lead-free solders are also available. These products are increasingly popular because of the Waste Electrical and Electronic Equipment (WEEE) and Restriction of Hazardous Substances (RoHS) directives from the European Union (EU). WEEE is designed to encourage the reuse, recycling and recovery of electrical and electronic products. RoHs requires the manufacturers of electronic and electrical equipment sold in Europe to demonstrate that their products contain only minimal levels of hazardous substances such as lead. 

Solders are available as pastes, powders, preforms, and solid wires. Melting range and conductivity are important specifications to consider. Some solders meet grade specifications from the American Bureau of Shipping (ABS), the American Welding Society (AWS), the British Standards Institute (BSI), or the Canadian Welding Bureau (CWB). Other meet requirements from the International Standards Organization (ISO), EURONORM (EN), the Resistance Welding Manufacturers Association (RWMA), or Lloyd’s Register of Shipping. The Unified Numbering System (UNS) is a specification established by the American Society for Testing and Materials (ASTM) and the Society of Automotive Engineers (SAE), a worldwide organization that also maintains Aerospace Materials Specifications (AMS) for filler materials. Organizations that approve or certify solders include Germany’s Deutsches Institut für Normung (DIN) and Norway’s Det Norske Veritas (DNV). Specialized solders that meet U.S. military specifications (MIL-SPEC) or Japanese Industrial Standards (JIS) are also available.