How to specify a high-frequency PCB

June 23, 2021 Editor

By Linda Liu, MKTPCB

As frequency increases, the effects of PCBs gain in importance for signal fidelity. Specifying PCB material, signal power and return planes, and signal layout all play a role.

A device’s high-frequency performance relies heavily on the electrical properties of the PCB used for mounting and connecting its circuit components. The extent of the PCB design influence intensifies exponentially with an increase in operational frequency.

The laminate materials for creating high-frequency boards have specific features, like dielectric constant (Dk), loss tangent (ε_r), and coefficient of thermal expansion (CTE). High-frequency laminates use more advanced composites compared to standard FR-4 materials. Let’s investigate the various high-frequency requirements, such as PCB laminate materials, trace width and spacing, placement of planes, component interconnections, and fabrication issues with unique materials that designers and manufacturers should consider.

What is a High-Frequency PCB?
The growing complexity of electronics frequently calls for faster signal transmission rates, hence, higher transmission frequencies. High-frequency PCBs typically carry signals frequency range of about 7 GHz, making it suitable for high-speed digital designs, radio frequency (RF), microwave, and smartphone use cases.

High-frequency PCBs require special materials — a slight alteration in the r value of the base materials can significantly affect the PCB impedance. Most designers and manufacturers use the Rogers dielectric materials because they have minimal dielectric and signal losses, less circuit fabrication costs, and they work well in quick-turn prototyping applications.

Characteristics of High-Frequency PCBs
The simplest way to understand high-frequency boards is by studying their characteristics. Here are four primary features of high-frequency PCBs.

They have a smaller and stable Dielectric Constant (Dk) (ranges from 2.1 to 5) to prevent delay in signal transmission. The lower the Dk value, the higher the frequency transmission rate.
The dissipation factor (DF) is small to avoid affecting the signal transmission rate. Therefore, try to work with a DF of between 0.0019 and 0.025 for quality signal transmission.
The boards ensure the thermal expansion rate of the laminates is the same as that of the copper foil. This will prevent the separation of copper foil when there is a temperature variation.
High-frequency PCBs ensure appropriate thermal resistance, impact endurance, chemical resistance, and peel-off resistance.

How to specify an RF PCB
Designing a PCB for optimum high-frequency performance requires that you understand the effects of board laminate materials, trace spacing and width, placement of planes, component interconnections, and fabrication issues. Let’s take a closer look at these design parameters.

You can use various materials to make high-frequency circuits. Though the primary materials are FR-4, you can use other materials that provide strong electrical performance. They include low-loss RF materials such as pure polytetrafluoroethylene (PTFE), ceramic-filled PTFE, hydrocarbon ceramic, and high-temperature thermoplastic.

While FR-4 has its disadvantages when applied in high-frequency circuits, you should acknowledge those disadvantages and develop performance tradeoffs for your design. The common FR-4 limitations are:

Dk stability — which can fluctuate from portion to portion and over frequency.
Loss factor — based on surface contaminants and the hygroscopic nature of the laminates.
Ability to endure manufacturing temperatures — Lead-free manufacturing temperatures are higher than standard soldering temperatures.
Thermal transmission — Even low-power, high-frequency PCBs generate heat.

When choosing the suitable PCB material for high frequencies, you must weigh the above parameters against product cost. The choice ranges from cost-effective FR-4 materials, with more loss and loosely controlled Dk values, to FR-4 materials with higher specifications, minimal loss, and well-specified dielectric constant.

You should effectively place your traces, ground planes, and the dielectric material to align the impedance transmission line. To achieve this, you need microstrips, striplines, coplanar waveguides, and differential pairs. Note that the trace width, dielectric thickness, dielectric constant, and copper thickness influence the impedance.

Because high-frequency signals are sensitive to noise, ringing, and reflections, you should carefully design them for impedance. The typical impedance is 50 Ω for single-ended and 100 Ω for differential transmission line, with control confines of ±10%. Figure 1 shows the cross section of a microstrip.

Figure 1. A trace’s width (W), thickness (t), and height (thickness) of the substrate (h) all affect loss tangent (&epsilon_r).
Source: https://www.isola-group.com/wp-content/uploads/PCB-Material-Selection-for-RF-Microwave-and-Millimeter-wave-Designs-1.pdf

Basically, this is a circuit trace with the high-frequency signals routed on aa board’s external layer.

In a stripline (Figure 2), the circuit trace with the high-frequency signals is routed on a board’s internal layer. The trace run between low-voltage reference planes (power and ground planes) above and below.

Figure 2. In a stripline, a circuit trace resides on a PCB’s inner layer.

Placement of Planes
Most high-frequency circuits are multilayer boards. These PCBs contain several laminates discretely etched, bored, and fused. Multilayer boards increase conductor layers, minimizing the required board space over double-sided boards.

Setting up the laminates is another significant step in a multilayer PCB. The stack determines the layers the PCB will eventually accommodate. At this point, you should define the layers that carry high-speed tracks and the allocate the ground and power layers.

Coating tracks conducting high-frequency signals in the ground and power layers reduce crosstalk and maintain a check on the PCB’s impedance. The PCB cost, however, increases proportionally with the number of layers — a design tradeoff.

High-Frequency Circuits mostly have a four or six-layer FR-4 design (Figure 3). Drilled and plated-through holes connect tracks on one layer to other layers. Complex structures use blind vias to link the external layers to the internal layer(s) and buried vias to merge the inner layers. The third option is the through via that goes through all the PCB layers. To achieve better connections, it is advisable to drill and plate-through all the vias. Note that via structures affect the PCB fabrication process and the cost of the end product.

Figure 3. A typical four-layer PCB stack-up has power and ground planes embedded in the board.

Understanding the dissipation factor
A good DF should be range from 0.0019 to 0.025 to avoid hindering a signal’s transmission quality. By reducing the DF, you will experience minimal signal dissipation and high signal flow (Figure 4).

Figure 4. A graph showing how the DF value affects the signal quality. (Q) is the quality factor and (D) is the dissipation factor as a function of frequency.
Source: https://www.researchgate.net/figure/Quality-factor-Q-and-dissipation-factor-D-as-a-function-of-frequency_fig2_285415398

Proper spacing
Properly spacing your PCB components improves its functionality, especially when it comes to PCB crosstalk and skin impact. Crosstalk is an unintended electromagnetic coupling between PCB traces. Crosstalk causes an unwanted coupling impact, and you can prevent it by creating the minimum distance between the plane and trace.

On the other side, skin impact/copper surface roughness is closely related to trace resistance. An increase in the trace resistance causes an increase in skin effect, making the board warm up. Therefore, when choosing the circuit trace, ensure that its measurements will not affect the PCB when the frequencies rise.

Considering the loss tangent
You should consider the molecular design of your substrate when analyzing its loss tangent. This will help you to know the impact of the molecular structure on your board at high frequencies.

Peel off Resistance
This is the ability of the solder mask to stick to the circuit board. You can apply the peel-off resistance factor to establish the best solder mask and copper foil for your application. A suitable material should display a high peel-off resistance. By selecting such materials, your copper foil will stick to the board for an extensive period. The peel-off resistance is essential when designing a High-Frequency circuit for high-temperature use cases.

Thermal expansion
This is the impact temperature leaves on the size of your board substrate. Each substrate displays its own temperature features, including the CTE. When choosing your PCB materials, ensure they have matching CTEs. Non-conforming CTEs can make your PCB substrate components expand at different rates during operation.

Conclusion
A High-Frequency PCB plays a vital role in applications experiencing high heat resistance. Creating a PCB design that follows international standards will ensure your High-Frequency circuit functions well, even in adverse conditions. As you have seen from the above discussion, there are several essential aspects of a High-Frequency board and the design process. To this end, we hope you can design a quality High-Frequency board using the tricks you have learned from this article.

Numerous companies produce and supply High-Frequency circuits, but few of them observe the international manufacturing standards. Always source your materials from reputable players no matter the price differences.

Linda Liu is the overseas marketing manager at MKTPCB, a leading PCB manufacturer that offers high-quality PCB products and services. Since 2012, she has established “first-of-its-kind” industry-changing and transformational businesses initiatives that increased revenue growth, brand exposure and market expansion for MKTPCB. Linda graduated from Western University with a bachelors degree in marketing.