Solder melts; the parts being joined do not

Soldering joins metal parts with a filler metal that melts below the parts' melting temperatures. It is closer to brazing than welding: the base metals do not melt. Electrical soldering connects wires, printed circuit boards (PCBs), and electronic components.

A good connection must be mechanically secure and conduct electricity. The molten solder must wet both surfaces: spread onto and bond with them, rather than sit on top as a ball. Clean surfaces, flux, and heat at both parts make that possible.

Start here, then follow Making and Inspecting Solder Joints and Soldering Wire Splices and Through-Hole PCBs.

Choose the tip for contact, not just access

A temperature-controlled station regulates the iron's tip temperature. A basic iron may have no temperature control; a cordless iron changes how it is powered, not how a joint is made. Check the tool's instructions and choose settings suitable for the solder and components.

Tip shape and size matter. A fine point reaches small spaces, but a chisel's broad face often transfers heat more effectively. Use the largest tinned contact face that fits the joint without touching neighboring connections. Interchangeable tips let one iron handle different work; change them according to the tool's instructions.

Supporting detail: Addendum 1 — What the iron's “sweet spot” means.

Match the solder and flux to electrical work

• Leaded solder: common tin-lead alloys melt at lower temperatures than common tin-copper or tin-silver-copper lead-free alloys. Leaded solder is still sold, but lead is restricted in many products. Follow the specified alloy for the work.
• Lead-free solder: common electrical alloys use tin with copper and/or silver. They may need a different heating process and can solidify with a duller finish. “Lead-free” does not mean “no fumes.”
• Diameter: fine solder wire makes it easier to meter a small amount into a small joint. Thick solder feeds more metal for the same length; it is easy to overfill a small pad.
• Flux: helps remove surface oxides during heating so solder can wet the metal. Rosin-core solder carries flux inside the wire; additional compatible electronics flux may help. Do not substitute plumbing acid flux.

Read the alloy and flux label, including any residue-cleaning instructions. See Chemicals and Safety Data Sheets for handling information.

Practice choosing solder

Support the work and control the heat and fume

Helping hands or a PCB vise hold the parts still while your hands hold the iron and solder. Keep a proper stand or cradle within reach. Use a damp soldering sponge or brass wool for tip cleaning, and have solder wick (copper braid) or a desoldering pump available for removing excess solder.

It gets hot. Really hot. Park the iron in its stand, and allow the work to cool before touching it. Use ventilation or fume extraction that keeps the plume away from your face and other people; most visible fume comes from flux. Wash your hands after handling leaded solder, and do not eat or drink at the bench.

SHOP RULE: NEVER SOLDER BATTERIES.

Supporting detail: Addendum 2 — Why the battery rule stays absolute.

Work on disconnected circuits. Follow Electrical Safety and Personal Protective Equipment; flux and fume handling belongs with Chemicals and Safety Data Sheets.

Supporting detail: Addendum 3 — Further reading on flux fume.

Clean, tin, and keep the working face coated

With the iron at working temperature, briefly clean the tip on brass wool or a damp soldering sponge. Apply fresh solder to coat its working face, then remove the excess on the cleaner. Leave a thin coating; do not wipe it bare and let it sit hot. Re-tin as needed and before storage as directed by the tool manufacturer.

Supporting detail: Addendum 4 — Why tinning helps heat transfer.

Heat both parts, then feed solder into the joint

Prepare the iron and bench using Soldering Equipment and Setup. The iron heats the parts; the heated parts melt the solder. Touch the tinned working face to both wires, or to both the pad and the component lead. Feed solder into the joint, away from the iron's contact point when access allows.

Supporting detail: Addendum 5 — A one-second count is a rhythm, not a temperature test.

Look for wetting and shape; shine alone is not a verdict

A good joint flows onto both surfaces. On a typical through-hole pad, look for a smooth transition from pad to lead, often a concave fillet (an inward-curving slope). Tin-lead joints are often shiny. A sound lead-free joint may be dull; dullness alone does not diagnose a cold joint.

Supporting detail: Addendum 6 — Why lead-free solder may look different.

Cold or poorly wetted

Solder balls up or fails to bond to one surface, often because the parts were not heated adequately. A rough or dull appearance can be a clue, but inspect the contact and shape too.

Disturbed

The connection moved while solidifying, leaving a cracked or disrupted joint. Secure it before reworking.

Too much solder

A large mound hides the connection and makes bridges easier to miss. Remove excess so the joint can be inspected.

Too little solder

The intended connection lacks coverage or a continuous bond. Reheat both surfaces and add only what is needed.

Bridge

Solder joins connections that should be electrically separate. Remove it before applying power.

Practice joint inspection

Correct the cause before reheating

Keep the circuit disconnected. Secure loose parts, clean and re-tin the tip, and use compatible flux if needed. Reheat both surfaces to remake a poorly wetted or disturbed joint. Use wick or a pump to remove excess solder or a bridge; do not pry at a pad while the solder is solid.

Repeated heating can damage components or lift copper pads from the board. If a joint still will not wet, let it cool and get help finding the cause. Inspect again after repair. A continuity check can help find an open connection or an unintended short, but it does not prove mechanical strength.

See Soldering Wire Splices and Through-Hole PCBs for splice support and through-hole work.

Make the splice mechanically sound before soldering

Solder should not be the only thing holding loose wires in position. Form a stable splice, support it during soldering, and provide strain relief so pulling or bending the cable does not load the soldered joint. Follow the wire and assembly requirements; these sketches show splice shapes, not a qualification standard.

• Lay-over or lap: stripped ends overlap alongside one another. Support and strain relief are especially important because the overlap does not lock itself together.
• Twist or pigtail: stripped ends twist together and leave the connection on the same side. Cover the joined end completely when insulating it.
• Western Union: an inline connection with each free end wrapped around the opposite wire. It is a commonly used, strong splice when properly formed; it is not automatically the best choice for every wire or assembly.

For conductor sizing, see Wire and Wire Gauge. Use the heating and inspection method on Making and Inspecting Solder Joints.

Practice identifying splices

Put heat-shrink on before closing the splice

Slide suitable heat-shrink tubing onto a wire before joining it, and keep it away from the soldering heat. After soldering, allow the joint to cool and inspect it. Move the tubing over all exposed conductor, overlapping intact insulation at both ends of an inline splice, then shrink it with the appropriate heat source according to its instructions.

Choose tubing whose unshrunk size passes over the splice and whose recovered size fits the wire. Avoid sharp wire ends that could puncture it. Ordinary heat-shrink provides insulation; do not assume it makes a waterproof connection or replaces strain relief.

Supporting detail: Addendum 7 — When pre-tinning helps.

On a PCB, heat the copper pad and component lead together

A pad is the exposed copper connection area on a printed circuit board. A component's lead is its metal leg. In through-hole assembly, the lead passes through a hole in the board. Insert the correct component in the correct orientation and support the board and component before heating.

Place the tinned working face against both the pad and lead. Feed solder into the heated connection and let it flow onto both, then remove solder and iron in that order. Hold still while it cools. Inspect the joint and the gaps to neighboring pads.

Where the component's assembly instructions call for trimming, cut the excess lead above the cooled joint without cutting into the solder fillet. Wear the required eye protection and control the clipped end so it does not fly away or remain on the board. Do not shorten connector pins that must still mate with another connector.

Soldering header pins is one bridge to Microcontroller Basics. A stray bridge changes the intended circuit connections; review Series and Parallel Circuits when tracing those paths.

Practice through-hole technique