How to Solder Correctly — the RC version
Every other soldering guide teaches circuit boards. RC is fat stranded wire onto battery connectors and motor tabs, next to a charged LiPo. Real temperatures, the failures that actually bite, and how to tell which one you have.
Updated Jul 22, 2026 · RC Crash Crew
Every soldering guide you have already found is about circuit boards. Almost nothing you solder on an RC model is a circuit board.
You will be joining fat stranded copper to battery connectors, to motor tabs, to other fat stranded copper — usually within arm's reach of a charged lithium pack. That is a different job, with different tools, different temperatures and a different set of ways to get hurt. This guide is about that job.
The rule that comes before every other rule
Unplug the battery. Every time. No exceptions.
A battery lead has two wires. The moment you cut or strip both of them with the pack connected, you are one slip away from touching them together — and a LiPo will happily dump hundreds of amps into a dead short. That is not a spark. That is a wire glowing orange, molten solder, and a pack that may catch fire minutes later when you think everything is fine.
If you are replacing a connector on a battery, cut one wire at a time, finish it completely, then do the other. Never have both conductors bare at once. Tape the one you are not working on.
Everything else in this guide is about making a good joint. This one is about not starting a fire.
Heat is not temperature — this is the thing beginners get wrong
A 25W iron set to 400°C and a 100W iron set to 400°C are both "at 400°C". Only one of them will solder a 12AWG battery lead.
Temperature is how hot the tip is. Heat is how much energy it can pour into the job before it cools down. Fat copper wire is a heat sink — it pulls warmth away faster than a small iron can replace it. The tip temperature collapses, the solder never properly melts against the wire, and you get a joint that looks finished and is not.
This is why "turn it up hotter" is usually the wrong fix. If your iron cannot keep up, more temperature just burns flux and insulation while the joint still fails. What you need is more mass and more power: a bigger tip, a bigger iron, and heat applied to the *work*, not to the solder.
The practical test: touch the iron to the wire, count. If solder will not flow within about three to five seconds, your iron is too small or your tip is too small for that job. Stop and change something — do not keep sitting on it, because that is how insulation melts and strands anneal.
What you actually need
| Job | Iron | Tip | Solder |
|---|---|---|---|
| Servo/signal wire, small connectors | 40–60W | 1.6mm chisel | 0.7mm |
| ESC/motor bullets, XT60 | 60–80W | 3mm chisel | 1.0mm |
| 10–12AWG battery leads, EC5, XT90 | 80–100W+ | 5mm chisel or hoof | 1.5mm |
A temperature-controlled station is worth more than any other tool on this list. A fixed-temperature "fire stick" from a hardware shop is the single most common reason a beginner's joints fail.
Also get: flux (a pen or a small tub of rosin paste), a brass wool tip cleaner, a damp sponge, helping hands or a vice, flush cutters, and heatshrink in several diameters. Flux is not optional on big wire — it is what lets solder wet oxidized copper.
Chisel tips, not conical. A conical tip touches the work at a point. A chisel tip touches along a face, and that contact area is what actually transfers heat.
Which solder, and the number that matters
| Solder | Melts at | Why you would use it |
|---|---|---|
| 63/37 tin-lead | 183°C (361°F) — one temperature | Easiest to learn on. Eutectic: goes straight from liquid to solid with no mushy stage, so a nudge while it cools is far less likely to ruin the joint. |
| 60/40 tin-lead | 183–190°C (361–374°F) | Very common, slightly cheaper. Has a small plastic range where it is neither liquid nor solid — move it during that window and you get a disturbed joint. |
| Lead-free (SAC305) | ~217–220°C (423–428°F) | Required in some places, sold everywhere. Needs a hotter iron, wets less readily, and looks duller even when correct — which makes judging a joint harder while you are learning. |
If you are learning and the law where you live permits it, 63/37 with a rosin core is the forgiving choice. Set your iron to roughly:
- 315–350°C (600–660°F) for leaded solder on small work
- 350–400°C (660–750°F) for lead-free
- 400–450°C (750–840°F) for 10–12AWG battery leads with a big tip
Those are starting points, not laws. If solder balls up and refuses to flow, you are too cold or short of flux. If flux boils off instantly and the joint browns, you are too hot.
The method
Soldering is two separate operations. Beginners try to do them at once and that is why it goes wrong.
1. Tin each piece separately
Tinning means wetting the surface with a thin coat of solder before joining anything.
- Twist the strands of the wire tightly. Do not let any wander.
- Add a little flux.
- Press the tip against the side of the wire, not on top of it — you want contact area.
- Feed solder into the wire, not onto the iron. If you melt solder on the tip and dab it, you get a blob sitting on cold copper. That is the classic cold joint.
- The solder should be sucked in and turn the copper silver along a short length. Stop there.
Do the same to the connector cup or the motor tab. Both sides tinned, both sides shiny.
2. Join them
Bring the two tinned parts together, apply the iron to the joint, and wait for both coats to melt and merge. Add a touch more solder only if the joint looks starved. Remove the iron and hold everything perfectly still until it goes dull-then-solid — a couple of seconds. Blowing on it does not help and can cause a grainy joint.
A finished joint should be shiny, smooth and slightly concave, with the shape of the wire still visible under it. A ball is too much solder. A dull gray lump is too little heat.
Heatshrink goes on FIRST
Slide the heatshrink onto the wire before you solder. Push it well back so the iron does not shrink it prematurely.
Everybody skips this once. You make a perfect joint on a battery lead, reach for the shrink, and realize the only way to get it on is to cut the joint you just made. It costs you the joint, a few millimeters of wire, and ten minutes.
Use shrink that fits the joint after soldering, not before, and shrink it with a heat gun or the *side* of the iron barrel — not the tip.
Job by job
Battery leads (10–14AWG)
The hardest thing on the model and the most dangerous. Big tip, high heat, plenty of flux, one wire at a time, heatshrink pre-loaded. Tin the wire and the connector cup separately, then bring them together. Expect three to five seconds of heat, not fifteen.
Watch for solder wicking up under the insulation. Solder traveling up the strands turns a flexible wire into a stiff rod, and the point where stiff meets flexible is where vibration will fatigue it and snap it — sometimes weeks later, mid-run. Keep tinning short and deliberate.
Bullet connectors (3.5mm / 4mm / 5mm / 6mm)
Hold the bullet in a vice or a bullet-holding block — not your fingers, and not pliers you are also trying to steer with. Fill the cup with solder, then push the tinned wire in while the solder is molten. Let it cool before moving it. Shrink over the base so no metal is exposed.
XT60 / XT90 / EC5
The housings melt. That is the whole difficulty. Plug the connector into a spare mating half while you solder it — the mating half holds the pins in alignment even if the plastic softens, so the connector still fits afterwards. Work fast, tin both sides first, and do not sit on it.
Rough current ratings: XT60 ≈ 60A, XT90 ≈ 90A, EC5 ≈ 120A continuous. Match the connector to what the model actually pulls, not to what fits.
Motor tabs
Motor tabs sink heat into the motor and the magnets do not enjoy it. Tin the tab quickly, tin the wire separately, join, get off. If the motor gets hot enough to be uncomfortable to hold, stop and let it cool before continuing.
Servo and signal wire
The opposite problem — tiny, and easy to melt. Lower temperature, small tip, fine solder, and be quick. These wires are usually 22–26AWG and will wick solder instantly.
What a bad joint looks like, and what it does to you
| Fault | How it looks | What it does in the model |
|---|---|---|
| Cold joint | Dull, gray, grainy or lumpy; solder sits on top rather than flowing in | Works on the bench, fails under load or vibration. Intermittent cut-outs, brownouts, an ESC that "randomly" resets. |
| Dry / starved joint | Not enough solder; wire and cup visibly separate under the coating | Same as above, but usually fails sooner. Often pulls apart by hand. |
| Too much solder | A shiny ball with the wire buried | Not electrically bad in itself, but hides whether the joint underneath is sound — you cannot inspect it. |
| Wicked insulation | Stiff wire for 10–20mm behind the joint | Fatigue fracture from vibration, days or weeks later, usually at the worst moment. |
| Burnt insulation | Browned, shrunken, sometimes split | Exposed conductor near other conductors. On a battery lead this is a short waiting to happen. |
| Lifted pad (on a board) | The copper pad has come away from the board | Usually terminal for that pad. Too much heat for too long. |
Diagnosing from the symptom: a model that cuts out over rough ground but is fine on smooth is telling you about a mechanical connection, not a tuning problem. A model that browns out only under hard acceleration is telling you about resistance in a high-current joint. In both cases, wiggle every soldered joint with the pack disconnected and look for one that moves, or that is stiff where it should flex.
Fixing one you have already ruined
You do not need to cut it off. Add flux, re-heat the joint until the existing solder flows properly, add a touch of fresh solder, and let it cool undisturbed. Most cold joints are cured by simply doing the heat properly the second time.
If it is contaminated, oxidized black, or full of burnt flux: remove the old solder with solder wick (braid) or a sucker, clean the copper, and start again. Do not build fresh solder on top of a bad joint — you get a shiny lump with a bad joint inside it.
What is not settled
Be suspicious of anyone who tells you these are simple.
- Leaded vs lead-free. Leaded is easier to work with and produces a more readable joint. Lead-free is legally required in some contexts and avoids the lead question entirely. Both make sound joints in competent hands. Hobby use is generally not covered by the regulations that push industry to lead-free, but check your own jurisdiction rather than assume.
- Tin the tip before storing, or not? Most makers say leave a blob of solder on the tip before you power down, to keep air off it. A minority prefer a clean tip. The one thing everyone agrees on: do not file or sand a plated tip — the plating is the tip.
- Sponge vs brass wool. A wet sponge thermally shocks the tip each wipe; brass wool does not, but is more aggressive mechanically. Plenty of good work is done with both.
Fumes, lead, and what is actually dangerous
Get this the right way round, because most casual advice does not.
The smoke you see is not lead. Lead does not vaporise at soldering temperatures — it boils at around 1749°C, and your iron is at 350. What is rising off the joint is burning flux, mostly rosin (colophony).
That flux smoke is the genuine respiratory hazard. Rosin fumes are a recognized cause of occupational asthma, and once sensitised, people stay sensitised. Do not lean over the joint and breathe it. Use a small fume extractor or a fan pulling the smoke away from your face, and work somewhere ventilated.
The lead risk is ingestion, not inhalation. It gets on your hands, then onto food, drink or your face. So: do not eat, drink or smoke at the bench, and wash your hands properly afterwards — soap and water, every time. Keep leaded solder away from children and pets.
And because this is RC: have a plan for a lithium fire before you need one. See LiPo battery safety.
Practice before it matters
Do not learn on a $120 ESC.
Buy a meter of cheap 14AWG silicone wire and a bag of XT60s, and make ten joints in a row. Cut each one open with flush cutters and look at the cross-section. You are looking for solder that has flowed *between* the strands, not a shell around the outside.
When ten in a row come out shiny, concave and flexible right up to the joint, you are ready to touch the model.
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*Figures in this guide: solder alloy melting points are standard values for 63/37, 60/40 and SAC305. Connector current ratings are manufacturers' nominal continuous ratings and vary by brand and wire gauge. Iron temperatures are working starting points, not specifications — your iron, tip and solder will shift them.*
*Last checked: 22 July 2026.*