Introduction →
Circuit board design can often fall into the segmentation pattern; while layout designers are well-acquainted with off-board I/O, less thought goes into the details of this connection. Cabling is an essential part of any electronic system, and a failure to account for design parameters results in unreliable and intermittent connectivity. Cable assembly manufacturers can greatly simplify the process by translating system designers’ needs into a cable that adheres to industry standards while meeting the operating requirements of the installation.

Considerations for Cable Manufacturing

Mechanical

  • Length and diameter (with tolerances)
  • Tensile strength
  • Weight
  • Minimum bend radius

Electrical

  • Voltage rating of insulation
  • Current rating of conductors
  • Shielding requirements
  • Capacitance

Environmental

  • Operating temperature range
  • Operating stressors (movement, flexing, abrasion, pressure, etc.)
  • Surrounding medium

The Basics of Cable Construction

Cable assembly has to balance the electrical and mechanical limits of the material and cable bundle while delivering the necessary performance characteristics and long-term reliability. At VSE, we start the cabling process by working with our customer requests for geometry and class (according to IEC 60228):

 

  • Class – Defines the conductor count: 1 for Class 1 (Rigid), 1 + 6n for Class 2 (Stranded), Class 5 (Flexible), and Class 6 (Extra Flexible). Wire count becomes so numerous in the Flexible Classes that exact count becomes less important. In terms of sizing, the nominal cross-section for Class 2 depends on the minimum number of strands for a particular conductor size, while Class 5 and 6 rely on the maximum component strand size of the conductor. Wires are defined purely by electrical and not physical characteristics: a nominal-diameter wire is a function of its resistance to control for supplier product differences.
  • Geometry – Cable construction can adopt different wire sizes and grouping arrangements to maximize flexibility. Smaller diameter wires can fill interstices, sub-cabling can ease cable layup and assembly, and fillers in unoccupied wire positions of the cable ensure a round shape for structural integrity.

Building Cables From the Inside-Out

Cable specification will incorporate the final assembly’s flexibility, shielding, insulation, and structure requirements. Requests for cable manufacturing should consider all of the following items for inclusion:

  • Conductors – The quantity, gauge, stranding, and insulation material are fundamental to the cable assembly. Customers may also wish to designate an industry standard (e.g., UL, MIL, etc.) to communicate design requirements easily.
  • Fillers – To promote a round shape in cables with cable assemblies that do not feature the minimum amount of wires, adding filters provides uniform flexibility for any possible bend direction. Plastic rods and fibers with similar mechanical properties to the wires are the most common selection; glass fiber is also available for high-temperature applications.
  • Sub-cables – The benefit of sub-assemblies in cable manufacturing is the ability to separate wires by function and related signaling. Sub-cabling must adhere to the same manufacturing processes as the overall cable and is a straightforward cost adder. However, the organization of cable branches can be invaluable in complex system assemblies.
  • Twist – To keep cables bound and regulate the tensile and compressive forces on wires during bending, the cable undergoes a helical twist. Notably, many cables comprise stranded wires that act as individual twisted pairs for EMI purposes; the cable twist must be careful to maintain this function. While cable construction can feature the same twist direction per radial layer, a contra-helical design alternating the twist direction by layer is preferable. For twisting specifications, the lay length (which indicates the axial distance per turn) should be approximately 8~16x the wire pitch diameter (wire pitch-to-pitch semi-circle diameter).
  • RF shielding – Shielding is an essential aspect of any well-functioning cable. Cabling options generally fall into one of two buckets:

    Metal-foil tape – Aluminum or copper foil taped to a thermoplastic backing, it contacts a drain wire connected to the shield net. The finished thickness is approximately 1 mil / .025 mm, but a thicker shielding up to 1.5 mil / .038 mm can be preferable for more robust shielding (a thinner shield tends to deform, causing cracking and peeling of the metal foil). Overall, foil shielding reduces the total flexibility of the cable.

    Braiding – The more durable of the two common shielding methods, braiding uses small-gauge wires directly integrated into the cable. It is standard in coaxial cables and, like metal-foil tape, is possible for multiple layers of a cable design. Minimum cable coverage should be at least 90% for adequate shielding, and a metal-foil/braiding combination can further improve results.

  • Jacket – As the outermost layer of the cable, it acts as a crucial barrier between the conductors and the surrounding environment. Material selection will differ according to the installation requirements: temperature, flexibility, abrasion rating, and susceptibility all contribute to the cable reliability. A thinner jacket offers the greatest flexibility, but the more noticeable “wireform” aesthetic may be less desirable in some high-visibility applications.

A Checklist for Cable Assembly Manufacturers

There’s much to juggle for cable manufacturing, which can quickly become overwhelming. In the interest of saving our customers time, we offer a simple checklist to determine materials and processing for optimal cable manufacturing:

☐ Number and gauge of wires?

☐ Does the cable require additional (i.e., MIL) specifications?

☐ Do wires or sub-cables require coding?

☐ Is there a tape barrier? What material properties does it need to exhibit?

☐ Is there an electrical shielding/mechanical barrier? What’s the minimum coverage necessary?

☐ What mechanical properties must the jacket possess? How thick does it need to be?

☐ What’s the maximum and minimum diameter of the cable for tolerance purposes

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Wrap Up System Integration With an Experienced Contract Manufacturer

Cable assembly manufacturers assist electronics designers by ensuring that cables meet all industry requirements and design specifications for their unique operating conditions. Signal and system integrity usually receive less consideration than the on-board design, but any off-board connections are just as essential to foster reliable communications. While cable manufacturing can seem like a further complication of electronics design, our engineers at VSE are committed to building electronics for our customers. With a few questions, VSE can ensure your cables suit the application and environment.

If you are looking for a CM that prides itself on its care and attention to detail to ensure that each PCB assembly is built to the highest standards, look no further than VSE. Contact us today to learn more about partnering with us for your next project.