Soldering Basics

Part of Basic Electrical Circuits

How to make reliable, low-resistance electrical joints using solder—the metallurgical technique behind all electronics construction.

Why This Matters

The quality of an electrical connection determines the reliability of an electrical system. A well-soldered joint has near-zero resistance, excellent mechanical strength, and will last for decades without maintenance. A poor connection—corroded, cold-soldered, or mechanically weak—creates resistance that generates heat, introduces intermittent faults that are nearly impossible to diagnose, and eventually fails completely.

Soldering is the technique by which two metal surfaces are permanently bonded using a lower-melting-point alloy (solder) that wets and bonds to both surfaces. Unlike welding (which melts the parent metals) or brazing (which operates at higher temperatures), soldering works at temperatures manageable with a simple charcoal or wood-fired soldering iron, making it accessible to any community with basic metalworking.

Soldering was practiced in ancient Egypt and has been central to electrical construction since the telegraph era. It requires no electricity to learn or practice. A person who can solder reliably can assemble and repair virtually any electrical device.

The Metallurgy of Solder

Solder is an alloy with a melting point lower than the metals being joined. The classic electrical solder is 60% tin, 40% lead (sometimes written 60/40 Sn/Pb), melting at approximately 183–190°C.

Why this alloy:

• Melting point low enough to avoid damaging components
• Surface wetting—it flows freely over clean copper and bonds chemically to it
• Mechanical strength adequate for electrical connections
• Low electrical resistance

Flux: Metal surfaces oxidize instantly in air. Oxide layers prevent solder from wetting the surface. Flux is a chemical that:

1. Removes existing oxide films by mild chemical attack
2. Protects the cleaned surface from re-oxidation during soldering
3. Improves heat transfer by coating the joint

Traditional fluxes:

• Rosin (colophony): Derived from pine resin. The standard electrical flux—mildly active when hot, non-corrosive when cool. Leave it on joints after soldering.
• Zinc chloride (killed spirits of salts): Zinc dissolved in hydrochloric acid. Highly active, removes heavy oxides. Must be thoroughly washed off after soldering—it remains corrosive and will attack copper over time.
• Tallow (animal fat): A mild flux adequate for plumbing; less effective for electrical work but usable on clean surfaces.
• Lemon juice, citric acid: Mild organic acids, adequate for light work on clean surfaces.

For electrical work, use rosin flux exclusively. For heavy copper plumbing or structural joints where appearance doesn’t matter, zinc chloride is effective but requires washing.

Fabricating a Soldering Iron

Copper bit iron (charcoal or gas heated): This is the original soldering iron, used from the 1800s through the early 20th century.

1. Take a copper bar or rod, approximately 15–20mm diameter, 8–10 cm long
2. Forge or file one end to a pyramid or wedge shape
3. Drill and tap the flat end to accept a steel rod handle
4. Attach a 30–40 cm steel rod handle (wooden scales can be added for comfort)

Using the copper bit iron:

1. Heat the copper tip in a charcoal fire until it is too hot to approach (approximately 400–450°C)
2. Remove from fire and immediately wipe the tip on a damp cloth or sal ammoniac block to clean it
3. Tin the tip: apply flux, then touch solder to the hot tip. The solder should flow freely and coat the tip in a silver film. If it balls up and doesn’t wet, the tip is too oxidized or too hot.
4. Work quickly—the iron cools as you use it. Return to the fire when the solder no longer flows freely.

Tinning: Keeping a thin coat of solder on the iron tip is essential. A tinned tip transfers heat efficiently; an oxidized tip cannot deliver enough heat to make a good joint.

Making Good Solder Joints

The cardinal rule: Heat the joint, not the solder. If you melt solder on the iron and touch it to the joint, you get a cold joint—solder that never properly wetted the base metal. Heat the joint until it is hot enough to melt solder on contact.

Procedure for a wire-to-terminal joint:

1. Clean all surfaces: strip insulation, twist strands tightly, scrape any oxide from the terminal with fine abrasive
2. Apply flux to the joint area
3. Mechanically pre-join if possible: wrap wire around terminal or through a hole
4. Apply the hot iron to the joint—hold it where it contacts both the wire and the terminal
5. After 1–2 seconds, touch solder wire to the joint (not the iron). It should flow immediately and wick into the joint.
6. Feed enough solder to fill the joint without building up an excessive blob
7. Remove the iron and hold the joint still until solder solidifies (2–3 seconds)

What a good joint looks like:

• Smooth, shiny surface (when using tin-lead solder)
• Concave meniscus shape—the solder fills the joint and tapers down toward the metal surfaces
• Wires are visible through a thin solder coat—solder fills around them rather than smothering them
• No blobs, no spikes, no dull granular texture

What a cold joint looks like:

• Dull, gray, granular surface
• Solder sits on top of the metals rather than wetting into them
• May be physically weak—can often be peeled off

Cold joints have high resistance (often several ohms compared to milliohms for a good joint) and are mechanically unreliable. Reheat any cold joint until the solder reflows, moving the iron to ensure both metals reach temperature.

Desoldering (Removing Existing Solder)

Solder wick: Copper braid saturated with flux. Place the wick over the solder joint, apply heat through the wick. Capillary action draws the molten solder into the braid. Remove iron and wick together; the joint is now largely solder-free.

Solder sucker: A spring-loaded vacuum pump. Heat the solder with the iron until liquid, then remove the iron and immediately press the solder sucker plunger to create suction. Draws solder away from the joint.

Reflowing: Sometimes solder joints are best removed by simply reheating them and removing the component or wire while the solder is liquid. This works for wire connections but risks damaging components that cannot withstand the heat.

Soldering Different Materials

Copper and brass: Solder bonds readily to clean copper and brass with rosin flux. The easiest metals to solder.

Tin-plated steel (tin cans, galvanized wire): The tin coating accepts solder well. Remove any galvanizing (zinc) first with dilute acid if present.

Aluminum: Extremely difficult. Aluminum forms a persistent oxide that prevents solder wetting. Requires special aluminum solder and flux, or the flux must be applied under the solder (scrubbing under molten solder) to exclude oxygen. Generally not recommended for field construction—use mechanical connections for aluminum.

Stainless steel: Use zinc chloride flux and work quickly. Wash thoroughly after soldering to prevent corrosion.

Silver: Bonds well. Often used for low-electrical-resistance relay contacts and precision instruments.

Common Mistakes and Remedies

Problem	Cause	Remedy
Cold joint (dull, weak)	Insufficient heat	Reheat joint properly
Solder won’t wet	Oxidized surface or no flux	Clean and re-flux
Solder bridges between conductors	Too much solder	Wick away excess
Burned insulation	Iron too hot or held too long	Work faster, lower temperature
Solder won’t stick to iron tip	Oxidized tip	Retinning: heat and flux the tip
Joint cracks with temperature cycling	Joint moved while cooling	Immobilize joint during cooling

With practice, producing consistently good solder joints takes only a few seconds each. The skill develops quickly and the rewards—reliable, low-resistance connections—benefit every electrical system built or repaired.