Soldering is used to join metals together without melting the pieces themselves. It shows up everywhere: assembling the circuit board inside your phone, connecting copper pipes under your sink, building jewelry, repairing car radiators, and creating stained glass art. The process works by melting a filler metal (solder) that flows into the gap between two pieces through capillary action, then hardens into a strong bond. Because the filler melts below 450°C, the base metals stay intact, making soldering ideal for delicate or heat-sensitive work.
How Soldering Works
Soldering joins two metal surfaces using a filler metal with a lower melting point than the parts being connected. A cleaning agent called flux is applied first to remove oxidation from the metal surfaces, which helps the molten solder flow evenly and bond tightly. Once heated, the liquid solder gets drawn into the narrow gap between the two pieces by capillary action, the same force that pulls water up a paper towel. When it cools, the solder locks the pieces together.
The most common traditional solder is a 63/37 tin-lead alloy, which melts at exactly 183°C. This “eutectic” composition transitions from solid to liquid at a single temperature rather than going through a pasty phase, making it easier to work with. Lead-free solders, now required in many applications, typically melt at slightly higher temperatures and use combinations of tin, silver, and copper instead.
Electronics and Circuit Board Assembly
The single biggest use of soldering is in electronics. Every circuit board inside a computer, phone, appliance, or vehicle has hundreds or thousands of solder joints holding components in place and creating electrical connections. Without soldering, modern electronics simply wouldn’t exist.
Several specialized methods handle different board designs. Wave soldering passes circuit boards over a wave of molten solder to attach components whose wire leads poke through holes in the board. Reflow soldering handles the tiny surface-mounted chips you see on modern electronics: solder paste is printed onto the board’s contact pads, components are placed by machine, and the whole assembly passes through an oven that melts the paste into permanent connections. Selective soldering targets individual components that can’t tolerate the heat of full-board methods. Intrusive soldering pushes solder paste into through-holes during a reflow cycle, combining elements of both approaches.
Hobbyists and repair technicians use handheld soldering irons to build or fix individual boards, replace damaged components, or prototype new designs. This is the type of soldering most people learn first.
Plumbing and Pipe Joining
Copper plumbing systems rely on soldered joints to create watertight seals. A copper pipe slides into a slightly larger fitting, and solder is melted at the joint with a torch. Capillary action pulls the molten solder into the gap between pipe and fitting, filling it completely. A joint with at least 70% fill of the capillary space is considered strong enough to handle maximum recommended pressures for residential and commercial systems.
Before 1986, plumbers commonly used a 50/50 tin-lead solder for these joints. The Safe Drinking Water Act banned lead-bearing solders from potable water systems, so modern plumbing solder is lead-free, typically a tin-silver or tin-copper alloy. If you’re working on older homes, any pipe joints made before that cutoff may still contain lead solder, which is one reason older plumbing sometimes gets replaced.
Jewelry Making
Jewelers use soldering to assemble rings, attach clasps, join chain links, and build complex metalwork from individual pieces. Unlike electronics soldering, jewelry work uses “hard” soldering with precious metal alloys that melt at higher temperatures and produce much stronger bonds. Silver solder, for example, is an alloy of silver, copper, and zinc. Gold solder is formulated to match the color and karat of the gold being joined.
Jewelry solders come in hard, medium, and easy grades, each melting at a progressively lower temperature. When a piece requires multiple solder joints, a jeweler starts with hard solder for the first joint, then uses medium and easy for subsequent ones. This prevents earlier joints from melting open when heat is applied to a new area. The result is that the solder nearly disappears into the finished piece, matching the surrounding metal in both color and strength.
Stained Glass and Decorative Art
Stained glass panels and lampshades are held together by solder. In the traditional method, shaped glass pieces are fitted into channels of lead came (H-shaped lead strips), and solder secures the intersections. The copper foil method, pioneered by Tiffany Studios, wraps each glass piece in thin copper tape, then solder joins the foiled edges together. This allows for finer detail, more complex curves, and stronger panels than lead came alone, which is why Tiffany-style lamps can feature thousands of tiny glass pieces in intricate patterns.
The process is straightforward: cut the glass to shape, wrap edges in copper foil, brush on liquid flux, and run solder along every seam with a soldering iron. The copper foil method doesn’t require cement to stiffen the finished piece, unlike traditional lead construction.
Automotive and Radiator Repair
Soldering plays a role in both vehicle manufacturing and repair. Wiring connections throughout a car’s electrical system are often soldered for reliability, since a soldered joint resists vibration better than a simple crimp in a high-stress environment. On the repair side, small leaks in copper or brass radiators can be patched by soldering the hole shut, a fix that’s often faster and cheaper than replacing the entire radiator when the damage is minor.
Dental and Orthodontic Work
Dentistry uses soldering to assemble and modify metal frameworks. Silver solders join wires in orthodontic appliances like retainers and space maintainers. In prosthetic dentistry, soldering connects sections of metal frameworks for bridges and partial dentures. Silver solder works well for orthodontics but tends to tarnish inside the mouth, so prosthetic applications typically use gold-based solders that resist corrosion better in that environment.
Metals That Don’t Solder Well
Soldering works best on copper, tin, brass, gold, and silver. Some metals resist it. Aluminum and aluminum bronze form tough oxide layers that prevent solder from bonding properly, and they also create galvanic corrosion problems with most solder alloys. Stainless steel and high-alloy steels have too much chromium oxide on their surfaces, requiring aggressive fluxes and careful preparation. Titanium, magnesium, cast iron, and chromium are very difficult to solder at all, typically requiring the surface to be pre-plated with a more solderable metal first. For these materials, welding or brazing is usually a better option.
Safety While Soldering
The main health concern with soldering isn’t the molten metal itself but the smoke produced when flux burns off. This smoke contains chemical compounds and fine particles that irritate your eyes and respiratory system. Working in a well-ventilated area is essential. A benchtop fume extractor with activated carbon filters or an exhaust snorkel positioned near the work pulls smoke away before you breathe it in. If you’re using leaded solder, washing your hands thoroughly after work prevents accidental lead ingestion, and gloves add an extra layer of protection.