Down to the Wire

An Introduction to Interconnect Wire and Cable. We are all familiar with the term wire and cable, and with the integral role it plays in the makeup of an interconnect harness. But just how much do we know really about this, the flexible backbone of the interconnect cable assembly? This quarter, QwikConnect is privileged to reprint, for your reading enjoyment, the Wire and Cable chapter, by Thomas Stearns, from McGraw-Hill’s Electronic Connector Handbook. And, if you enjoy it just half as much as we did, then it follows we enjoyed it fully twice as much as you. In any case, may we suggest that the complete volume, edited by Robert S. Mroczkowski, would make an excellent addition to your interconnect library? Wire, according to IEEE definition, refers to a conductor plus its insulation, if any. A bare wire, therefore, is equivalent to a conductor. Electrical connections to a wire are, of course, made to the conductors. The insulation must be removed prior to (crimped and wrapped) or during (insulation displacement) the termination process for such connections. FIGURE 1: Schematic examples of (a) jacketed, (b) ribbon, (c) shielded transmission line, and (d) coaxial cables. Cables are of two general types: multiwire cables to provide mechanical protection for the wires and/or to facilitate wire handing (e.g. jacketed and ribbon cables) and cables intended to provide specific electrical performance characteristics such as shielded transmission line or coaxial cable. Some cable constructions, of course, incorporate both benefits. Examples of such cables are provided in Fig.1. which illustrates a multiconductor jacketed cable, and Fig.1b which shows a ribbon cable configuration. These cables provide no high-performance electrical characteristics. The cables in Figs. 1c and 1d do provide such benefits. The shielded transmission line ribbon cable illustrated in Fig.1c has controlled impedance, through the use of controlled spacing and conductor diameters as well as the dielectric material. Protection against electromagnetic interference is provided by the shield which surrounds the cable. The coaxial cable illustrated in Fig. 1d offers the same benefits but in a different geometric configuration. Tickets Please The function of a conductor is, of course, to provide a conductive path between two subsystems of an electric or electronic system. Conductors are always metallic to take advantage of the intrinsic high electrical conductivity of metals as well as their advantages in fabrication and mechanical strength. A conductor may consist of two elements: the base metal of the conductor and, in many cases, a coating or finish that provides corrosion protection or specific performance characteristics. The most common materials are as follows: Copper Copper is by far the metal of choice for conductors, because it has an unmatched combination of conductivity, formability, and cost. Other conductor materials include copper alloys and aluminum. These conductors are chosen for their combined properties of conductivity, tensile strength, weight, and cost with the intended application determining which factor is most significant. Finishes include tin, silver, and nickel, the choice of finish depending on the application requirements – in particular, temperature and frequency. Copper is easily drawn to a wide range of sizes and provides the requisite flexibility for installation and use in a variety of applications. The upper limit on conductor diameter is determined by the flexibility requirements, while the lower limit is due to the reduction in wire tensile strength, usually for manufacturing/handling reasons. Termination to copper conductors by any popular means (mechanical or solder) is straightforward and reliable. Copper is used in both annealed and hard tempers, which strongly influence tensile strength, electrical conductivity, and resistance to flexural failure. Copper Alloys Copper alloys are used for applications requiring increased tensile strength, but they always involve a reduction in electrical conductivity, even at low levels. Examples of high-strength alloys include cadmium-copper (C16200) and cadmium-chromium-copper (C13500). Selection of an appropriate conductor alloy is a complex task encompassing consideration of cost, weight, conductivity, metallurgical stability, and suitability for the desired termination technique. Table 1 includes a compilation of electrical and mechanical characteristics of copper alloys used as conductors in wire and cable. The electrical properties are listed in two ways: the material resistivity, in microhms per cm, and percent IACS, the conductivity relative to the International Annealed Copper Standard. Aluminum Aluminum has promising weight/conductivity characteristics but, compared to copper, is difficult to terminate. These characteristics lead to consideration of aluminum in applications where weight savings are important, such as aerospace and automotive, but at an increased attention to termination technologies. Some electrical and mechanical properties of aluminum and aluminum alloys are also given in Table 1 From a termination viewpoint, aluminum conductors are difficult to terminate for two reasons. First, aluminum forms a strongly bonded insulating oxide film that can cause problems for both soldering and mechanical termination technologies. Soldering requires the formation of an intermetallic compound between the tin and the metal of the conductor which is inhibited by the hard-to-remove oxide layer. The oxide layer also inhibits mechanical electrical connections due to its thickness and adherence. Second, aluminum is susceptible to creep–material flow under pressure–which must be accounted for in the design of mechanical termination technologies. This tendency to flow requires that a spring member be included in any aluminum termination so that the residual force necessary to ensure the integrity of the connection can be maintained as the aluminum flows. Aluminum also has low fatigue strength and therefore has limited flexural capability. The combination of these negative characteristics limits the use of aluminum conductors. The widest application of aluminum conductors is in heavy power cables, buried and overhead (with steel reinforcing cores for overhead applications), interior distribution runs, and buss bars. Conductor Finishes Copper conductors are coated, plated, or clad with several metals to provide increased resistance to corrosion or improved temperature capability. Among the most common finishes are tin, silver, and nickel. Both tin and silver provide protection to the copper during the application of the wire insulation where high temperatures are unavoidable and certain insulations interact with the copper leading to insulation breakdown and copper corrosion. Nickel is primarily used to improve high-temperature capability. TABLE 1 Selected Properties of Conductor Materials Material Conductivity (% IACS) Tensile Strength (KSI) Elongation (%) Copper   C11000   Annealed   Hardened   C13500   Sp. Temp.   C16200   Annealed 102 96 88 90 34 68 65 42 25 10 10 25 Aluminum   EC   A905056 62 29 15 45 15 15 Tin finishes provide corrosion protection due to the self-limiting oxidation characteristic of tin. Tin also improves termination performance both in soldering, because tin is a component of the solder, and in mechanical permanent connections such as crimping and insulation displacement connections (IDS), where the ease of oxide displacement enhances the creation of the desire metallic interface. Tin-finished conductors are used in many commercial applications, but at a cost penalty compared to bare conductors. The high resistivity of tin, however, limits its use to lower-frequency applications where skin effects are neglible. Skin effects refers to the fact that the current in a conductor is carried closer to the surface of the conductor as the frequency of the current increases. Silver, in contrast, is beneficial in high-frequency applications where its high conductivity enhances the effective conductor conductivity. Silver is a common conductor finish in coaxial wire and cable applications where its benefits compensate for its higher cost. Nickel improves the high-temperature capability of copper conductors by providing protection against oxidation at up to 250° C. Nickel finishes have negative effects on both solderability and ease of mechanical termination, although both effects can be dealt with by proper processing. Conductor Constructions Conductors are formed from single or multiple strands referred to as solid or stranded conductors, respectively. Each has its advantages and limitations. Solid Conductors are lower in cost and have lower bulk resistance than stranded conductors of the same cross section. The lower resistance, in combination with higher heat dissipation, leads to solid conductors having a higher current rating for a given wire size. However, they may have reduced flexibility and flexural endurance and are more susceptible to damage during insulation stripping than stranded conductors. A nick or scratch in the surface of a solid conductor may propagate through the entire cross section with continued stress, leading to conductor failure. With stranded conductors, damage is isolated to the single damaged strand. Stranded conductors are used primarily because of their superior flexibility and flexural endurance compared to solid conductors. These benefits, however, are dependent on the strand configurations used in construction of the wire, a number of which are illustrated in Figs. 2 and 3. Strand configurations vary in complexity from bunched to rope stranding. In bunch stranding, all the conductors are treated as a unit without any control over the relative positions of the individual strands (Fig. 2a). Bunch stranded conductors exhibit the highest level of flexibility and may or may not have a twist, or lay, during construction. There are several stranding configurations in which a center conductor is surrounded by outer layers of conductors (Fig. 2b); the number of strands in each layer increases by six in such constructions. In rope constructions, used for very high strand counts, individual conductor strands are replaced by multiple conductor bundles, generally bunch stranded, in similar configurations (Fig. 2c). FIGURE 2: Schematic illustration of (a) bunched, (b) concentric, and (c) rope stranding constructions. Concentric stranded configurations, illustrated in Fig. 2b, are distinguished from one another by the lay, or twist, given to the conductors during manufacturing, as illustrated in Fig. 3. In unilay constructions, (Fig. 3a), the conductor lay has a single pitch, but the conductor layers alternate in the direction of the twist. In “true” concentric constructions (Fig. 3c), both the direction and pitch of the twist vary with the layers. The reason for the variation in lay pitch and direction is to control the relative positions of the conductors and thereby the overall dimensions and stability of the conductor bundle. Unilay constructions have the greatest flexibility but the least control over the bundle stability and dimensions. True concentric constructions have the greatest control over these factors. Clearly, such dimensional control entails an increased cost, and the more complex conductor constructions are used only when performance benefits justify the added cost. The benefits of improved dimensional control are of particular significance in wire intended for insulation displacement connection (IDC) applications. FIGURE 3: Schematic illustration of selected concentric conductor constructions: (a) unilay, (b) equilay, and (c) true concentric. Stranded constructions also affect flexibility and electrical resistance. Decreasing the pitch of the lay, for example, results in reduced flexibility, while electrical resistance is increased due to the increase in the overall length of the conductor strands because of the conductor twist. Stranded conductors may be bonded together by a coating of tin that is applied over the full conductor length. If such coatings are applied to pretinned strands, the result is called overcoated copper. If done to bare copper conductors, the result is a slightly lower cost form called top coated. All versions show reduced flexibility and greater tendency to fracture with vibration and mechanical abuse. They also exhibit greater dimensional control and stability of conductor positions within the wire bundle, which improves IDC processing. We’ve Got It Covered Wire insulation performs both mechanical and electrical functions, with the relative importance depending on the application. Insulation requirements also vary, depending on whether wire or cable applications are being considered. In addition to these factors, the chemical and thermal stability of the insulation may also be an important consideration. These performance characteristics are, of course, dependent on the materials, structure, and dimensions of the insulation. A variety of polymeric materials in various modifications are used as wire and cable insulation. Mechanical Considerations The mechanical requirements on wire and cable insulation are related to protection of the conductors against abrasion and handling stresses. Insulation hardness, flexural/tensile strength, toughness, and coefficient of friction are material properties that affect these performance characteristics. For discrete wires, the magnitude of mechanical stresses is generally less than for cable, simply due to size differential and number of conductors included. Mechanical properties, therefore, are arguably more important for cables than for wires. For small-gauge wires, the insulation may be the major contributor to the tensile strength of the wire. The mechanical properties of the insulation take on different aspects when the requirements for mechanical permanent connections are considered. For crimped connections, the ability to repeatably and reliably strip the insulation from the conductors is an important consideration. For IDC connections, the displacement of the insulation as the wire is inserted into the IDC slot is critical. Electrical Considerations There are two aspects to the electrical requirements for wire and cable insulation, depending on the frequency of the application. For low-frequency applications, the function of the insulation is primarily exactly that: insulation, i.e., the ability to ensure that the conductor cannot short against the other conductors or conductive elements in the equipment. For high-frequency applications, the dielectric properties of the insulation take on increased importance due to their effects on signal propagation speed and the characteristic impedance of the wire and cable. These characteristics depend on the dielectric constant and dimensions (particularly the thickness) of the insulation. For high-frequency applications, foamed insulations, to decrease the dielectric constant by the addition of air, are used to increase signal propagation velocities. Chemical/Thermal Considerations Chemical considerations include stability against cleaning solvents, oxidation at elevated temperatures, outgassing of additives, and flammability. These characteristics for the most part depend on the materials of manufacture. Thermal considerations include the effect of temperature on the hardness and tensile/flexural strength of the insulation and, for soldering applications, stability against short term high-temperature exposures. Because these effects depend strongly on the polymer structure, they are primarily materials dependent. Insulation Materials There are two basic classes of insulation applications: the primary insulation, applied directly on the conductor to provide electrical insulation, and secondary insulation, in the form of jackets, to provide mechanical protection of the conductors within the cable and facilitate wire handling. The requirements are similar but with different emphasis, depending on the application. Figure 4 provides an example of a jacketed cable with the primary and secondary insulations indicated. A limited selection of materials are discussed due to the broad range of insulation materials and applications. FIGURE 4: Schematic illustration of a jacketed cable indicating both primary and secondary insulations. Primary Insulations. Major primary insulation materials include polyvinylchloride (PVC), polyolefins, and fluorinated hydrocarbons. PVC is dominant in commercial and low-frequency applications, whereas polyolefins and fluorinated hydrocarbons, especially foamed versions, are used in high-frequency applications. Polyvinylchloride. The ubiquity of PVD arises from its combination of reasonable cost and good overall properties due to a broad range of formulation possibilities. The formulation may be as much as 50 percent plasticizers and additives, leading to a range of flexibility, toughness, and abrasion resistance. In addition, some PVC insulations are radiation cross linked for improved abrasion resistance. PVC has high dielectric strength and insulation resistance, but dielectric/capacitive losses limit its high-frequency potential. Depending on the formulation, PVC can cover the operating temperature range of –55 to 105°C, although –20 to 80°C is a more typical range. Polyolefins. The polyolefins include polyethylene (PE), polypropylene (PP), and copolymers of these materials. The properties of PE insulations are variable by controlling the density of the polymer. Both low- and high-density formulations are widely used. Low-density polyethylene (LDPE) is a tough, flexible polymer with a low dielectric constant, which makes it suitable for high-frequency applications, particularly in foamed forms to further reduce the effective dielectric constant. High-density polyethylenes have better abrasion resistance and toughness than LDPE and a slightly higher temperature capability of 90°C, compared to 80°. Fluorinated Hydrocarbons. Three of the major fluorinated hydrocarbons used as primary insulations are polytetrafluoroethylene (PTFE), polyfluroethylene-propylene (PFEP) and polyvinidenefluoride (PVDF). All of these materials have improved temperature capability, high electrical performance, and mechanical properties, compared to the previous materials. These advantages however, come at an increased cost, both in materials and processing, which limits their applications to those where the superior performance is required. Table 2 summarizes some of the major properties of interest for these primary insulation materials. The values given are nominals, and the manufacturer and manufacturing processes can lead to significant variations around these values. TABLE 2 Selected Propertes of Primary Insulation Materials Material Volume Resistivity (ASTM D257) (W-cm) Dielectric Constant (ASTM D150) (nominal) Voltage Breakdown (ASTM D149) (V/mil) Hardness (Shore) Abrasion Resistance Temp Range(°C) PVC, Standard 1011 7 500 D85/90 Fair 120/80 PE 1018 2.5 600 D45/60 Good -60/80 PP 1015 2.2 650 D50 Excellent -40/105 PTFE 1018 2.1 600 D60 Excellent -70/250 PFEP 1018 2.1 600 D60 Excellent -70/250 PVDF 1015 5.5 250 D70/80 Excellent -40/150 Secondary Insulations The following materials are used primarily as secondary insulation, or jackets, in cables, although they may be primary insulations in larger wire sizes, in some cases. Polychloroprene. Polychloroprene (more familiar by the trade name Neoprene™) is produced in a variety of formulations to meet different application requirements. As with PVC, the polymer may contain 50 percent additives, including reinforcers, plasticizers, antioxidants, and vulcanizing agents to promote cross linking and improve mechanical characteristics. Polychloroprenes have a desirable combination of mechanical properties, especially toughness, as well as being flame resistant and resistant to oils and solvents. Ethylenepropylene Rubber. Ethylenepropylene rubber (EPR) is a polyolefin copolymer with a good combination of electrical and mechanical properties. In addition, it is weather resistant and imperious to alkalis, acids, and many solvents. Polyurethane. Both thermoplastic and elastomer formulations of polyurethane are used as jacket materials. The electrical properties of polyurethanes are not adequate for use as primary insulations, but their mechanical properties, particularly tear strength and shock resistance, make them good mechanical insulators. Polyurethanes are also used as primary insulation when applied from a solution to provide thin insulation to small-diameter wires. Thermoplastic Elastomers. Thermoplastic elastomers (TPEs) have a block copolymer structure that provides a combination of properties and processibility, resulting in a useful jacket material. TPE has good low-temperature flexibility, a low and stable dielectric constant, and very good elongation characteristics. Polyamides. Polyamides are available in a variety of formulations that share similar properties. Advantages of polyamides in cabling include low friction coefficient, toughness, and solvent resistance. Disadvantages include moisture absorption and a reduction in cable flexibility. Table 3 summarizes some of the properties of these secondary insulation materials. Wire You Making It That Way? Manufacturing processes for wire and cable have reached a mature level of sophistication and automation, resulting in high quality at low cost. All of the current high-production methods and equipments function continuously, many with in-line process control and quality testing. TABLE 3 Selected Properties of Secondary Insulation Materials Material Volume Resistivity (ASTM D257) (W-cm) Voltage Breakdown (ASTM D149) (V/mil) Hardness (Shore) Abrasion Resistance Temp Range(°C) PCR 1011 150/600 1A50 Excellent -30/90 ERR 1017 900 A30/50 Good -40/80 PU 1011-14 500 A80 Excellent -50/80 TPE 1016 500 A95 Good -40/125 PZ 1014 450 D85 Excellent -40/120 Conductors An overview of wire conductor manufacture begins with the feedstock. The overwhelming choice is copper, sometimes alloyed for increased mechanical properties. Feedstock is drawn to the desired conductor size by repeated passes through drawing dies with appropriate annealing stages. Conductors for particular applications may be either single or multiple stranded, depending on flexibility requirements. The equivalent cross-sectional area, and therefore the current capacity and resistivity, may be built up in a wide range of stranding patterns, as discussed earlier, at a small cost penalty, when greater flexural endurance and reduced forming force are desired. Insulation Most primary conductor insulation is applied by extrusion, a low-cost method that is adaptable to a wide range of sizes, shapes, and materials. The bare conductor, or conductor bundle, is fed through the center of an extrusion die. A pump forces heated polymer through the die in a surrounding fashion so that the emerging conductor is fully coated with insulation. Multiple conductors can be fed through a specially shaped die to produce multiconductor cables. In this case, the extrusion process both insulates the conductors and locks then in to a known desired placement. Unlike other insulating methods, extrusion allows an unlimited range of cross-sectional shapes. Therefore, extruded insulations are carefully tested for conductor concentricity and minimum wall thickness. FIGURE 5: Schematic illustration of (a) shielded and (b) coaxial cable constructions. An important manufacturing consideration during extrusion is the circularity or diametral uniformity of the conductor or conductor bundle. Nonuniform conductors are more difficult to coat with a consistently uniform thickness of insulation by extrusion. For best control of insulation thickness, which is particularly important in multiconductor cables, concentric stranded or solid conductors are desirable. The effect on nonuniform insulation thicknesses can accumulate over the width of a ribbon cable and complicate termination processes, particularly for preloaded IDC connectors. Other methods for insulating conductors are solution or dip coating, tape wrapping, and braiding. Dip coating is used with polymers that can be dissolved in a solvent to produce a so-called varnish. Once coated onto the conductor and dried, such polymer coatings can be cross-linked or cured for enhanced thermal and chemical resistance. Examples of dip coated insulations include polyesters, polyimides, and polyurethanes. In addition to easy process control and low cost, major advantages to dip coating include the ability to create well controlled thin insulations and adaptability to soldering (because the thin insulations is evaporated by the soldering heat). Dip coating is most effective on solid conductors because of difficulty in extracting the carrier solvent to produce a void-free insulation over stranded conductors. Tape wrapping is a popular process for applying high-performance, non-soluble or non-extrudable dielectrics such as polyimides and fluorocarbons. In this application technique, wrappings are spirally wound onto the conductor or cable and subsequently fused or otherwise bonded to form an insulation layer. Braided insulators also consist of spirally wrapped multiple insulators, but generally fibers instead of tapes and are wrapped in an over/under pattern. A variety of materials (including cotton and linen) and braiding patterns are used for commercial wires. Braided insulations may be jacketed or overcoated with extruded coverings such as rubber for better abrasion resistance, application of identification markings, and improved handling properties. Extremely high-temperature insulation can be achieved when glass fibers are used. Shielding Conductive jackets or shields are applied to insulated conductors for two reasons. First, the conductive jacket may be a simple shield intended to provide grounding or protection from external electromagnetic interference. In addition to providing protection from external electromagnetic interference, shielding is also used to control internal electromagnetic radiation, which can cause interference with conductors or equipment if it is allowed to leave the cable. Second, shielded cable can be made to have a controlled impedance (a coaxial cable) to provide improved electrical performance in high-frequency applications. Shielded coaxial cable cross sections are schematically illustrated in Fig. 5. While all coaxial cables are shielded, not all shielded cables are coaxial. The two constructions are distinguished by more demanding tolerances on concentricity and dimensions for coaxial cable as compared to shielded cable. A Cable for All Seasons Each wire and cable application has a unique mix of cost, mechanical, electrical, and life requirements. When the volume of usage in an application grows large enough, these requirements achieve generic status, and the wire form is said to be application specific. Such application specifications include service temperature, insulation, conductor type, cable construction, and intended use. Typical examples of such categories with their most critical properties follow. Military Endurance under severe environmental stresses such as high and low temperatures, flexure, radiation, and mechanical stresses are characteristics of this class. Conductors are generally copper, with a finish to enhance corrosion resistance. Insulation materials will be selected to provide high levels of chemical and thermal stability as well as mechanical protection and support. A full range of wire sizes is used. Electrical Electrical applications include home and white goods applications. Such applications are generally high voltage (110/220 V) and relatively high currents, in the range of amperes to tens of amperes. These considerations affect the necessary electrical insulation thickness and conductor size, respectively. Cost is also an important factor, and a variety of extruded vinyls and polyethylene formulations are used. While some electrical applications involve soldering, mechanical connections dominate in these applications. Crimped connections have been the dominant permanent connection technology for many years, because most applications used discrete wires, but IDC applications are increasing steadily as discrete wires are replaced by flat cabling. Flat cable usage is increasing for several reasons, among them being enhanced wire handling, elimination of wiring errors, space savings, improved high-frequency performance, and mass termination capability using IDC technology, all of which lead to lower-cost manufacturing. Two general types of flat cable exist, ribbon cable, and flat flexible cable (FFC). Ribbon cables are generally made up of round conductors in a variety of ways, including extruded, laminated, and braided constructions, while FFC cables are generally constructed by laminating copper conductors to a flexible polymer backing using a number of technologies. Figure 6 provides examples of ribbon and FFC cables. Such cables are also used in electronic applications, primarily ribbon cable, as jumpers between subassemblies and where flexing during application is a requirement. FIGURE 6: Schematic illustration of (a) ribbon, and (b) FFC cable constructions. Electronics In electronics, cable applications far exceed those of discrete wire due to the large number of input/output signals typical of such applications. In addition to the multiconductor requirement, however, many electronics applications also require attention to electrical performance characteristics such as crosstalk and controlled impendance. When multiple conductors in a cable carry signals, the electromagnetic fields surrounding the conductors may couple into adjacent conductors, producing crosstalk, an unwanted signal in a ground conductor or distortion of a signal in an adjacent signal conductor. As the operating frequencies increase, cables and connectors must be considered as transmission lines where controlled impedance is an important consideration. Controlled impedance is necessary to minimize losses in signal amplitude through impendance mismatch and the associated signal reflections. Crosstalk. Protection against crosstalk can be provided in several ways, depending on the frequency requirements of the application. In increasing order of effectiveness, some commonly used techniques to reduce cable crosstalk include: Physical separation Twisted pair Braided shields Taped shields Solid shields The effectiveness of physical separation depends simply on the inverse square law of decreasing electromagnetic field strength. When space is available, physical separation can be effective, but recent designs with steadily decreasing conductor spacings limit the opportunities for using this technique. Twisted pair constructions are realized by simply twisting pairs of current-carrying wires around one another. This simple construction results in a cancellation of some portion of the external magnetic fields around the conductors which, in turn, reduces the coupling between conductors. A more effective method, however, is to split a signal between two conductors and invert their polarity, creating a balanced pair, which provides very good cancellation of the fields of the pair. Twisted pair, and shielded twisted pair to isolate adjacent pairs, can provide acceptable electrical performance at frequencies up to a few megahertz and a few gigahertz, respectively. In braided shields, fine-gauge conductors are braided, in a variety of configurations over the insulation. The coverage of the insulation by the braid depends on how the braiding is done and the size of the braid conductors. Braided shields are limited in their maximum frequency by the openings between braid conductors. As the application frequency increases, the electromagnetic fields can “leak” through the openings in the braid. Improving braid coverage has a negative effect on cable flexibility, which may also limit its usage. In taped shields, conductive tape (frequently metallized mylar) is wrapped aournd the insulation with some overlap of the wraps. Coverage is significantly improved over braiding. Multiple layers provide improved performance at increased cost. For very high frequencies, even the good coverage provided by taping is insufficient. In such cases, solid shields are used. Solid shields are difficult to form into harnesses and to terminate, which limits their use to applications where maximum performance is required. Controlled Impedance. For this discussion, cable for electronic applications is limited to those in which the frequencies are such that the cable must be considered as a transmission line. Coaxial cable is discussed as an example of the considerations important in such applications. Figure 5b serves as a reference for the following discussion. Two performance parameters are of major interest: the propagation velocity and characteristic impedance of the cable. Propagation velocity is important from a power transfer viewpoint, to efficiently couple power and minimize reflections. Two approximate expressions for these parameters, applicable to coaxial cable constructions, follow: The propagation velocity, Vp is given by: Where = the dielectric constant of the insulation. The characteristic impedance Z0 is given by: where = as above D = inner diameter of the outer conductor d = outer diameter of the inner conductor Note that both parameters depend on the dielectric constant of the insulation. The characteristic impedance also depends on geometric factors. The insulation material is obviously important, and low dielectric constants are desirable from a signal propagation viewpoint, which is why foamed insulations are often used to take advantage of the low dielectric constant of air. The requirement to control tolerances is also apparent. Concentricity is assumed in Eq. (2), and variations in D and d will affect the characteristic impedance. As miniaturization continues to reduce cable sizes, tolerance control becomes increasingly important. The conductor size affects one other parameter, signal attenuation. Small conductors suffer greater attenuation (due to resistive losses) than do large conductors. The correct cable choice depends on unique application requirements. Examples of controlled impedance cable constructions are provided in Fig. 7. Figure 7a illustrates antenna cable, Fig. 7b microstrip transmission line cable, and Fig. 7c a twinax cable. Automotive The combination of low voltage/high current loading with severe environmental stress, both thermal and mechanical, with demanding cost pressures makes automotive wire and cable a unique category. Wires intended for use between battery and load have large conductors but reduced temperature stresses. Under-hood applications, such as ignition harnessing and sensor leads, may be exposed to high ambient temperatures (150ºC) is a common requirement) and high mechanical stresses, particularly shock and vibration. Crimped connections are dominant in such applications. Passenger compartment applications have lower electrical and environmental requirements and are trending toward ribbon and FFC cables for ease of manufacturing. Shocking News About Cables The voltage rating of a wire or cable is determined primarily by the dielectric withstanding voltage of the insulation and its thickness. However, for high-voltage applications, geometric factors that may lead to corona discharge must also be considered. FIGURE 7: Schematic illustration of selected controlled impedance cable constructions: (a) antenna lead, (b) microstrip, and (c) twinax. The current rating of a wire or cable is generally characterized by the temperature rise at rated current. As such, it depends on Joule heating, which varies as I2 –R and, therefore, depends on the resistance of the conductor. Conductor resistance, in turn, depends primarily on the conductor material and dimensions and, to a lesser extent, on geometry. Current capacity increases with conductor cross-sectional area. For a given cross section, however, flat or rectangular conductors will have a higher current capacity than round, due to slightly greater heat dissipation and the associated effect on temperature rise with current. Heat dissipation will also depend on the insulation, both material, and thickness. The attenuation, or voltage drop, along a cable depends on the same factors.

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