Differential Pair Routing Walkthrough

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Differential signaling is a common technique used in high-speed PCB design to transmit signals with lower EMI and increased noise immunity compared to single-ended signaling. It works by sending a signal over two complementary traces, called a differential pair. The receiver looks at the difference between the two traces rather than their absolute voltages.

Table of Contents

Benefits of Differential Signaling

Using differential pairs provides several key advantages:

  • Lower EMI – The equal and opposite currents flowing through the two traces create electromagnetic fields that cancel each other out, resulting in lower EMI emissions.
  • Increased noise immunity – Noise coupled onto the pair will affect both traces equally and be rejected by the receiver. This makes differential signaling less susceptible to external noise sources.
  • Controlled impedance – With the proper board stackup, a differential pair can be constructed to have a controlled 100Ω characteristic impedance. This matches well with many high-speed transceivers and cables.
  • No return path needed – Single-ended signals require a solid return path to establish a voltage reference. Differential signals return through their complement trace, removing the need for a ground plane.


Differential signaling also comes with some downsides:

  • More routing channels used – Each differential signal requires two traces instead of one, using up more routing resources.
  • Skew concerns – The two traces that make up a differential pair must be length matched to avoid signal skew. This can be challenging for complex routing.
  • Ground noise – Return currents are more concentrated for differential pairs. This can increase ground bounce if not mitigated.

Overall, the improved signal integrity is worth the tradeoffs for most high-speed designs. Let’s look at how to implement good differential pair routing.

Differential Pair Routing Requirements

To achieve optimal signal quality, there are a few important requirements to follow when routing differential pairs:

Match Lengths

  • The two traces in a differential pair must be length matched to within a certain tolerance, usually 10-15 mils.
  • Matching the lengths keeps the signals in phase and minimizes skew.
  • Consider using serpentine routing if needed to match lengths.

Balance Spacing from Other Nets

  • Each trace should have equal spacing from adjacent signals and ground planes.
  • Balanced spacing helps ensure any coupled noise affects both traces equally.

Avoid 90° Bends

  • 90° bends should be avoided where possible.
  • The impedance discontinuity can reflect signals and cause distortion.
  • Use two 45° bends instead for necessary turns.

Maintain Proper Spacing

  • High-speed signals require wider trace spacing than typical traces.
  • A good rule of thumb is 2-3x the trace width for impedance control.
  • Wider spacing also reduces crosstalk between neighboring signals.

Follow these guidelines carefully and your differential pairs will have superb signal integrity from end-to-end.

Stackup Considerations

Proper board stackup design is critical in facilitating good differential routing. Here are some key stackup guidelines for high-speed differential signals:

Place Near Inner Layers

  • The differential pairs should be routed on inner layer(s) whenever possible.
  • This shields the pairs from external noise sources.

Ensure Adequate Isolation

  • Allow enough prepreg thickness between the pair routing layer and adjacent planes.
  • 10-12 mils is usually sufficient for isolation.

Include Ground Planes

  • Include ground planes above and below the differential layer.
  • The ground planes act as low impedance return paths.

Watch Glassweave Skew

  • Glassweave in prepreg can cause etch variation and skew.
  • Using prepreg without glass controls this better.

Model in 3D EM Simulator

  • Model the entire stackup in your EM simulator to verify impedance tolerance.
  • Make tweaks as needed to board thickness and materials.

With the right stackup configuration, achieving your target 100Ω differential impedance will be straightforward.

Length Matching Techniques

Length matching the two traces in a differential pair is one of the most critical steps in routing. Here are some techniques to match lengths accurately:

Tune Serpentine Sections

  • Use wider serpentine sections to finely tune and match lengths.
  • Keep the radii as large as possible to minimize impedance changes.

Mirror Routing

  • Route the two traces as mirror images to ensure matching.
  • Any skew from glassweave will equally affect both traces.

Matched Bends

  • When turning corners, make sure both traces have identical bend geometry.
  • This maintains matched delay through bends.

Limit Compression Bends

  • Avoid routing bends that compress or extend a trace.
  • The change in delay can throw off length matching.

Simulate before Final Layout

  • Simulate differential traces in your SI tool and plot delays.
  • Make tuning tweaks until the delays match closely.

With smart use of these techniques, you can achieve virtually identical routing for a differential pair.

Crossing Other Signals

Crossing single-ended or differential signals with a differential pair needs special care to prevent problems:

Use Perpendicular Crosses

  • Cross other signals at perfect 90° angles.
  • Anything other than perpendicular introduces skew.

Alternate Layer Transition Location

  • When transitioning layers, move the top trace to bottom, and vice versa.
  • This ensures magnetic coupling is equal on both traces.

Space Widely at Crosses

  • Use 3-4x typical trace spacing when crossing other nets.
  • Wider spacing reduces coupling and noise injection.

Avoid Same-Layer Crossing

  • Avoid crossing differential pairs on the same layer when possible.
  • Use vias to transition to another layer first.

Filter Unavoidable Aggressors

  • If crossing a very high-speed aggressor is unavoidable, filter it.
  • Small ferrite beads can help prevent coupled noise.

Paying attention to crossover best practices is one of the key factors in achieving clean “eye” signals at the receiver.

Redistributing Differential Pairs

There will often come routing situations where a differential pair needs to be “redistributed” for some section of the path:

Plan Ahead with Stub Length Budget

  • The stubs between original and redistributed pairs must be well controlled.
  • Budget no more than 500-1000 mils of stub length for each trace.

Watch for Skew During Propagation

  • Differences in glassweave or vias can cause skew during trace propagation.
  • simulate the routing to catch any unexpected skew issues.

Break Out Carefully from Pair

  • Allow each trace to diverge from the main pair at the same angle.
  • This maintains balance and controlled impedance.

Avoid Length Mismatch Near Ends

  • Any length mismatch between tails at Driver or Receiver can greatly impact signal quality.
  • Take special care to match at the very ends.

Bring Back Together Gently

  • As the two traces come back together, try to avoid sharp turns.
  • Gentle bends will help in rejoining with matching lengths.

With good planning and awareness, it is possible to redistribute differential pairs when needed without degrading the signal.

Differential Vias

Vias are required when transitioning differential pairs between layers. Here are some tips on using vias with differential signals:

Space Vias Tightly

  • Use minimum clearance between via pads and void the anti-pad.
  • This minimizes via inductance.

Place Vias Close Together

  • Keep via barrels as close together as allowed for coupling.
  • Close coupled vias act like a single double-sized via.

Extend Pads for Controlled Impedance

  • Size via pads larger to maintain 100 ohm differential impedance.
  • Model in EM simulator to determine required pad size.

Plate Vias Where Possible

  • Plated barrel vias have lower inductance and resistance than unplated.
  • But even unplated works fine for small via counts.

Implement Stitching Vias

  • Adding 4-6 closely spaced vias provides redundancy if one via fails.
  • This “via stitching” technique improves yield and reliability.

Vias allow necessary layer transitions while maintaining short and direct routings. Follow these guidelines and your vias will perform exceptionally.

Advanced Routing Techniques

For complex dense boards, it helps to keep some advanced differential routing techniques handy:

Coupled Serpentines

  • Intertwined serpentine traces in open areas can match lengths in tight spaces.
  • Allows matching despite obstacles for one trace.

Adjacent Pours for Shielding

  • Use ground or power planes next to the pair for added shielding.
  • The planes act as a co-planar waveguide.

Non-Manhattan Geometries

  • Allow 45° traces and arcs in your router settings.
  • This enables more elegant and matched routing.

Tuning with Fill Shapes

  • Carefully placed copper fills can fine-tune trace lengths.
  • Best used towards ends of traces to match.

Review Tuning in SI Tool

  • Check any manual tuning in your signal integrity tool.
  • Look for matched delays and proper impedance.

Don’t be afraid to get creative with these tips when tackling dense boards with challenging differential routing!

Eye Pattern Analysis

Once routing is complete, it is critical to simulate the differential channels and evaluate the “eye” pattern at the receiver. The eye provides valuable insight into the signal quality:

Check Eye Height and Width

  • The eye height indicates the noise margin. More is better.
  • The eye width indicates timing margin. Wider is better.

Look for Over/Undershoot

  • Significant over/undershoot indicates impedance discontinuities.
  • This can distort the eye and close it prematurely.

Watch for Asymmetry

  • The eye should be symmetrical about the center crossing.
  • Asymmetry implies skew problems between the two traces.

Evaluate at Max Length

  • Make sure to simulate at longest path length.
  • Longer paths are more susceptible to distortion.

Iterate to Improve Eye

  • If the eye is not open and clear, revisit routing and make tuning tweaks.
  • Keep iterating until the eye is as clean as possible.

With some experience reading eyes, you can easily assess how well your differential pairs are routed and make improvements.

Final DRC Checks

As a final verification step before manufacturing, here are some key DRC checks to run on differential pairs:

Verify Matched Lengths

  • Run length check on the traces looking for no more than 10-15 mil mismatch.

Check Trace Spacing

  • Ensure consistent spacing between traces and to adjacent signals.

Confirm Proper Layer Usage

  • Validate no segments ended up on an incorrect layer.

Review Bend Geometry

  • Check that all bends are identical between traces.

Look for any Vias Too Close

  • Ensure via spacing meets requirements for the fabricator.

Double Check Minimum Hole Size

  • Confirm vias and through holes meet minimum size rules.

By diligently running through these DRC checks, you can catch any last minute errors and ensure your differential signals meet all physical requirements.


That concludes our detailed walkthrough on proper differential pair routing techniques! To summarize:

  • Follow length matching, spacing, bend, and termination rules
  • Employ a well designed board stackup
  • Utilize crossing and redistribution best practices
  • Carefully design any required vias
  • Analyze eye diagrams to assess signal quality
  • Run thorough DRC checks before manufacturing

These steps will help you achieve robust, high-speed differential signal transmission on your PCBs. With the exponentially growing speeds seen in today’s designs, understanding excellent differential routing practices is an indispensable skill for any PCB layout engineer.

Frequently Asked Questions

What is the typical trace width for differential signals?

For common 100Ω differential impedance, a typical width would be 3-5 mils (0.15mm). Wider traces around 5-6 mils help reduce resistance. The required width depends on dielectric material and spacing.

Should I route diff pairs on outer layers if the board isn’t high speed?

Lower speed diff pairs can be routed on outer layers if needed for routing density. The risk of EMI is lower at slower edge rates. But inner layers are still preferred when possible.

How can I determine how many diff pairs a layer can accommodate?

As a rough estimate, you can expect 3-4 differential pairs per inch of layer width – but this varies by trace width and spacing. It’s recommended to utilize layer stack planning tools to determine precise pair count.

Are acute angle bends always bad?

Sharp acute bends below 30 degrees can cause noticeable impedance discontinuities so should be avoided where possible. But a narrow acute bend will degrade signals much less than a 90 degree bend. Some acutes are unavoidable.

Is length matching always required when redistributing diff pairs?

The stubs between original and redistributed pairs should be matched within ~250 mils or so. But the redistributed segments can take completely separate paths as long as rejoin points are matched. So full redistribution length matching may not be required.