The weakest link – have you considered the cabling?

18 May 2015

Identifying possible causes for failure, and the prevention thereof, is critical for systems in aerospace and defence applications, particularly where extreme shock and vibration are prevalent, as well heat, dust, moisture or chemicals. 

It is often said that a chain is only as strong as its weakest link and, for that very reason, engineers tend to focus their reliability testing on the interconnects.

Connector manufacturers are continually developing products that meet these demands, but perhaps another key element to consider, which can be overlooked, is the choice of cable terminated onto the connector.  High reliability and consistency of these cables throughout the operational life of these systems are crucial.  For example, Temp-Flex solutions by Molex are designed to specifically meet these needs.

The most common problems experienced by designers when using copper cable are its location and the fastening method used to the routing medium.  Tight space constraints can cause interference from adjacent cabling and close proximity to electronic components can cause RFI and EMI disruption.

Fibre, on the other hand, is immune to RFI and EMI, and has the added advantages of transmitting larger amounts of data over longer distances with less loss, as well as a smaller diameter and weighing less than its copper counterpart.  There are drawbacks to using fibre however; apart from considering E/O conversion, choosing the incorrect bend radius and over-tightening restraints can result in macro stress on the cable-to-contact termination during operation and micro stress on the fibre itself.  Not following best practice in terms of cleaning and inspection during installation and maintenance can also lead to failures.

The aerospace and defence industry as a whole is well aware of these issues and the particular nuances each specific design and working environment brings.  Accordingly, a need exists for an alternative solution.

To address the industry demands, Molex manufactures Temp-Flex specialty insulated wire and cable for harsh environments: high temperature; high flexure; abrasives; corrosive chemicals and exposure to body fluids.  Coatings include high performance fluoropolymer coatings and biomedical materials with standard wire sizes from 50 to 26 AWG, with constructions designed to meet demanding customer specifications.  For example, some Temp-Flex products include a 0.0005” pin-hole free extruded wall of insulation over wires finer than a human hair.

Achieving these demanding customer specifications place an emphasis on how the cable is manufactured.  For example, is the diameter of the cable at the start of the reel the same as in the middle and at the end?  The mechanical consistency of Temp-Flex cables is tested along the whole length with documentation available to validate electrical performance.

Consistently monitored coaxial cable manufacturing processes are used to maintain tight mechanical tolerances, yielding class-leading electrical performance of Temp-Flex flexible RF cable, flexible microwave cable, low inductance cable and coaxial ribbon cable.  Low loss, flexible microwave cable uses the Temp-Flex-patented monofilament technology, field-proven to enhance performance at high frequencies; they are designed to be flexible, yet rugged, while maintaining consistent electrical performance.

The FEP flat ribbon high temperature, high flex, abrasion resistant cables use a chemically-inert insulation material.  This low dielectric FEP material is extruded, creating electrical and mechanical performance advantages.

The Velocity of Propagation (VoP) of an electrical or electromagnetic signal is the speed of transmission through a physical medium such as a coaxial cable or optical fibre; there is a direct relation between VoP and wavelength of the signal, and the VoP of a cable can change with temperature and age.  Often stated as a percentage of the speed of light, VoP can also be stated as time-to distance.  To explain, a cable with a VoP of 0.85 would transmit a signal at 85% of the speed of light.  The VoP is a direct function of the dielectric constant of the insulation material.  A twisted pair cable, which typically has a lower VoP (such as 0.65) due to its insulation material choice, can transmit a signal at 65% of the speed of light.

In certain applications speed is of the essence, which is why the VoP is pre-defined and normally specified.  In satellite applications, for example, engineers will tend to specify low loss RF signalling cabling with high radiation resistance.  Temp-Flex coaxial cable solutions are insulated with a fluoropolymer dielectric, with a helically-wrapped silver-plated copper flat wire shield and braid sizes to achieve superior shielding effectiveness.  The standard solid core (low loss) construction coaxial cable delivers 70% VoP; well-suited for satellite applications, the air enhanced dual monofilament design (ultra-low loss) achieves 85 to 88% VoP and improved insertion loss at high frequencies.

For ground-based radar applications, on the other hand, the system designer requires tightly controlled differential signalling to minimise skew; here, tightly controlled twisted pairs are used.  And in digital soldier applications, where high speed quadrax and twisted pair data lines and power in a round and robust cable are required, cabled primaries with custom fillers and shield offer a flexible robust solution.

By maintaining a laboratory for mechanical and electrical cable testing – up to 50 GHz – Molex designs and simulates cable constructions to customers’ specific electrical requirements.  In addition, to ensure product consistency and maintain tight mechanical tolerances, all processing equipment and tooling are fabricated in the company’s fully equipped machine and fabrication shops.

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