Solder

Solder is a fusible metal alloy used to join metal parts by melting the alloy so it flows into the joint and solidifies, forming a metallurgical bond. In electronics, soldering uses low-melting alloys to bond components to substrates without melting the base metals.

Two broad families exist: leaded solders (tin–lead) and lead-free alternatives. Common leaded alloys include 60/40 Sn/Pb and 63/37 Sn/Pb (the latter is eutectic and melts at about 183°C). Lead-free solders, such as SAC305 (tin–silver–copper), melt around 217–221°C and require different fluxes and heat profiles.

Flux cleans oxides and improves wetting. Rosin-based fluxes are common in electronics; water-soluble fluxes are more active but may require rinsing. The choice of flux depends on the application and cleaning requirements. Flux residues can vary in corrosiveness and may be left in place or cleaned, depending on the flux type and specifications.

Process: apply flux, heat the joint with a soldering iron or hot air, and feed solder to the joint so it wets the surfaces by capillary action. Remove heat promptly to avoid damage. Common issues include cold joints and bridging; desoldering tools help with correction.

Applications and safety: Solder is used in electronics and plumbing, among other fields. Lead exposure risks and environmental regulations such as RoHS have driven a shift to lead-free solders. Proper ventilation, handling, and disposal are advised, and some high-reliability work requires validated processes and inspection.