Several types of wire conductors are used to connect electrical systems. Common ones include single wires, twisted pairs, coaxial cable, flexible flat cable, and bundled wires. Single or multiconductor cables are feasible when few connections are needed. However, as product complexity increases, direct wiring becomes costly and unwieldy and other interconnection systems become necessary.
Flat cables, sometimes called ribbon cables, contain numerous conductors embedded side by side in flexible strips of plastic insulation. These cables can be mass terminated at low cost. Flat cables carry more current and usually weigh less than round cables having conductors of corresponding size. Nevertheless, flat cables are more commonly used to carry digital signals rather than power. In digital applications, cable qualities that affect signal degradation are more important than current ratings. Cable surge impedance, for example, should match that of connected circuits. Also, if cable capacitance is too high, it can cause crosstalk that garbles data. High-speed signals call for flat cables with low transmission loss.
Conductors in flat cable can be round or rectangular. But rectangular-wire cable costs more, so it is generally used only where cables are repeatedly flexed. Rectangular-wire cable often connects electronic modules inside enclosures with components mounted on a hinged door.
The most commonly used flat cables contain round wire on 0.05-in. spacing. Conductor sizes are awg 26 or 28, solid or stranded, or size 30 solid. Standard flat-cable connectors accommodate this range of wire sizes. Some cables also contain flexible woven-metal ground planes or woven-metal shielding. Larger conductors and bigger spacings are specified to carry more current.
Flat cable is available in almost 20 different widths, containing from 9 to 64 conductors. Grooves in the insulation delineate conductors. These grooves allow flat cables to be split into two or more segments where cables terminate at different locations.
The most common insulation is polyvinyl chloride (PVC), rated for operation in temperatures of up to 105°C. PVC-insulated cables generally connect electronic modules inside enclosures.
Some flat cable, however, has polyethylene insulation with a PVC coating. Polyethylene has lower transmission line losses than PVC. The PVC outer layer protects the polyethylene from physical damage and meets flammability requirements. Though more costly than PVC cable and limited to temperatures of 75°C, dual-insulated cables are suitable for connections outside of enclosures.
Flat cables are rather stiff transversely but quite flexible lengthwise. Tensile loads imposed on cables after installation are uniformly distributed over the wires and insulation; thus, wire size generally is selected to meet electrical rather than mechanical needs.
Flat cables with conductors on 0.05-in. spacing generally are rated at 1 A for either 150 or 300 V. The current rating is limited more by the connectors than by thermal limitations of the cable. However, flat cable rarely carries this much current or voltage. A high voltage drop is one reason. An awg 26 conductor, for example, has a resistance of approximately 40 mΩ/ft at room temperature. Thus, 1 A flowing through a 25 ft length produces a 1-V drop.
Flat cable has excellent properties for conveying high-speed digital signals. This quality is maintained in service because the separation between cable conductors remains constant.
Surge impedance is more important than current ratings for data transmission. Good impedance match between cable and connector is also important, especially for fast rise-time pulses. Surge impedance for PVC-insulated 0.05-in. cable ranges from 125 Ω for awg size 30 conductors to 93Ω for size 26. Impedance of polyethylene-insulated cable ranges from 110Ω for size 30 to about 80Ω for size 26.
Capacitance also is important. Capacitance of a signal-carrying conductor in PVC varies from 13 pF/ft for awg size 30 conductors to 15.1 for size 26. For polyethylene, the range is from 15.5 to 20 pF/ft. Signals in one flat-cable conductor may interfere with those on an adjacent conductor. This so-called crosstalk is covered by ratings generally specified for a 10-ft length in percent of the signal strength of the adjacent conductor. Crosstalk is usually specified for signals having different rise times. Measurements are made with cables connected as triplets.
Near-end crosstalk (the end where the interfering signal is introduced) for PVC cables runs from about 5 to 6% and far end from 6 to 7% with 3-ns rise-time signals. For 7-ns signals, near-end crosstalk runs from about 3 to 4%; far end about 3%. For polyethylene-insulated cables, 3-ns crosstalk runs from about 4.5 to 6% near end and far-end crosstalk from 3 to 4%. For 7-ns signals, these values are 4 to 6% and 2%.
Flexible flat cables:
Flexible circuits play an important role in the integration of electronics into consumer devices by enabling electronics to be placed in spaces too small or contorted for rigid boards. The cables can be purchased in standard quantities, sizes, and spacings in a variety of insulating materials. They can also be specified for custom applications.
Flexible circuits are typically fabricated by photographic or screening processes. Features are printed on long strips of copper foil laminated on plastic film. Unwanted copper is etched away. Holes are then pierced or drilled. A plastic cover is often applied, after which, individual circuits are stamped out of the strip.
Material selection depends on many factors. For example, many flexible circuits are bent, rolled, or folded only once. Requirements for such applications are less stringent than for circuits which flex many times. Cost also is a factor. Polyimide, random-fiber aramid, and polyester are commonly used materials.
Polyimide is the most costly but has the highest temperature rating at 250°C. Polyimide film is available in 0.5 to 5-mil thicknesses but 1 and 2-mil films are most common. Polyimide has excellent dielectric characteristics, chemical resistance, and flexibility. But it is fairly hydroscopic. Consequently, polyimide flexible circuits generally require a drying operation just prior to being soldered into place. Otherwise, absorbed water, when heated, can cause blistering and delamination.
Initial tear strength of polyimide is high. But tears in the material, once started, propagate easily. This propagation, however, can be offset somewhat by suitable design techniques.
Random-fiber aramid has a consistency more like heavy paper than plastic film. It is more hydroscopic and temperature sensitive than polyimide. But it costs about 85% less and its tear strength is higher. Random-fiber aramids used for flexible circuits are typically 2, 3, or 5 mil thick.
Polyester is the least expensive of the three, about one-third that of random-fiber aramid. It has good chemical, mechanical, and electrical characteristics, and moisture absorption is much lower than that of polyimide or random-fiber aramid. However, its maximum temperature is only 95°C. Flexible circuits made with polyester often distort and delaminate during soldering unless extreme care is used. Consequently, polyester circuits are rarely wave soldered and are typically used with insulation-piercing connectors. Film is available in thicknesses to 14 mil.
Flexible circuits are often plated to protect conductors or meet interconnection requirements. Solder is the least expensive and the most commonly used plating material. Typically, a 60/40 mixture is electroplated over all exposed conductor surfaces. When only the pads are plated, the solder is applied by rolling or dipping.
Gold-plated conductors are used for some applications. In most cases, conductors are plated with nickel prior to gold plating to prevent copper migration into the gold. But nickel is brittle and cracks when flexed repetitively. Copper migration, however, is not a problem where gold plating is at least 30 ∝in. thick.