Soldering a motor for a DIY robot comes down to five things: tin the pads and wires first, use a hot iron (around 350°C / 660°F) for a short contact time, keep the joint small and shiny, add strain relief, and solder a capacitor across brushed DC motors to suppress electrical noise. Motor terminals are the hardest joints on a small robot because the tabs act as heat sinks, the plastic end bell melts easily, and the wires move constantly once the machine drives. This guide covers the gear, the joint-by-joint technique for common hobby motors, how to deal with the brush noise that resets microcontrollers, and the mistakes that show up later as intermittent faults rather than immediate failures.
Table of Contents
- Gear and settings that matter
- Soldering the motor terminals step by step
- Why brushed motors need a capacitor, and where it goes
- Strain relief and routing, the part most builds skip
- Diagnosing a joint that went wrong
- When not to solder
- Frequently Asked Questions
Gear and settings that matter
A temperature-controlled iron is the single biggest upgrade. A fixed-wattage "fire stick" either cannot deliver enough heat into a motor tab or runs so hot it melts insulation; an adjustable station lets you set a working temperature and hold it. For leaded 60/40 or 63/37 solder, roughly 325–350°C is a common working range; lead-free alloys generally need 30–40°C more because they melt higher. Tip shape matters more than wattage.
A chisel or bevel tip ("D" or "C" series) carries far more heat into a brass motor tab than a fine conical tip, which is why a 60 W iron with a needle tip can fail at a joint a 40 W iron with a 2.4 mm chisel handles easily. Keep the tip tinned and wipe it on brass wool rather than a dry sponge. A short shopping list for motor work: Stranded wire is not a preference here. Solid core work-hardens and snaps at the joint under the vibration a driving robot produces, usually after the build is closed up and hard to reach.
- Temperature-controlled iron, 40 W or more, with a chisel tip
- 0.6–0.8 mm rosin-core solder (leaded is easier to learn on; ventilate regardless)
- Flux pen or paste, for oxidized tabs and tinned stranded wire
- 22–26 AWG stranded silicone-insulated wire, not solid core
- Helping hands or a vise, flush cutters, wire strippers, heat-shrink tubing
Soldering the motor terminals step by step
Work on the motor before it is in the chassis. Decide which terminal is positive for your intended "forward" direction and mark it; on brushed DC motors polarity only sets the spin direction, so a swap is a reversed wheel rather than damage, but tracking it saves firmware guesswork later. The sequence that gives clean joints: Keep contact time short.
The commutator brushes and the plastic bearing housing sit millimetres behind the tab, and a long dwell can soften the end bell or anneal the spring contact inside. If a joint does not take within about three seconds, stop, let the motor cool, add flux, and try again with a bigger tip rather than a longer hold. Small motors with pierced tabs — the common yellow TT gearmotors and N20 micro gearmotors use them — are easier if you thread the wire through the hole, bend it back, then solder. The mechanical hook carries the load and the solder only has to carry current, which is what solder is good at.
- Clamp the motor so the terminals face up and nothing can shift.
- Strip 3–4 mm of insulation, twist the strands, and tin the wire until the solder wicks in and the bundle stays flexible.
- Flux and tin each motor tab: touch the iron flat against the tab, wait for it to accept solder, then add a thin pre-tinned layer.
- Lay the tinned wire against the tinned tab, press with the iron for one to two seconds, and watch the two tinned surfaces merge.
- Remove the iron and hold the wire still until the joint goes solid and matte-free.
Why brushed motors need a capacitor, and where it goes
Brushed DC motors generate electrical noise continuously. As the commutator segments break contact, they arc, and that arcing both injects high-frequency hash onto the power rails and radiates into nearby wiring. On a robot this shows up as a microcontroller that resets when the motors start, an I²C sensor that stops responding under load, or a servo that twitches during acceleration. Ceramic capacitors across the motor shunt that noise close to where it is made.
The standard hobby approach uses three 0.1 µF ceramics: one directly across the two terminals, and one from each terminal to the motor case, soldered to a tab or mounting lug that is electrically bonded to the metal body. Many hobby motors ship with these already fitted; if yours has small disc capacitors soldered to the terminals, leave them in place. Three practical constraints are worth knowing before you add them: Capacitors reduce conducted and radiated noise; they do not fix a shared-ground problem. If the logic and the motors draw through the same thin wire, separate the supply paths or add a dedicated capacitor bank near the motor driver as well.
- Use ceramic, not electrolytic. An electrolytic capacitor across a motor that reverses direction is being reverse-biased, which it is not built for.
- Keep the leads short. Long leads add inductance and undo much of the filtering.
- Do not add capacitors across the *output* of a brushless ESC. Brushless motors are commutated electronically and are driven by fast switching waveforms, and capacitance on those phase wires can confuse or damage the controller.
Strain relief and routing, the part most builds skip
The joint that fails is almost never the one that looked bad on the bench. It is the one that flexed ten thousand times. A robot's motor wires move whenever the chassis vibrates, and the stiff transition where solder has wicked up into the strands becomes a stress concentration point that eventually cracks internally while still looking intact. Three layers of protection, cheapest first. Slide heat-shrink over each joint so it covers both the solder and a few millimetres of insulation, which stops the wire bending sharply right at the metal.
Then anchor the wire pair to the motor body or chassis with a dab of hot glue or a cable tie, so any pull lands on the anchor rather than the tab. Finally, route with a service loop — a small amount of slack — so a steering or suspension movement does not tug the joint. Hot glue is also the usual answer for motors whose tabs are too delicate to survive repeated reconnection, such as N20 gearmotors. Solder the joint, test it electrically, then bury the tab and the first stretch of wire in glue. Note the trade-off plainly: it is now difficult to rework, so verify polarity and continuity before you commit.
Diagnosing a joint that went wrong
Judge a joint by appearance first. A good one is shiny (with leaded solder), concave where the wire meets the tab, and shows the wire's outline through a thin layer rather than sitting inside a ball.
A dull grey, lumpy, or spherical blob usually means the tab never reached temperature and the solder bonded to the wire only — a cold joint, which conducts just well enough to pass a bench test and fail under current. A short checklist when a motor behaves intermittently: Common root causes worth ruling out before you blame technique: an oxidized tab that needed flux, a tip too small to deliver heat, insulation that melted back and left strands touching the case, and movement during cooling — which produces a crystalline, fractured joint that often looks fine from one angle.
- Tug test: pull each wire firmly. A joint that moves or releases was never sound.
- Continuity with a multimeter, then wiggle the wire while watching the reading for dropouts.
- Resistance across the terminals: a brushed motor reads a low value, often single-digit ohms, and varies as you turn the shaft. An open reading at every shaft position points at a broken internal connection rather than your solder.
- Watch for a motor that runs warm and weak with no load, which can indicate solder or flux that bridged into the motor body or a partially shorted winding.
- Reflow anything suspect: add flux, re-heat until the solder flows properly, and let it cool undisturbed.
When not to solder
Soldering is not always the right joint. If you expect to swap motors during development, crimped connectors — JST-PH or XT30 depending on current — give a reusable connection and move the fragile point to a socket you can replace. Crimping done properly is mechanically stronger than solder for wire-to-wire service connections, because it does not create a stiff section where the wire flexes.
High-current paths deserve attention to wire gauge as much as joint quality. A stall current several times the running current flows through that joint every time the robot pushes against a wall, so size the wire for stall rather than nominal draw and keep the solder joint short and well-wetted so it does not become the hot spot. If the motor connects to a driver board with screw terminals, use ferrules on the stranded wire rather than tinning the ends. Tinned stranded wire under a screw terminal cold-flows over time, the clamp loosens, and the connection heats up — a slow failure mode that is easy to install and hard to find later.
Frequently Asked Questions
Can I use a soldering iron on a brushless motor's phase wires?
Yes — brushless phase wires solder like any other stranded wire, usually to bullet connectors or directly to the ESC pads. The difference is that you must not add noise-suppression capacitors on those three wires; the ESC drives them with a switching waveform that capacitance interferes with.
Do I need lead-free solder?
Not for a personal hobby build in most jurisdictions; RoHS-style restrictions target products placed on the market. Leaded 60/40 or 63/37 melts lower and wets more easily, which makes it more forgiving on heat-sinking motor tabs. Ventilate your workspace either way — the fumes you smell are flux, which is an irritant regardless of alloy, and wash your hands after handling leaded solder.
My microcontroller resets when the motors start. Is that a soldering problem?
Possibly, but check power first. Motor inrush and stall current pull the supply voltage down, and a shared battery with thin wiring is the usual cause. Separate the motor and logic supply paths, add bulk capacitance at the driver, and fit the 0.1 µF ceramics at the motor before re-examining joints.
How do I attach wires to a motor with no terminals, only pads?
Tin the pad with a chisel tip and minimal dwell, tin the wire separately, then rejoin the two tinned surfaces in one short press. Pads have less thermal mass than tabs, so they need less time, and they lift permanently if over-heated — practise on a dead motor first if you have one.
