What to consider when specifying cable assemblies for high reliability applications

By Marco Enge   |   August 10, 2021

Building high-reliability equipment for applications such as aerospace, defence, motorsport, and life-critical systems involves integrating high-reliability components from many sources. While it is relatively straightforward to source high-reliability versions of processors and memories, it may be more difficult to source apparently simple system components such as cable assemblies to the right specifications. This is because cable assemblies are likely to be custom parts, highly optimised to deliver the right trade-off between size, connection density and mass, but only ever produced in prototype or relatively low volumes. Achieving high-reliability cable assemblies when there is so little opportunity for process learning and optimisation during manufacture is a challenge, especially for organisations that lack experience in the discipline.

The issue of cable-assembly reliability is increasingly important as electronics plays a growing role in many markets. In aerospace, electronics is used for flight systems, engine control, in-flight cabin amenities and ground control stations. Autonomous guidance systems are enabling greater use of unmanned aerial, underwater and ground vehicles.

The commercialisation of space is leading to the launch of many more traditional and small satellites, all of which need onboard electronics to control their platforms and operate their payload.

Motorsport is becoming ever more sophisticated, demanding improved engine and transmission control, as well as more complex cameras and telemetry to enable deeper spectator engagement.

And in the industrial sector, devices that would have been bench-bound a decade ago are now being reworked to enable portable and remote operation. All of these need high-reliability connector and cable assemblies but will never be made in mass volumes.
 

Defining high reliability for connectors and cable assemblies

What do we mean by a high-reliability connector or cable assembly?

High-reliability connectors must withstand normal operating conditions such as their rated electrical loading, as well as more extreme environmental factors such as vibration and shock, and / or rapid changes in operating temperatures, extended wear and increased impact, and high levels of strain on cables that must be properly managed by strain-relief systems.

Vibration can be constant and steady or vary in amplitude and frequency. Shock is a single sudden impact. In both cases, the electrical signal through the connection should be uninterrupted.

Thermal specifications for high-reliability connectors are typically more challenging than those for commercial-grade parts, which may be expected to operate from -25°C to +85°C, and industrial-grade parts, which may be expected to operate from -40°C to +105°C. A high-reliability connector range will have a lower operating temperature limit of -55°C or -65°C, and an upper limit of +125°C or +175°C. These operating temperatures affect the choice of materials used in manufacture.

High-reliability connectors and cable assemblies also need to be more robust than commercial parts, so that they can survive higher levels of wear and greater impacts when in service. Again, these requirements inform the design and construction of connectors and cable assemblies.

Another part of this robustness is the ability for cable assemblies to handle more strain than would happen in a commercial environment. This can be due to harsher operating environments, or simply because cables have been forced through tight bends in densely packed equipment in a way that imposes a continuous strain on them.

These issues can be countered with strain-relief features on the connectors, screws to hold mated connectors together, and the use of potting compounds to hold cables in place in the back of a connector.
 

The additional challenge of space

There are further considerations for parts that need to work in space. These include the effects of working in vacuum, dealing with cosmic and thermal radiation, the costs of lifting mass into space, the constraints on system volume, and the very high levels of vibration experienced at launch.

Systems being launched into space also face a very high cost of failure and are very difficult, if not impossible, to repair. Most organisations that launch devices into space take out some sort of insurance to cover launch risks and operating failures, but a track record of reliability issues in space-equipment designs may lead to higher premiums over time.

Operating in space creates challenges that many designers might not otherwise consider unless they were specifically brought to their attention.

For example, the lack of air in a vacuum to act as an insulator will reduce the voltage at which a current in a conductor will ‘flash over’ to the nearest metal part. Vacuum will also cause materials in a connector or cable assembly to give off any gases that are dissolved, trapped, frozen or absorbed within them, contaminating surrounding equipment.

Cosmic radiation becomes a greater source of electromagnetic interference in space than at ground level, and so needs more comprehensive management. And thermal radiation is a greater challenge, especially if a satellite is exposed to direct sunlight or complete darkness during orbit, also known as thermal cycling.
 

Make vs buy

Organisations that need to use high-reliability connector and cable assemblies in their products must decide whether to build them themselves or buy them from a trusted supplier.

At heart, this is an issue of risk management and mitigation: do you believe that by doing the work in-house you will maintain the oversight and quality control needed to have confidence in the capabilities of the assembly, or do you think it would be better to use an external supplier which has the skills, experience and tooling on hand to do the work? It's a difficult balance to strike.

The flow chart below (developed by Harwin), can help designers work out whether to make or buy cable assemblies.

If your application is relatively straightforward and off-the-shelf solutions are available, then the question is whether you can get the right volume of the assemblies at the right time for your project timeline. If so, it may make sense to just buy the assemblies in.

If not, then consider whether to do the work in-house. If your organisation has the experience in crimping wires to contacts, assembling contacts in connector shells, and securing them in place using potting compound, the next question is whether you also have the right tools to do this work for the connectors you have chosen, or the budget to acquire the right tools.

If yes, the next question is the volume of parts you plan to make. Do you want to make or outsource prototype and production volumes? It might be quicker to turn fast prototypes inhouse, but more convenient to outsource to a trusted supplier.

If you want to do the work in-house, and you have the skills, experience and tooling on hand, one further question to consider is whether you have the test equipment, or the budget to acquire it, necessary to do the quality control associated with high-reliability manufacturing. If not, then again, you may have to turn to outsourced manufacturing.

The make vs buy decision is also informed by other issues, such as the quality of your relationships with potential suppliers, whether you need the parts to be certified to a sector-specific standard, and the value you put on flexibility.

Image courtesy of Harwin

One further consideration is the technical challenge involved in producing cable assemblies at the quality needed to deliver high reliability. Even something as apparently simple as crimping and assembling a connector has its subtleties.
 

The technical challenges of cable crimping and assembly

It’s easy to assemble a cable badly - anyone can do it. Doing it well takes attention to detail at all stages. The first step is to use the right tooling for the specific connector type that you are working with – something ‘close enough’ from another job won’t deliver the required results.

For example, wires that are being prepared for insertion into a connector contact need to be stripped to the right length using a measuring gauge, to ensure that bare wire is not left exposed between the contact and the insulation. The wire also needs to be properly dressed to stop any strands escaping from the contact and creating a potential short.

Crimping needs to be done by applying equal force around the barrel of the contact, to ensure the wire is firmly held and to avoid the contact being distorted by unevenly applied force. Similarly, the crimped contact must then be carefully inserted into the connector housing to ensure that it engages properly with any internal mating lugs.

If a contact needs removing, a special extraction tool should be used to remove it without damaging the connector housing. There are even correct and incorrect ways of securing and labelling wire bundles.

If you are interested in exploring a make vs buy decision and what it takes to produce properly assembled, standards-compliant high-reliability cable assemblies in anything from prototype to production volumes, find out how in our new webinar with Harwin. Register now to secure your spot.

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About Author

Marco Enge

As a Senior Product Manager Marco is responsible for product marketing and strategy for interconnect...

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