PCB Leads and Wires: Selection and Termination

The Connection Everyone Forgets

Most of a design review is spent on the circuit, and the connection between the board and the rest of the product is treated as a detail. It is not. A solder joint between a stranded wire and a plated hole, a crimped terminal in a connector and an insulation displacement contact each have their own current rating, their own failure mode and their own assembly requirement, and the joint that fails in the field is more often the wire than the circuit.

The purpose of this note is to set out the choices, from the gauge and the insulation to the way the wire is attached and how the joint is protected from being pulled.

Wire Gauge and Current

Wire is specified by its cross sectional area, expressed in the American wire gauge system, in square millimetres or in strands of a given diameter. The current capacity depends on the area, the permitted temperature rise and the environment: a wire in free air carries more than the same wire in a bundle, and a wire in a hot enclosure carries less.

Two practical points follow from the tables. The first is that the current rating of a wire is defined for a temperature rise above ambient, so the rating has to be reduced for the ambient inside the product. The second is that the resistance matters as much as the rating where the cable is long: a supply that is correct at the board can be below the minimum at the far end if the voltage drop along the wire was not calculated.

Where the wire is short and the current is high, the gauge is chosen by the voltage drop rather than by the current rating, and the connector becomes the limiting component. A terminal rated for ten amperes used on a board carrying fifteen will pass the functional test and fail in service.

Insulation

• PVC. The general purpose insulation: cheap, flexible, adequate to 105 degrees Celsius. It releases corrosive fumes if it burns, which matters in a sealed enclosure.
• Silicone. Flexible over a wide temperature range, up to about 180 degrees Celsius, and it does not stiffen in the cold. It is used near heaters and in appliances.
• PTFE. Thin, lightweight, high temperature and chemically inert. It is used where the wire has to be small, where the temperature is high and in medical and aerospace harnesses.
• XLPE and cross linked materials. Better thermal performance than PVC at moderate cost, common in automotive harnesses.
• TPU and TPE. Tough and abrasion resistant, used where the cable is dragged or flexed repeatedly.

The insulation also sets the stripping method. PTFE does not cut cleanly with the usual blades and needs a hot stripping process or a precision stripper, and the wrong tool damages the conductor or leaves a nick that becomes a fatigue crack.

Termination Methods

Soldered directly into the board. The wire is inserted into a plated hole and soldered. It is the cheapest connection and the weakest mechanically, because all of the load is carried by the solder joint and the pad. Where it is used, the wire should pass through the hole and the pad should be large, and the joint should be strain relieved.

Soldered to a pad on the surface. Used where the wire leaves the board at an angle and the hole is not convenient. The pad is large and, where the wire can move, a second mechanical anchor point is added.

Crimped terminal in a connector. The wire is crimped into a metal terminal that fits a connector on the board. The crimp is the most reliable connection available when it is made with the correct tool, because the metal is deformed around the strands and forms a gas tight joint. A poor crimp, made with pliers or with the wrong die, is worse than a solder joint.

Insulation displacement. The wire is pressed into a slotted contact that cuts through the insulation and grips the conductor. It is fast and repeatable, and it is used in ribbon cable and in some appliance wiring, with the limitation that the contact is designed for a specific insulation type.

Push in and spring terminals. A tool free connection for mains wiring in lighting and drive products, where the installer needs a fast, repeatable termination. The board carries the terminal block and the layout has to leave the insertion and the cable access.

Board to board connectors and press fit. Where two boards are joined, a press fit pin inserted into a plated hole gives a gas tight connection without solder, and it is used on backplanes where a solder joint would be impossible to rework.

Our notes on PCB assembly describe how the wired and connectorised assemblies are built.

Strain Relief

The failure mode that matters is a pull or a repeated flex at the point where the wire meets the board. Three measures prevent it.

Anchor the wire to the board. A cable tie through a hole, a clip, or a strain relief gland on the enclosure takes the load and leaves the joint unloaded. This single measure solves most field failures of wired connections.

Keep the bend radius generous. A wire bent sharply at the joint concentrates the fatigue in one place. The radius should be several times the cable diameter, and the wire should leave the board perpendicular to the edge where possible.

Pot or coat the joint. A compliant adhesive over the joint, or a conformal coating, spreads the load and prevents moisture from reaching the solder. It is common in products that see vibration or humidity.

Account for the temperature cycle. A wire that expands more than the board will flex the joint on every power cycle. Where the temperature swing is large, the wire is routed with a service loop that absorbs the movement.

Reliability

The tests that find these problems are a pull test on the joint, a flex test where the wire is bent repeatedly at the joint, a vibration test with the harness fitted, and a thermal cycling test with the wire installed. The pull test is performed on a sample from production, and the acceptance value is a specified force applied in the direction the wire leaves the board.

The visual criteria matter as much: a solder joint with the wire inserted through the hole and wetted on both sides, a crimp with the conductor visible in the inspection window and no strands cut, and a connector fully seated with any locking feature engaged. Our notes on quality management describe the inspection standards, and our notes on turnkey PCB assembly cover the option of buying the board with the harness already fitted and tested.

Wire Routing and the Board Layout

The wire is often the largest antenna on the product, and it is also the largest path for conducted interference.

Keep supply and signal wires apart. A switching supply wire running beside a sensor wire couples into it. Separate them, or use twisted or shielded cable where the requirement is strict.

Route the return with the signal. A pair of wires carrying a signal and its return has a small loop area and couples far less than two wires routed separately.

Give the connector a defined entry. The wire enters the board at an angle, and the layout should leave the space for it so that the harness does not press against components or pull on the joint.

Place the connector so that the wire does not cross the board. A wire routed over the components is a service problem and a coupling risk, and the fix is a layout decision made before the board is fabricated. Our notes on PCB design and layout cover the connector placement rules, and our notes on PCB manufacturing describe the board that carries them.

FAQ

How do I choose the wire gauge? From the current, the permitted temperature rise and the ambient inside the product, then checked against the voltage drop over the length of the run.

Is a crimp better than a solder joint? A correctly made crimp is more reliable and more repeatable, especially where the joint moves. A crimp made with the wrong tool is worse than either.

How can I stop wires breaking at the board? Anchor the wire to the board or the enclosure so that the joint carries no load, keep the bend radius generous and add a compliant coating over the joint.

What is checked in production? A pull test on a sample, a visual check of the joint and the crimp, and a continuity test of every connection in the harness.

Conclusion

The wire is part of the circuit and deserves the same care as the tracks on the board. Choose the gauge from the current and the voltage drop, choose the insulation from the environment, terminate with the right tool for the connector, anchor the wire so that the joint carries no load, and test the joint with a pull and a flex. Most of the field failures attributed to a circuit are really failures of the connection to it.