Introduction
As electronic devices push to faster switching speeds and higher frequencies, PCB designers face greater challenges. Printed circuit boards serving RF, microwave and high-speed digital applications require specialized design practices to ensure signal integrity and avoid unintended radiation.
This article provides an in-depth guide to PCB design for high frequency applications covering:
- PCB materials selection criteria for high frequency
- Component selection and layout considerations
- Routing techniques for high frequency signals
- Smart component placement guidelines
- Critical high speed layout strategies
- Stackup design for high frequency boards
- Modeling and simulation best practices
- Example multi-GHz PCB design walkthrough
- Prototyping and design validation recommendations
- Guidelines for designing testability
- Common high frequency design pitfalls to avoid
By mastering these PCB design principles, electrical engineers can fulfill the exacting demands of cutting-edge wireless, telecom, defense and digital systems operating above GHz frequencies.
PCB Material Selection Considerations
Selecting the optimal PCB substrate is the foundation of any high frequency layout. Key material selection criteria include:
- Permits faster signal propagation speed
- Reduces cross-talk between tightly routed traces
Controlled Dielectric Thickness
- Consistent thickness avoids electrical discontinuities
- Thinner dielectrics improve impedance control
Low Loss Tangent
- Reduces signal loss and distortion
- Select materials tested through mmWave frequencies
Tighter Dielectric Tolerances
- Minimizes impedance variability from material variations
- ±5% to ±10% dielectric tolerance common
Thermal Stability
- Maintains stable electrical properties over temperature
- Reduces impedance shifts during operation
Moisture Resistance
- Prevents electrical performance degradation
- Requires materials with low moisture absorption
Advanced PCB materials like Rogers or Taconic RF laminates offer the essential properties needed for designing high frequency PCBs.
Component Selection and Layout
The first step in any successful high frequency PCB layout is component selection and placement planning:
Select Components Rated for High Frequency
- Review datasheets to confirm HF suitability
- Beware of marginal components not fully characterized
Choose Component Packages with Low Inductance and Parasitics
- Avoid long leads
- Favor low-profile SMT packages
- Be mindful of parasitic capacitance
Position Noise-Sensitive Components Judiciously
- Keep away from high-speed lines and interfaces
- Provide shielding if needed
Locate Components for Short Routing
- Place components with high-speed interactions nearby
- Minimize overall trace lengths
Getting the right components in the right locations from the start enables optimum routing.
High Frequency Routing Techniques
With components placed, connecting them demands precision routing:
Impedance Control
- Use impedance calculators to set trace width/spacing
- Account for reference plane proximity
- Maintain consistency across matching nets
Minimize Vias
- Each via adds inductance degrading high frequency response
- Route critical traces on same layer if possible
Eliminate Right Angles
- Use 45° beveled corners instead
- Reduces reflections and ringing
Symmetric Routing
- Match routing for differential pairs
- Controls skew within pair
Shielding
- Enclose critical signals between ground planes
- Adds ground guard traces to isolate noise
Bypass Capacitors
- Sprinkle bypass caps near components
- Suppress noise and transients
Strict adherence to sound routing practices prevents signal degradation.
Component Placement Guidelines
Meticulous component placement is mandatory:
Bypass Capacitors
- Place immediately adjacent to power pins
- Use multiple capacitors for wide frequency range
Decoupling Capacitors
- Surround ICs with interspersed capacitors
- Different values target various frequencies
Voltage Regulators
- Position adjacent to power-hungry ICs
- Minimizes IR drops through board
Crystals and Oscillators
- Locate near IC with short traces
- Adds ground guard traces for isolation
Connectors and Interfaces
- Place at board edge with clear routing paths
- Avoid antennas, sensitive components
EMI Filters
- Insert strategically to dampen emissions
- Often place ahead of connectors
Every component on a high frequency PCB influences signal integrity and must be scrutinized.
Critical High Speed Layout Strategies
In addition to individual routing practices, overarching layout strategies are mandatory:
Partitioning
- Segregate board into zones
- Digital, analog, RF, antenna, high speed areas
Symmetrical Architecture
- Match component placement
- Maintain uniform shape and routing
Short Interconnections
- Keep overall routing compact
- Eliminate excess stubs
Termination
- Strategically terminate lines
- absorbs incident wavefronts
Ground Fill Connectivity
- Maximize ground pour connectivity
- Avoid ground islands
Layer Usage
- Use layers judiciously based on needs
- Transition across layers intelligently
Test Points
- Include coaxial test points
- Facilitate validation and troubleshooting
Every layout technique applied should serve the singular goal of signal integrity.
PCB Stackup Design
For high frequency boards, the layer stackup itself requires special attention:
Thinner Dielectrics
- Enables fine features and lines
- Tighter spacing and impedance control
More Layers
- Permits enclosure of critical nets
- Dedicated power and ground layers
- Low impedance returns beneath traces
Buried and Blind Vias
- Provides isolation between layers
- Avoids stubs from unused vias
Dielectric Selection
- Use consistent dielectric material throughout
- Important for homogenous properties
Differential Routing
- Cores with thicker dielectrics
- Thinner dielectrics above and below
- Centers differential lines for consistency
Embedded Passives
- Integrate capacitance within layers
- Provides localized decoupling
The cross-section design choices ultimately dictate attainable miniaturization and performance.
Modeling and Simulation
Applying modeling and simulation techniques prevents surprises:
** Material Property Simulation**
- Model dielectric constant, loss tangent and characteristics
** Transmission Line Analysis**
- Evaluate losses, reflections, terminations
- Ensure impedance tolerances
** Signal Integrity Modeling**
- Perform circuit, IBIS and 3D EM analysis
- Verify timing, noise margins, eye diagrams
Power Integrity Modeling
- Simulate ground bounce, rail collapse, resonances
- Check voltage levels during transients
EMI/EMC Analysis
- Model emissions and susceptibility
- Assess shielding and external interference
Accurate modeling provides confidence prior to hardware.
Example Multi-GHz PCB Design Walkthrough
Consider a dual channel 10Gbps serial link PCB operating at 6.25 GHz:
Stackup
- 8 layer board with thick cores, thin prepregs
- Differential microstrip lines routed on inner layers
Partitioning
- High-speed digital, power, analog, clocking, power
- Clear separation between zones
Materials
- Low-loss laminate: Rogers RO4350B, εr=3.48
- Low-loss prepreg: Rogers RO4450F, εr=3.23
Routing
- Matched 100 ohm diff pairs + ground traces
- Minimal vias, 45° corners, shielding ground traces
Bypassing
- 100nF caps near each IC power pin
- Smaller high freq. caps interspersed
Termination
- AC-coupled single-ended interconnect
- Source/load termination resistors
Validation
- Time/frequency domain modeling
- Verify eye diagrams, jitter, stability
This example shows how a variety of techniques combine to address high frequency design needs.
Prototyping Recommendations
Given the greater likelihood of issues, prototyping takes on heightened importance:
- Build multiple incremental prototypes
- Incorporate board instrumentation like test points
- Perform careful impedance measurements
- Execute signal integrity testing beyond compliance
- Thermally cycle boards while monitoring performance
- Verify EMI/EMC including radiated emissions
- Be prepared to modify layout based on results
- Allow sufficient time and budget
Thorough prototyping and validation provides confidence prior to release.
Designing for Testability
Special considerations are required to test high frequency designs:
Coaxial Connectors
- Small form factor connectors like SMP or SMA
- Facilitate attaching lab equipment
Test Points
- strategically placed vias or pads
- 0201 package size resistors limit loading
Probe Pads
- Provide access for high frequency probes
- Include ground pads in close proximity
Boundary Scan
- Include test features on ICs
- Verify connectivity and basic function
Built-In Instrumentation
- On-board oscillators, PLLs, counters
- Add monitor nodes and output signals
By planning testability up front in the design process, characterization and troubleshooting is straightforward.
Common High Frequency Design Pitfalls
Despite best efforts, even experienced designers must remain vigilant against some common missteps:
- Selection of inadequate PCB materials
- Failure to provide shielding for sensitive devices
- Incomplete isolation between circuit zones
- Allowing impedance discontinuities
- Poor stackup choices that jeopardize SI
- Excessive vias without impedance control
- Lack of terminating transmission line stubs
- Insufficient decoupling capacitors
- Inadequate consideration of grounding needs
- Forgetting EMI mitigation strategies
- Attempting to route before placement planning
Forewarned is forearmed against these potential pitfalls.
Frequently Asked Questions
Here are some common high frequency PCB design questions:
Q: What are some good stackup guidelines for data rates above 5Gbps?
Use at least 6 layers. Route critical nets on inner layers with thick cores and thin dielectrics. Enclose nets between ground planes. Include 10-20% blank margin border.
Q: How can I estimate appropriate line impedance values?
Use calculators or equations to determine single-ended or differential pair impedances based on dielectric constant, trace dimensions, and reference planes.
Q: What PCB finishes provide the best high frequency signal integrity?
Immersion silver and annealed copper (oxidation resistant) offer minimal skin effect losses at high frequencies.
Q: What are some techniques to reduce crosstalk on densely routed boards?
Shielding ground traces, ground vias near traces, routing orthogonally, wider spacing, lower dielectric constant materials.
Q: When should I avoid vias on a high frequency design?
Minimize vias on clock nets or matched-length nets. Use same-layer jogs instead if possible.
Conclusion
Designing PCBs for multi-GHz applications requires adopting specialized layout practices tailored to the unique needs and challenges. By combining sound high frequency design principles, engineers gain the ability to successfully implement designs operating at the limists of speed and frequency – enabling cutting-edge RF, microwave and high-speed digital systems across countless end applications.
10 Ways for High Frequency PCB Layout
If the frequency of the digital logic circuit reaches or exceeds 45 MHz to 50 MHz, and the circuit operating above this frequency already accounts for a certain amount (for example, 1/3) of the entire electronic system, it is usually called a high frequency circuit. High-frequency circuit design is a very complex design process, and its wiring is critical to the overall design! Master the following ten methods, you will be less detours in high-frequency circuit design.
1. Multi-layer board wiring
High-frequency circuits board tend to have high integration and high wiring density. The use of multi-layer pcb boards is both necessary for wiring and an effective means to reduce interference. In the PCB Layout stage, a reasonable selection of the printed board size of a certain number of layers can make full use of the intermediate layer to set the shielding, better achieve the near grounding, and effectively reduce the parasitic inductance and shorten the transmission length of the signal, and at the same time All of these methods are advantageous for the reliability of high-frequency circuits by reducing the crosstalk of signals and the like.
According to the data, the four-layer board is 20dB lower than the noise of the double-panel. However, there is also a problem. The higher the PCB half-layer number, the more complicated the pcb manufacturing process and the higher the unit cost. This requires us to select the appropriate number of PCB boards for PCB layout. Proper component layout planning and proper routing rules to complete the design.
2. The less the lead bend between the high-speed electronic device pins, the better.
The lead wire of the high-frequency circuit wiring is preferably a full line, which needs to be turned, and can be folded at a 45-degree line or a circular arc. This requirement is only used to improve the fixing strength of the copper foil in the low-frequency circuit, and in the high-frequency circuit, the content is satisfied. One requirement is to reduce the external transmission and mutual coupling of high frequency signals.
3. The shorter the lead between the pins of the high-frequency circuit device, the better.
The radiant intensity of the signal is proportional to the length of the trace of the signal line. The longer the high-frequency signal lead, the easier it is to couple to the component close to it, so for data such as signal clock, crystal, DDR, High-frequency signal lines such as LVDS lines, USB lines, and HDMI lines are required to be as short as possible.
4. The less alternating between the lead layers between the pins of the high-frequency circuit device, the better.
The so-called “the least alternating between the layers of the leads is better” means that the fewer vias (Via) used in the component connection process, the better. According to the side, a via can bring about a distributed capacitance of about 0.5pF, and reducing the number of vias can significantly increase the speed and reduce the possibility of data errors.
5. Pay attention to the “crosstalk” introduced by the parallel lines of the signal lines.
High-frequency circuit wiring should pay attention to the “crosstalk” introduced by the parallel lines of the signal lines. Crosstalk refers to the coupling phenomenon between signal lines that are not directly connected. Since the high-frequency signal is transmitted along the transmission line in the form of electromagnetic waves, the signal line acts as an antenna, and the energy of the electromagnetic field is emitted around the transmission line, and an undesired noise signal generated between the signals due to the mutual coupling of the electromagnetic fields Called Crosstalk.
The parameters of the PCB layer, the spacing of the signal lines, the electrical characteristics of the driver and receiver, and the termination of the signal line all have a certain impact on crosstalk. Therefore, in order to reduce the crosstalk of high-frequency signals, it is required to do the following as much as possible during wiring:
Inserting a ground or ground plane between two lines with severe crosstalk can allow isolation and reduce crosstalk under the conditions allowed by the wiring space.
When there is a time-varying electromagnetic field in the space around the signal line, if parallel distribution cannot be avoided, a large area “ground” can be placed on the reverse side of the parallel signal line to greatly reduce the interference.
Under the premise of wiring space permission, increase the spacing between adjacent signal lines, reduce the parallel length of the signal lines, and the clock lines should be perpendicular to the key signal lines and not parallel.
If the parallel traces in the same layer are almost unavoidable, in the adjacent two layers, the direction of the traces must be perpendicular to each other.
In digital circuits, the usual clock signals are signals with fast edge changes, and the external crosstalk is large. Therefore, in the PCB design, the clock line should be surrounded by ground lines and more ground holes to reduce the distributed capacitance, thus reducing crosstalk.
For the high-frequency signal clock, try to use the low-voltage differential clock signal and cover the ground. You need to pay attention to the integrity of the package.
Do not leave the unused input terminal, but ground it or connect it to the power supply (the power supply is also ground in the high-frequency signal loop). Because the suspended line may be equivalent to the transmitting antenna, grounding can suppress the emission. Practice has proved that using this method to eliminate crosstalk can sometimes be effective immediately.
6. The power supply pin of the integrated circuit block increases the high frequency decoupling capacitor
A high frequency untwisting capacitor is added to the power supply pin of each integrated circuit block. Increasing the high frequency decoupling capacitor of the power supply pin can effectively suppress the high frequency harmonics on the power supply pin to form interference.
7. Ground wire of high frequency digital signal and ground of analog signal are isolated
When analog ground lines, digital ground lines, etc. are connected to the common ground line, high-frequency turbulent magnetic beads should be used to connect or directly isolate and select a suitable place for single-point interconnection. The ground potential of the ground of the high-frequency digital signal is generally inconsistent, and there is often a certain voltage difference between the two directly. Moreover, the ground of the high-frequency digital signal often has a very rich harmonic component of the high-frequency signal. When the digital signal ground and the analog signal ground are directly connected, the harmonics of the high-frequency signal interfere with the analog signal by means of ground-line coupling.
Therefore, in general, the ground of the high-frequency digital signal and the ground of the analog signal are to be isolated, and the method of single-point interconnection at a suitable position or the interconnection of high-frequency turbulent magnetic beads can be adopted.
8. Avoid loops formed by traces
Do not form a loop as much as possible for all types of high-frequency signal traces. If it is unavoidable, make the loop area as small as possible.
9. Must ensure good signal impedance matching
During the transmission of the signal, when the impedance does not match, the signal will reflect in the transmission channel, and the reflection will overshoot the synthesized signal, causing the signal to fluctuate around the logic threshold.
The fundamental way to eliminate the reflection is to make the impedance of the transmitted signal match well. Since the difference between the load impedance and the characteristic impedance of the transmission line is larger, the reflection is also larger. Therefore, the characteristic impedance of the signal transmission line should be equal to the load impedance as much as possible. At the same time, it should be noted that the transmission line on the PCB should not be abrupt or corner, try to keep the impedance of each point of the transmission line continuous, otherwise there will be reflection between the segments of the transmission line. This requires the following wiring rules to be observed when performing high-speed PCB routing:
USB Wiring Rules: USB signal differential routing is required. The line width is 10 mils, the line spacing is 6 mils, and the ground and signal lines are 6 mils apart.
HDMI cabling rules: HDMI signal differential routing is required, linewidth is 10mil, line spacing is 6mil, and the spacing between each pair of HDMI differential signal pairs exceeds 20mil.
The LVDS routing rules require LVDS signal differential traces with a linewidth of 7 mils and a line pitch of 6 mils. The purpose is to control the HDMI differential signal pair impedance to 100+-15% ohm DDR routing rules. The DDR1 routing requires that the signal should not pass through the hole as much as possible. The signal line is equal in width and the line is equidistant from the line. The line must meet the 2W principle to reduce crosstalk between signals. For high-speed devices with DDR2 and above, high-frequency data is required. The lines are equal in length to ensure impedance matching of the signals.
10. Maintain the integrity of signal transmission
Maintain the integrity of signal transmission and prevent “ground bounce” caused by ground segmentation.
