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The Rog Blog is contributed by John Coonrod and various other experts from Rogers Corporation, providing technical advice and information about RF/microwave materials.

Selecting PCB Materials For High-Speed Digital Circuits

January 22, 2015

Digital circuits continue to conquer higher speeds, with components such as microprocessors and signal converters routinely performing billions of operations per second. True, high-speed digital circuits can be flawed by such things as impedance discontinuities in transmission lines and poor plated-through-hole (PTH) interconnections between layers on multilayer circuit boards. But they can also be hurt by less-than-ideal choices of printed-circuit-board (PCB) materials for those high-speed-digital circuits. Which leads to the question: “What are the key parameters to consider when selecting a PCB material for a high-speed-digital circuit application?”

Analog circuit designers have learned to judge PCB materials by a number of important material parameters related to performance, such as dielectric constant (Dk) and dissipation factor (Df). These material parameters can also serve as yardsticks when comparing different circuit materials for high-speed digital circuit applications. In fact, it can be helpful to understand how high-speed digital signals are related to high-frequency analog signals when considering different PCB materials for those digital signals.   

As digital applications have continued to gain in speed, some of the general-purpose PCB materials typically selected for fabricating those circuits, such as FR-4, fall short in performance for various reasons. In many ways, the demands placed on a circuit material by high-speed digital circuits and their signals are similar to what is needed from those PCB materials by analog microwave and millimeter-wave signals.

For example, a high-speed 10-Gb/s digital signal is a square-wave signal that can be viewed as a combination of different, but related, sine waves. A high-speed 10-Gb/s digital signal is comprised of different-frequency signal components, including a fundamental-frequency tone at 5 GHz, a third-harmonic signal at 15 GHz, a fifth-harmonic signal at 25 GHz, and a seventh-harmonic signal at 35 GHz (and, typically, harmonic signal components even higher than that).

Maintaining the signal integrity of a digital signal, and the sharpness of its rise and full times, is the equivalent of transferring millimeter-wave signals (the harmonics) with low loss and distortion. A PCB material capable of maintaining the signal integrity of high-speed digital signals at 10 Gb/s should also be capable of handling analog millimeter-wave signals through about 35 to 40 GHz with low loss and distortion. PCB material parameters that are critical to analog millimeter-wave circuit performance will also be important as guidelines for choosing PCB materials for high-speed digital circuits.

The PCB parameters that can be used for guidelines when choosing circuit materials for high-speed digital applications include Dk, dissipation, loss, and even dielectric thickness. The dielectric constant, Dk, of a PCB material has long been a guiding parameter for both analog and digital circuits since it is so closely related to the impedance of the circuits that will be fabricated on that material. Changes in a PCB material’s Dk, whether as a function of frequency, as a function of temperature, or for other reasons, can adversely affect the performance of broadband high-frequency analog circuits as well as high-speed digital circuits because it will change the impedances of transmission lines in unexpected ways. In particular, these unwanted changes in Dk and impedance result in distortion to the higher-order harmonics making up a high-speed digital signal, with loss of digital signal integrity. In general, PCB materials with low and stable Dk values with frequency and temperature will support high-speed digital circuits with low distortion of the higher-order harmonic signal components, as revealed by measurements with clean and clear eye diagrams for those high-speed digital circuits.

Dispersion is a PCB material characteristic closely related to Dk. All PCB materials exhibit some amount of dispersion, which refers to the change in Dk as a function of frequency. A circuit material with minimal change of Dk with frequency will exhibit minimal dispersion, a good characteristic for high-speed digital circuits. Dispersion can be caused by a number of different circuit material traits, including the polarity of the dielectric material, the loss of the material, and even how the surface roughness of the copper conductor affects the PCB material loss at higher frequencies. If a PCB material exhibits different Dk values for the different harmonic signal components comprising a high-speed digital signal, it will cause losses and even shifts in frequency for those harmonics, resulting in degradation of the high-speed digital signals.

PCB signal losses at increasing frequencies, especially at the higher frequencies needed by a high-speed digital circuit’s higher-order harmonic signal components, can suffer excessive losses to the amplitudes of those higher harmonic signals, resulting in distortion to those high-speed digital signals. As noted in many earlier blogs, losses in a PCB can come from a number of different causes, including the dielectric material and the copper conductors.

The length of a high-speed digital circuit on a PCB material can also have a great deal to do with maintaining the integrity of those high-speed digital signals. Circuit losses for any PCB material are a function of frequency and will increase with increasing frequencies. A PCB material with acceptable losses within a bandwidth closer to the fundamental-frequency tone of a high-speed digital circuit, such as 5 GHz as in the earlier example, and perhaps even with low loss at the third-harmonic signal component, such as 15 GHz, may have excessive loss at the fifth- and seventh-harmonic signal components of that high-speed digital signal. In addition, signal losses are additive with length: a signal experiencing a loss of, for example, 0.5 dB per inch at 5 GHz for the first inch of a 10-inch-long high-speed digital circuit, will suffer loss of 5 dB at 5 GHz across the length of the circuit.

Depending upon the circuit’s dielectric losses and copper conductor losses, the total loss across the length of the circuit can be considerably higher for the high-speed digital signal’s higher-order harmonic signal components than for the lower-order harmonic tones. For some circuit materials, the loss for a 10-in.-long circuit may be 10 dB or more at the fifth- and seventh-harmonic signal components of a high-speed digital signal, resulting in considerable distortion to the high-speed digital signal transferred across that PCB material.

As noted, changes in a PCB’s transmission-line impedance from changes in Dk can cause distortion in high-speed digital signals. But when working with PCBs for high-speed digital circuits, attention should be paid to physical details as well. Such things as right-angle bends in transmission lines can affect performance. A right-angle bend represents a change in the effective width of the transmission line, resulting in an impedance discontinuity, and an increase in the capacitance at that portion of the transmission line. The use of mitered 45-deg. bends can minimize the impedance discontinuity and minimize the reflections of the signal passing through that junction.

The choice of PCB material for high-speed digital circuits can be guided by the speed of those digital circuits, with such material characteristics as loss and dissipation factor (Df) targeted for lower values at higher frequencies. Circuit materials with medium to low loss are suitable for digital circuits to 10 Gb/s, while lower-loss circuit materials are usable for digital circuits to about 25 Gb/s, and circuit materials considered to exhibit extremely low loss are well suited for the fastest digital circuits, such as operating at 50 Gb/s and faster. In terms of circuit material Df, typical values might be 0.010 to 0.005 for applications to about 10 Gb/s, 0.005 to 0.003 for applications to about 25 Gb/s, and 0.0015 or less for circuit applications to 50 Gb/s and faster.

As an example, RO4003™PCB material from Rogers Corp. (www.rogerscorp.com) is a ceramic filled hydrocarbon laminate with woven glass reinforcement and a Dk of 3.38 at 10 GHz through the thickness (z axis) of the material. It offers impressive Dk consistency over frequency, and is rated for Dk variations of only ±0.05. The Df is only 0.0027 through the z axis at 10 GHz. With its low and consistent Dk value, the material has been developed for broadband analog applications through millimeter-wave frequencies and low-distortion, high-speed digital applications through 25 Gb/s. In support of those digital applications, the material features extremely tight dielectric thickness tolerance and is compatible with multilayer PCB applications.

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