Almost nobody is taught to solder. They are shown it, once, and then they copy what they saw.
What they usually saw was this: heat the iron, melt a little solder onto the tip, carry it over to the board like a spoonful of soup, and press it down. And it appears to work. The part stays put. The blob is shiny. The board might even switch on.
A surprising number of those joints are not joints at all. They are beads of metal resting on a surface they never bonded to, held in place by nothing but their own shape. Some fail immediately. The more troublesome ones work perfectly on the bench and then fail three weeks later, or only when the board is warm, or only when it is knocked.
This is not a talent problem and it is not a steady-hands problem. It is a mental model problem. If you believe soldering is a kind of metal glue, then every decision that follows from that belief is reasonable, and almost every one of them is wrong.
So this first lesson does not teach you any technique at all. It gives you the model. Everything in the twelve lessons that follow is easier once you have it, and most beginner mistakes stop being mysterious and start being obvious.
By the end of this lesson you will be able to explain what physically happens inside a solder joint, recognize the shape of a joint that worked, and name the single habit that causes most beginner failures.
1.1 Three ways to join two things
The aim of this section is to place soldering next to the two things people usually confuse it with, so that you know what kind of process it is before you know how to do it.
There are broadly three ways to join two pieces of material, and it is worth being precise about which one you are doing.
Glue introduces a different substance between two parts and grips them. The parts themselves are unchanged by the process. Nothing is shared between them, and the join is only ever as good as the adhesive. Pull hard enough and the adhesive lets go, usually all at once.
Welding melts the two parts into each other. They stop being two separate things and become one continuous piece of metal. It is enormously strong, and it is completely unsuitable for electronics, because melting a resistor is the same as destroying it.
Soldering sits between the two, and it borrows the useful half of each. You introduce a third metal, an alloy called solder, which melts at a far lower temperature than the parts do. That molten alloy then forms a genuine metallic bond with the surface of both parts. The parts never melt, so a delicate component survives the process, but the connection that results is metal joined to metal rather than something merely stuck on.
That middle path is precisely why electronics uses it. You need a connection that carries electricity as though the two parts were one continuous piece of metal, and you need to make it without cooking anything.
Which leads to the definition worth memorizing:
Soldering joins two metal parts by melting a third, softer metal that bonds to the surface of both, without melting the parts themselves.
There is one consequence of that definition that beginners routinely miss. Your solder joint is doing two jobs at once. It is the mechanical anchor holding the component to the board, and it is also part of the electrical path, with the current genuinely running through it.
Those two jobs can come apart, and that is the trap. A joint can be mechanically convincing, gripping the part perfectly firmly, while carrying almost no current at all. This is why "it looks like it is stuck on there" is not a test of anything, and it is why the rest of this course spends so much time on what a joint should look like rather than on how firmly it holds.
The mechanical job has limits too. Solder will anchor a resistor or an LED indefinitely and never complain. It is not a structural fixing, though, and anything that takes real force, a socket that gets plugged and unplugged, or a wire that gets tugged, needs mechanical support of its own as well as solder. Lesson 11 deals with that.
Figure 1.1: Glue, welding and soldering compared. Only soldering leaves both parts intact while still forming a metal-to-metal bond.
1.2 Wetting: why solder grips at all
The aim of this section is to give you the one piece of physics that explains nearly every soldering failure you will ever have. If you take a single idea from this lesson, take this one.
When molten solder touches metal that is hot enough and clean enough, it does something specific: it spreads. It runs out across the surface into a thin layer with a shallow, sloping edge, and it climbs up anything vertical that it is touching. That behavior has a name, wetting, and it is the visible evidence that a bond is forming.
What is happening underneath is that atoms of the solder and atoms of the metal underneath are interdiffusing at the boundary, forming a very thin layer of a new alloy that belongs to both. That layer, which is far too thin to see, is the joint. Everything else is just the solder that got it there and the shape it froze into.
When the surface is not ready, the solder does the opposite. It pulls itself into a ball and sits there with a steep, rounded edge, touching the metal but bonded to none of it. There are only two reasons this happens, and you can do something about both:
- The metal is not hot enough. Solder that melts on the iron and then lands on a cool pad cannot wet it. The heat has to be in the parts, not just in the solder.
- The metal is not clean. Metals grow a layer of oxide in air, and oxide is not something solder can bond to. This is the entire reason flux exists, and why the solder you buy already has flux running through the middle of it. Lesson 3 deals with flux properly.
The everyday comparison is water. Pour water onto clean glass and it spreads into a film. Pour the same water onto a freshly waxed car and it beads up and rolls off. Same water, opposite behavior, and the only variable is the surface it landed on.
So here is the reframing that makes the rest of the course make sense. The skill of soldering is not the skill of applying solder. It is the skill of preparing metal so that the solder wants to spread. Get the metal hot enough and clean enough and the solder does the actual work for you, quickly and rather beautifully. Fail to, and no amount of care with the solder itself will rescue it.
Figure 1.2: Solder that wets spreads out with a shallow edge. Solder that cannot wet pulls into a ball with a steep edge and bonds to nothing.
1.3 How the board actually sits while you work
The aim of this section is to explain the arrangement you will see in every figure in this course, because it looks upside down until you have done it once.
Through-hole components, which is nearly everything you will solder as a beginner, are pushed in from one face of the board and soldered on the other.
The sequence is: you push the component's legs down through its holes from the top, so the body of the part sits flush against the board. Then you turn the whole board over. The component is now underneath, held in place by the board itself, with its legs poking up towards you through the holes. The face you are now looking at, the one you are about to solder, is the opposite face from the one the component sits on.
This has two consequences that matter.
First, each leg gets exactly one joint, on the side facing you. Not one on each side. Solder does wick down into the plated barrel of the hole and that is a good sign, but you are building one joint per leg, and you build it from one side.
Second, it explains why every figure in this course is drawn with the component body below the board and the iron working on the top face. That is not an illustrator's convenience. It is the view you will genuinely have, with the thing you are working on facing you and the component hidden underneath.
Figure 1.3: The component sits flush on the far face and you solder the side facing you. Each leg gets one joint, not one on each side.
1.4 The shape of a finished joint
The aim of this section is to give you a target. You cannot judge your own work without knowing what success looks like, and the good news is that success has a very distinctive silhouette.
A joint that wetted properly is low, wide, and hollow-sided. It flows outwards from the leg and down onto the pad, with flanks that curve inwards rather than bulging out, and it stops cleanly at the edge of the pad rather than spilling past it. Picture a small volcano, or the way solder pools around the base of a pin: it rises from a wide base and tucks in as it climbs.
The individual parts you are looking at are worth naming, because the rest of the course uses these words:
- The pad is the exposed metal ring on the board's surface around the hole. On amomii boards it is tinned and looks silver. It is the surface your solder has to wet, and it is the single most important surface in the whole operation.
- The plated through hole is the metal-lined barrel running through the board, connecting the pad on one face to the pad on the other.
- The leg, or lead, is the component's wire, running up through that barrel.
- The fillet is the shaped body of solder you have just made, joining the leg to the pad.
And now the part that saves people a great deal of anxiety. Shine is not the test. Shape is the test.
A correct joint made with modern lead-free solder is often slightly dull, and can look faintly grainy or textured. That is normal and it is not a fault. A joint made with leaded solder is usually noticeably brighter. Beginners who have only ever seen photographs of leaded joints frequently conclude that their perfectly sound lead-free work has failed, and reheat joints that were fine. Lesson 3 goes into the real differences between the two, honestly, including the ones nobody likes saying out loud.
Judge the silhouette. A tall, rounded, shiny ball is the shape to distrust, however good it looks.
Figure 1.4: The anatomy of a finished through-hole joint. Note the low, wide profile and the hollowed flanks flowing out to the edge of the pad.
1.5 The four beats of making a joint
The aim of this section is to give you the sequence, so that you can follow along when Lesson 8 slows it right down and puts an iron in your hand. Nothing here is something you need to practice yet.
A single joint has four beats, and it takes about three seconds once you have the feel of it.
One: the iron goes on first. Always first, and on its own. The tip touches the leg and the pad at the same time, so that both come up to temperature together, and it sits in the angle where they meet rather than balanced on top of the leg. A tip with a little solder already on it does this far better than a bare one, for a reason section 1.6 comes back to. Nothing else happens for about a second. No solder yet.
Two: feed the solder in. The rule is that the solder must touch the hot metal, not the iron's tip. The easiest way to guarantee that is to feed it in from the opposite side of the joint to the iron, into the angle where the leg meets the pad. On a crowded board there is not always room for that, and it does not greatly matter: what matters is that the joint melts the solder rather than the tip melting it. If the metal has come up to temperature, it melts the instant it touches.
Three: take the solder away, and leave the iron exactly where it is. This is the beat most people skip, and skipping it is why their joints look like lumps. As soon as enough solder has gone in, pull the wire out of the joint. The iron stays on.
Over the next half-second you will see the solder stop being a lump and start being a shape. It wets out: running around the leg, flowing outwards across the pad until it reaches the edge, sinking down into the barrel of the hole, and pulling its own surface into that low, hollow-flanked profile from section 1.4. That is the bond forming, and it is the same wetting from section 1.2 happening in front of you.
You are not placing the solder or shaping it. You are holding the heat there and letting it go where it wants to go. Lift the iron at the same moment as the solder and you stop the process halfway through.
Four: iron away, then hold still. Once it has wetted out, lift the iron. Then do nothing at all for a second or so while the joint freezes. Moving a joint while it is solidifying leaves a fractured, crystalline structure inside it that can look perfectly acceptable from the outside and be electrically unreliable.
So the order, and it matters at both ends: iron on first, solder off first.
And then you trim the leg
Look at what you have made and you will notice the leg is still standing proud of the joint, often by a centimeter or more. That is correct. It is exactly what your board looks like the first time you solder it, and it is not a sign you have done anything wrong.
You do not cut the legs beforehand. You push the part in with its legs at full length, because that length is what holds it in place when you turn the board over, and you deal with it afterwards.
Once the joint has cooled, take a pair of flush cutters and snip the leg off just above the top of the fillet. Flush cutters have one flat face, so laid against the joint they cut close and leave a short tidy stub rather than the sharp diagonal spike that ordinary side cutters produce.
Two things worth knowing before you do it. A trimmed leg leaves at speed, and offcuts are remarkably good at finding an eye, so either hold the waste end as you cut or point it away from your face. And cut once, cleanly. Twisting and tugging at a leg to work it off puts all of that force straight into the joint you have only just made.
Then step back and look at the board, because this is the moment it stops looking like a project and starts looking like a product.
Figure 1.6: Trimming comes last, after the joint has cooled. Flush cutters lie flat against the joint and leave a short stub rather than a spike.
Figure 1.5: The four beats. The iron goes on first and comes off last. Watch beat three in particular: the solder is withdrawn while the iron stays on the joint, and that is the moment the fillet wets out into shape.
1.6 The mistake almost everyone makes
The aim of this section is to name the specific habit that causes more failed beginner joints than everything else put together, and to explain why it fails, because "do not do that" is not a reason.
The mistake is melting solder onto the iron's tip and carrying it to the board.
It is an entirely understandable thing to do. The tip is right there, it is hot, and solder touched to it melts instantly and obediently. The board, by comparison, is cold and fiddly and does nothing when you touch it. So people load up the tip with a nice bead and take it over.
Here is why it fails, and there are two separate reasons.
The first is heat. That bead has been sitting on the tip losing energy, and it lands on a pad that has had no chance to warm up at all. It is cool solder arriving on cold metal, and cool solder on cold metal cannot wet.
The second reason is the one people never hear, and it matters more. Your solder has flux running through the core of it, and flux is what strips the oxide off the metal so that solder can bond to it. Flux is consumed by heat. Melt solder on a 350 °C tip and hold it there, and the flux has burned off and evaporated before the solder ever reaches the board. What lands on the pad is solder with its cleaning agent already spent.
So you arrive with cool solder, on cold metal, carrying no active flux, and then wonder why it beads up. That is a cold joint, and it is the single most common fault in beginner work.
The fix is not more skill or a steadier hand. It is one change to the order of operations:
Heat the metal first, and let the hot metal melt the solder.
The test is pleasingly simple. If the joint is hot enough to melt solder on contact, it is hot enough to bond to it. If it is not, then no quantity of solder is going to help, and adding more will only hide the problem under a bigger blob.
What this rule does not mean
It does not mean your tip should be bare and dry. Very much the opposite, and the over-correction causes its own problems.
A working iron always carries a thin film of solder on its tip. That is what tinned means, and Lesson 6 is entirely about keeping it that way. That film matters here for a reason that is easy to miss. A clean, dry tip touching a round component leg makes contact at very nearly a single point, and heat crosses a single point extremely slowly. A tinned tip flows into the gap instead, making genuine contact with the leg and the pad at once, and the joint comes up to temperature in about a second rather than five.
So the distinction is not "solder on the tip, yes or no". It is what the solder is for:
- A film of solder on the tip, there to carry heat into the joint. Correct, and necessary.
- A payload of solder on the tip, carried over to become the joint itself. The mistake.
One is a bridge for heat. The other is a delivery of cool, spent solder. They look superficially alike and they are opposites.
Figure 1.7: Left, solder melted on the tip and carried over, landing cool and fluxless on a cold pad. Right, both metals heated first so the joint itself melts the solder.
What this sets up
You now have the model, and every remaining lesson is in service of it.
Getting the metal hot enough is why your choice of iron matters, and why a tip that has been neglected transfers heat so badly that good technique cannot rescue it. That is Lessons 2 and 6. It is also why some pads on a real board take far more heat than others, which is Lesson 7, reading the board.
Getting the metal clean enough is what flux does, and it is why solder is manufactured with flux inside it rather than sold separately. That is Lesson 3.
Recognizing whether it wetted is how you check your own work by eye, before you power anything up and before you reach for a meter. That is Lesson 9, good joint bad joint.
None of it is difficult. It is simply specific, and now you know what it is being specific about.
Frequently asked questions
Is soldering the same as welding? No. Welding melts the parts themselves and fuses them into a single piece. Soldering melts only the added solder, and the parts stay solid throughout. Soldering also happens at a far lower temperature, which is exactly what makes it safe for electronic components.
Does solder conduct electricity as well as copper? Not quite, but it does not need to. A solder joint is very short and has a generous cross-section, so its resistance is negligible in any normal circuit. A properly wetted joint behaves as though the parts were continuous metal.
Why did my solder ball up instead of spreading out? The metal was either not hot enough or not clean enough. Those are the only two causes. Heat the joint for another second before feeding solder in, and make sure your iron's tip is clean and freshly tinned.
Do I need to melt the component? No, and you must never try. If a component gets hot enough to melt, you have destroyed it. Only the solder melts.
Should there be solder on my iron's tip when I touch the joint? A thin film of it, yes, and that is not the same thing as carrying a blob over. The film is what lets heat cross from the tip into the leg and the pad, because a dry tip touches a round leg at almost a single point. What you must not do is melt the joint's worth of solder onto the tip and transport it. See the end of section 1.6, and Lesson 6 for how to keep a tip tinned.
Does the solder joint hold my component on, or is it just an electrical connection? Both. It is genuinely the mechanical anchor as well as part of the circuit. The catch is that those two jobs can come apart: a joint can grip the part firmly and still carry almost no current, which is why you judge a joint by how it looks rather than by how solidly it feels.
Will more solder make a stronger joint? No. Strength comes from the bond at the surface, not from the volume of metal sitting on top of it. Too much solder actively hides whether the joint wetted, which turns a visible fault into an invisible one.
My joints look dull rather than shiny. Have I done something wrong? Probably not. Lead-free solder normally sets slightly dull and can look faintly textured. Judge the shape rather than the shine, and see Lesson 3.
How hot does the joint need to be? Comfortably above the melting point of your solder. In practice most work is done with the tip somewhere around 300 to 350 °C, though it depends on your solder, your tip and what you are heating. Lesson 2 covers choosing a temperature properly.
Next
Lesson 2: Choosing your soldering iron. What actually matters in an iron and what does not, temperature control, power, tips, and why the thing you plug it into decides how hot it can get.
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