How do I solder electronics without making a mess of it?
Tinning the tip, heating the joint rather than the solder, and recognising a cold joint. Plus lead-free, flux, extraction, and the handful of things that make the difference between a joint that lasts and one that fails intermittently.
- Difficulty
- intermediate
- Time
- 10 min
- Read
- 5 min
- Safety
- warning
Short answer
Get the tip clean and tinned. Heat the joint — the pad and the leg together — for a second or two, then feed solder into the joint, not onto the iron. It should flow and form a shiny concave fillet. Remove the solder, then the iron, and let it cool without moving. A dull, blobby, convex joint is a cold joint and it will fail.
Soldering electronics is a small skill with a very sharp learning curve: about twenty minutes of deliberate practice takes most people from making blobs to making proper joints. Almost all of the difficulty is in two ideas — a clean tinned tip, and heating the work rather than the solder.
Safety
Step by step
- Set up before you start.A stand with a brass wool ball or a damp sponge, ventilation or a fume extractor, good light, and the work held rather than held in your hand. A third-hand tool or a small vice is worth more than a better iron.
- Get the tip clean and tinned.Wipe the hot tip on brass wool, then melt a little solder onto it so it is bright and silver. A dull, blackened tip transfers almost no heat, and this is the reason most beginners think their iron is too weak.
- Set a sensible temperature.Hot enough to melt solder quickly, not so hot that it burns flux instantly and lifts pads. Lead-free solder needs a higher setting than leaded. If a joint takes more than a few seconds, the temperature or the tip is wrong rather than the technique.
- Heat the joint, not the solder.Touch the tip so it contacts both the pad and the component leg at once, for a second or two. Then feed the solder into the joint on the opposite side from the iron. If you melt solder onto the iron and dab it on, you get a blob stuck to a cold joint.
- Watch for the flow.Good solder flows and wets both surfaces, forming a smooth, shiny, slightly concave fillet like a tiny volcano. That shape is the sign that both surfaces reached temperature.
- Remove in the right order.Solder away first, then the iron. Then leave it alone for a couple of seconds — a joint moved while the solder is solidifying comes out grainy and cracked.
- Recognise a cold joint.Dull, grey, blobby, ball-shaped rather than flowed, and often with a visible line where the solder failed to wet the pad or the leg. It may work today and fail intermittently for years, which makes it the worst kind of fault.
- Know what flux is doing.It removes oxide so the solder can wet the metal. Most electronics solder has a flux core, which is why it works straight from the reel; extra flux paste helps on oxidised or awkward joints. Clean rosin residue off afterwards where appearance or corrosion matters.
- Desolder properly when you need to.Solder wick — braid — pressed onto the joint with the iron on top pulls the solder into it. A desoldering pump works for through-hole. Levering a component out while heating one leg lifts pads off the board, and a lifted pad is a much harder repair.
- Wash your hands afterwards.Particularly with leaded solder, and always before eating. It is a small habit and it is the standard advice.
Tips
- A cheap temperature-controlled station outperforms an expensive fixed-temperature iron for this work, and a good tip matters more than the badge on the handle.
- Practise on a scrap board or a practice kit before working on anything you care about. The skill arrives quickly and it arrives through repetition, not reading.
- Keep the tip tinned when you put the iron down and before you switch it off. A tip left bare and hot oxidises and stops transferring heat, and that is what kills tips.
Common mistakes
- Carrying solder to the joint on the iron — The flux burns off on the way, so the solder arrives without the thing that lets it wet the surface. It is the defining beginner error and it produces cold joints every time.
- Using a dirty or untinned tip — Oxide is an insulator. The iron can be at the right temperature and still deliver almost no heat to the joint.
- Using plumbing solder or acid flux on electronics — Acid flux corrodes the board and the joint over time. Electronics needs rosin or no-clean flux-cored solder, and nothing from the plumbing shelf.
- Holding the iron on a pad for a long time — Pads lift off the board with prolonged heat and once lifted the repair is much harder. If it is not flowing in a few seconds, stop and fix the cause.
If it doesn't work
The solder will not stick to the pad
Cause: Oxidation, contamination, insufficient heat, or a cold tip — Fix: Clean and tin the tip, add a little flux to the joint, and make sure the tip is contacting both the pad and the leg. Adding more solder is not the answer.
A joint that works intermittently
Cause: Almost always a cold joint or a hairline crack in the solder — Fix: Reflow it: add a touch of flux and fresh solder and heat until it flows properly. Intermittent faults on old equipment are very often exactly this.
A bridge of solder between two pads
Cause: Too much solder, or the tip dragging — Fix: Solder wick lifts it cleanly. Adding flux first makes the wick work far better.
Questions people ask
Leaded or lead-free?
Leaded solder flows more easily at a lower temperature and is easier to learn on, and it is restricted for commercial products and is a health consideration. Lead-free needs more heat and better technique. For hobby repair, either is used; wash your hands and ventilate whichever you choose.
Do I need a fume extractor?
You need the fume away from your face. Rosin flux fume is a recognised respiratory sensitiser, and a small fan or extractor is a cheap way to deal with it properly.
Can I solder surface mount components?
Yes, with a fine tip, flux, tweezers and magnification, and it is a step up in difficulty. Hot air rework is the usual approach for anything with hidden or many pins.
Is it safe to repair mains equipment?
Only if you are competent to work on it, it is fully isolated, and any capacitors have been safely discharged. Mains-side repairs are a different risk category from low-voltage electronics.