HOW-TO GUIDE
How to Solder a Wire to a Small PCB Through Hole
By the OhmSavvy Research Team ·
Hot tips, fumes and flying wire ends
- Park the iron in its stand between joints; the tip runs well above 300 °C.
- Ventilate the bench: flux smoke irritates the airways. Wash your hands after handling leaded solder.
- Wear eye protection when trimming; clipped wire ends fly.
Choose a wire gauge that fits the PCB hole
Work backwards from the hole. Small boards and breakout modules mostly use finished holes of about 0.8 mm to 1.0 mm, sized for component leads and 2.54 mm headers. As a typical rule, aim for a hole roughly 0.2 mm to 0.4 mm wider than the conductor so solder can flow down the gap.
| AWG | Conductor diameter (nominal) | Typical hole it fits | Common use |
|---|---|---|---|
| 30 | 0.25 mm | 0.6 mm and up | Wire-wrap, mods, bodge wires |
| 28 | 0.32 mm | 0.6-0.8 mm | Ribbon cable, fine signal wiring |
| 26 | 0.40 mm | 0.8 mm | Sensors, servo leads, I2C runs |
| 24 | 0.51 mm | 0.8-1.0 mm | Signal and low-current power |
| 22 | 0.64 mm | 1.0 mm | Hook-up wire, LED strips |
| 20 | 0.81 mm | 1.2 mm and up | Small power leads |
These are solid-equivalent diameters; a stranded bundle measures slightly more, so test-fit a tinned end first. For wires carrying real current, size the wire for the load, not the hole.
1Strip the right length
The bare length should equal the board thickness plus about 1 mm to 2 mm of protrusion. On a standard 1.6 mm board that means stripping about 3 mm. Too much and bare copper above the board can touch a neighboring pad; too little and the strands never reach the solder side. Use the stripper notch for your gauge so no strands are cut, and leave roughly 0.5 mm to 1 mm between insulation and board.
2Twist and tin the wire
Twist the strands tightly in the direction of their lay, then touch the iron under the wire and let a little solder run into the strands, binding them into one smooth rod rather than leaving a blob on the end. Tinning stops stray strands fanning over the pad. If the tinned end will not pass through the hole, it is carrying too much solder.
3Insert the wire from the component side
Push the tinned end through from the side where parts sit, so the joint forms on the opposite face. A slight bend in the tail stops it dropping out. Clamp the board in a holder so nothing moves while the solder sets.
4Heat the pad and the wire together
Lay the side of the tip against the copper pad and the wire at once; a small bead of solder on the tip acts as a heat bridge. After about a second, feed solder where wire meets pad, away from the tip, so it flows down into the hole. Typical setpoints are 320 °C to 350 °C leaded and 340 °C to 370 °C lead-free; see our soldering temperature guide.
5How much solder to feed
Less than most beginners expect: on a 0.8 mm hole with a 1.5 mm pad, a few millimeters of 0.6 mm or 0.8 mm solder is usually enough. Stop once the fillet covers the pad while the wire outline still shows. Withdraw the solder, then the iron, and hold the wire still while the joint freezes.
01
Strip
Board thickness + 1-2 mm
02
Tin
Twist, then bind strands
03
Insert
From the component side
04
Solder
Heat pad and wire together
05
Trim + anchor
1 mm tail, relieve strain
What a good through-hole wire joint looks like
| What you see | What it means | Fix |
|---|---|---|
| Smooth, shiny, concave cone around the wire | Solder wetted both pad and wire | None needed |
| Round ball sitting on the pad | Too much solder or the pad never got hot | Wick off excess, reheat with flux |
| Bare copper ring visible on the pad | Not enough solder or poor wetting | Add flux and a little more solder |
| Dull, grainy surface | Wire moved while cooling or too little heat | Reflow and hold still |
On a plated hole, a small solder ring on the component side shows the barrel filled. For more on judging fillet shape, see how to use a soldering iron.
6Trim the tail
Trim the end to about 1 mm with flush cutters, flat face toward the board, in one clean snip. Twisting the cutters can crack a fresh joint. Trimming before soldering also works if enough still pokes through for a fillet.
7Add strain relief
A solder joint is rigid and the wire beside it is not, so bends concentrate where the solder stops and strands break there. Support the wire so the joint never takes the load:
- Loop the wire up through a spare or dedicated strain-relief hole first.
- Run a bead of hot glue or neutral-cure silicone over the insulation and board.
Keep the heat off thin insulation
Which soldering iron and tip to use for small through holes
Any temperature-controlled iron works; the tip matters more. A 1.5 mm to 2 mm chisel moves more heat into a pad than a needle-point conical and still fits between close holes. The tip types guide compares the shapes; for wire-to-wire splices, see how to solder wires together.
Irons from our soldering reviews
USB-C PD powered, about 12 s heat-up, OLED display. Compact for small boards and field work.
100W station on the same T18 tips as the FX-888D; a rotary dial and five presets let you save a setting for larger pads.
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Frequently asked questions
Should a wire be soldered on the top or bottom of a PCB?
Push the wire in from the component side, the face where parts sit, and make the fillet on the opposite solder side. The insulation and bend stay on one face and the joint on the other, so the board supports the wire. On a plated hole, solder drawn up the barrel shows as a small ring on the component side.
My wire is too thick for the PCB hole. What can I do?
Use the next thinner gauge if the current is low, as it is for most signal and sensor wiring. If you need the thicker wire, solder it to a header pin or a short tail of thinner wire first. Never drill out a plated hole: the drill removes the copper barrel linking the pads.
Can I solder a wire to a PCB pad without putting it through the hole?
You can tack a tinned wire onto a pad's surface, but it is much weaker. The thin copper pad and its adhesive take all the pull, and a sharp tug can lift it. Passing the wire through the hole lets the barrel and both pads share the load.
How long should the iron stay on a through-hole joint?
Typically two to four seconds for a small pad and thin wire with a suitable tip. If solder has not flowed by about five seconds, lift off, let it cool and fix the cause: a bigger tip, a slightly higher setpoint or fresh flux. Holding on longer mostly melts insulation.
Do I need extra flux when soldering a wire to a PCB?
Rosin-core solder carries enough flux for a clean new board and freshly tinned wire. Add a little liquid or paste flux when the pad is old, tarnished or reworked, or the tinned strands have dulled. Clean the residue off with isopropyl alcohol unless the flux is a no-clean type.
