Introduction to Soldering
An optional, adult-supervised skill for making circuits permanent.
The question
How do you turn a breadboard circuit into something permanent that won’t fall apart?
What you’ll discover: what soldering is, the safety rules that come first, and how to make a good joint. Soldering is optional: nothing else in Electronics School requires it.
First, the safety rules
Soldering is only for builders the supervising adult judges ready, always with that adult right there. A soldering iron’s tip is far hotter than an oven: hot enough to burn instantly.
- A soldering iron plugs into the wall. It’s the one exception to our batteries-only rule, so an adult sets it up and it’s never left on and unattended.
- Hold the iron like a pencil, by the grip only. If it falls, let it fall. Never try to catch it.
- Hold parts with pliers, tweezers or a clamp: component legs get hot right away.
- Always put the iron back in its stand. Never lay it on the table. Unplug it when you’re done and let it cool in the stand.
- Wear safety glasses, long cotton sleeves and closed-toe shoes. Hot solder can spit, and clipped wire ends can fly.
- Ventilate: work in a well-aired room with your head to the side of the work, not over it. A small fan can draw the smoke away from your face, without blowing it at anyone else. Flux smoke can trigger asthma.
- Use lead-free solder, keep food and drinks away from the bench, and wash your hands after soldering.
- Clear the area of paper and anything that burns, and keep the iron’s cord away from the hot tip.
- If you get a burn, cool it under cool running water for 20 minutes and tell the adult.
In simple terms
BeginnerMetal glue that conducts
Soldering joins metal parts with a small amount of melted metal called solder. When it cools, it holds the parts together and conducts electricity. It’s how components are fixed to the circuit boards inside almost every electronic device.
Most solder for electronics comes as thin wire with flux inside. Flux cleans the metal surfaces as the solder melts, so it flows and sticks properly. The smoke you see while soldering comes mostly from the flux.
Making a good joint
- Clean the hot tip on a damp sponge or brass wool, and melt a little solder onto it (called tinning). A shiny tip transfers heat well.
- Touch the tip so it heats both the component leg and the copper pad at the same time, for about two seconds.
- Feed solder into the joint, where the leg meets the pad, not onto the iron. It should melt and flow around the leg.
- Remove the solder, then the iron. Don’t move anything for a few seconds while it cools.
- Trim the extra leg with flush cutters, holding the end so it doesn’t fly off.
A good joint is shiny and shaped like a small cone or volcano around the leg. A dull, lumpy ball that doesn’t hug the pad is a “cold joint,” and it may not conduct reliably: reheat it with a touch more solder.
Go deeper
IntermediatePractice before projects
Start on a practice board: inexpensive “learn to solder” kits come with a board, resistors and LEDs, and making 20 practice joints teaches more than any page can. Through-hole parts (with legs that go through holes in the board) are the easiest to learn on.
A temperature-controlled iron with a stand is worth having. Tips come in different shapes; a small chisel tip suits most beginner work.
AdvancedWhy lead-free, and why temperature matters
Traditional solder was a tin-lead alloy that melts at about 183 to 190 °C (361°F to 374°F). Lead is toxic, so modern electronics mostly use lead-free alloys of tin with small amounts of silver and copper, which melt at roughly 217 to 227 °C (423°F to 441°F), so irons are typically set higher for them. Lead-free solder can be a little harder to work with, but it avoids lead on your hands and bench.
Too little heat gives cold joints; too much, or heating too long, can lift copper pads off the board or damage heat-sensitive parts. Heat the joint quickly and well, then get out.
Quick check
Where does the iron go whenever you’re not using it?
Back in its stand, never on the table.
Should you melt the solder onto the iron or into the joint?
Into the joint: heat the leg and pad, then feed solder where they meet.
What does a good solder joint look like?
Shiny and cone- or volcano-shaped around the leg.
Words to know
- Solder
- A metal alloy melted to join and electrically connect parts.
- Flux
- A cleaning chemical inside solder wire that helps it flow.
- Tinning
- Coating the iron tip or a wire with a little solder.
- Cold joint
- A dull, poorly flowed joint that may not conduct reliably.
Learn more
- University of Toronto: Soldering Guidelines (PDF): safety practices for soldering
- Lesson 17: Troubleshooting
- Back to Electronics School
Electronics School is part of the Discovery Zone. Every activity here runs on small batteries or a USB cable: never wall outlets. The one plug-in tool is the optional soldering iron, set up by an adult.