The soldering iron (or soldering pencil) is the tool that heats a joint to melt the solder alloy and electrically fix the components.
Many beginners are afraid of it (will I burn myself? Will I break the chip?). Others think soldering is simply “melting metal on top of things.” In reality, soldering requires technique and care.
Let’s look at the basic technique for obtaining reliable joints and avoiding board damage.
Required Equipment
You don’t need a €300 soldering station to start, but please, avoid €5 “bargain bin” soldering irons that plug directly into the wall. They have no temperature control and will burn your boards.
The Soldering Iron
Look for one with adjustable temperature.
- Power: Between 40W and 60W is ideal.
- Tip: Use a fine tip (conical or small screwdriver shape). Large tips are for plumbing, not electronics.
The Solder Alloy
Solder wire is not pure tin.
- 60/40 Alloy (Lead): The classic (60% Tin, 40% Lead). It melts at a low temperature (≈190°C) and flows beautifully. It’s the best for learning, although lead is toxic (wash your hands afterward).
- Lead-Free: The mandatory standard in industry (RoHS). It’s harder to use: melts at a higher temperature (≈220°C), is less shiny, and is more difficult to “wet.”
Solder wire has a hollow core filled with flux (Rosin). Flux is an acid that cleans oxide from metals when heated.
Without flux, solder won’t stick. If your solder is very old or poor quality, the flux will have dried out and soldering will be a nightmare.
Learn to solder without needing external flux. Some people slather on flux as if there’s no tomorrow. Generally, with good quality solder, there’s no need to add extra flux.
The Soldering Technique
A good solder joint should be shiny, smooth, and shaped like a concave volcano. Follow these four steps:
Step 1: Clean
Clean the soldering iron tip on the damp sponge or brass wire cleaner. The tip should be bright/silver, not black. A dirty tip doesn’t transfer heat.
Step 2: Heat the Joint
Don’t melt solder on the iron tip and then try to stick it onto the wire (that’s called “gluing,” not soldering).
- Touch the soldering iron tip SIMULTANEOUSLY to both the board pad and the component lead.
- Wait 2 or 3 seconds for both parts to heat up.
Step 3: Apply Solder
- Without removing the iron, bring the solder wire to the opposite side of the joint (not directly to the iron tip).
- If the lead and pad are hot, the solder will melt upon touching them and flow to cover everything perfectly.
- Apply the right amount (enough to cover the pad, but without forming a ball).
Step 4: Remove
- Remove the solder wire first.
- Remove the soldering iron second.
DON’T BLOW ON IT! Let it cool naturally for a couple of seconds. If you blow on it or move the component while it’s cooling, you ruin the crystalline structure.
The Hall of Horrors (Common Mistakes)
Learning to identify a bad solder joint is as important as making a good one.
Cold Joint
Public enemy number one.
- Appearance: Grayish, dull, rough, or ball-shaped without touching the pad.
- Cause: The component moved while cooling, or (most common) you didn’t heat the pad enough and the solder spilled on top without fusing to the copper.
- Consequence: Intermittent connection. Your circuit will work sometimes and not others.
Solder Bridge
Two close pins are joined together with solder.
- Cause: Using too much solder or a tip that is too large.
- Consequence: Short circuit. If you connect VCC and GND, smoke will appear.
Lifted Pad
The copper ring on the board detaches and breaks.
- Cause: Excessive heat (keeping the iron on for 10 seconds) or applying brute force with the tip.
- Consequence: Irreparable open circuit (you’ll have to make jumper wires).
The Ball (Over-soldering)
A round mountain of solder that looks like a water drop.
- Cause: Not knowing when to stop adding solder.
- Consequence: Not electrically severe, but it can hide a cold joint underneath and makes it easy to create bridges.
Safety and Maintenance
- Always tin the tip: When you finish working and are about to turn off the iron, leave a large drop of solder covering the tip. This protects it from oxidation (you sacrifice that solder to save the tip).
- Ventilation: The smoke released is not lead (lead doesn’t boil at 300°C), it’s the flux burning. It is irritating to the lungs and eyes. Work in a ventilated area.
- Safety Glasses: Sometimes, when cutting a wire, it flies off. Or a drop of solder jumps. Protect your eyes.
