We're all aware of the far-ranging uses and necessity for cables within electronics. With their widespread use and cables' inherent sensitivity to electromagnetic energy, the shielding methodology of a cable can be even more important than the conductor and jacket specifications.
A shielded cable is a specific type of passive electrical cable comprised of at least one insulated conductor, encased in a common conductive layer of material. The shield itself can be made from a wide range of materials depending on the type of cable and the application. These materials include, but are not limited to, braided copper strands, steel, aluminum, copper tape, or even conductive polymers.
Shielded cables generally have an insulating exterior jacket that rests over top of the shield, which itself is usually made of an extruded polymer. Many common off the shelf bulk wire designs offer shielded and unshielded versions, but custom shielded wire builds are common as long as you can accept the minimum order quantities.
Braided Shielding in a Cable Assembly
Shielded Cables: Breaking Things Down
The primary benefit of employing a shielded cable is that the shielded layer acts as a Faraday cage, which is a common design practice used to manage pesky electromagnetic fields. The shielding layer helps by preventing unwanted coupling into the cable assembly from an outside source, but also helps reduce the overall radiated emissions that emanate from cables. This fact helps improve the likelihood of passing the various EMI/EMC tests during qualification by minimizing unwanted interference with other devices.
Implementing a shielded cable offers numerous improvements to signal integrity and emissions, but this type of cable does come with some drawbacks that must be considered when searching out the proper use condition for your shielded cable.
The most immediate impact is cost - shielded cables are more expensive than their unshielded counterparts due to the addition of a shield layer which results in more materials required. For example, on a recent quotation for a 20 foot, 3 conductor, 22AWG cable that was quoted with and without a shield, the addition of the shield added approximately 20% to the price. Shielded cables are also physically larger than unshielded cables, which could be problematic if you're working on a cabling architecture where space is at a premium. Since there is more material between the outer jacket and conductors, the outer diameter must naturally increase. For projects where the max jacket diameter is a concern, foil wraps can be a thinner shielding alternative when compared to a braided shield.
Finally, the shielding layer is notoriously fragile and susceptible to damage and must maintain proper electrical contact with a properly referenced ground to remain effective. Any gaps in the shielding or an ineffective termination to ground will render the shielding useless. While these drawbacks may sound unappealing, many customers and engineers would prefer a bulking more expensive cable than risk failing emissions testing when qualifying the cable to EMI/EMC requirements.
Once you have confirmed that a shielded cable is required, there are multiple types of shielded cables to choose from depending on your needs. These include foil shield cables and braided shield cables, both of which bring with them several advantages and potential drawbacks that you need to be aware of moving forward.
What is Foil Shielding on a Cable?
Foil shielding is extremely common and refers to a cable where the shield is made out of a foil layer, which itself is mostly comprised of aluminum or another effective conducting material. In certain situations, the foil may also be made from a thin layer of copper as well. Some cables employ a metallized foil like mylar which is spooled like tape and then wrapped on to the perimeter of the wire jacket in a helical pattern forming the shielding layer. The outer jacket is then applied using similar techniques. Depending on your provider, a wide range of materials and wire sizes can be utilized to help develop a solution that is best suited to fit your unique needs.
The major advantage of foil shielding has to do with the fact that this is the only shielding option that provides total coverage (100%) of the conductors. This means that these types of cables can stand up to even the highest frequency RFI applications, which is particularly critical in data cables. Typically, these types of cables are used with high frequencies greater than 100 MHz. Likewise, foil shielding is lightweight and inexpensive, making it very quick and easy to mass produce as well.
What is Braided Shielding on a Cable?
Braided shielding is also sometimes referred to as "mesh shielding" due to the thin wires used during its construction. These types of cables use a tightly woven lattice of thin material, usually tin or copper, to totally encase the shielded cable assembly in every way.
Unlike foil shielding, braided shielding does not offer 100% coverage of the cable itself. Coverage typically varies from between 70% to 95% depending on the cable itself. This mostly has to do with the braid pattern. Braid patterns will limit coverage to a maximum of about 95% and gaps in those braids limit effectiveness at very high frequencies. However, it does offer several other benefits that often make up for this potential limitation.
For starters, many agree that braided shielding offers more versatility than its foil counterpart. Braided shielding is both flexible and strong, making it ideal for a wider array of potential applications. Likewise, braided shielding works best in low frequency environments - though it does work on the high end of the spectrum, too. Usually, braid shielded cables are used in situations where you're working with low to mid-range frequencies of up to 100 MHz.
The manufacturing for braided shielding cables is quite similar to foil shielding, though there are naturally a few key differences. Layers of individual conductive metal strands are crossed over the cable core or insulated conductor depending on your needs. They are then encased in the jacket for maximum protection. As was true with foil, different wire sizes can be used during the bulk manufacturing process, and it is compatible with many different cable diameters.
Additional Considerations: Right Solution, Right Job
The most important thing to understand about all of this is that both foil and braided shielding cables are completely viable options depending on what you’re trying to accomplish. They both have their unique advantages and disadvantages, but one is not better than the other. As always, you’ll need to start with the specifics of your job, including what you’re trying to do, and how you need to do it. Once you know the goal you’re working towards and the rigid conditions you must adhere to, you’ll have everything you need to pick the type of cable shielding that works best for your project.
For example, if flexibility is one of your major concerns braided shielding would undoubtedly be the way to go. Not only does it offer a higher degree of flex life, but its mechanical strength is one of its core benefits and one that would be hard to replicate through other means. Indeed, foil shielding would not be recommended at all in this case, as it’s not ideal for continuous flexing applications. Braided shielding provides much better flexibility, while still offering the structural integrity and low resistance that you can depend on.
If you’re working with high frequency applications, however, 100% coverage is an absolute necessity, meaning that foil shielding isn’t just the better option, but it’s essentially the only one. The reverse is true when working at lower frequencies where braided shielding would be ideal. Your decision will start to get a bit more difficult to make once you begin working with mid-range frequencies as both resistance and coverage become priorities, but the larger needs of your project should still heavily impact the ultimate type you choose.