How to Select Flat Cables

Image credit:  E-Z-Hook; Lumberg Automation; Samtec Inc.

Flat cables are used in computers for internal connections to peripherals. They consist of highly stranded, individually-insulated conductors that are laid parallel and then fused in a flat, flexible ribbon form.  Flat cables are also referred to as ribbon cables or FFCs (flexible flat cables).

Advantages of Flat Cables

Flat cables distribute the mechanical load over the total width of the cable, improving heat dissipation and eliminating the need for large conductors. Unlike conventional round wire cables, they do not require redundant insulating materials, fillers, and tapes. Because the conductor spacing is fixed and the geometry is constant, electrical characteristics such as impedance, capacitance, time delay, cross talk, and attenuation remain constant.

Specifications

When selecting flat cables, it is important to consider specifications related to conductors, insulation material, pitch and cable shielding.

Conductors

A cable conductor consists of a wire or combination of wires that are not insulated from each other, and facilitate the passage of electric current through the cable.  Conductor-related specifications include material, size, and number of conductors.

• Conductor material includes various conductive metals.  Copper is the most widely used material, and features excellent corrosion resistance and high thermal conductivity, but relatively low strength-to-weight ratio.  Aluminum is less dense than copper, and is often used as a power conductor.
• Conductor size is measured in American Wire Gauge (AWG) in the United States.  The higher the gauge number, the smaller the diameter and thinner the wire.  Thicker wires have less resistance and are able to carry more current.  Outside of the U.S., conductors are measured in mm².
• Number of conductors is usually determined by the cable's application or intended use.

Jacket and Insulation Material

Suppliers manufacture cable using a variety of jacket and insulation material in order to reduce the leakage of current from the conductor.  The cable's application is an important consideration when selecting insulation material. 

• Neoprene is a synthetic rubber well-suited to chemical applications.  It is resistant to oil, flames, and chemicals.

• Silicon rubber provides superior flexibility but lacks toughness and fluid resistance.

• Thermoplastic is an excellent insulating material.  Polyethylene (PE), polypropylene, polyvinyl chloride (PVC), Teflon®, and Tefzel® are types of thermoplastic material.  While each material has its own unique advantages and disadvantages, most are highly flexible and provide resistance to moisture or chemicals.

Cable Pitch

Cable pitch is a specification unique to flat cables.  The pitch is defined as the distance between the cable's conductors.  Cable pitch is almost always uniform, but some flat cables may feature varying distances between conductors.  Common pitches include 0.5 mm, 1 mm, 1.25 mm and 2 mm.

Pitch is calculated by dividing the cable's width by the total number of conductors.

Shielding

Flat cables may feature a type of electromagnetic shielding material, which is wrapped around the cable underneath the outer jacket.  Shielding serves to prevent electrical noise from affecting the transmitted signal, and to reduce electromagnetic radiation emission from the cable itself.  Shielding is usually comprised of metal braiding, metal tape or foil braiding.

Cutaway image of a shielded cable.  Image credit:  Tkgd2007

References

Alpha Wire - Understanding shielded cable

Elco - A history of flat flexible cable (FFC)

Gore - Selecting the right cable system for your environment

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