Soldering Wire Splices and Through-Hole PCBs

Make the splice mechanically sound before soldering

Solder should not be the only thing holding loose wires in position. Form a stable splice, support it during soldering, and provide strain relief so pulling or bending the cable does not load the soldered joint. Follow the wire and assembly requirements; these sketches show splice shapes, not a qualification standard.

• Lay-over or lap: stripped ends overlap alongside one another. Support and strain relief are especially important because the overlap does not lock itself together.
• Twist or pigtail: stripped ends twist together and leave the connection on the same side. Cover the joined end completely when insulating it.
• Western Union: an inline connection with each free end wrapped around the opposite wire. It is a commonly used, strong splice when properly formed; it is not automatically the best choice for every wire or assembly.

For conductor sizing, see Wire and Wire Gauge. Use the heating and inspection method on Making and Inspecting Solder Joints.

Practice identifying splices

Put heat-shrink on before closing the splice

Slide suitable heat-shrink tubing onto a wire before joining it, and keep it away from the soldering heat. After soldering, allow the joint to cool and inspect it. Move the tubing over all exposed conductor, overlapping intact insulation at both ends of an inline splice, then shrink it with the appropriate heat source according to its instructions.

Choose tubing whose unshrunk size passes over the splice and whose recovered size fits the wire. Avoid sharp wire ends that could puncture it. Ordinary heat-shrink provides insulation; do not assume it makes a waterproof connection or replaces strain relief.

When pre-tinning helps

Some wires and terminals are pre-tinned: a small amount of solder wets the surface before final assembly. This can help the final joint flow, but too much solder makes a wire end stiff and bulky. Follow the connection's assembly method and still heat both parts when making the final bond.

On a PCB, heat the copper pad and component lead together

A pad is the exposed copper connection area on a printed circuit board. A component's lead is its metal leg. In through-hole assembly, the lead passes through a hole in the board. Insert the correct component in the correct orientation and support the board and component before heating.

Place the tinned working face against both the pad and lead. Feed solder into the heated connection and let it flow onto both, then remove solder and iron in that order. Hold still while it cools. Inspect the joint and the gaps to neighboring pads.

Where the component's assembly instructions call for trimming, cut the excess lead above the cooled joint without cutting into the solder fillet. Wear the required eye protection and control the clipped end so it does not fly away or remain on the board. Do not shorten connector pins that must still mate with another connector.

Soldering header pins is one bridge to Microcontroller Basics. A stray bridge changes the intended circuit connections; review Series and Parallel Circuits when tracing those paths.

Practice through-hole technique