Key material performance parameters are used to determine suitability of PCB materials for high frequency PCB applications. These include dissipation factor (Df), and dielectric constant (Dk).
For digital circuits, and their steadily rising speed requirements, choosing the wrong PCB materials can have devastating results (such as poor plated-through holes or impedance discontinuities). Material selection based on these parameters should be considered to avoid such problems in high-speed digital circuit applications.
High-speed digital signals use a square-shaped wave made up of a combination of several sine waves comprising different frequency signal components. These types of signals include a fundamental-frequency signal, a third-harmonic signal, a fifth-harmonic signal, a seventh-harmonic signal, and a host of other odd-harmonic signals of higher frequencies. Therefore, for a digital signal to maintain its integrity and the sharpness of its rise and fall times, it is necessary to transfer millimeter-wave signals with the lowest possible loss and distortion.
For example, a PCB material capable of handling analog signals of millimeter wavelengths of 25-100 GHz, with low loss and distortion, should be able to maintain the signal integrity of a high-speed digital signal at 10 Gbps.
FR-4 is the most popular circuit board material, but it fails to perform satisfactorily for analog microwave and millimeter-wave signals due to insertion loss and distortion.
Dissipation factor (or loss tangent) of the circuit material defines its support for high-speed digital applications where insertion loss is critical.
“Mid-loss” substrates with dissipation factors of 0.010 or less cause high insertion-loss at Gbps for digital signals.“Low-loss” materials with dissipation factors of 0.005 or less are considered more suitable. However, high-speed digital designs are rising to the 25-28 Gbps range, so even better, very low-loss materials (dissipation factors in the range of 0.003 or less ) are now required. Faster speeds expected in the future, will require even better “ultra-low” loss materials with dissipation factors of 0.0015 or less.
Dissipation factor, is a critical factor to be considered when loss is an important concern and signal distortion requires minimization to preserve signal integrity. Where circuit materials are transporting mixed signals, or a combination of different technologies such as RF/microwave signals and high-speed digital signals, the materials dissipation factor indicates how complex signal routing performs. Simply put, dissipation factor is the measure of the tendency of the dielectric material to absorb energy from the waveform passing through it.
Selecting a PCB material with a low dissipation factor ensures that the high-speed circuits fabricated on that material will exhibit minimal losses. While dielectric is very important, other losses can also be significant depending on circuit configuration and thickness. One such loss is conductor loss, which depends on frequency, material dielectric constant, conductor finish, conductor thickness, and the surface roughness of the conductor.
PCB materials with low dissipation factors tend to be more expensive than materials with comparatively higher dissipation factor values. As always, there is “no-free-lunch” leaving engineers with a tradeoff between price and performance.
Dielectric constant of a PCB material affects the impedance of circuits fabricated on that material. Dielectric constant changes with frequency, temperature, and other factors, affecting performance of high-speed digital circuits as it changes the impedance of transmission lines on the PCB adversely, and in unexpected ways.
Changes in impedance and dielectric constant distort higher-order harmonics found in high-speed digital signals causing loss of digital signal integrity. Therefore, for low distortion of higher-order harmonic signal components, PCB materials with dielectric constants which are low, and stable with frequency and temperature.
Another important characteristic of PCB material’s is dispersion. Dispersion is closely related to its dielectric constant. It refers to a change in dielectric constant with frequency. All PCB materials exhibit this to some degree. Ones with minimal changes in dielectric contact with frequency, also display minimal dispersion. Low dispersion is a good characteristic for high-speed digital circuits.
Dispersion causes signal losses at higher frequencies. If these losses are at the higher-order harmonic signal components needed by a high-speed digital circuit, its higher harmonic signals may suffer excessive losses in their amplitudes, causing significant distortion of the high-speed digital signals.
The length of the track carrying a high-speed digital signal on the PCB can also affect the integrity of that signal. Signal losses increase with the length of travel of the signal. A 5 GHz signal traveling a distance of 10 inches, loses only 0.5 dB loss per inch for the first inch, but is likely to suffer a loss of 5 dB across the entire length of travel.
Total losses for the length of travel for high-speed digital signals are considerably higher for higher-order harmonic frequencies. A 5 GHz signal may only lose a low amount of the fundamental and third-harmonic signals, but experience much higher losses for the fifth- and seventh-harmonic signal components. Some PCB materials may introduce losses of up to 10 dB or greater for the fifth- and seventh-harmonic signal components of the 5 GHz signal as it traverses a 10-inch distance which would be considerable distortion to a high-speed digital signal.
Apart from the losses due to dissipation, dielectric constant, and dispersion of PCB materials, physical details can also distort high-speed digital signals. Examples include: right-angled bends in transmission lines (changing the effective width of the transmission line). This causes a discontinuity of impedance of the transmission line, resulting in an increase in the capacitance. Generally, design engineers avoid such discontinuities in the signal path by using mitered 45-degree bends or arcs to minimize reflections that high-speed signals passing through.
PCB materials are available with various performance levels, usually representing tradeoffs between price and performance. Circuit material performance primarily depends on dielectric constant, as the most important comparison parameter.
Various sizes, thicknesses, and dielectric constants are used to make material selections. Suppliers characterize their materials in terms of dielectric constant in the z-axis or in the x-y plane and at a typical test frequency (such as 1 or 10 GHz). High speed circuit design often requires channels closely matched in phase or amplitude. Manufacturers specify materials with dielectric constant tolerance across the circuit board to help designers minimize variations in performance in transmission lines and other circuit structures.
Summary
PCB substrate materials for high-speed digital signals must be able to handle signals rich in harmonic content. Maintenance of signal integrity requires the selected material to accurately reproduce the harmonic content without undue distortion. Parameters such as dielectric constant, provide guidance into the capability of the material to maintain consistent impedance characteristics which is critical to achieve high signal integrity in high-speed digital circuits.
