Solders

Solder is a fusible metal alloy used to join two metal surfaces by melting at a temperature lower than the base metals. When the filler metal flows into the joint and cools, it forms a mechanical and electrical connection. Soldering is commonly used in electronics, plumbing, metalworking, and jewelry.

There are two broad categories: soft solders and hard solders. Soft solders melt well below 300°C and are typically tin-based. The most common electronics solder is tin–lead, historically 60/40 or 63/37 tin–lead, with the eutectic 63/37 melting at about 183°C. Due to health and environmental concerns, lead-free alternatives are widely used, such as tin–silver–copper alloys (for example SAC305, roughly Sn96.5–Ag3.0–Cu0.5) with melting points around 217–221°C. Other lead-free options include SnCu and various Sn–Ag alloys.

Hard solders have higher melting ranges and include silver solder and other alloys used for jewelry, copper plumbing, and metalwork. These often require higher temperatures and different fluxes and are sometimes called brazing when used to join nonfusing metals.

Flux plays a key role by removing oxides from surfaces to improve wetting and flow of the molten solder. Flux types include rosin, resin, and water-soluble formulations; some leave no-clean residues.

Applications and considerations: solder joints should wet the surfaces well for a strong bond. Poor wetting can cause cold joints or tombstoning in electronics. Lead-containing solders are restricted in many regions; lead-free solders are now standard in consumer electronics and many plumbing applications. Safety concerns include exposure to fumes and lead hazards, necessitating proper ventilation and handling.