Choosing the Proper Components in Your Cable Assembly ...

Sep. 09, 2024

Choosing the Right Components for Your Cable Assembly or Wire Harness

By Epec Engineered Technologies

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In any electronic or electrical system, the cable assembly or wire harness plays a critical role, yet it often becomes one of the last elements to consider. It's essential to address cable assembly requirements early in the design process to specify the most effective solution for the intended application. Various components will influence the assembly design, and each choice fundamentally affects the performance of the overall cable assembly or wire harness. Utilizing an assembly that is incorrectly designed—either over or under—can lead to significant performance drawbacks for the entire system.

Key questions to address include understanding the environment in which the assembly will operate. For example, if the cable assembly will be installed dynamically—which means it will undergo bending or flexing—its components must be capable of withstanding such conditions. This same consideration applies if the assembly is expected to endure harsh environments, cleanroom settings, or exposure to sunlight and moisture.

This article will explore the different factors to contemplate when designing a cable assembly or wire harness. This includes selecting appropriate materials to meet installation expectations, determining termination methods based on the type of signals transmitted, and identifying necessary safety certifications related to environmental conditions.

Conductor or Circuits

The number and type of circuits or conductors included in an assembly hinge on its specific application. There are numerous options for conductive materials, conductor stranding, and plated finishes.

Copper is the predominant and most adaptable conductor material available. It can be coupled with various coatings designed to prevent corrosion and enhance the termination process. In situations where a conductor’s breaking strength is critical, alternatives like copper-clad steel and copper alloys are available. These materials sustain copper's conductivity while incorporating elements such as steel or alloys (e.g., cadmium, chromium, and zirconium) to boost both flexibility and strength. In niche applications, stainless steel can serve as a conductor, but it requires plating to enhance its conductivity due to its inferior natural conductivity compared to copper.

Uncoated copper is vulnerable to atmospheric corrosion; thus, many conductors receive protective plating—often tin—to maintain durability and aid termination. For assemblies exposed to elevated temperatures, coatings like silver and nickel provide reliability at temperatures of 200°C and 260°C, respectively.

Conductor designs may consist of a single solid strand or multiple smaller strands. Stranded conductors were developed to mitigate the rigidity of solid wires and come in various configurations; the application will dictate the choice of construction. For a given conductor size, a higher number of thinner strands results in greater flexibility.

Figure 1: An illustration of a stranded conductor (right) versus a solid conductor (left); both examples feature bare copper without additional plating.

Each conductor in the cable assembly should be specifically designed for its expected use. For instance, if a conductor's role is to deliver power, the anticipated current must be considered to determine an appropriate conductor size. Conversely, for conducting signals, both the signal's speed and the length of the assembly will dictate the optimal conductor construction.

Primary Insulations/Sheaths

The choice of insulators, whether Thermoset or Thermoplastic, is also determined by the specific needs of the cable assembly's application. With a broad range of available materials aimed at differing environments, critical considerations include the assembly's operating temperature, the type and level of anticipated voltage, the expected harshness of the installation environment, and the types of chemicals or fluids it may encounter.

Materials can be selected to allow functioning in temperature extremes from -65°C to +200°C or even higher. When assessing temperature conditions, it's essential to verify whether the assembly will be static or dynamic—i.e., subject to movement under elevated temperatures. Assemblies in dynamic conditions at extreme temperatures require rugged compounds for effective operation.

The current or voltage that the cable assembly must carry will also influence the insulation choice. Certain insulation types cannot manage high currents over extended durations.

If high-speed signals will be transmitted, insulation can be "foamed" through chemical or gas injection processes—creating air bubbles in the insulation that facilitate signal transmission without impedance.

Primary insulation can be color-coded or marked for identification in multi-conductor cables. Coloration can be achieved by incorporating a colorant in the compound during primary extrusion, while marking can involve printing numbers or applying stripes, which can be longitudinal, spiral, or ring-banded.

Twisting/Cabling of Conductors

Originally invented by Alexander Graham Bell, twisting wires effectively cancels out Electromagnetic Interference (EMI) from external sources. Telecommunications are often impacted by noise problems when pairs within the same cable are adjacent over long distances, leading to crosstalk between them. However, twisting pairs mitigates this noise effect as they are only proximal during half the twist. For communication or data signal transmission, twisted pairs are highly recommended to minimize noise and EMI disruptions.

Additionally, conductors or twisted pairs are cabled together—a manufacturing process that helically wraps conductors or pairs for several benefits including enhanced flexibility and a round aesthetic for the finished assembly.

Figure 2: The above example showcases a cable utilizing conductors that are not configured as twisted pairs.

Shielding

To further manage both EMI and RFI, overall shields are applied post-cabling of conductors/pairs. Various shielding options exist, each with respective advantages and disadvantages. A **foiled shield**, a popular choice, consists of metalized foil adhered to polyester backing. These offer low costs, reasonable flexibility, and good performance at high frequencies, but less so at lower frequencies.

Another option is a **braid shield**, made up of numerous small wires that are braided around the cable core. While this option presents higher costs and challenges in termination compared to foil, it excels at low-frequency shielding and provides enhanced flex life.

Lastly, a **spiral shield** employs small wires wrapped around the cable core, providing excellent flexibility and endurance but weakness against high-frequency noise and termination difficulties.

For optimal EMI/RFI protection, a combination of foiled and braid shields is advisable, offering superior safeguarding across frequencies while remaining easy to terminate, although usually at a higher cost.

Figure 3: An illustration demonstrating a combination of foil and braid shield used to control EMI/RFI.

Outer Sheath

Once the cable core is completed, an outer sheath or jacket is applied to serve as protective covering. This jacket must typically meet flame-retardant standards per UL and/or CSA requirements. It may also need to be physically robust, chemically resistant, and flexible to accommodate movements throughout its lifespan.

The material utilized for the outer jacket varies based on the assembly's intended installation environment, with PVC being the most broadly utilized due to its cost-effectiveness. Other options include urethane-based compounds for rigorous service applications, elastomer-based materials suitable as rubber substitutes, and fluorocarbons known for their fire resistance and toughness.

Connectors/Strain Reliefs

Cable assembly designers face numerous options regarding the selection of connectors. The type and speed of the transmitted signal typically dictate connector choice. For example, if an assembly is transmitting power, a crimp connector may be appropriate; however, if high-speed signals are involved, solder or welded connections are ideal. Connector options may also be limited based on the pre-existing equipment where the assembly will connect.

The designer must also determine whether strain relief is essential. Strain relief establishes a transition between the cable and termination area, preventing applied loads from jeopardizing terminations. Various strain relief designs are available, including solid and segmented options, with the latter offering better bend relief but increased cleanliness challenges in sterile settings.

Safety Certification

When designing a cable assembly or wire harness, one of the foremost concerns is location. This encompasses the region where the assembly will be employed, as well as the applicable safety certifications and environmental standards it must meet.

Numerous regulatory and safety organizations oversee electrical device examinations across the globe. For instance, North America relies on UL and CSA as primary standard agencies, while many other regions adhere to IEC, CEE, or CENELEC guidelines as a foundational basis for regulatory compliance.

Additionally, environmental standards impose restrictions on hazardous substances within products. Directives such as REACH and RoHS limit hazardous substance use, alongside the WEEE Directive, which dictates the disposal and recycling processes for electronic goods nearing the end of their usability.

Alongside safety and environmental criteria, some assemblies may require compliance with industry standards based on performance benchmarks. Depending on the application, cable assemblies may need to adhere to specifications like HDMI, SFP+, QSFP, TIA/EIA 568-C.2, or receive endorsement from independent testing agencies such as ETL.

Conclusion

In summary, several critical factors must be evaluated when designing a cable assembly or wire harness for specific applications. Every design specification must be deliberated prior to manufacturing commencement. Overlooking any of these vital elements may diminish the performance of the finished product and alter costs if design assumptions are unbalanced. If your product requires a cable assembly, engaging in a thorough discussion with your manufacturer regarding the full application is best practice to determine the optimal cable assembly solution tailored for your needs.

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