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ENGINEERING AND DESIGN
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PHYSICAL CHARACTERISTICS OF THE PCB
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13.1 CLASSES OF PCB DESIGNS
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Printed circuit boards (PCBs) or printed wiring boards (PWBs) can be divided into two general classes which have common characteristics based on their end functions. These two classes have very different materials and design requirements and functions and, as a result, need to be treated differently throughout the design and fabrication processes. The first class contains analog, RF, and microwave PCBs such as are found in stereos, transmitters, receivers, power supplies, automotive controls, microwave ovens, and similar products. The second contains digital-based circuitry such as is found in computers, signal processors, video games, printers, and other products that contain complex digital circuitry. Table 13.1 lists many of the characteristics of each class of PCBs.
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TABLE 13.1 Characteristics of RF/Analog versus Digital-Based PCBs RF, microwave, analog PCB Low circuit complexity Precise matching of impedance often needed Minimizing signal losses essential Small circuit element sizes often essential Only 1 or 2 layers High feature accuracy needed Low/uniform dielectric constants needed Digital-based PCB Very high circuit complexity Tolerant of impedance mismatches Tolerant of lossy materials Small circuit element sizes desirable Many signal and power layers Moderate feature accuracy needed Dielectric constant secondary
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13.1.1 Characteristics of Analog, RF, and Microwave PCBs As can be seen from Table 13.1, the materials, design, and fabrication needs of this class of PCBs are markedly different from those of PCBs commonly referred to as digital.
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Circuit complexity is low because most components used have two, three, or four leads. This is due to the high usage of resistors, transistors, capacitors, transformers, and inductors.
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PRINTED CIRCUITS HANDBOOK
Traces, pads, and vias often act as inductors, capacitors, and coupling elements in the actual circuit. Their shapes may have a material effect on overall circuit performance. For example, the lead inductance and capacitance in a transistor collector circuit wire may act as the resonant components for an RF amplifier or it may degrade performance if it is unwanted. Figure 13.1 shows the impedance of traces as a function of their capacitance.
FIGURE 13.1 Trace capacitance vs. trace impedance, based on L0 = 8.5 nH/in. (Prepared by Ritch Tech, 1992.)
Two traces running side by side may be used to couple a signal from one circuit to another as is done in directional couplers of microwave amplifiers. (This same coupling in a digital circuit may result in a signal getting into a neighboring circuit causing a malfunction.) A series of conductors running side by side may function as a band-pass filter. Proper performance of filters, as well as most other wideband RF circuits, depends on all frequencies traveling with equal speed through the structures. To the extent that this is not true, frequencies that arrive later distort the signal being processed. This is called phase distortion.
Figure 13.2 illustrates the dielectric constant of various PCB materials as a function of frequency. Notice that some materials exhibit a dramatic decline in dielectric constant as frequency increases.The speed with which a signal travels through a dielectric is a function of the dielectric constant. Figure 13.3 illustrates signal velocity as a function of dielectric constant. From these two graphs it can be seen that using a dielectric material with a nonuniform dielectric constant in RF applications may result in severe phase distortion because the higherfrequency components arrive at the output before the lower frequencies.
A trace in a power supply circuit may be expected to carry several amps without significant heating or voltage drop. Its resistance may even be used as a sense element to detect current flow. Similarly, handling large currents with insufficient copper in a trace may result in a voltage drop that degrades circuit performance. Figure 13.4 illustrates trace resistance of a copper trace as a function of its width and thickness. Figures 13.5 and 13.6 illustrate conductor heating as a function of width, thickness, and current flow. PCBs used in consumer electronics tend to share lower circuit complexity with RF and analog PCBs. However, the performance demands are far lower. The need for the lowest possible cost offsets this. To achieve the cost objectives, every effort is made to keep all connections on a single side and to form all holes in a single operation by punching. This
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