RF PCB Stackup Guide

Key Takeaways

• Proper RF PCB stackup design is essential for maintaining signal integrity, reducing crosstalk, and minimizing electromagnetic interference (EMI) in high-frequency analog designs. 

• Decoupling and signal integrity are crucial considerations in RF PCB stackup design. Incorporating internal power, ground planes, and adjacent placement provides effective decoupling and supports power integrity for high-speed digital interfaces.

• Material selection plays a significant role in RF PCB stackup design. The chosen material should exhibit low signal losses, heat dissipation capabilities, and consistent performance across a wide frequency range.

RF PCB stackups are almost always multilayer, with at least four and frequently more layers.

Engineering an intelligent RF PCB stackup is crucial for ensuring the integrity of high-frequency analog signals, preventing crosstalk, and minimizing electromagnetic interference (EMI). Whether it's a complex or simple project, designing a good stackup definition can greatly enhance your RF circuit's long-term reliability, performance, and cost-effectiveness. We’ll discuss best practices, examples, and material selection in engineering your RF PCB Stackup.

RF PCB Stackup General Practices 

RF PCB stackup design ensures signal and power integrity, especially in RF designs where clean analog signals are essential. The isolation between critical circuit blocks is a primary concern in RF PCB stackup design, as it prevents interference between different RF regions and ensures the separation of analog and digital signals. This requires careful consideration of both layout and stackup design. 

Another important aspect is decoupling, which involves providing sufficient decoupling between power and ground planes to maintain power integrity. When designing an RF PCB stackup, it is recommended to use at least a 4-layer configuration with internal power and ground planes. Placing the power and ground planes in adjacent internal layers allows for effective decoupling and power integrity, while components can be conveniently placed on a surface layer adjacent to the ground plane. 

The chosen PCB stackup significantly influences the accessibility of power and ground connections in the layout and affects the available space for routing signals. Moreover, the RF PCB material must meet specific requirements for high-frequency operation, including low signal losses, the ability to dissipate excess heat generated by the board, and consistent performance across a wide frequency range.

Planning Your RF PCB Stackup

When designing an RF PCB stackup, the layer count is an important consideration. Low-layer count boards are typically inadequate for RF PCBs as they lack sufficient space for power, ground, and signal layers while meeting target impedance requirements. Therefore, a higher layer count board is necessary.

For 2-layer boards, the limited space makes it impractical to accommodate printed circuits and impedance-controlled traces unless all routing is done as coplanar lines. This restriction poses challenges for RF signal placement and routing, making 2-layer boards unsuitable for RF PCB designs.

On the other hand, 4-layer boards are slightly better. In this configuration, all RF signals and components can be placed and routed on a single layer, while the bottom layer can be utilized for power, digital signals, and other peripherals. Alternatively, the bottom layer can be used for lower-frequency printed RF circuits. If two signal layers are required, the two internal layers can be designated for placing ground (GND) to provide isolation and impedance control on both layers.

For optimal RF PCB performance, six or more layers are often recommended. This stackup allows for controlled impedance routing on the outer surface layers while the plane layers provide shielding between signal layers. Additionally, placing the power and ground planes adjacent to each other on the same side of the core enhances decoupling thanks to increased interplane capacitance. To ensure a consistent reference potential throughout the board, the ground planes should be interconnected using stitching vias.

RF PCB Stackup Examples

It is important to avoid the mistake of closely coupling the planes in the center of the PCB with large dielectrics between the signal layers and planes. While this configuration may enhance interplane capacitance, it does not contribute to signal integrity, crosstalk reduction, or electromagnetic compatibility (EMC). This is why using a 4-layer board for your stackup is recommended at a minimum.

To enhance the EMC performance of a four-layer board, it is recommended to position the signal layers as close as possible to the planes, ideally with a spacing of less than 10 MIL. Additionally, using a large core with a thickness of approximately 40 MIL between the power and ground planes while keeping the overall substrate thickness at around 2 MIL is advisable. This close coupling between the traces and planes will minimize crosstalk between traces and help maintain acceptable impedance levels.

The RF components and traces are positioned on the top layer, followed by a ground plane and a power plane. On the bottom layer, all non-RF components and traces are placed. This configuration ensures minimal interference between RF and non-RF elements. The presence of the adjacent ground plane enables a direct and efficient path for the ground return current.

The concept behind this PCB stackup is to facilitate a mixed-signal design approach by incorporating digital signals on two signal layers and RF signals on another signal layer. Placing the RF signal on the top layer allows 50 Ohm impedance routing with narrow trace widths and complete isolation from the digital sections. The bottom layer can be utilized for controlled impedance differential pairs catering to digital interfaces. Ultimately, the power/ground plane combination ensures the necessary decoupling for power integrity, effectively supporting high-speed digital interfaces.

Ensuring Your RF PCB Stackup Retains Signal Integrity

Analog components are susceptible to noise, which can propagate into the input and affect downstream signals. This sensitivity arises from their operation in the linear regime. In contrast, digital components operate at saturation, requiring larger noise levels to cause errors. Proper stackup design and layout ensure isolation, preventing interference between high-frequency analog signals, low-frequency analog signals, and digital signals. In high-speed PCB designs, minimizing signal loss is crucial. Placing high-frequency signal planes adjacent to ground planes, placing power planes adjacent to ground planes for enhanced decoupling, and routing low-frequency signals on non-critical layers help optimize signal integrity.

A Note on RF PCB Grounding

For each RF layer, it is advisable to have a dedicated ground plane. This ground plane is positioned directly below the corresponding layer to ensure the shortest possible current flow path.

Furthermore, the ground plane should be continuous without any breaks. The presence of breaks in the ground plane can create shorter paths for the current to return, which is undesirable. Therefore, maintaining the integrity and continuity of the ground plane is of utmost importance.

Material Selection for Your RF PCB Stackup

RF PCBs is specifically manufactured using materials that meet the requirements for high-frequency operation. These materials must exhibit low signal losses, maintain stability throughout high-frequency operations, and possess the ability to dissipate heat efficiently. Consistency in the values of dielectric constant (DK), loss tangent (tan δ), and coefficient of thermal expansion (CTE) over a wide frequency range is also crucial. Typically, the dielectric constant values for these boards range from 3 to 3.5, while the loss tangent values fall between 0.0022 and 0.0095 within the frequency range of 10-30 GHz.

In terms of substrate material, it is recommended to utilize polytetrafluoroethylene (PTFE) or ceramics. During high-speed PCB operations, these materials should demonstrate consistent dielectric constant, loss tangent, and CTE characteristics. Ceramic substrates such as Rogers are particularly well-suited for RF applications among the various options. However, it's important to note that these ceramic substrates are more fragile, often contain toxic components, and as a result, are considerably more expensive to manufacture. 

Consequently, there is a considerable incentive to utilize FR4 as the substrate material for its cost. Still, it is worth noting that FR4 has a speed limit of around 10 GHz due to attenuation. Achieving reliable operation at or near 10 GHz can be challenging and depends on factors such as the specific type of FR4 used and the capabilities of the fabrication house. Similarly, Rogers substrates produced by low-quality fabricators may underperform compared to FR4 from high-quality fabricators.

Take your RF PCB stackup design to the next level with Cadence AWR software. With its powerful simulation and analysis capabilities, Cadence AWR software provides the tools to optimize signal integrity, minimize crosstalk, and ensure EMC compliance in your RF designs. From stackup planning to impedance control, Cadence AWR software offers a comprehensive solution for achieving high-performance RF PCB stackups.

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