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PHYSICAL CHARACTERISTICS OF THE PCB
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FIGURE 13.2
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Dielectric constants versus frequency of various PCB materials.
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FIGURE 13.3 Signal velocity as a function of dielectric constant. (Prepared by Shared Resources, Inc., 1991.)
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eliminates both drilling and plating. The substrate material system is often resin impregnated paper, the lowest-cost substrate system for electronic packaging. Summarizing, successful RF and analog design depends heavily on the properties of the materials used and on the physical shapes of the conductors and their proximity to each other
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FIGURE 13.4 Trace resistance vs. trace width and thickness. (Prepared by Ritch Tech.)
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FIGURE 13.5
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Temperature rise vs. current for 1-oz copper.
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rather than on the ability to handle very large numbers of circuits simultaneously. Hand routing or connecting of the individual parts coupled with manipulating the shapes of individual copper features are essential parts of this design process. For these reasons, the design tools and design team must be chosen to meet these needs. Physical layout tools that provide convenient graphical manipulation of PCB shapes are a must.
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13.1.2 Characteristics of Digital-Based PCBs Compared to RF and analog PCBs, digital-based PCBs have complex interconnections, but are tolerant of rather wide feature size and materials variations.
PHYSICAL CHARACTERISTICS OF THE PCB
FIGURE 13.6
Temperature rise vs. current for 2-oz copper.
They are characterized by very large numbers of components, often numbering in the hundreds and sometimes the thousands. Digital components often have large numbers of leads, as high as 400 or more. This high lead count stems from logic architectures that have data and address buses as wide as 128 bits or more. To connect PCBs with these wide data buses, digital systems often contain board-to-board connectors with as many as 1,000 pins. Digital circuits have increasingly fast edges and low propagation delays to achieve faster performance. Edge rates as fast as 1 ns are now encountered in devices destined for products as common as video games. Table 13.2 lists edge speeds of some commonly used logic families, edge rate being the time required for a logic signal to switch from one logic level to the other (switching speed). Propagation delays, the time required for a signal to travel through a device, are decreasing along with edge rates.
TABLE 13.2 Typical Logic Family Switching Speeds Logic family STD TTL ASTTL FTTL HCTTL 10KECL BICMOS 10KHECL GaAs Edge speed, ns 5.0 1.9 1.2 1.5 2.5 0.7 0.7 0.3 Critical length, in 14.5 5.45 3.45 4.5 7.2 2.0 2.0 0.86
These fast edges and short propagation delays lead to transmission line effects such as coupling, ground bounce, and reflections that can result in improper operation of the resulting PCB. Table 13.2 illustrates the degree to which a fast switching signal will couple into a neighboring line as a function of the edge-to-edge separation and the height of
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the signal pair above the underlying power plane. The critical length listed in Table 13.2 is the length of parallelism between two traces at which the coupling levels in Fig. 13.7 are reached.
FIGURE 13.7
Trace-to-trace coupling. (Prepared by Shared Resources, Inc.)
The digital circuits themselves are designed to function properly with input signals that vary over a relatively wide range of values. Figure 13.8 illustrates the signal levels for a typical logic family, in this case ECL. The smallest output signal from an ECL driver is the difference between VOLmax and VOHmin or 0.99 V. The smallest input voltage to a device at which the logic part is designed to work properly is the difference between VILmax and VIHmin or 0.37 V. The difference between these two levels, the noise margin of 0.62 V is available to counteract losses in the wiring and the dielectric and from other sources such as coupling and reflections. From this it can be seen that digital logic has a high tolerance of losses and higher immunity to noise. This tolerance of noise and losses makes it possible to have trace features and base materials that introduce substantial losses and distortion while still achieving proper operation. It is this relatively high tolerance of distortion that makes it possible to manufacture economical digital PCBs. Summary. The large number of connections in digital PCBs generally requires multiple wiring layers to successfully distribute power and interconnect all the devices. As a result, the design task is heavily weighted on the side of successfully making many connections in a limited number of routing layers while obeying transmission line rules. The base materials need to have characteristics that result in a PCB that is economical to fabricate and able to withstand the soldering processes while preserving high-speed performance. Compared to RF PCBs, losses in the dielectric tend to be of little concern for digital PCBs. The actual shapes of conductors, pads, holes, and other features have little effect on performance. (For detailed treatment of these topics, see Howard W. Johnson and Martin Graham, High Speed Digital Design: A Handbook of Black Magic, Prentice-Hall, New York, 1993.) The PCB design system and the design skill set for digital PCBs must be optimized to ensure accuracy in making large numbers of connections while successfully handling the high speed requirements of the system.Achieving this in a reasonable amount of time demands the use of a CAD system that contains an automatic router for use in connecting the wires.
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