Flexible Flat Cables: Having it your way

Need a super flexible flat cable? Need a 36AWG ribbon cable with 21 conductors all in pink? No problem.

Off the shelf flat cables are often available in limited sizes and flexibility which creates problems for designers who need more robust or flexible flat cables. Off the shelf flat cables are normally made of PVC and the conductor stranding is 7 or 19 strand. For applications requiring high flex cables, resistance to chemicals or flexibility off the shelf cables may fail to meet your expectations. Off the shelf cables do not give options for:

• Flexibility and hi-flex applications
• Odd number of conductors
• Uncommon sizes
• Unusual configurations
• Custom colors
  
  

Flexible flat cables are ideal for hand held devices, automation and robotic applications. Engineers and product designers do have options by working with custom wire and cable manufactures to design exactly what they need. Custom wire and cable manufacturers can offer a variety of sizes, configurations and flexible flat cable options. There are two basic types of flat cables:

Ribbon cables - Ribbon cables are multiple conductors connected in a parallel configuration where the conductor spacing is controlled. Silicone ribbon cables are a popular choice for applications where an ultra flexible ribbon cable is needed. Using silicone along with high strand count conductors and/or alloys for added strength and resistance to fatigue for hi flex applications will add to a ribbon cable's flexibility other flexible material are TPE, Polyurethane, and even PVC (Calmont's Superflex PVC).

Top 3 Choices for Ultra Flexible Wire or Cable

Flexible wire in terms of how limp it is starts with the wire design options. The choices you make can greatly impact how flexible the wire or cable will be. The right choices can make the difference between flexible wire and a super flexible wire. Designing a flexible wire or flexible cable begins with the design. We start at the core, conductor stranding and construction.

1. Conductor stranding: Conductors are made up of fine strands of wire assembled together into several configurations. The more strands in the construction, the more flexible and resistant to repetitive flexing the conductor will be. Conductor stranding options range from a solid strand (one strand) to over a thousand strands depending on the wire gauge. The construction of conductor stranding is important as well. Stranded conductors can be manufactured in a variety of configurations, the most common being concentric (true concentric, equilay concentric, unidirectional concentric, and unilay concentric), bunched and rope.

Conductor stranding with rope construction makes for a more flexible wire. This construction has the advantage of increasing flexibility by using a larger number of finer strands while maintaining a tighter diameter tolerance than a simple bunched construction. Ropes are more evident in the larger AWG sizes, such as 8 AWG and larger, but there are also many applications that require the flexibility of rope constructions in the smaller gauges. Constructions vary and can contain hundreds or thousands of strands. To achieve the most flexible wire design, be sure to use the higher stranded conductors. For flexible wire with repetitive flexing applications, consider using an alloy for fatigue resistance.

2. Insulation: Flexible wire and cable requires a flexible insulation material. There are many choices for insulating materials. The most flexible wire insulation is silicone, together with the right conductor stranding can be as limp as a wet noodle. While silicone is the most flexible wire insulator, it does have poor abrasion resistance. Choosing the right material will depend on your application and use, consult your wire and cable manufacturer to determine which material will work best for your application. Flexible insulation and jacket materials include TPE/TPR, polyurethane and even PVC, Calmont's Superflex® uses a special PVC formula for increased flexibility. When choosing an insulation or jacket material, consider your requirements. Is your application requiring sterilization? Is abrasion resistance required? Temperature, chemical contact and bio-compatibility are other requirements to consider. Calmont's insulation and jacket material chart is a helpful tool.

3. Cable construction: Cable construction is another component for super flexible cables. Cable lay length and shielding can increase the cable's flexibility. The cable lay is how the conductors are cabled together, the wires can run parallel or be twisted around each other. The length between each twist can be adjusted for maximum cable flexibility. Shield options include spiral, braid and Mylar or foil wrap. The application is important when considering shield types. For flexible cables which undergo repetitive flexing, spiral and foil type shield are not good options as the shield coverage will be compromised over time.

If you want a more flexible cable design, contact Calmont Wire & Cable to learn more about material choices.

Conductor Stranding: Poor designs lead to breakage.

Conductor stranding is an important component for applications with hi flex cycles or vibration. The higher the conductor stranding the better the resistance to stress caused by flexing. Custom cables with high strand count conductors are the best choice for these types of applications. Higher conductor stranding also makes for an ultra flexible wire when using the right insulation. Discussing the flex requirements with your wire and cable manufacturer is critical to choosing the right cable construction.

Are there circumstances when conductor stranding doesn't help with wire breakage and loss of conductivity? Yes! Recently, I was asked what type of wire I would recommend for an application that required the wire to be soldered onto a PCB. This person was repairing PCBs experiencing conductor breakage at the solder point due to vibration and he thought the wire was to blame. This is a common problem when the wire is soldered onto PC boards especially now with the use of lead free solders.  Applications such as aerospace that are subject to vibration and movement of the wire can cause stress at the solder point.  The problem has nothing to do with conductor stranding or flexibility of a wire, while it is true that high strand conductors and the use of alloy materials adds strength to the conductors. The reason for this has nothing to do with the conductor stranding or how flexible a wire is, but the termination. Termination with connectors is recommended to solve this problem. Cable assembly and contract manufacturers understand this problem and are using edge cards and other types of connectors for termination with newer PCB applications. The problem is more prevalent among legacy boards that are being repaired where retrofitting connectors is usually not an option. Often times a strain relief cannot be installed due to the nature of the application. The solution to repair this type of problem with the old boards may be as simple as using an epoxy to secure the wire to the board to prevent stress at the termination point.

Having the right strain relief is important especially with handheld devices. The lack of or using the wrong type of strain relief can actually cause a stress point and product failure. Strain reliefs can vary in sizes and materials, consult your cable assembly house or contract manufacturer for guidance. I had a customer experiencing conductor breakage on his handheld tool and sent us a sample for analysis. Though his design used high strand count conductor strands, we found the conductor stranding to be damaged at the strain relief, it was discovered the strain relief design had been changed to a spring which caused added strain to the conductors. He changed his design and the returns on his product ceased.

The moral to the story is that though high stranded conductors do resist the stresses of flexing, the cause of the problem may not be in the cable design. Take the time to consult your cable manufacturer and the contract manufacturer for the best material options for your device.