Embedded-cHow-ToBeginner · 4 min read

How to Route Differential Pairs on PCB: Step-by-Step Guide

To route differential pairs on a PCB, keep the two traces close and parallel with controlled spacing to maintain impedance. Match their lengths precisely and avoid sharp bends to ensure signal integrity and reduce noise.

📐

Syntax

Routing differential pairs involves these key steps:

Trace Pair: Two traces carrying complementary signals.
Spacing: Controlled gap between the traces to maintain impedance.
Length Matching: Ensuring both traces have equal length to avoid timing issues.
Routing Style: Keep traces parallel and avoid sharp angles.

pseudocode

RouteDifferentialPair(trace1, trace2, spacing, length_tolerance)
{
  // trace1 and trace2 are the two signal lines
  // spacing is the gap between traces
  // length_tolerance is max allowed length difference
  KeepTracesParallel(trace1, trace2, spacing);
  MatchTraceLengths(trace1, trace2, length_tolerance);
  AvoidSharpBends(trace1);
  AvoidSharpBends(trace2);
}

💻

Example

This example shows how to route a differential pair with 5 mil spacing and length matched within 2 mils.

pseudocode

trace1 = CreateTrace(start=(0,0), end=(100,0))
trace2 = CreateTrace(start=(0,5), end=(100,5))
spacing = 5  // mils
length_tolerance = 2  // mils

RouteDifferentialPair(trace1, trace2, spacing, length_tolerance)

// Output: Two parallel traces 5 mils apart, length matched within 2 mils, no sharp bends

Output

Two traces routed parallel with 5 mil spacing and length difference less than 2 mils

⚠️

Common Pitfalls

Unequal Lengths: Causes signal timing mismatch and degrades signal quality.
Incorrect Spacing: Leads to impedance mismatch and noise.
Sharp Bends: Create reflections and signal loss.
Crossing Other Signals: Increases crosstalk and interference.

pseudocode

/* Wrong way: Unequal length and wide spacing */
trace1 = CreateTrace(start=(0,0), end=(100,0))
trace2 = CreateTrace(start=(0,10), end=(90,10))  // shorter length and 10 mil spacing

/* Right way: Equal length and controlled spacing */
trace2 = CreateTrace(start=(0,5), end=(100,5))  // matched length and 5 mil spacing

📊

Quick Reference

Keep differential traces close and parallel.
Maintain consistent spacing for impedance control.
Match trace lengths within a few mils.
Avoid sharp 90° bends; use 45° or curved bends.
Route away from noisy signals to reduce interference.

✅

Key Takeaways

Keep differential pair traces close and parallel with controlled spacing.

Match the lengths of both traces precisely to maintain signal timing.

Avoid sharp bends to prevent signal reflections and loss.

Maintain consistent impedance by controlling trace width and spacing.

Route differential pairs away from noisy signals to reduce interference.