What is RF PCB Design?
RF PCB design refers to the process of designing and laying out printed circuit boards (PCBs) specifically for high frequency and wireless applications that use radio frequency (RF) signals. RF PCBs are used in a wide range of products including cell phones, WiFi routers, Bluetooth devices, radars, and wireless communication equipment.
Designing PCBs for RF is more challenging than regular PCB design because the high frequencies used introduce various signal integrity issues and electromagnetic interference (EMI) that must be carefully mitigated. This requires specialized RF design techniques, component selection, board materials, and manufacturing processes to ensure optimal performance, reliability, and regulatory compliance.
Key Considerations in RF PCB Design
There are several critical factors that must be considered when designing PCBs for RF applications:
- Signal integrity – maintaining clean, undistorted signals
- Impedance control – matching impedances to minimize reflections
- EMI/EMC – minimizing radiated and conducted emissions and susceptibility
- Power integrity – providing clean, stable power to active components
- Thermal management – dissipating heat to prevent overheating
- Mechanical requirements – fitting the PCB into the enclosure and passing environmental tests
- Manufacturing constraints – ensuring the PCB can be reliably and cost-effectively produced
Failing to properly address any of these areas can severely degrade the performance of the end product. Therefore, following RF design best practices is crucial.
RF PCB Material Selection
One of the most important decisions in RF PCB design is selecting the right dielectric material for the substrate. The substrate material largely determines the electrical properties of the PCB at high frequencies.
Key Properties of RF Substrates
The most critical properties to consider when choosing an RF substrate are:
- Dielectric constant (Dk) – affects the impedance and wavelength
- Dissipation factor (Df) – determines the dielectric losses
- Thermal coefficient of Dk – affects impedance stability over temperature
- Moisture absorption – impacts Dk stability in humid environments
- Thickness – determines impedance and mechanical strength
In general, RF substrates should have a stable dielectric constant across the frequency range of interest, low dissipation factor to minimize losses, and low moisture absorption for environmental stability. Thicker substrates provide better mechanical strength but make impedance control more difficult.
Common RF Substrate Materials
Here are some of the most commonly used RF substrate materials and their typical properties:
Material | Dk | Df | Thermal Coefficient of Dk (ppm/°C) | Moisture Absorption (%) |
---|---|---|---|---|
FR-4 | 4.5 | 0.02 | +400 | 0.15 |
Rogers 4003C | 3.38 | 0.0027 | +40 | 0.06 |
Rogers 4350B | 3.48 | 0.0037 | +50 | 0.05 |
Rogers RT/duroid 5880 | 2.20 | 0.0009 | -125 | 0.02 |
Rogers RO4000 | 3.38-3.48 | 0.0027-0.0037 | +40 to +50 | 0.06 |
Isola I-Tera MT40 | 3.45 | 0.0031 | -11 | 0.10 |
PTFE/Teflon | 2.1 | 0.0002 | -400 | Nil |
Polyimide | 3.5 | 0.002 | -300 | 1.20 |
FR-4 is the most common and lowest cost material but has the worst RF properties. PTFE has excellent properties but is expensive and difficult to manufacture. The Rogers materials offer a good balance of performance and cost for most applications.
RF Component Selection and Placement
Choosing the right components and placing them optimally on the PCB is critical for RF performance. Passive components like resistors, capacitors, and inductors used in RF designs must be specifically rated for high frequencies. Capacitors should have low equivalent series resistance (ESR) and inductors should have high self-resonant frequency (SRF) and quality factor (Q).
Active RF components like power amplifiers and low-noise amplifiers are also specially designed for high frequency operation. They often have unique packages with thermal pads and exposed grounds to provide low inductance grounding and heat dissipation.
Component placement is just as important as selection. In general, RF components should be placed as close as possible to the device pins they are connected to in order to minimize trace lengths and parasitic inductance. Decoupling capacitors in particular must be placed very close to the power pins. Ground return paths must also be carefully considered.
All component footprints should be carefully reviewed to ensure they match the manufacturer’s recommended land pattern. Improper footprints can cause solderability and reliability issues.
Common RF Components and Their Functions
Component | Function |
---|---|
Power amplifier | Amplifies the RF signal to the desired output power level |
Low-noise amplifier | Amplifies weak RF signals while minimizing added noise |
Mixer | Converts signals between different frequencies |
Oscillator | Generates a reference frequency for the system |
Bandpass filter | Allows signals in a certain frequency band to pass while rejecting others |
Balun | Converts between balanced and unbalanced signals |
RF switch | Routes RF signals between different paths |
Directional coupler | Couples a defined amount of the signal to another path |
Power divider/combiner | Splits or combines RF signals with specific power ratios |

RF PCB Stackup Design
The PCB stackup refers to the arrangement of copper and dielectric layers in the board. Proper stackup design is crucial for maintaining the desired impedance of the transmission lines and minimizing crosstalk and EMI.
Best Practices for RF PCB Stackups
- Use dedicated signal and ground layers to create controlled impedance traces
- Provide ground reference planes adjacent to each signal layer
- Use thicker dielectrics between signal layers to reduce crosstalk
- Keep power and ground planes close together to create bypass capacitance
- Use multiple ground vias to provide low-inductance paths for return currents
- Avoid splitting ground planes as this can cause return path discontinuities
- Minimize use of vias as they create impedance discontinuities and radiate EMI
Here is an example 6-layer RF PCB stackup:
Layer | Material | Thickness (mils) |
---|---|---|
Top copper | Copper | 1.4 |
Prepreg | FR-4 | 6 |
Ground plane | Copper | 1.4 |
Core | FR-4 | 47 |
Power plane | Copper | 1.4 |
Prepreg | FR-4 | 6 |
Bottom copper | Copper | 1.4 |
This stackup provides 50 ohm controlled impedance traces on the outer layers referenced to the adjacent ground and power planes. The thick core layer reduces broadside coupling between the planes.
Transmission Lines for Controlled Impedance
Transmission lines are PCB traces designed to carry high frequency signals with controlled impedance to prevent reflections. The characteristic impedance of a trace is determined by its geometry and the surrounding dielectric properties.
Types of Transmission Lines
- Microstrip – a trace on the outer layer above a ground reference plane
- Stripline – a trace embedded between two ground reference planes
- Coplanar waveguide – a trace with ground planes on both sides on the same layer
- Grounded coplanar waveguide – coplanar waveguide with a ground plane below
For most applications, microstrip and stripline are the most commonly used transmission line types as they are simpler to fabricate. Coplanar waveguides have lower dispersion but are more difficult to route.
Calculating Transmission Line Impedance
The characteristic impedance of a transmission line can be calculated using formulas that take into account the trace width, dielectric thickness, and dielectric constant. Here are the formulas for microstrip and stripline:
Microstrip:
Z0 = (87/sqrt(εr+1.41)) * ln(5.98h/(0.8w+t))
Stripline:
Z0 = (60/sqrt(εr)) * ln(4h/(0.67π(0.8w+t)))
Where:
– Z0 = characteristic impedance in ohms
– εr = dielectric constant
– h = dielectric thickness in mils
– w = trace width in mils
– t = trace thickness in mils
These formulas assume the trace thickness is much less than the width and dielectric thickness, which is usually the case. More precise calculations that account for this can be done using field solvers.
Most PCB design tools have built-in impedance calculators that will automatically determine the required trace width for a given stackup and impedance.
Trace Width and Spacing Guidelines
Here are some general guidelines for sizing and spacing RF PCB traces:
- Use 50 ohm impedance for most RF traces
- Avoid using traces narrower than 5 mils as they can be difficult to manufacture
- Maintain at least 3x the dielectric thickness between parallel traces to minimize crosstalk
- Provide at least 20 mil spacing between traces and board edges to prevent excess fringing
- Use curved or 45°; bends instead of 90°; bends to minimize impedance discontinuities
- Avoid routing traces under components or vias to prevent parasitic coupling
Power Integrity Considerations
Maintaining power integrity is critical in RF designs as noise on the power supply can mix with the RF signals and cause intermodulation distortion and other issues. The power distribution network (PDN) must provide a low impedance path for both DC and high frequency current.
PDN Design Best Practices
- Use large, unbroken power and ground planes to minimize DC resistance and inductance
- Place decoupling capacitors close to ICs to provide a local high frequency current reservoir
- Use multiple capacitor values (decade spacing) to decouple a wide frequency range
- Connect decoupling capacitors to power/ground with short, wide traces and multiple vias
- Minimize power plane cuts and necks that can increase inductance
- Isolate noisy digital circuitry from sensitive analog/RF circuitry using separate power islands
- Filter power inputs to the PCB using a PI filter (inductor-capacitor)
Decoupling Capacitor Selection
Choosing the right decoupling capacitors is key to providing a low impedance over the frequency range of interest. The main parameters to consider are:
- Capacitance value – determines the impedance at lower frequencies
- Equivalent series resistance (ESR) – determines the impedance at the resonant frequency
- Equivalent series inductance (ESL) – limits the effective frequency range
- Voltage rating – must exceed the maximum voltage on the power rail
Ceramic capacitors are most commonly used for decoupling as they have low ESR and ESL. Smaller package sizes like 0201 and 01005 provide the lowest inductance. Typical decoupling values used are 0.1uF, 0.01uF, and 1000pF with voltage ratings of 16V, 25V or 50V.
Thermal Management Techniques
RF components like power amplifiers dissipate a significant amount of heat that must be effectively removed to prevent overheating and damage. The PCB design plays a key role in transferring heat away from these components.
PCB Thermal Design Best Practices
- Place thermal vias under component thermal pads to transfer heat to internal copper planes
- Use large ground planes on outer layers to spread heat across the board
- Connect multiple ground planes together with a dense via array to increase thermal conductivity
- Use thick copper planes (2oz or more) to improve heat spreading
- Provide an unbroken copper pour around hot components to act as a local heatsink
- Place hot components near board edges or corners to maximize convective cooling
- Avoid placing hot components near temperature-sensitive components like crystals
Thermal Via Design
Thermal vias are used to transfer heat from the top side of the PCB to the internal or bottom side planes. They should be placed in a grid pattern under the device thermal pad. Here are some guidelines for thermal via design:
- Use as many vias as will fit under the thermal pad (typically 20-100)
- Via hole size should be as large as possible while still meeting manufacturing constraints
- Vias should have a plating thickness of at least 1 mil to minimize thermal resistance
- Vias should extend through all available copper planes for maximum heat spreading
- Use thermally conductive via fill material if available
Electromagnetic Compatibility (EMC) Design
Meeting EMC requirements is a critical part of RF PCB design. Emissions radiated by the PCB must be below the limits set by regulatory agencies like the FCC. The PCB must also have sufficient immunity to external electromagnetic interference (EMI).
EMC Design Best Practices
- Minimize current loop areas by placing decoupling capacitors close to ICs
- Provide a continuous ground plane on all layers to minimize radiated emissions
- Route high speed signals on inner layers between ground planes to contain emissions
- Avoid routing high speed traces near board edges where they can radiate
- Terminate unused IC pins to ground or power to prevent floating inputs
- Use shielding cans over sensitive RF components to prevent interference
- Filter all incoming/outgoing signal and power lines using ferrite beads and/or capacitors
- Provide ESD protection on all external connectors using Tvs Diodes or similar devices
Grounding and Shielding Techniques
Proper grounding and shielding is essential for good EMC performance. Here are some key techniques:
- Use a solid ground plane on the bottom layer to provide a low-impedance return path
- Connect ground planes on different layers together with a dense via array
- Use stitching vias along the edges of the board to provide a continuous grounding path
- Provide a series of closely spaced vias (via fence) around the perimeter of RF compartments to isolate them
- Use shielding cans over sensitive RF components with the cans soldered to the ground plane
- Provide a low-impedance connection between the PCB and chassis ground using finger stock or conductive gaskets
- Route noisy traces like clocks and switching power supplies away from sensitive analog/RF traces
Design for Manufacturing (DFM)
Designing an RF PCB that meets performance requirements is only half the battle – it must also be manufacturable. Involving the PCB fabricator and assembler early in the design process can help identify and resolve any potential manufacturability issues.
DFM Guidelines for RF PCBs
- Use standard PCB materials and thicknesses whenever possible to reduce cost and lead time
- Minimize the number of unique hole sizes and plated through-hole components
- Provide adequate spacing between components for assembly clearance
- Avoid blind and buried vias as they increase cost and decrease yield
- Use surface mount components whenever possible as they are easier to assemble
- Provide fiducials on the PCB for machine vision alignment
- Add test points for key signals to facilitate debugging and testing
- Follow the IPC guidelines for annular rings, solder mask expansion, and other manufacturing allowances
Panelization Considerations
PCBs are typically manufactured in panels containing multiple copies of the same design. Panelization can impact RF performance due to the additional coupling between boards. Here are some panelization best practices:
- Use a solid copper border around each board in the panel to minimize coupling
- Route scoring lines between boards to make depaneling easier
- Avoid placing sensitive RF components near the scoring lines
- Add mouse bites or V-grooves along the board edges for easy snap-out
- Place test coupons in the panel for impedance and material property verification
PCB Layout and Routing
Once the schematic is complete and the stackup and component placement are defined, the final step is to lay out the PCB and route all the traces. RF PCB layout and routing requires careful attention to detail to maintain signal integrity and prevent electromagnetic interference (EMI).
RF PCB Layout Best Practices
- Minimize trace lengths, especially for high-speed signals like clocks and RF
- Route traces on appropriate layers based on their function (e.g. power on power layer, signals on signal layer)
- Provide ground reference planes adjacent to signal layers
- Avoid routing traces through gaps in the ground plane as this creates a discontinuity
- Use 45° bends instead of 90° bends to minimize reflections
- Keep traces as straight as possible and avoid unnecessary bends
- Avoid routing traces under components as this can cause parasitic coupling
- Provide adequate clearance between traces and components/vias
- Use differential pair routing for high-speed digital signals
- Minimize the use of vias, especially for RF signals
- Follow the manufacturer’s recommended PCB layout for RF components and modules
Routing Techniques for RF Signals
RF signals require special consideration during routing to maintain signal integrity and minimize losses. Here are some key techniques:
- Use microstrip or stripline transmission lines for controlled impedance
- Keep RF traces as short as possible to minimize losses
- Avoid using vias for RF signals as they create discontinuities
- Use ground stitching vias along RF traces to provide a continuous ground reference
- Avoid routing RF traces near board edges where they can radiate