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Basic guide to custom cable design

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Custom cable design is a complex subject, but the most important cable design concepts can be understood simply. This guide breaks crucial design elements down to their basics and offer insight into cable design pitfalls to avoid. We also offer tips for thinking about cable standards and design for manufacturability.

Why is cable design important?

If your product needs a cable assembly, and there’s not an off-the-shelf cable available to meet your needs, you’ll need a custom cable. Your cable design will impact everything from cable performance and production cost to production timelines and manufacturing processes.

This shouldn’t intimidate you, though. Cable design can be quite simple, and custom cable suppliers make great resources if you run into any roadblocks.

How to know you’re getting it right

When designing a custom cable assembly, you’ll want to feel certain you’re making the right decisions. Sometimes what is right or wrong isn’t entirely clear. But there’s three questions worth keeping in mind to guide your process. They should keep you on the right track.

The first is: What performance does my product need from its cable assembly?

Make sure that the cable you design meets the needs of your product. If it doesn’t, it isn’t worth manufacturing. These performance characteristics will shape the electrical specs crucial to defining your cable design.

The second is: Will my cable encounter unique service conditions or restrictions?

Maybe the cable needs to rest inside a very small or oddly-shaped product. Maybe your application requires the cable to operate in a vacuum, perform without stopping for years or last through inclement weather. These restrictions will guide the physical characteristics you prioritize in your cable design.

The third is: Can my cable be any more straightforward?

This is the least crucial of the three questions, but trust us: Standardizing design components as much as possible will make a big difference when your design is manufactured at scale — both in terms of cost and the strength of your supply chain.

Keep the above questions in mind to make sharper decisions during your cable design process.

Custom cable design elements

While all custom cable is different, the elements that make up cables are ubiquitous. Your design will have a conductor, protection for that conductor, and at least one terminal or connector.

Specifying the conductor

A conductor’s primary characteristics are its material, plating, flexibility and gauge.

If you’ve ever updated electrical configurations in an old house, you’ve probably participated in the widespread shift from use of aluminum wire to copper wire, driven by safety. These days, almost all wire in electronics applications uses tinned copper (copper covered with tin). Bare copper is often used in power cords and automotive applications but degrades much quicker than tinned copper when exposed to moisture, oxygen or high temperatures.

Of course, there are cases in which something other than tinned copper is used. Silver-plated copper offers a safer option in high-temperature applications, where tinned or nickel-plated copper struggle with increased resistance.

As a less common example, one of our customers requires bare copper since connections need to be ultrasonically welded, and tinned copper doesn’t weld to itself as well as bare copper. In their case, designing to limit environmental degradation is important.

Conductor flexibility is ultimately determined by the size of your parts or some physical needs of your application. If your cable needs to operate in motion, it will need to be flexible. Stranded cable offers more flexibility than solid cable, and high strand count contributes to high flexibility.

Gauge, on the other hand, may be influenced by an electrical parameter you need to meet. Gauge measures a conductor’s thickness, listed as an American wire gauge (AWG) value. The thicker a conductor, the more current it can carry.

Specifying connectors and terminals

Specifying connectors and terminals shouldn’t rival rocket science for complexity, yet we see manufacturers make a lot of avoidable mistakes here. There are a few primary considerations to keep at hand:

  • You should match plating materials on a pair of mating terminals 
  • Mating strength and mating cycle ratings should match the cable’s end application 
  • You should specify terminals built to handle the gauge of conductor you are using 

Corrosion will occur anywhere two dissimilar metals make contact — it’s unavoidable. But choosing materials wisely can reduce the amount of corrosion where a conductor meets a terminal. Most terminals are tin-plated brass or phosphor bronze, since they play nice with common crimp and conductor materials.

Replacing tinned copper throughout your part with silver-plated copper wiring (more corrosion-resistant) is expensive. But if high reliability is critical, it makes sense to solder your standard tinned conductor to a nickel-plated conductor before it reaches the terminal to protect it from corrosion.

If you’re using gold-plated terminals, you’ll notice the gold is rated for longevity as well. Since gold is a soft metal, it wears down over time. Consider what level of performance and endurance your cable needs before selecting a gold-plated terminal.

Mating/unmating force denotes the amount of force required to make and break a connection. Some connections are meant to separate at low levels of force to avoid equipment damage (like a headphone cord detaching from a laptop when you stand up, so that the laptop isn’t pulled off the table). Others are meant to remain secure and even withstand vibration, motion or unexpected pulling (like in critical equipment that can’t lose power).

A connector’s mating cycle rating is based on the number of times one can disconnect and reconnect a cable before its performance degrades. For instance, the USB charger for your AirPods probably has a very high mating cycle rating. But often, cable used in industrial applications isn’t meant for repeated unplugging and reconnecting.

Before specifying a terminal, think about your product’s end-use and how you need it to perform. Specify accordingly, and you’ll avoid over-designing your cable or sacrificing important performance specs.

And when it comes to the gauge range a terminal is rated for, select a terminal rated to handle the gauge of conductor you are connecting to it. This sounds self-explanatory, but we’ve seen cables designed otherwise plenty of times. If you want to trust a connection, make sure to use the parts as intended.

Multi-Tek employee interacting with production software

Sometimes, manufacturers want to run two widely different gauges of wire into the same connector. This is usually when the cable carries both signal and power, and they’re operating with a space restriction that prevents using two separate connectors.

We’ve been asked to just crimp the connection as tight as it’ll go, then solder it in to keep it from slipping out. Our recommendation when wires with a large gauge disparity are needed in the same connector is to select the connector with the widest available gauge range. You’ll be doing yourself (and your cable manufacturer) a favor.

Specifying insulation

Insulation is used to preserve the integrity of a cable’s conductor. What you need from insulation will likely derive from environmental considerations and details related to your product’s end-use. When specifying insulation, pay attention to four main characteristics:

  • Temperature rating 
  • UV rating 
  • Breakdown voltage rating 
  • Flex rating 
reel of wire

An insulation material’s temperature rating refers to the highest temperature at which the insulation is capable of protecting the conductor. For instance, Teflon typically carries a temperature rating around 200°C, making it preferable to PVC insulation for high-temp applications.

An insulator’s UV-rating denotes its resistance to UV light. While PVC becomes brittle after prolonged exposure to sunlight, polyurethane (PUR) offers sturdier protection.

Large amounts of electrical current aren’t always safe for insulation materials. An insulator’s breakdown voltage notes the voltage at which the current will zap straight through the insulation. Paper, Teflon and glass have particularly high breakdown voltage, but not every application will push an insulator to its voltage limit.

Flex rating is an important consideration for cables in constant motion. Flex rating denotes the number of times a material can undergo a certain type of motion (such as torsional, side-to-side or others) before degrading. In robotics parts and when constant motion is a cable feature, we like to use PUR as an insulator.

Your application may not need high performance in every area from an insulator. Some parts require special attention to some other characteristics, like chemical resistance or abrasion resistance. In any case, choose insulation that’s built to handle your application. Any less, and you risk part failure. Any more, and your material costs will add up quickly.

Specifying shielding

Cables need shielding to protect their signal from electromagnetic interference (EMI).  A shielding material covers the conductor and connects to a ground to protect the conductor from any stray electromagnetic energy that may leak from elsewhere in a product. Sometimes, plugging up a source of EMI proves more challenging than reinforcing a signal cable with shielding.

Manufacturers use two main types of shielding in most custom cable projects. The first is Mylar foil shielding, which is a metalized plastic that can be wrapped around individual wires or around a complete wire braid.

The other type of shielding is a metallic braid shield, usually tinned copper braided around a conductor. It’s not as lightweight but offers more flexibility and a better flex life than Mylar foil shielding. The different levels of braid shielding refer to the percentage of the wire bundle that is covered by the shield.

5 Common cable design mistakes to avoid

Tons of decisions and microdecisions go into a cable design. It can feel like there’s a million places something can go wrong. But product engineers who avoid these common pitfalls will keep their design on the right path.

1. Specifying unnecessarily strange components

If you absolutely need that rare connector, then your manufacturer will just have to make the most of a tough situation. But if you have the luxury of choice when it comes to components, choosing something standard will keep your supply chain consistent and your material costs under control.

We see this most often with companies specifying a hard-to-get connector series or overengineered multi-conductor cable where a standard cable will do.

2. Mismatching wire AWG and terminal size

We mentioned this above, but it’s worth repeating here. Too many manufacturers specify terminals that don’t match the size of their wire. Sometimes this comes from a lack of understanding of size descriptions and standard best practices. Other times engineers don’t consider all the factors that play into a wire’s size.

Say you’re using 18 AWG wire, and you’ve specified a terminal rated for 18 – 22 AWG wire sizes. Your wire is on the large end of what that terminal is rated for, but the terminal should still work, right? Maybe not. If your wire is covered with thick insulation, it may not fit properly in the terminal.

3. Specifying unnecessarily precise dimensions and tolerances

Sometimes tight tolerances are unavoidable. But in most of the cases that we see an extremely tight tolerance on a drawing, we’re looking at a case of overdesign. And these decisions can prove costly.

A cable with tighter tolerances than a comparable product can cost as much as ten times to produce. Consider that cost before falling into this common trap.

4. Calling out a specific manufacturer when a standard part will do

Specifying a specific manufacturer when unnecessary handicaps your supply chain and produces a design that is less viable for production at scale. In some cases, particular components are necessary, but manufacturers often request particular parts when generic ones will offer more consistent supply and indistinguishable performance.

5. Using vague descriptions

This mistake is the polar opposite of the overspecification of tolerances and components. We sometimes see drawings with instructions like “use wire as needed,” with no suggestion of the wire’s function, intent or desired performance. That’s just not enough for a cable manufacturer to work from.

Start with the basics (voltage, temperature, flexibility and gauge). A little bit of information goes a long way in helping a manufacturer implement your design.

Designing custom cable for manufacturability

Design for manufacturability comes down to ensuring that your part can be built consistently and economically. We covered the topic in our video series “Unshielded.”

Design for manufacturability (DFM) doesn’t mean sacrificing cable performance. For a cable to be manufacturable, it must meet all your needs. That is non-negotiable. DFM involves consideration of the manufacturing process. Is this design easy to build with standard tooling? Does it require a manufacturer to keep specialized inventory?

If you keep manufacturing in mind during design, you’ll avoid many of the common pitfalls listed above, and you’ll design a product that a supplier can produce cheaply at a high quality standard.

Are cable standards relevant?

The most common set of cable standards is the IPC/WHMA-A-620 standard. The standard has three classes, grouped by how critical the cable assembly’s performance truly is. Most general consumer products adhere to Class 1 standards, where “the major requirement is the function of the completed assembly.”

Many industrial products adhere to Class 2, where “continued performance and extended life is required, and for which uninterrupted service is desired but not critical.”

Class 3 is reserved for cable assemblies used in high-risk medical applications or in military technology, where “continued performance or performance on-demand is critical, equipment downtime cannot be tolerated, end-use environment may be uncommonly harsh, and the equipment must function when required.”

If you are working with a supplier that builds custom cables to an IPC/WHMA-A-620 Class 2 standard (like Multi-Tek), you don’t need to specify a certain level of crimp quality or solder precision. Your supplier meets that standard on all their products. As such, quality concerns like the security of a solder joint are less a design concern than a manufacturing concern. They should play a role in selecting a supplier, but they don’t need to complicate your design process.

What about UL listed and recognized parts?

A part’s UL marking denotes its status related to part safety and sustainability. UL operates beyond cable and electrical components and certifies parts in countless industries.

UL recognized parts have been tested for general safety, and UL listed parts have undergone more stringent testing for specific applications. If you’re interested in specifying UL listed components, make sure they’re listed for your particular application.

Your cable supplier may obtain a UL recognition for your custom cable assembly. This can make obtaining a UL listing for your product easier and provide more supply chain transparency to your customers.

UL part lists and guidelines can feel like a puzzle. We made a video guide outlining the subject. Don’t hesitate to reach out to your cable supplier with any questions or for any assistance.

Has a cable assembly expert seen your design?

As with any product design, a second set of eyes can make a huge difference.

At Multi-Tek, our cross-disciplined teams identify and resolve design problems before they have a chance to impact your end product. We’ll work together to catch inconsistencies, make recommendations for material swaps and ensure an efficient manufacturing process.

If you have any cable design questions, reach out. We’d love to talk shop and specs.