PCB design can quickly become complicated. When designers optimize thousands of component placements and routes, it’s easy to miss the forest for the trees. High-frequency PCB design (typically 1 GHz and up) introduces additional complications, as signal behavior changes dramatically and the transmission medium becomes more impactful to signal integrity. High-frequency PCB design guidelines help ensure that board performance properly accounts for the differences compared to standard-frequency layouts.
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High-Frequency PCB Design Guidelines
Material Considerations
The material properties of the substrate during signal transmission have a pronounced effect on high-frequency signals. There are two primary factors to consider when selecting substrate materials:
- The dielectric constant (εr, κ, or Dk) is a relative measure of material insulation compared to a vacuum. Signals can travel faster on materials with a lower dielectric constant, as these materials are closer approximations of vacuum conditions (for signal propagation).
- The loss tangent (tan(δ)) quantifies the dissipation of energy in the substrate, i.e., energy losses originating from the signal and absorbed by the substrate during transmission. At higher frequencies, more energy dissipation produces attenuation, affecting signal trigger levels and disrupting communications.
FR4, the standard material family used for PCB manufacturing, has a below-average high-frequency response. Therefore, manufacturers and designers may wish to explore other options when optimizing performance. Ceramics and polytetrafluoroethylene (PTFE, AKA “Teflon”) exhibit lower dielectric constants and loss than typical FR4 laminates.
Pre-Emphasis
Most transmission media (e.g., copper traces, coaxial cables, etc.) impart low-pass filter characteristics to the signal – higher-frequency signals receive more attenuation than lower-frequency signals. Digital signals, with their fast rise/fall times, approximate square waves due to high-frequency harmonics present at the logic-level transition; the stable low- and high-level signals are primarily composed of low-frequency signals and largely unaffected. The low-pass characteristics during logic-switching events increase signal rise times and reshape the theoretically ideal square wave into a rhombus shape, increasing error by shrinking the time the low- and high-level signals exist between transitions. A secondary effect known as the skin effect causes current to concentrate on the surface of conductors with increasing frequency, effectively reducing the area of current flow and increasing resistance.
Pre-emphasis can combat the deleterious effects of high-frequency signal degradation due to the transmission media. By increasing the signal amplitude near the transition regions (i.e., higher absolute amplitude for the rising and falling edges), the signal scales to compensate for the low-pass response. After propagating through the low-pass media, the signal (given the proper pre-emphasis levels) appears unaffected, and the waveshape better approximates a square with sharp edges and ample time for high- and low-logic level states.
Layout Considerations for High-Frequency Boards
Decoupling capacitors will also be essential to high-frequency applications. Nanofarad-and-down capacitors filter the high-frequency response, with smaller-value capacitors capable of filtering out higher-frequency noise on power nets. These capacitors will connect in parallel to other, larger-value capacitors to form an expansive filter network that removes higher-order harmonics and other noise from the power nets for enhanced power integrity during operation. Spatially, decoupling capacitors should be as close as possible to the power net and ground pins as possible without compromising the IC fanout. If the fabrication uses a via-in-pad process, a direct drop-down to the appropriate reference plane from the pads of the decoupling capacitor is best.
While necessary in many applications, ferrite beads are far less commonplace in design than decoupling capacitors. However, they provide excellent high-frequency filtering capabilities on power nets due to their ability to block high-frequency noise originating from the power net source. The ferrite bead maintains power integrity when combined with an appropriately sized decoupling capacitor to filter low-frequency noise. Proper termination, circuit layout, and filter networks can eliminate enough noise to make the ferrite bead extraneous; ensure its performance is necessary before opting for its inclusion.
Your Contract Manufacturer Is On Your Wavelength
High-frequency PCB design guidelines ensure that the board and layout properly synchronize with the challenging physical demands of high-frequency waveforms. While all boards have to deal with noise and filtering, these challenges compound with high-frequency transmissions. If you need an experienced PCB contract manufacturer, look no further than VSE. Our engineers are committed to building electronics for our customers, including high-frequency designs. We’ve been realizing life-saving and life-changing boards for over forty years with our valued manufacturing partners.
