Hook splice on stranded wire
A hook splice is still the most reliable way to join two stranded wires by hand: it survives a pull test that a lap splice fails, and it keeps the joint thickness low enough to fit under heat shrink. The mechanic is simple - interlock the strands so the joint carries load in tension before any solder is applied.
Procedure
- 1
Strip 12 mm of insulation and twist each stranded end in the direction of the existing lay.
- 2
Bend each tinned end into a J with a 4 mm leg and hook the two J's together.
- 3
Pinch the interlock closed with flat pliers so the strands bear on each other, not on solder.
- 4
Hold the wire horizontally, place a 1.6 mm conical tip on top of the hook at 330 degrees C, and feed wire into the far side.
- 5
Remove the solder as soon as the hook surface turns bright, then lift the iron and let the joint cool untouched for three seconds.
- 6
Slide heat shrink over the joint and shrink from the centre outward.
Workshop notes
Strip length and hook geometry
Strip 12 mm from each wire for a 1.0 mm cross-section, twist each end tight in the lay direction, then bend each into a J with a 4 mm leg. Hook the two J's together and pinch them closed with flat pliers. The interlocked length should be 6 to 8 mm: shorter and the joint concentrates stress in one spot, longer and the hook will not close without crushing strands.
Solder the hook, not the insulation
Bring a 1.6 mm conical tip down onto the top of the hook with the wire lying horizontal, and feed 0.8 mm Sn63Pb37 into the far side of the joint. Solder should wick through the interlocked strands and stop at the insulation edge, visible as a slight swelling. If solder creeps under the insulation, the wire becomes a stiff hinge and will break at the boundary; if it stops mid-hook, the joint is only half-wetted.
Why eutectic alloy matters here
Sn63Pb37 has no pasty range, so the joint solidifies instantly on cooling and stays bright. Lead-free SAC305 in a hook splice stays pasty for about two seconds longer and any movement during that window produces a ring of micro-cracks that is invisible under the insulation. For field repairs, the eutectic alloy is worth the RoHS exception; for production, use a crimp instead.
Practice drill
Make ten hook splices on 22 AWG stranded wire, then pull each to failure with a luggage scale. A correct splice fails at 12 to 16 kgf or in the wire away from the joint. Any splice that pulls apart inside the hook means the strands were not interlocked before soldering.
Failure modes
| Symptom | Cause | Treatment |
|---|---|---|
| Joint breaks inside the hook under load | Strands laid side by side rather than interlocked, so solder alone carried the load | Re-strip, twist each end tight, form true J hooks and pinch them closed before any heat is applied |
| Wire snaps just outside the insulation | Solder wicked under the insulation and created a rigid-to-flexible hinge point | Shorten the strip to 10 mm, keep the tip on the top of the hook only, and add adhesive-lined heat shrink that spans 10 mm either side |
| Joint looks dull and grainy | Wire moved while the alloy was still pasty, or the stranded wire was oxidised | Clean the strands with alcohol and a light abrasive before twisting, and clamp the wire so it cannot move during the three-second cooling window |
Supplies
- 0.8 mm Sn63Pb37 rosin-cored solder
- ROL0 no-clean flux pen
- 2:1 adhesive-lined heat shrink, 4 mm
- Isopropyl alcohol wipes
Tools
- Station at 330 degrees C
- 1.6 mm conical tip
- Wire strippers rated for 22 AWG
- Flat-nose pliers and a luggage scale for pull testing
Safety
Stranded wire splices are frequently live in field repairs. Isolate the circuit, discharge any bulk capacitance, and confirm with a meter before the iron comes near the conductor.
Neighbouring cards
- Diagnosing and repairing a cold joint 340 °C / 2.5 s
- Through-hole fillet baseline 340 °C / 2.0 s
- Wick cleaning a through-hole pad without lifting it 350 °C / 1.5 s
- Drag soldering a 0.1 in header 360 °C / 1.2 s