What a good joint looks like
A correct joint is a smooth concave cone — it flows up and around the component lead and out onto the pad, like water wetting a surface. With leaded solder it's noticeably shiny. Lead-free is duller by nature, which is normal and not a defect.
The key word is wetting. Solder should look like it has flowed onto and bonded with the metal, not like it's been dropped there and set. That single visual distinction is most of the skill.
Diagnosing by appearance
Dull, grainy, rough surface (with leaded solder)
This is a cold joint — the classic defect. It means the metal never got hot enough for the solder to properly bond, so you have solder resting against the parts rather than joined to them.
Cold joints are dangerous precisely because they often work. They conduct just enough to pass a test, then fail later with vibration, temperature change, or time. A device that develops an intermittent fault months after you built it is frequently a cold joint.
Cause: not enough heat, or heat applied for too short a time. Fix: reheat the joint until the solder visibly flows and reshapes itself, then remove the iron and let it cool undisturbed.
A ball or blob sitting on top of the pad
Solder has failed to wet the surface, so surface tension pulled it into a ball. It's touching, not bonded.
Cause: usually a dirty or oxidised surface, sometimes insufficient heat on the pad. Fix: clean the pad and component lead, add fresh solder (which brings fresh flux with it), and make sure you're heating the joint rather than the solder.
Too much solder — a dome you can't see the lead through
Not necessarily a bad connection, but it hides problems and risks bridging to a neighbouring pad. You want to be able to see the shape of the lead through the joint.
Fix: remove the excess with desoldering braid and redo it. Less solder than feels right is usually correct.
A bridge between two pads
Solder connecting two pads that shouldn't be connected. On fine-pitch parts this is easy to do and easy to miss visually.
Fix: desoldering braid across both pads, with a little fresh flux. Then check with a multimeter's continuity function — a bridge you can't see is exactly the kind of thing that costs an evening.
A cracked ring around the lead
Almost always caused by movement while the joint was cooling. Solder cools within a second or two, and disturbing it during that window fractures the bond.
Fix: reheat and, this time, hold everything still until it has fully solidified. This is precisely what a helping-hands stand or small vice is for.
The technique that fixes most of it
Almost every defect above comes back to one error: melting solder onto the joint instead of heating the joint until it melts the solder.
The correct sequence:
- Touch the iron to both the pad and the lead at once, so the tip bridges the two. Both need to reach temperature.
- Count about a second or two. Resist the urge to rush.
- Feed solder into the joint — against the pad and lead, not onto the iron tip. If the joint is hot enough, the solder flows in and spreads on its own. If it doesn't flow, the joint isn't hot enough.
- Remove the solder, then the iron. In that order.
- Don't move anything until it's set.
If you take one thing away: solder touching the iron tip and dripping onto a cold pad is what produces cold joints. The joint has to do the melting.
Things that quietly sabotage you
A dirty tip
An oxidised, blackened tip transfers heat poorly, and you'll blame yourself for what's actually a tool problem. Wipe on brass wool frequently, and keep the tip coated in a little fresh solder when idle — this is called tinning, and it's the single biggest factor in how long a tip lasts.
Wrong temperature
Too cold and the solder won't flow. Too hot and flux burns off instantly, tips degrade fast, and you risk lifting pads off the board. Fixed-power irons without temperature control are frequently too hot, which is why the bench guide recommends a temperature-controlled iron specifically.
Old solder, or no flux
Flux is what cleans the metal so solder can bond; it's already inside the core of decent electronics solder. Very old solder, or solder intended for plumbing, will fight you. Use thin rosin-core electronics solder, and add a little flux for anything awkward.
The wrong tip
A tip much smaller than the joint can't deliver heat fast enough, so you hold it there too long and cook the component. Bigger tips are often the right answer, which surprises people.
Testing your work
- Look at it — ideally with magnification. Shiny, concave, lead visible through it.
- The tug test — gently pull the component. It shouldn't move at all.
- Continuity check — multimeter from the joint to the other end of that trace. Also check adjacent pads to confirm you haven't bridged them.
Do all three. Visual inspection alone misses bridges, and continuity alone misses a mechanically fragile joint that will crack later.
Practise on something worthless
Get a scrap board — a dead appliance, an old PC card, anything with through-hole parts — and make thirty joints on it. Deliberately make a cold one so you can see the difference next to a good one. Half an hour of that beats reading about it, and it costs nothing.
Gear that helps
A temperature-controlled soldering iron is the one tool where quality genuinely changes the outcome. Add thin rosin-core solder, desoldering braid for fixing mistakes, and a helping-hands stand so nothing moves while joints cool.