Cable assembly design considerations

Most cable assembly problems are decided before anything is cut or crimped. A jacket that hardens in the cold, a conductor sized for current but not for voltage drop, a solder joint where the cable flexes, a drawing that does not say where length is measured from — each looks minor on paper and becomes expensive in the field. This guide walks through the decisions in roughly the order they should be made, from the requirements to the documentation that lets an assembly be built the same way every time.

Start with the requirements

A good requirements set describes what the assembly must survive and do, not only what it should look like. The questions below cover the information an assembly supplier needs to propose a construction; unanswered questions are not a problem in themselves, as long as they are recognised as open rather than silently assumed.

1. 01Function and interfaces: what does the assembly connect, and which mating connectors, pinouts and polarisation are fixed by the equipment at each end?
2. 02Electrical load: current per circuit (continuous and peak), voltage between circuits and to earth, signal types (power, analogue, digital data, RF).
3. 03Signal integrity and EMC: controlled impedance, twisted pairs, shielding requirements and how the shield is terminated at each end.
4. 04Mechanical duty: fixed installation, occasional handling, continuous flexing in a cable chain, or torsion on a robot axis.
5. 05Environment: temperature range, oils, coolants, cleaning agents, UV, water and the required IP rating of the mated connectors.
6. 06Length and routing: overall length and tolerance, breakout lengths, minimum bend radius available in the installation.
7. 07Regulatory and market: RoHS/REACH, flame-retardance or halogen-free requirements, industry-specific standards and approvals required for the end product.
8. 08Workmanship and test: acceptance class, electrical tests and whether test records are required per assembly.
9. 09Identification: labelling, traceability level (batch or serial) and customer part number.
10. 10Volume and life cycle: prototype quantities, series volumes, expected product life and tolerance for alternative components.

Conductors, insulation and jacket

Conductor sizing

Conductor cross-section is set by three separate limits, and the largest of the three wins. Temperature rise: the current a conductor can carry depends on insulation temperature rating, ambient temperature and how many loaded conductors share the jacket — a multicore cable carries less per conductor than a single wire in free air. Voltage drop: long runs at low voltage are often limited by drop, not heating. Mechanical: very small cross-sections are fragile and may fall below the range of the intended crimp contact. Conductor stranding matters as much as size: IEC 60228 distinguishes solid (class 1), stranded (class 2), flexible (class 5) and extra-flexible (class 6) conductors, and anything that moves needs the finer classes.

Insulation and jacket materials

The jacket protects everything inside it from the environment, and in many assemblies it is the first component to fail. Material families have characteristic strengths, but properties vary considerably between compounds within each family; the cable manufacturer's datasheet for the actual compound governs temperature range, oil resistance and flex performance.

Shielding

Foil shields give full coverage and are effective at high frequencies but tolerate little flexing; braids give lower coverage, better low-frequency performance and better flex life; combined foil and braid is common in data and instrumentation cables. A shield is only as good as its termination: a full 360° connection to the connector housing performs far better than a drain wire or pigtail. The details are in shielded vs unshielded connectors.

Bend radius and flex life

Cable manufacturers state a minimum bend radius, usually as a multiple of the cable's outer diameter and often with separate values for fixed installation and for continuous movement. The value for movement is always larger. Bending tighter than specified stretches conductors on the outside of the bend, compresses insulation on the inside and, in shielded or coaxial cables, changes the geometry that sets impedance and shielding.

Flex life — the number of movement cycles a cable survives — is not a single number but the result of a specific test: bend radius, travel, speed, acceleration and the failure criterion all matter. Values from one manufacturer's test cannot be compared directly with another's. Cables for cable chains are designed for bending in one plane; cables for robot dress packs must also withstand torsion, which is a different construction. Specify the movement type, and ensure the motion happens in the cable, not at the connector.

Terminations, overmoulding and strain relief

IEC 60352-2 (current edition 2024) applies to crimped connections on stranded wires from 0.05 mm² to 10 mm² and solid wires from 0.25 mm to 3.6 mm diameter, and gives tests for judging their stability under mechanical, electrical and climatic stress. It does not cover crimping of coaxial cables: RF connector crimps follow the connector manufacturer's instructions for strip dimensions, ferrule and die. In production, crimp quality is controlled by verifying crimp height on set-up and at intervals, by pull testing, and by periodic cross-sections.

Overmoulding encapsulates the connector-to-cable transition in a moulded body. It adds sealing, strain relief and a robust handle, but it is permanent and needs a tool. The overmould compound must bond to, or mechanically lock onto, the cable jacket — similar material families bond well, fluoropolymers poorly — and moulding pressure and temperature must not damage internal terminations. Strain relief in non-moulded assemblies comes from cable clamps in the backshell or housing, heat-shrink boots or cable glands; the clamp should grip the jacket, not the conductors, so that pulling on the cable never loads the contacts.

Labelling should survive the life of the assembly: printed heat-shrink sleeves or laminated labels rather than paper. A label typically carries the part number with revision and a date or batch code; where traceability matters, a serial number links the assembly to its test record.

Workmanship and testing

IPC/WHMA-A-620, Requirements and Acceptance for Cable and Wire Harness Assemblies, is the industry-consensus workmanship standard for crimped, mechanically secured and soldered interconnections in cable and harness assemblies (revision E was released in 2022). It defines three product classes — Class 1 general electronic products, Class 2 dedicated service products, and Class 3 high-performance or harsh-environment products — with progressively stricter acceptance criteria. State the class on the drawing; if no class is stated, the supplier and customer may assume different ones.

Electrical testing of every assembly is normal practice for continuity and shorts and is inexpensive with an automated tester. Mechanical checks — crimp height, pull tests, visual inspection to the agreed class — are usually done at set-up and on a sampling basis. For a new design, a first-article inspection against the drawing confirms that the documentation and the product agree before series production starts.

Documentation

Connector and contact choices for the ends of the assembly follow the same logic as any connector selection — see how to select an industrial connector, and for coaxial assemblies RF connector selection and signal integrity. An experienced cable assembly specialist can help turn an incomplete requirement into a buildable construction.