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41 Cable harnesses | Focus aircraft, which rely for their power on solar cells across their wings and the interconnections with other onboard systems. Although the composition of flexible circuits varies between product lines, they broadly consist of four main elements. First, a base material – typically a dielectric substrate film – provides the system’s mechanical flexibility, strength and durability, and as a layer on which to install conductors. The conductors (the second element) provide the power and data connections needed by the application of course. Third, adhesives bond the conductors to the base material and laminate them, as well as bond other laminates or substrates together depending on the configuration of the flexible circuit. Lastly, an insulating cover coat is laid over the surface of the other materials. This seals against moisture and dust, as well as protecting against heat, abrasion and mechanical stress to the conductor during bending. As mentioned, flexible harnesses can be designed in various ways. One of the most common and cost-effective approaches is a single-sided circuit, which consists of just one conductor layer and may or may not have a protective coating. Double-sided flexible circuits incorporate conductors on both sides of the base material, typically with protective insulation covering both sides, and use plated through-holes to enable connectivity between the layers. Multi-layer circuits contain three or more layers of conductors, with adhesives and protective lamination between them depending on application- specific requirements for flexibility and protection. This approach often leads to the best ratio of connection density to cost, and is accordingly popular in the aerospace and defence sectors. Also, a patented technique for varying the thicknesses of conductors enables sculpted flexible circuits. These could be designed thinner to accommodate bends or folds along the length of the harness, or thicker at ends where leads for soldering to connectors will be needed. A few different base layer materials are available, each with important properties that are key to which is best suited to a given application. These closely mirror the requirements of conventional cable insulations: in addition to high flexibility, it is vital that the base layer provides tensile strength, thermal stability, chemical resistance and moisture protection. Polyimide and polyester are popular as base layers, as they are highly flexible and strong materials, and provide decent dielectric strength and chemical resistance. They also resist thermal expansion and contraction, with polyimide able to sustain temperatures up to 105 C and polyester operating normally up to 185 C. Aramids and fluorocarbons can both be used as base layers up to 220 C, and provide high tensile strength. Aramids have a much lower thermal expansion coefficient, however, while fluorocarbons are more flexible. Like traditional cabling, most conductors in flexible circuits are made from copper, although aluminium, gold and silver can also be used. Further options exist though: nickel for example offers a route with easy welding and low heat resistance; nickel-chromium can be applied as a high-resistance circuit, and phosphor bronze is useful for corrosion- resistant contacts. Polyesters and polyimides are used for adhesives as well as base layers, as are acrylics and modified epoxies. Polyesters often make the least expensive adhesive, while polyimides and modified epoxies are more costly but have far higher temperature resistance. Acrylics have the highest temperature resistance, as well as the greatest overall adhesive strength (or ‘peel strength’). Also, polyimides and acrylics offer better chemical resistance and dielectric strength than polyesters and modified epoxies. The uppermost protective layers of flexible circuits are typically polyester films coated in polyester adhesives, or polyimide films bonded with either acrylic or epoxy. The cover film is often selected to be the same as the base layer material. Polyethylene naphthalate and polyethylene terephthalate are also common, as are coatings such as acrylated epoxy and acrylated polyurethane, which are typically dispensed over the circuit as a liquid before being rapid-cured by UV radiation. Unmanned Systems Technology | June/July 2020 Various configurations and material options are available for optimising flexible circuits (Courtesy of Trackwise)