Introduction to Differential Trace Impedance
Differential trace impedance is a crucial concept in highspeed PCB design. It refers to the characteristic impedance of a pair of traces routed in parallel that carry equal and opposite signals. The impedance of the differential pair depends on the geometry and spacing of the traces as well as the properties of the dielectric material.
Maintaining a consistent differential impedance is important to ensure signal integrity and minimize reflections, crosstalk, and electromagnetic interference (EMI). Typically, differential pairs are routed over a solid reference plane, such as a ground plane, which provides a clear return path for the signals.
However, in some cases, it may be necessary or desirable to route differential traces without a reference plane. This can present challenges in terms of controlling the impedance and ensuring proper signal propagation. In this article, we will explore the concepts and techniques related to differential trace impedance without a reference plane.
Factors Affecting Differential Trace Impedance
Several factors influence the characteristic impedance of differential traces:
 Trace width and thickness
 Spacing between traces
 Height above the reference plane (if present)
 Dielectric Constant (Dk) of the PCB material
 Dissipation factor (Df) of the dielectric
 Copper roughness
The differential impedance can be calculated using formulas that take into account these parameters. One common approximation is:
Z_diff = 2 * Z_0 * (1  Kc)
Where:
– Z_diff is the differential impedance
– Z_0 is the characteristic impedance of a single trace
– Kc is the coupling coefficient between the traces
The coupling coefficient Kc depends on the spacing between traces relative to the height above the reference plane. Closer spacing and greater height above the plane increase the coupling.
Challenges of Routing Without a Reference Plane
When differential traces are routed without a continuous reference plane underneath, several issues can arise:

Uncontrolled impedance – The lack of a reference plane makes it difficult to precisely control the characteristic impedance of the traces. The impedance can vary along the length of the traces due to changes in the effective dielectric constant and coupling.

Increased crosstalk – Without a reference plane to constrain the fields, there is more opportunity for crosstalk between adjacent trace pairs. The coupling between traces can lead to signal degradation and noise.

EMI and radiation – Differential traces without a reference plane can act as antennas, radiating electromagnetic energy and causing EMI issues. The lack of shielding provided by the reference plane allows the fields to escape and potentially interfere with other circuits.

Reduced noise immunity – The reference plane helps to shield the differential traces from external noise sources. Without this shielding, the traces are more susceptible to picking up noise from the environment.
Despite these challenges, there are situations where routing differential traces without a reference plane may be necessary, such as in flex circuits or in areas with limited layer stackup. In these cases, careful design and simulation can help to mitigate the issues.
Design Considerations
When designing differential traces without a reference plane, several key factors should be considered to minimize the impact on signal integrity:

Trace geometry – The width, thickness, and spacing of the traces should be carefully chosen to achieve the desired impedance and minimize coupling. Wider spacing between trace pairs can help to reduce crosstalk.

Dielectric material – Using a PCB material with a higher dielectric constant (Dk) can help to confine the fields closer to the traces, reducing the impact of the missing reference plane. However, higher Dk materials also have higher dissipation factors, which can increase losses.

Routing topology – The routing pattern of the differential traces can affect the impedance and coupling. Keeping the traces as symmetrical and balanced as possible helps to maintain a consistent impedance along the length. Avoid abrupt changes in direction or spacing.

Shielding and guarding – In some cases, it may be possible to use shielding or guarding techniques to mitigate the effects of the missing reference plane. This can involve adding grounded traces or planes adjacent to the differential pairs to provide some level of field containment.
Simulation and Modeling
Given the complex interactions between the differential traces and the surrounding environment, simulation and modeling are essential tools for designing and optimizing differential impedance without a reference plane.
Electromagnetic field solvers, such as 2D or 3D finite element analysis (FEA) tools, can be used to accurately model the behavior of the differential traces in the absence of a reference plane. These simulations take into account the full geometry and material properties of the PCB Stackup.
By running simulations with different trace geometries, spacings, and dielectric materials, designers can explore the impact on impedance, coupling, and signal integrity. The simulation results can help to identify potential issues and guide the design optimization process.
Some key metrics to analyze in the simulation results include:

Impedance profile – Plotting the characteristic impedance along the length of the differential traces can reveal any variations or discontinuities that may cause reflections.

Coupling – Measuring the coupling between adjacent differential pairs can help to quantify the crosstalk and determine if it falls within acceptable limits.

Fields and radiation – Visualizing the electric and magnetic field distributions around the traces can provide insights into the potential for EMI and radiation.

Sparameters – Extracting the scattering parameters (Sparameters) of the differential traces allows for frequencydomain analysis of the signal integrity, including insertion loss, return loss, and crosstalk.
By using simulation and modeling techniques, designers can make informed decisions about the differential trace geometry and layout to achieve the best possible performance without a reference plane.
Manufacturing Considerations
When fabricating PCBs with differential traces without a reference plane, there are several manufacturing considerations to keep in mind:

Impedance control – The PCB Manufacturer must be able to tightly control the trace geometry and dielectric properties to maintain the desired impedance. This may require specialized equipment and processes, such as impedancecontrolled etching or laser direct imaging (LDI).

Material selection – The choice of PCB material is critical for achieving the desired dielectric constant and dissipation factor. The manufacturer should have experience working with the specified material and be able to maintain consistent properties across the panel.

Registration and alignment – Without a reference plane, the registration and alignment of the differential traces becomes more critical. Any misalignment or skew between the traces can affect the impedance and coupling. The manufacturer should have tight tolerances for layertolayer registration.

Testing and validation – Postmanufacturing testing and validation are important to ensure that the differential traces meet the specified impedance and perform as expected. This may involve impedance testing, timedomain reflectometry (TDR), or other signal integrity measurements.
Close collaboration between the PCB designer and the manufacturer is essential to ensure that the differential traces without a reference plane are fabricated correctly and meet the required specifications.
Best Practices and Guidelines
Based on the challenges and considerations discussed above, here are some best practices and guidelines for designing and routing differential traces without a reference plane:

Minimize the length – Keep the differential traces as short as possible to minimize the impact of the missing reference plane. Longer traces are more susceptible to impedance variations and crosstalk.

Maintain symmetry – Route the differential traces as symmetrically as possible, with equal length and spacing between the pairs. Avoid any unnecessary bends or jogs that can introduce discontinuities.

Use wider spacing – Increase the spacing between differential pairs to reduce the coupling and crosstalk. The spacing should be balanced against the desired impedance and the available board space.

Consider the stackup – If possible, choose a PCB stackup that minimizes the distance between the differential traces and the nearest reference plane. This can help to partially mitigate the effects of the missing plane.

Simulate and optimize – Use electromagnetic simulation tools to model and optimize the differential trace geometry and spacing. Iterate the design until the desired impedance and signal integrity metrics are achieved.

Add shielding or guarding – If necessary, consider adding shielding traces or planes adjacent to the differential pairs to provide some level of field containment and reduce crosstalk.

Validate with measurements – After fabrication, perform thorough measurements and testing to validate the impedance and signal integrity of the differential traces. Use TDR or VNA measurements to characterize the traces and identify any issues.
By following these best practices and guidelines, designers can successfully route differential traces without a reference plane while minimizing the impact on signal integrity and performance.
FAQ
Q1: What is differential trace impedance?
A1: Differential trace impedance refers to the characteristic impedance of a pair of traces routed in parallel that carry equal and opposite signals. It depends on the geometry and spacing of the traces, as well as the properties of the dielectric material.
Q2: Why is a reference plane important for differential traces?
A2: A reference plane, such as a ground plane, provides a clear return path for the differential signals and helps to control the impedance of the traces. It also shields the traces from external noise sources and reduces EMI and radiation.
Q3: What are the challenges of routing differential traces without a reference plane?
A3: Routing differential traces without a reference plane can lead to uncontrolled impedance, increased crosstalk, EMI and radiation issues, and reduced noise immunity. The lack of a reference plane makes it difficult to precisely control the characteristic impedance and coupling between traces.
Q4: How can simulation and modeling help in designing differential traces without a reference plane?
A4: Electromagnetic field solvers can accurately model the behavior of differential traces without a reference plane, taking into account the geometry, spacing, and material properties. Simulations can help to optimize the trace design, analyze impedance variations, and assess crosstalk and signal integrity.
Q5: What are some best practices for designing differential traces without a reference plane?
A5: Some best practices include minimizing the trace length, maintaining symmetry, using wider spacing between pairs, considering the PCB stackup, simulating and optimizing the design, adding shielding or guarding if necessary, and validating the fabricated traces with measurements.
Conclusion
Designing differential traces without a reference plane presents unique challenges for signal integrity and impedance control. The lack of a continuous reference plane can lead to impedance variations, increased crosstalk, EMI, and reduced noise immunity.
To successfully route differential traces without a reference plane, designers must carefully consider the trace geometry, spacing, dielectric material, and routing topology. Electromagnetic simulations and modeling are essential tools for optimizing the design and assessing the impact on signal integrity.
Manufacturing considerations, such as impedance control, material selection, registration, and testing, are also critical for ensuring the fabricated PCB meets the desired specifications.
By following best practices and guidelines, such as minimizing trace length, maintaining symmetry, using wider spacing, and adding shielding or guarding when necessary, designers can effectively mitigate the challenges of routing differential traces without a reference plane.
Differential trace impedance without a reference plane requires careful design, simulation, and validation to achieve optimal signal integrity and performance. With the proper techniques and considerations, it is possible to successfully implement differential routing in the absence of a continuous reference plane.
I hope this detailed article provides useful information on differential trace impedance without a reference plane. Let me know if you have any further questions!