RF Circuit Board Design

RF PCBs need to meet a range of design guidelines and techniques. These include the use of native 3D design tools, as well as electromagnetic simulation and modeling.

For example, the thickness of layers needs to be determined to achieve a target impedance value (usually 50 ohms). Different materials are used to satisfy requirements for electrical performance and thermal properties.

Materials

RF circuit boards operate at high frequencies, and as such require special materials. These materials are often referred to as RF PCB material or microwave circuit board material, and come in a range of thicknesses, dielectric constants, and impedance values. It is important to understand how these specifications relate to circuit performance, especially for RF and microwave applications.

The most common RF PCB material is FR-4, which is typically used for lower-frequency digital designs, but does not offer the best electrical performance at higher frequencies. Its dissipation factor and dielectric constant increase as frequency increases, causing significant insertion losses and varying impedance across the board. Fortunately, there are a variety of low-loss RF PCB material options available. These materials are more stable, with a lower dissipation factor and a much flatter dielectric constant than traditional glass epoxy materials.

Another consideration when selecting RF PCB material is the thermal properties. These need to be good in order to maintain the integrity of signals that are running close to each other. For this reason, a high-quality RF PCB will use multiple layers of different materials. This enables the designer to fine-tune the balance of cost, performance and thermal management.

A multi-layer RF PCB will typically include thicker Rogers core materials on the outer layers, and lower-cost epoxy glass laminates on the inside layers. Using these different layers allows the designer to create a hybrid stackup that offers the best of both worlds.

Design

A key step in RF PCB design is to determine the characteristic impedance of a particular material system and its layer thicknesses. This is done by analyzing the impedance of each trace and its width, which can be found using formulas that incorporate complex dielectric constants. This information can then be used to assign a trace width that produces a desired impedance value in the RF area.

The RF circuit board should also have a high number of ground vias to reduce parasitic ground inductance caused by current loops. It is recommended that the ground plane be placed on rf circuit board the component layer directly underneath the RF IC. This should be plated with the maximum number of via holes permitted by other layout considerations. The ground vias should also be buried as deep as possible to provide a thermal heat sink for the signal and power lines.

Signal lines that carry high-speed signals should be routed on a different layer than RF signals, to prevent coupling problems. This can be accomplished by inserting decoupling/bypass capacitors. Dedicated layers should also be used to route VCC and power supply lines. These should be capped with appropriate bypass capacitors to minimize digital noise and RF emissions. Finally, traces linking the RF components should be kept as short as possible, properly spaced and arranged orthogonally on adjacent layers.

Vias

Vias are holes through the PCB that allow for electrical interconnection between layers. There are three via types: through-hole, blind, and buried. Through-hole vias are visible from the surface of the PCB and can be used to connect all layers of the board. Blind and buried vias are not visible and can only be accessed by laser drilling, plating, and lamination. Depending on the type of via, additional processes may be required to improve either thermal performance or assembly yield. These may include epoxy hole filling, secondary solder masking, or a combination of both.

Stitching vias are important in RF PCBs because they provide a path to ground for high-speed signals. They also help to shorten the signal path between components, which can improve signal integrity. In addition, stitching vias can help reduce the cost of multilayer PCB fabrication by reducing the amount of copper required.

When designing your rf circuit board, make sure to clearly state your via requirements. Failure to do so can delay your quote, affect your bare PCB production time, or cause assembly problems during manufacturing. For example, if you specify a plated through-hole via but do not provide the correct openings in your Gerber file, the contract manufacturer may not be able to create it. This can result in a failed board or expensive troubleshooting and rework.

Stackup

The choice of PCB layers is critical for RF applications. A correct stackup helps the board distribute energy, support signals at high speeds, and eradicating electromagnetic interference. It also ensures that the circuit board is able to handle electrical currents and power. For this reason, it’s important to follow the rules of stackup design.

The standard RF PCB stackup consists of two or three copper layers with different thicknesses and dielectric properties. This type of stackup offers the best compromise between cost and performance. It also ensures a good isolation between traces and components, and provides a ground plane RF Circuit Board Supplier and power supply decoupling. In addition, it allows for a better distribution of currents.

Another important consideration when choosing a PCB stackup is the material used to create it. Generally, low-loss laminates are preferred for RF PCBs that operate above 5 GHz WiFi and Bluetooth frequencies. These materials are more expensive than standard FR4, but they offer superior results. They are also more suitable for etching and soldering.

The PCB stackup should be designed to achieve the desired characteristic impedance (Zo) and differential impedance (Zdiff) as specified by the component datasheets. This is done using a vector loop model that represents the variation in tolerances of individual dimensions. The model also includes the effects of geometric variation and dimensional variations.