Everything so far has been preparation. This is the lesson where you actually do it.
The good news is that a solder joint takes about three seconds and there are only four movements. The reason this course has spent seven lessons getting here is not that soldering is hard, it is that doing it badly also takes three seconds and looks quite similar, and without the preparation you would have no way of telling which one you had done.
So read this one with the iron in front of you. Then do it about twenty times on something that does not matter.
By the end of this lesson you will be able to make a sound through-hole solder joint, know what each second of it should look like, and recognize in the moment when it is going wrong rather than afterwards.
8.1 Before you switch on
The aim of this section is a four-item check that prevents most first-attempt frustration.
- Is the board unpowered? No batteries, nothing plugged in. Lesson 5.
- Is the stand where it always lives, on your dominant side? You are about to need it without looking.
- Is the light coming from the side? Not from behind your head. You need shadows on the joint to read its shape.
- Is the work held? Tape, a third hand, a vice, or a book. You have two hands and this needs three.
Then switch on and set 320 °C, or 340 °C if you are on lead-free. See Section 6.6.
When it reaches temperature, tin the tip. Wipe on brass wool, immediately load fresh solder, so the working face is bright and wet. Lesson 6. This is not optional and it is not maintenance: without it, the first joint will not heat.
8.2 How to hold everything
The aim of this section is to set up the physical geometry, because most beginner awkwardness is a grip problem rather than a technique problem.
Hold the iron like a pen. Not like a knife, not in a fist. Close to the end of the handle, resting in the web of your hand, with your fingers doing the aiming. It should feel light.
Rest your hand on something. The bench, the edge of the board, your other wrist. A hand floating in mid-air shakes; a hand resting somewhere does not. This is the single biggest improvement available to someone who thinks they have unsteady hands.
Hold the solder in your other hand, about 3 to 4 cm from the end. That distance matters more than it sounds. Thin wire has no stiffness in it, so gripping much further back means the wire simply buckles the moment you press it against a joint, and you end up pushing a spring rather than feeding metal. Hold it close and you can place the end exactly where you want it. Feed it forward through your fingers as it is consumed, and re-grip when you run short rather than starting with a long floppy length.
Approach the joint from the side, not from above. The tip should come in at a shallow angle so that its face lies into the angle where the leg meets the pad, rather than poking down at it. Section 2.3 explained why that contact geometry is everything.
And the solder comes in from the other side. The rule from Lesson 1 is that the solder must touch hot metal, not the tip. Feeding it in from the opposite side of the joint is simply the easiest way to guarantee that.
Figure 8.1: Iron like a pen, hand resting on something, tip in from the side at a shallow angle, solder in from the opposite side.
8.3 The four beats
The aim of this section is the technique itself, in the order it happens, with what to look for at each moment.
This is Section 1.5, now with an iron in your hand. The order matters at both ends: iron on first, solder off first.
Beat one: the iron goes on, alone
Lay the tip into the angle where the leg meets the pad, touching both at once. Not balanced on top of the leg. Not resting on the pad beside the leg. In the corner between them.
No solder yet. This is the beat people skip and it is the one that makes the joint work.
What to look for: almost nothing visible, which is why it feels wrong. If you are using a flux-cored approach on a previously soldered joint you may see a wisp of smoke. Otherwise, just count. About one second, longer on a big pad.
What is happening: the leg and the pad are coming up to temperature together. The film of solder on your tip is bridging the gap and carrying heat across. See Section 6.2.
Beat two: feed the solder in
Touch the end of the solder wire to the joint, on the far side from the iron, right in the corner where leg meets pad.
What to look for: if the joint is hot enough, the solder melts on contact and disappears into the joint immediately. That instant is your temperature test, and it is the most useful feedback in the whole process. If instead the solder sits there as a solid wire against the metal, or beads and rolls, the joint is not hot enough. Take the solder away, leave the iron there another second, and try again.
⛔ Do not touch the solder to the iron's tip to "get it started". That is the film-versus-payload mistake from Section 1.6, and it lands cool, fluxless solder on cold metal.
How much: less than you think. For a typical small joint, two to three millimeters of 0.7 mm wire. Feed it in, watch, stop. It is far easier to add a little more than to remove too much.
Beat three: the solder comes off, the iron stays
This is the beat almost everyone skips, and skipping it is why joints come out as lumps.
As soon as enough solder has gone in, pull the wire away. Leave the iron exactly where it is.
What to look for, and this is the moment worth learning to see: over about half a second the solder stops being a lump and becomes a shape. It wets out, running around the leg, flowing outward across the pad until it reaches the edge, sinking down into the hole, and pulling its own surface into that low, hollow-flanked profile from Section 1.4.
You will see it happen. It is quick and it is unmistakable once you know to look, and it is genuinely satisfying. That is the bond forming.
If you lift the iron at the same moment as the solder, you stop this halfway and freeze a lump.
Beat four: the iron comes off, then nothing moves
Once it has wetted out, lift the iron away cleanly. Sideways and up, not dragged along the board.
Then do nothing for a second or two. Do not move the board, do not nudge the component, do not blow on it. It is freezing, and a joint disturbed while it freezes sets fractured inside while looking perfectly acceptable outside. Lead-free needs this second more than leaded does, because of the pasty range in Section 3.1.
Then put the iron back in the stand. Every time, without looking, because it lives in one place.
Figure 8.2: The four beats. Iron on first and alone, solder fed to the joint, solder away while the iron stays and it wets out, then iron away and hold still.
8.4 What it should look like, and how long it should take
The aim of this section is to give you the two numbers and the one shape to judge against.
Time on the joint: two to four seconds in total. One second heating, one feeding, one wetting out. On a big pad or a ground plane, up to five or six. See Section 7.5.
⛔ If you are still there at ten seconds, stop. Lift off, let it cool, and think about why. Almost always it is one of three things: the tip is not tinned, the pad is on a plane, or the iron is not recovering. Sitting on a joint hoping is how components die and pads lift.
The shape you are aiming for: low, wide, with slightly concave sides, flowing right out to the edge of the pad and stopping there. Like a small volcano hugging the leg.
And the thing to distrust: a tall, rounded ball. That is the shape of solder that was placed rather than drawn on, and it is what you get from the mistake in Section 1.6.
Figure 8.3: Two to four seconds is normal. Past ten, stop and find the cause rather than pressing on.
⚠️ Shine is not the test. Shape is the test. A correct lead-free joint is dull and slightly textured, as Section 3.2 explained. If you reheat a perfectly good lead-free joint because it looks dull, you will eventually lift the pad.
Then trim the leg, with flush cutters, once it has cooled. Section 1.5 covered this: the leg stands proud until now, which is correct, and you snip it just above the fillet. Hold the offcut or point it away from your face.
8.5 The five things that go wrong on a first attempt
The aim of this section is a live troubleshooting list, so you can diagnose in the moment rather than after ten failures.
The solder will not melt when it touches the joint. The joint is not hot enough. In order of likelihood: your tip is not tinned, so no heat is crossing; the tip is only touching the leg and not the pad; or that pad is on a ground plane. Re-tin first, always, before adjusting anything else.
The solder melts but sits in a ball and will not spread. Classic non-wetting. The metal is not hot enough or not clean. Leave the iron on a moment longer and feed fresh solder, which brings fresh flux with it. If it still refuses, the surface may be oxidized and wants a touch of flux from a pen.
The solder went onto the tip instead of the joint. You touched the wire to the iron. It is the most natural mistake in the world because the tip melts it obediently. Wipe the tip, re-tin, and touch the solder to the far side of the joint next time.
The joint is a big shiny ball. Too much solder, or lifted before it wet out. Both are fixable: reheat the joint with a clean tip and let it sit a second without adding anything. It will often flow out and settle into shape by itself. If there is genuinely too much metal, Lesson 10 shows you how to take some away.
The component moved and the joint went crusty and dull. Disturbed while freezing. Reheat it, let it flow, and this time do not touch anything for two seconds. On lead-free this is easy to do by accident.
8.6 Practice before you build
The aim of this section is to argue for twenty minutes of boredom that will save you an afternoon of regret.
Do not make your first ever joints on the thing you care about.
Make twenty joints on something disposable. A cheap practice board, an old dead appliance board, some stripboard, or the least important corner of your kit.
What to practice, in order:
- Make ten joints and just watch beat three. Ignore whether they are pretty. Learn to see the moment it wets out. That is the skill.
- Make ten more and time them. Aim for consistency, three seconds each, not speed.
- Deliberately make one bad one. Melt solder on the tip, dab it on a cold pad, and look at the result. Seeing a cold joint you made on purpose is worth more than reading about one, and it means you will recognize it later.
Then go and build. And when you have built something, Lesson 9 shows you how to grade your own work, which is the lesson that turns twenty joints into actual skill.
What this sets up
Lesson 9, good joint bad joint, is the inspection. You now have joints to inspect.
Lesson 10, desoldering and rework, is how to undo anything in this lesson that went wrong. Nothing you have just done is permanent.
Lesson 11, wires and connectors, is the same skill applied where there is no board to hold things for you.
Frequently asked questions
How do you solder for the first time? Tin the tip, lay it into the angle where the component leg meets the pad so it touches both, wait about a second, then touch solder to the joint on the far side from the iron. When enough has flowed in, take the solder away but leave the iron for another half second while it wets out. Then lift the iron and keep everything still while it cools.
How long should you hold a soldering iron on a joint? Two to four seconds for a normal joint, up to five or six on a large pad connected to a ground plane. If you are still there at ten seconds, stop and work out why rather than continuing.
How much solder should I use on a joint? Very little. Two to three millimeters of 0.7 mm wire for a typical small joint. You are aiming for a low, wide shape that reaches the edge of the pad, not a ball.
Should I put solder on the iron first? Only a thin film, to carry heat, and that is tinning. Never a payload of solder carried over to become the joint. That lands cool with its flux already burned off, and it is the most common cause of cold joints.
Why does my solder ball up and not stick? The metal is not hot enough or not clean. Leave the iron on longer and feed fresh solder, which brings fresh flux. If it still resists, add flux from a pen.
Which do I remove first, the solder or the iron? The solder. Then leave the iron on for another half second while the joint wets out and takes its shape, then lift the iron. Removing both together freezes it halfway.
When do I cut the component legs? After soldering, once the joint has cooled, with flush cutters, just above the joint. The leg standing proud before that is correct and is what holds the part in while you work.
Next
Lesson 9: Good joint, bad joint, and checking your work. A gallery of every common fault, what causes each one, and how to check a whole board before you power it up.
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