Soldering basics

Learn

What you’ll learn

• The tools you need and how to stay safe
• The technique for a good joint
• How to spot and fix bad joints

Safety first

A soldering iron tip is about 350 °C. Always return it to its stand, work in a ventilated space (don’t breathe the fumes), wear safety glasses, and wash your hands afterwards. Under-14s should solder with an adult.

Tools

Tool	Notes
Temperature-controlled soldering iron	300–350 °C for most work
Lead-free solder with flux core	0.6–0.8 mm diameter is easy for small parts
Damp sponge or brass wool	To clean the tip
Helping hands or a vice	To hold the work
Flush cutters	To trim legs
Desoldering pump or braid	To fix mistakes

The technique

1. Tin the tip: melt a little solder on the clean tip so it shines.
2. Touch the tip to both the pad and the component leg at the same time.
3. After 1–2 seconds, feed solder into the joint (not onto the tip). It should flow around the leg.
4. Remove the solder, then the iron. Total time about 2–3 seconds.
5. Don’t move the joint while it cools.

Good and bad joints

Joint	Looks like	Fix
Good	Shiny (lead-free is slightly duller), smooth volcano/cone shape	—
Cold	Grey, lumpy, balled up	Reheat with a touch of fresh solder
Bridge	Solder joining two pads	Drag away with a clean tip, or use braid
Too little	Pad not covered, gap visible	Add a little more solder

Examples

Example: soldering header pins to a sensor board

1. Push the header’s long pins into a breadboard to hold them straight.
2. Place the sensor board on top.
3. Solder one pin, check the board is straight, then solder the rest.
4. Inspect each joint; check for bridges with the multimeter’s continuity mode.

Example: joining two wires

Strip 5 mm, slide on heat-shrink tubing, twist the wires together, solder them, then slide the tubing over the joint and shrink it.

Practice

Practice boardEasy

On a cheap practice PCB, solder 10 resistor legs. Photograph them and compare with the good/bad joint table.

DiagnoseMedium

A joint is grey and lumpy and the LED flickers when you tap the board. What’s wrong, and how do you fix it? A cold joint. Reheat it with the iron touching both pad and leg, and add a little fresh solder so it flows.

Header testHard

Solder headers onto a module, then use continuity mode to prove that no neighbouring pins are bridged.

Quick check

1. Where do you feed the solder?

Onto the tip onlyInto the heated joint (pad and leg)Onto the spongeAnywhere

2. A grey, lumpy joint is probably…

PerfectA cold jointA bridgeOverheated plastic

3. Why solder one header pin first?

To check alignment before doing the restIt’s fasterTo save solderNo reason

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