Lesson 3: Solder and flux, explained

Most people buy solder the way they buy masking tape. It is the cheap consumable, the thing you grab without reading the label, and the choice feels roughly as consequential as choosing a color.

It is not. The reel you buy decides how much heat you need, how forgiving the process is, what a finished joint looks like, and what you should and should not do with your hands afterwards. And the most important ingredient in it is not the metal at all.

This lesson covers two things. The first is a choice with a genuine trade-off in it, one that a lot of writing on this subject fudges. The second is flux, which almost nobody thinks about and which does more to determine whether your joints work than anything else you will buy.

By the end of this lesson you will be able to choose a solder deliberately rather than by accident, explain what flux is doing inside it, and answer the "is soldering going to poison me" question with something better than a shrug.

3.1 What solder actually is

The aim of this section is to explain why solder melts the way it does, because that behavior is the whole reason one type feels easier than another.

Solder is an alloy, a mixture of metals chosen to melt at a far lower temperature than the things you are joining. Copper melts at 1085 °C. Solder melts somewhere near 200 °C. That gap is what lets you make a metal joint without destroying a component.

The behavior that matters is what happens between solid and liquid.

Most alloys do not melt at a single temperature. They soften over a range: below the lower figure they are solid, above the upper figure they are fully liquid, and in between they are a slushy, pasty mixture of the two. That in-between state is where a joint can be disturbed and freeze wrong, and it is why "hold it still while it cools" was one of the four beats in Lesson 1.

Some alloys, however, have one exact recipe where that range collapses to a single point. Those are called eutectic, and they go straight from solid to liquid with nothing in between. A eutectic solder is noticeably nicer to use, because the moment of vulnerability is much shorter.

Hold onto that idea. It is the real reason behind most of what follows.

Figure 3.1: A eutectic alloy (left) goes straight from solid to liquid at one temperature.
A non-eutectic alloy (right) passes through a pasty range where the joint can be disturbed.

3.2 Leaded and lead-free, told straight

The aim of this section is to give you both halves of a real trade-off, including the half that is awkward to say.

For most of the history of electronics, solder was tin and lead, usually 63% tin and 37% lead. That particular ratio is eutectic, melting cleanly at 183 °C.

Modern solder is mostly lead-free, usually a tin, silver and copper alloy such as SAC305. It is not eutectic. It melts across roughly 217 to 221 °C, and other lead-free alloys sit a little higher again.

Here is the part that gets fudged, and we are not going to fudge it.

Leaded solder is easier to use, and it looks better

That is simply true, and pretending otherwise helps nobody:

• It melts around 35 °C lower, so you need less heat in the joint and there is less risk to the component.
• It is eutectic, so there is no pasty phase to get caught in. It sets instantly and cleanly.
• It wets more readily. It flows out across a pad with less encouragement.
• It sets bright and shiny, which happens to be what most people have been taught a good joint looks like.
• It is more forgiving of imperfect technique, which matters enormously when your technique is new.

If the only consideration were ease of use and appearance, leaded solder wins, and it is not close.

It is also lead

Lead is a cumulative toxin. It does not leave your body readily, the effects build over time, and they are worse for children and during pregnancy. That is not scaremongering, it is the reason it was legislated out of most consumer electronics manufacturing in the first place, through RoHS.

But it is worth being precise about where the risk actually is, because the popular belief is wrong and the wrong belief leads people to take the wrong precautions.

The lead does not reach you through the smoke. It reaches you through your hands.

Lead does not meaningfully vaporize at soldering temperatures. What you can see rising off a joint is flux smoke, which is a genuine respiratory irritant and worth extracting, but it is not lead fume. Section 3.5 covers that properly.

The real route is ingestion. You handle leaded solder, a fine residue ends up on your fingers, and then you eat a cookie, rub your eye, or use your phone. That is it. That is the whole mechanism, and it is entirely manageable:

• Wash your hands with soap after soldering, every time, before you touch food or your face.
• Do not eat or drink at the bench.
• Keep offcuts contained rather than letting them roll about on a surface you also eat at.
• Do not let children handle it, and be more careful again if you are pregnant.

So which should you use

That is genuinely your decision, and it depends on things we do not know about you.

Leaded is a reasonable choice if you are an adult, working occasionally, in your own space, with no children around, and you will actually wash your hands. It is easier, and easier means you will learn faster and enjoy it more.

Lead-free is a reasonable choice if you would rather not have a cumulative toxin on your bench at all, if there are children in the house, if you are working somewhere shared, or if you will be doing a lot of this. It also matches what commercially made boards are assembled with.

What we would not do is tell you that lead-free is "just as good and better for you" and leave it there. The first half of that sentence is not quite true, and being told a comfortable half-truth is a poor way to be taught anything.

The practical consequence, whichever you pick

If you use lead-free, adjust your expectations rather than your technique:

• Set the iron a little hotter, typically 20 to 30 °C above where you would run leaded.
• A correct joint looks dull, and can look faintly grainy. This is not a fault.
• Hold still slightly longer, because there is a pasty range to get through.

That third point is the one that damages boards. A beginner using lead-free sees a dull joint, decides it has failed, reheats it, sees it dull again, reheats it again, and eventually lifts the pad off the board. Shape is the test, not shine, exactly as Lesson 1 said. Lesson 9 shows you the shapes.

Figure 3.2: Both of these joints are correct. Leaded solder sets bright, lead-free sets dull and slightly textured. Judge the silhouette, never the shine.

3.3 Flux: the ingredient that actually decides it

The aim of this section is to introduce the thing that does the real work, and that most beginners do not know they already own.

Lesson 1 said solder must wet the metal to bond, and that it cannot wet metal that is dirty. Here is the specific dirt.

Every bare metal in air grows a thin layer of oxide almost immediately. It is invisible, it is only molecules thick, and it is chemically a completely different substance from the metal underneath. Solder cannot bond to it. Left alone, oxide is a lid on the surface, and molten solder will slide over it and ball up.

Flux is a mild chemical that strips that oxide off and keeps it off while you work. It becomes active with heat, cleans the surface just ahead of the solder arriving, and shields the hot metal from the air so that it does not simply re-oxidize while you are still working on it.

Without flux, soldering barely works at all. With it, it feels almost effortless.

It is already in your solder

You do not have to buy flux to start, because it is inside the solder wire. Cut a piece and look at the end: running down the middle are one or more cores of solid flux. As the solder melts, the flux is released a fraction ahead of it, cleaning the surface the solder is about to land on.

That is a genuinely elegant piece of design, and it is why "solder" and "flux" feel like one product to most people.

It also explains the failure in Section 1.6 precisely. Melt solder onto a hot tip and hold it there, and the flux is consumed and gone before the solder ever reaches the board. What lands on your pad is solder with its cleaning agent already spent, which is why it beads up and refuses to bond. Flux is a one-shot ingredient, and it is spent by heat.

Types you will see on a label

• Rosin based, traditionally from pine resin. The classic, effective, and the source of the smell.
• No-clean, the most common thing sold to hobbyists now. Leaves a small amount of residue that is designed to be harmless and can be left alone. It is usually slightly shiny and slightly sticky, and it is not a fault.
• Water-soluble, more aggressive, cleans well, but the residue is corrosive and must be washed off properly. Not what you want as a beginner.

Buy rosin-cored or no-clean. That is the whole decision.

Buying flux separately

Eventually you will want a flux pen or a small pot of flux paste, and Lesson 4 covers it. The moment it becomes obviously worth having is when you start reworking joints, because a joint you are reheating has no fresh flux left in it at all. That is Lesson 10.

Figure 3.3: Oxide is an invisible lid on the metal that solder cannot bond to. Flux strips it away just ahead of the solder and shields the surface while you work.

3.4 Choosing a diameter

The aim of this section is to save you from a decision that looks technical and is not.

Solder wire is sold by diameter, usually somewhere between 0.5 mm and 1.5 mm, and the only thing it really controls is how much solder arrives per second while you feed it in.

• Thick wire delivers a lot quickly. Good for big joints and thick wires, easy to overdo on small ones.
• Thin wire delivers slowly and gives you fine control, but you feed a lot of it and it can run out mid-joint.

For through-hole electronics, 0.7 mm or 0.8 mm does nearly everything, and if you own one reel that is the one to own. It is fine enough for small pads and quick enough not to be tedious on larger ones.

Buy a small reel, not a bargain bulk spool. Solder does not really go off, but a beginner's needs change fast and a large spool of the wrong choice is a sad object.

Figure 3.4: Solder wire in cross section. The flux runs in cores down the middle of the metal and is released a fraction ahead of it.

3.5 What the smoke actually is

The aim of this section is to replace a common and unhelpful belief with an accurate one, because the wrong belief leads to the wrong precaution.

Ask most people what the smoke coming off a solder joint is, and they will say lead.

It is not. Lead boils at over 1700 °C and does not meaningfully evaporate at soldering temperatures. There is essentially no lead in that smoke, even when you are using leaded solder.

What you are seeing is vaporized flux, mostly rosin, and it deserves respect on its own terms. Rosin fume, or colophony, is a recognized respiratory sensitizer. The word matters: a sensitizer is not simply an irritant. Repeated exposure can cause some people to develop a lasting sensitivity, including occupational asthma, and once sensitized the reaction can be triggered by very small amounts thereafter.

That is a real occupational hazard for people who solder all day. For someone building a kit at a kitchen table on a Sunday, it is a manageable one:

• Do not put your face directly over the joint, which is exactly what everyone does while concentrating.
• Work somewhere ventilated, with a window open.
• Move the smoke away from you, even with a small fan. Anything that stops it rising into your face is doing the main job.

Lesson 5 covers ventilation properly, including where a filtered extractor is and is not worth the money.

The useful summary: the fume risk comes from flux and is about your lungs; the lead risk comes from your hands and is about what you eat. Different hazards, different precautions, and the precaution most people take addresses the wrong one.

3.6 Reading the label

The aim of this section is to make a reel of solder decodable in about ten seconds.

A typical description reads something like: Sn60Pb40, 0.8 mm, rosin core, 2% flux.

• The alloy. Sn is tin, Pb is lead, Ag is silver, Cu is copper. Sn63Pb37 is the eutectic leaded one. If you see no Pb anywhere, it is lead-free.
  • SAC305 means 96.5% tin, 3% silver, 0.5% copper. The common lead-free choice, and the one most guides assume.
  • Sn99.3Cu0.7 or SAC0307 are tin-copper alloys with little or no silver. Silver is expensive, so a lot of hobby reels and starter kits use these, and seeing an unfamiliar code is not a sign you have bought something dubious. They melt a few degrees higher than SAC305, around 227 °C, and are slightly less willing to flow. Perfectly usable. Give them a little more heat and a little more patience.
• The diameter. See section 3.4. You want 0.7 or 0.8 mm.
• The flux type. Rosin core or no-clean. Avoid water-soluble to begin with.
• The flux percentage. Usually 1.5% to 2.5%. Anything in that band is normal, and more is not better.

⛔ One thing to avoid outright: solder sold with no flux type stated at all, or "solder wire" with no alloy given. It is cheap for a reason, and cheap solder is the one economy in this hobby that makes everything else harder.

What this sets up

Lesson 4, the rest of the kit, includes a flux pen and the other things that become obviously worth having once you know why.

Lesson 5, safety and your bench, takes section 3.5's ventilation point and makes it practical.

Lesson 9, good joint bad joint, is where "shape not shine" stops being a slogan and becomes a gallery you can check your own work against.

Frequently asked questions

Is leaded solder better than lead-free? It is genuinely easier to use and it sets brighter, because it melts about 35 °C lower and is eutectic, so there is no pasty phase. It is also lead. Both statements are true, and which matters more depends on your circumstances rather than on physics.

Are solder fumes dangerous? The smoke is vaporized flux, not lead. Rosin fume is a respiratory sensitizer and worth not breathing, especially repeatedly. Keep your face out of the plume, ventilate the room, and move the smoke away from you.

Does lead get into the air when soldering? Essentially no. Lead does not vaporize at soldering temperatures. The route into your body is your hands and then your mouth, which is why washing your hands matters more than anything else you can do.

Why does my lead-free solder look dull? Because that is what a correct lead-free joint looks like. It is not a cold joint. Judge the shape: low, wide and hollow-flanked is right, whatever the finish.

What is flux and do I need to buy it? Flux strips the invisible oxide layer off metal so that solder can bond. You do not need to buy any to start, because it runs in cores down the middle of your solder wire. You will want a flux pen once you start reworking joints.

What size solder should I buy? 0.7 mm or 0.8 mm covers almost all through-hole work. Buy a small reel rather than a large one.

Can I still buy leaded solder? In most places, yes, for hobby and repair use. RoHS restricts what can be sold in finished consumer products, not what an individual may use at home. Check your own local rules.

Next

Lesson 4: The rest of the kit, and what can wait. Stands, third hands, magnification, mats and cutters, grouped by when you will actually want them rather than as a shopping list.

Soldering Fundamentals is free and always will be. No sign-up, no gate, no catch.