Most people who learn to solder on a circuit board end up doing most of their actual soldering somewhere else entirely: joining two wires, putting a plug on the end of a cable, repairing a headphone lead, wiring a switch into a project.
And it is a different job. Not harder, but different enough that board technique applied unchanged produces bad results.
Three things change. Nothing holds anything still, because there is no board doing that work for you. The wire itself carries heat away in a way a component leg does not. And the joint has a mechanical life ahead of it: a board joint sits still forever, while a wire joint gets pulled, bent and dragged for years.
That last one is the important one, and it is why this lesson spends as much time on the things around the joint as on the joint itself.
By the end of this lesson you will be able to tin a wire, make a joint that will not fail when it is tugged, use heat shrink properly, and know which connector jobs are worth soldering at all.
11.1 Tinning a wire
The aim of this section is the preparation step that everything else depends on, and that is skipped more often than any other step in this course.
Stranded wire is dozens of fine strands twisted together. Left bare, it frays, splays, refuses to stay where you put it, and only some of the strands ever end up in the joint.
Tinning solves all of that at once: you flow solder into the strands so they become one solid, tidy, conductive piece.
How to do it
- Strip the insulation. Only as much as you need, typically 5 to 8 mm for a small joint. Use a stripper set to the right gauge, not your teeth and not a knife, both of which nick strands.
- Twist the strands gently in the direction they are already laid. A twist or two, not a corkscrew.
- Lay the iron under the wire, not on top. Heat rises through the strands, and the underside is where the tip has most contact.
- Feed solder onto the top of the wire, on the opposite side from the iron. The same rule as everywhere else in this course: the solder touches the hot metal, not the tip.
- Watch it soak in. The solder will run into the strands and along the wire. Stop as soon as the exposed length is filled.
What good looks like: the strands have disappeared into a solid, slightly shiny rod. It is still flexible right up to the insulation. You can see the individual strands as texture, not as separate wires.
Two things to avoid
⛔ Solder running up under the insulation. If solder creeps up inside the sleeve, that section of wire becomes rigid. A rigid section next to a flexible one is a stress concentration, and it is precisely where the wire will eventually snap. Keep the heat short and stop as soon as the exposed part is filled.
⛔ A fat blob. You want the strands filled, not coated. A blob will not fit into whatever you are about to put it in.
Figure 11.1: Iron underneath, solder fed onto the top. The strands fill and become one solid rod, with no solder wicking up under the insulation.
11.2 The joints worth knowing
The aim of this section is to give you three joints and be clear about which to use when, rather than presenting a catalog.
Both wires should be tinned first, in every case.
The lap joint
The everyday one. Lay the two tinned ends alongside each other, overlapping by roughly the length you stripped, and heat them together until the solder in both flows and merges.
Good for: almost everything. Quick, low profile, easy to cover with heat shrink.
Its weakness: it relies entirely on the solder for mechanical strength, so it needs strain relief, which is Section 11.4.
The hook joint
Bend a small hook in each tinned wire, link them like two links of a chain, squeeze them closed, and solder.
Good for: anything that will be pulled. The mechanical link takes the load rather than the solder.
Its weakness: bulkier, and harder to hide neatly under heat shrink.
The Western Union splice
Cross the two wires, wrap each end tightly around the other in opposite directions, then solder the whole assembly.
Good for: the strongest inline joint you can make with wire alone. The name comes from telegraph line work, where a joint had to survive weather and tension.
Its weakness: bulky, fiddly, and complete overkill for a project box. Worth knowing it exists for the day you need it.
What about just twisting them together?
⛔ Twisting two wires together and dropping solder on the outside is not a joint. It looks like one. The solder coats the outside of the bundle and never penetrates, so the actual electrical path is still strand-on-strand contact, which oxidizes over time and becomes intermittent.
Tin them first, then join. That is the difference between a repair that lasts and one you will be redoing in six months.
⛔ And never use a twisted-and-taped joint in anything mains powered. That is a different discipline with different rules and it is outside this course entirely.
Figure 11.2: Three joints. The lap for everyday work, the hook where it will be pulled, and the splice where it must not fail.
11.3 Heat shrink, and the mistake everybody makes once
The aim of this section is a short, complete method, including the step that gets forgotten exactly once by every single person who has ever done this.
Heat shrink tubing is plastic sleeving that contracts when heated, typically to half its diameter, forming a tight insulating cover over a joint.
The method
- ⛔ Cut a piece and slide it onto one of the wires BEFORE you solder. Push it well down, away from the heat.
- Make your joint.
- Let it cool completely.
- Slide the tubing over the joint, centered, so it overlaps the insulation on both sides.
- Heat it evenly, turning as you go, until it grips.
Step one is the one everybody forgets. You will make a beautiful joint, reach for the heat shrink, and realize there is no way to get it on. Everyone does it once. Some people do it twice.
⚠️ The trick that saves you: get into the habit of sliding the tube on immediately after stripping, as part of preparation, not as a separate step you might remember later.
Figure 11.4: Everybody does this once. Slide the tube on when you strip the wire, not when you remember it.
Heating it
- A heat gun is the right tool and gives an even, tidy result.
- A hairdryer works for thin tubing and takes longer.
- A soldering iron, in an emergency, and only in a very specific way. ⛔ Never the tip. A tinned tip is coated in solder, so touching it to the tubing melts a hole, splits it, and leaves plastic burned onto the tip that you then have to clean off before you can solder anything else. What you can do is hold the bare metal barrel or shaft of the iron near the tubing, a few millimeters clear, and rotate the wire slowly. Radiated heat does the job. No contact at any point.
- ⛔ Never a lighter. It scorches, it distorts, and it looks like exactly what it is: a shortcut.
Sizing it
Choose a size that slides on with a little room to spare and shrinks tight over the finished joint. Too tight and you cannot get it on; too loose and it will not grip even fully shrunk.
Ratio matters: 2:1 is common and adequate for most inline joints. 3:1 is worth having when going over something lumpy like a connector shell. Adhesive-lined shrink has glue inside that flows when heated and seals as well as covers, which is genuinely worth it for anything that might get damp.
11.4 Strain relief, which matters more than the solder
The aim of this section is the point of the whole lesson, and the reason wire work differs from board work.
Here is a thing worth stating plainly:
A solder joint is a good electrical connection and a poor mechanical one.
Section 1.1 said solder is genuinely the mechanical anchor for a component on a board. That works because the component never moves. A wire is different: it gets pulled, bent, coiled, stepped on and yanked out by the cable rather than the plug.
Solder is a relatively soft, brittle metal. It does not tolerate repeated flexing. It work-hardens and cracks, and it cracks exactly where the flexible part meets the rigid part, which is the edge of your beautiful joint.
So the joint must never be the thing taking the strain. Something else has to.
How to actually do it
- Heat shrink over the joint and well onto the insulation both sides. This is the minimum and it does a surprising amount, because it stops the wire flexing right at the joint edge.
- A service loop. Leave a little slack inside the enclosure so a pull on the outside takes up slack rather than loading the joint. Costs nothing.
- Anchor the cable to the enclosure, with a cable gland, a P-clip, a zip tie through a hole, or the molded strain relief a connector housing provides. Anchor before the joint, so any pull stops at the anchor.
- Do not overtin the wire near the joint. Rigid sections next to flexible ones are where cracks start, which is why Section 11.1 warned about wicking under the insulation.
- Support both sides. A joint anchored on one side only just moves the problem along a bit.
⚠️ Diagnostic worth knowing: an intermittent fault that changes when you wiggle the cable is almost always a cracked wire joint or a broken strand next to one, not a component. This is why almost every headphone lead in the world dies at the plug.
Figure 11.3: The joint must never take the strain. Anchor the cable before the joint, then leave slack after the anchor.
11.5 Connectors
The aim of this section is to be honest about when soldering a connector is the right choice and when it is not.
Not every connector should be soldered, and knowing which is which saves a lot of frustration.
Crimp, and be honest about it
Most modern connectors are designed to be crimped, not soldered. Dupont pins, JST, XH, Molex, spade terminals, ferrules. A correct crimp is a gas-tight cold weld, it is mechanically stronger than solder, and it flexes without cracking.
⚠️ Soldering a crimp terminal is usually a downgrade, and it introduces exactly the rigid-next-to-flexible problem from Section 11.4. If a terminal has a barrel designed for a crimp tool, use a crimp tool. A cheap ratcheting crimper is not expensive and it is the right answer.
This is worth saying because the instinct, once you can solder, is to solder everything.
amomii's own Bolt Board modules use solderless XH connectors for exactly this reason, on kits aimed at younger builders. Not everything should be a soldering job.
Connectors that genuinely want solder
- Barrel jacks and DC power connectors, which usually have solder cups.
- Audio jacks, 3.5 mm and 6.35 mm, which have solder tabs.
- Panel-mount switches and potentiometers, with solder lugs.
- Screw terminals and pin headers on a board, which are just board joints. Lesson 8.
Technique for a solder cup or lug
- Tin the cup or lug first, and tin the wire. Both.
- Hold the connector, in a third hand or a vice. Both your hands are busy.
- Heat the cup and let the solder in it melt, then feed the tinned wire in.
- Hold still while it cools. Same rule as everywhere.
Why tin both first: you are then just melting two tinned surfaces together, which is quick. Trying to heat a cup, feed solder, and hold a wire simultaneously is a three-handed job you will lose.
⚠️ Slide the heat shrink on before you do any of this. Connectors are exactly where you will forget.
What this sets up
Lesson 12, surface mount, is the last technique lesson and is deliberately awareness rather than instruction.
Lesson 13, what to build next, is where all of this becomes a thing you own.
Frequently asked questions
How do you solder two wires together? Tin both ends first so the strands become solid, then overlap them and heat until the solder in both flows and merges. Slide heat shrink onto one wire before you start, and cover the joint once it has cooled.
Why should I tin a wire before soldering it? Bare stranded wire frays and splays, and only some strands end up in the joint. Tinning fuses the strands into one solid conductor that stays where you put it and joins reliably.
Do I have to use heat shrink? It insulates the joint and, just as importantly, stops the wire flexing right at the joint edge, which is where solder cracks. Electrical tape is a poor substitute; it unwinds and goes sticky.
Why do my soldered wire joints keep breaking? Almost certainly strain, not a bad joint. Solder is brittle and cracks where flexible meets rigid. Anchor the cable before the joint, leave a service loop, and run heat shrink well onto the insulation both sides.
Should I solder crimp connectors? No. Crimp terminals are designed for a crimp tool, and a correct crimp is stronger than solder and flexes without cracking. Soldering one usually makes it worse.
What is the strongest way to join two wires? A Western Union splice, where each wire is wrapped tightly around the other before soldering. It is bulky and rarely necessary; a lap joint with proper strain relief is fine for almost everything.
Why does solder run up under the insulation? Too much heat for too long. It makes that section rigid, and the wire will eventually snap where the rigid part meets the flexible part. Keep the heat brief and stop as soon as the stripped section is filled.
Next
Lesson 12: A quick look at surface mount. What those tiny parts are, why we did not start you there, and what it actually takes to work on them.
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