PCB Cable Assembly: Crimp, Strain Relief and Routing Practice

Most products do not end at the edge of the board. A PCB cable assembly joins the circuit to the outside world, and it usually carries the signals that are least forgiving: the power feed, the differential pair that runs to a sensor, or the ribbon that connects two boards inside an enclosure.

Because the assembly crosses a boundary between disciplines, it is frequently under-specified. The board is designed carefully, the cable is chosen for its current rating, and the joint between them is decided at the last moment. This article covers the termination methods, the mechanical rules and the testing that make a cable assembly as reliable as the board it serves.

Termination Methods and Where Each Fits

Crimping is the default for stranded wire. A correctly formed crimp welds the strands and the terminal into a single metallurgical structure, and it survives vibration in a way that a soldered joint of the same size does not, because solder wicks into the strands and creates a rigid section next to a flexible one. The critical parameter is the crimp height, not the appearance of the terminal.

Soldering remains appropriate for a wire to board joint where a connector would consume too much height, and for fine wires in low volume production. It requires strain relief at the joint, a defined solder amount, and a check that the solder has not wicked past the insulation, because the wicking is what turns a flexible wire into a brittle one. Insulation displacement and welded splices fill the remaining niches.

Crimp Quality, Tooling and Inspection

A crimp is only as good as the tool that made it. Hand tools with a ratchet mechanism are acceptable for prototypes and repairs, but volume production needs a press with a defined applicator and a pull test that is performed on samples from each setup. The pull test result belongs in the process record, and it should be repeated after every tool change or wire lot change.

Inspection looks at three things: the crimp height against the specification, the position of the wire in the terminal, and the condition of the strands. A crimp that is too tall leaves voids, while one that is too short cuts the strands, and both look acceptable in a photograph. The barrel should show the wire at the correct insertion depth, with no strand outside the insulation grip.

Strain Relief, Routing and Bend Control

Every cable experiences a force at the point where it leaves the connector, and the purpose of strain relief is to move that force away from the electrical joint. A moulded boot, a cable tie anchored to the chassis, or a clamp around the jacket all achieve this, provided the cable is not pinched and the joint is not the thing being pulled.

Routing decides how long the assembly will survive. Cable ties are placed so that the bend radius stays above the minimum for the wire gauge, the cable is kept away from sharp edges and from any surface that gets hot, and any connector that carries a high speed signal has its pair length preserved to the end of the run. On the board side, the connector footprint and the mounting holes should be designed so that the cable can be dressed without straining the solder joints.

Testing the Finished Assembly

A cable assembly is tested for continuity, for the resistance of each conductor against its own specification, and for the insulation between adjacent conductors. Where the assembly carries high speed signals, the test includes impedance and, on a differential pair, skew, because a cable that is electrically correct may still fail to pass a signal that a short test cannot exercise.

The mechanical test is a pull on the cable, not on the terminal, applied at the specified force and repeated after thermal cycling where the assembly goes into an outdoor or automotive product. gopcb supports these programmes by delivering boards with the connector footprints, mounting features and panel processes that volume cable assembly requires, so the termination and the board are designed as one unit.

Connector Selection and the Board Interface

The connector is the part of the assembly that the designer has least control over and the part that most often decides whether the product passes its environmental tests. A locking latch, a polarisation key and a contact system rated for the number of mating cycles the product will actually see are worth more than a lower unit price, because a connector that works loose in the field generates a failure report that no amount of board level testing will explain. The current rating should be derated for the number of contacts carrying current simultaneously, since the rating quoted in a datasheet usually applies to a single contact in free air.

On the board side, the interface is a mechanical problem as much as an electrical one. Connector shells that carry weight or cable torque should be anchored to the chassis rather than held only by their solder joints, the pads should be sized for the mechanical load as well as the current, and the mounting holes and keep-outs should be defined together with the cable route so that the assembly can be dressed inside the enclosure without pinching the wires. Where the connector carries a high speed pair, the pin assignment is chosen so that the pair stays adjacent through the connector body and the two conductors are routed with matched lengths to the point where the cable begins.

Documentation and Inspection Standards

A cable assembly is defined by a drawing that shows the wire gauge, the insulation colour and material, the cut length, the strip length, the terminal part number, the crimp height, the pull test value and the connector orientation. Without those details the assembly house will make a reasonable choice, and the reasonable choice is rarely the one the mechanical engineer had in mind when the enclosure was designed.

The inspection standard matters for the same reason. Whether a terminal is acceptable at the upper limit of its crimp height tolerance, whether a strand may be folded back under the insulation grip, and whether a slight discolouration of the insulation from the stripping tool is a defect are all questions that a written standard answers once instead of at every shipment. Where the assembly is safety related, the standard is frequently an industry specification rather than an internal one, and the drawing should name it so that there is no ambiguity at the receiving inspection bench.

Related reading: PCBA development process, board outline and mounting design, PCB design quality characteristics, and PCB manufacturing processes.

FAQ

Is a crimped joint better than a soldered one? For stranded wire in a vibrating environment, yes. A crimp keeps the strands flexible and forms a joint that does not embrittle, while solder wicks into the strands and creates a stress point beside the joint.

How is a crimp inspected? By crimp height measurement with a micrometer, a pull test on production samples and a visual check of the strand position. Photographs alone cannot confirm a good crimp.

Why does the bend radius matter so much? Because bending concentrates strain at the point of the bend. Below the minimum radius the insulation and the conductors are permanently deformed, and the failure appears later as an intermittent open circuit.