TL;DR:
Soldering joins metal parts with a melted alloy. Heat the joint (pad and pin) with the iron, then feed solder to the joint, not to the iron tip.
A good joint is shiny, smooth and cone-shaped. A dull, lumpy or ball-shaped one is a bad joint that may fail.
Work with ventilation and eye protection, keep the iron in its stand, and treat the tip as a burn hazard.
Breadboards and jumper wires are fine for a first test, but a robot shakes. Loose jumpers fall out, and the robot stops with no obvious reason. Solder makes a joint that is both electrical and mechanical. Learning to make good ones is a small skill with a large payoff.
What solder does
Solder is a metal alloy that melts at a fairly low temperature. Common lead-free solder melts at roughly 217 to 227 °C, and traditional tin-lead solder at about 183 °C. Most solder for electronics has flux in its core. Flux cleans the metal and helps the solder flow, which is what lets it bond.
The solder does not glue the parts. It wets the surfaces, flows into the gap and, as it cools, forms a metal bond with both. The metal surfaces must therefore be hot enough to melt the solder themselves. That is the rule behind everything that follows.
What you need
A temperature-controlled iron with a stand. Around 350 °C is a typical setting for lead-free solder on small joints; follow your iron's and solder's guidance.
Solder with flux core, thin (about 0.5 to 0.8 mm) for small pads.
A damp sponge or brass wool to wipe the tip, and a small amount of tip tinner.
Safety glasses, a fume extractor or a fan blowing away from you, and a heat-proof mat.
Helping hands, a clamp or a breadboard to hold the work, and flush cutters.
Worked example: soldering a header to a sensor board
You have a small sensor board with six holes along one edge and a strip of six header pins at 2.54 mm (0.1 inch) spacing. The goal is six clean joints with no solder bridges.
Set up: Glasses on. Fan or extractor on. Iron in its stand and heating. Tin the tip lightly and wipe it.
Hold the pins steady: Push the short ends of the header into a breadboard so the long ends stand up. Lay the sensor board on top, so the pin ends poke through the holes.
Tack one end pin: Touch the tip so it contacts both the pin and the copper pad at the same time. Count about one second. Feed a little solder to the joint on the side away from the iron, so the pin and pad melt it. Remove the solder, then the iron.
Check the alignment: Is the header straight against the board? If not, reheat that one joint and nudge it while the solder is molten. Do not push on hot metal with your fingers.
Solder the other pins: Repeat for each pin. Each joint should take about two to three seconds in total. Let the first joint cool a moment while you move to a pin further away, so the part does not overheat.
Inspect: Each joint should look shiny and form a small smooth cone from the pin to the pad. Look for a bridge between neighbouring pins, which is solder joining two pads.
Test with a meter: Switch a multimeter to continuity. Check that each pin connects to its own trace and that neighbouring pins do not beep together. Do this with the board unpowered.
The key habit is heat the joint, not the solder. If you melt solder on the iron tip and drop it on a cold pad, it forms a ball that sits on top without bonding. That is a cold joint: it looks dull or lumpy and can fail with vibration. Heat first, and let the pad and pin melt the solder.
How much solder? Enough to fill the hole's edge and cover the pad, but not a blob. If you made a bridge between two pins, wipe the tip clean, touch the bridge, and the molten solder will usually pull onto the tip. Solder wick can remove the rest.
Safety: burns, fumes and eyes
Hot tip, hot fumes: The tip is at several hundred degrees and can cause a serious burn. Always return the iron to its stand, never lay it down, and unplug it when you finish. Wear safety glasses, because molten solder and flux can spatter. Work with ventilation or an extractor so you do not breathe the flux smoke. If you use leaded solder, do not eat or drink while working and wash your hands afterward. Keep the work area clear of paper and keep long hair tied back.
Cut leads with flush cutters pointed away from you, since clipped ends can fly off. Never solder a circuit that is powered or connected to a battery. Be extra careful around lithium polymer packs: a slip that shorts a battery lead can start a fire, so leave pack wiring to a practised helper or use proper connectors.
Where it goes wrong
Cold joint. Dull and grainy. Reheat with fresh flux and a little extra solder.
Too much heat. Lifted pads and burnt boards come from holding the iron on too long. Keep it to a few seconds per joint.
Dirty tip. Black oxide on the tip stops heat reaching the joint. Wipe and re-tin.
Bridges. At 2.54 mm pitch, they come from too much solder or a tip that is too big.
Pin not seated. A header that is crooked is hard to fix after it cools, so check before soldering the rest.
How real robots use it
Robot electronics are mostly soldered or connected with locking connectors. Soldered joints near motors are often supported with strain relief, since a joint can crack if a wire flexes at it. Which parts you end up soldering depends on what you chose, as in choosing sensors for a first robot, and the current those wires carry comes from your power budget.
Try it yourself
Practise on a cheap scrap perfboard before using a real sensor. Solder 20 joints on it, then look at each under good light or with a magnifier and sort them into good and bad. By the last row they should all be shiny cones taking about two seconds each.
Frequently asked questions
Is lead-free solder harder to use?
It melts at a higher temperature and flows a little less easily, so it needs a good iron and clean tips. Technique is the same.
Why add flux if the solder has flux in it?
The core flux is enough for new parts. Extra flux helps when fixing a bad joint or working on oxidised metal.
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