PCB Impedance Control

As signal frequencies increase, controlling the impedance of PCB traces becomes critical. Impedance mismatches cause signal reflections, ringing, and data errors. This guide explains what impedance control is, why it matters, and how to design and verify controlled impedance traces.

What Is Characteristic Impedance?

Characteristic impedance (Z0) is the ratio of voltage to current of an electromagnetic wave traveling along a transmission line. For a PCB trace, it depends on:

• Trace width (W)
• Trace thickness (T) — copper weight
• Dielectric thickness (H) — distance to reference plane
• Dielectric constant (Dk / Er) of the substrate material
• Trace geometry — microstrip (outer layer) vs stripline (inner layer)

Z0 is measured in ohms and is a property of the trace geometry, not the signal itself.

Why Control Impedance?

Signal Reflection

When a signal encounters an impedance discontinuity (change in Z0), part of the signal energy is reflected back toward the source. The reflection coefficient is:

Gamma = (Z_load - Z_source) / (Z_load + Z_source)

A 10% impedance mismatch causes ~5% signal reflection. At high frequencies, these reflections create:

• Ringing: Oscillations on the signal edge that can trigger false logic transitions
• Overshoot/undershoot: Voltage excursions beyond supply rails, potentially damaging ICs
• Data errors: In high-speed serial links, reflections degrade the eye diagram

When Is Impedance Control Needed?

Rule of thumb: Control impedance when the signal rise time is less than twice the propagation delay of the trace.

Practical guideline by interface:

Impedance Types

Single-Ended (SE) Impedance

A single trace referenced to a ground plane. The most common controlled impedance type.

• Standard value: 50 ohm (RF), 60 ohm (general digital)
• Tolerance: +/-10% is standard, +/-5% for high-speed

Differential Impedance

Two traces (a differential pair) carrying complementary signals. The differential impedance is the impedance between the two traces.

• Zdiff ≈ 2 x Z_odd (where Z_odd is the odd-mode impedance of each trace)
• Standard values: 85 ohm (USB3), 90 ohm (USB2), 100 ohm (HDMI, Ethernet, LVDS)
• Coupling matters: Tighter spacing between traces increases coupling and reduces Zdiff

Coplanar Impedance

A trace with ground copper on both sides on the same layer. Used for RF and high-frequency designs where ground return path on the same layer is beneficial.

Trace Geometries

Microstrip

• Trace on an outer layer with a reference ground plane below
• Higher propagation speed (~60% of light speed)
• Slightly higher radiation/EMI
• Easier to probe and debug

Embedded Microstrip

• Outer layer trace covered by solder mask or prepreg
• Dk of solder mask affects impedance (typically Dk~3.5-4.0)
• Most common configuration in real boards

Stripline

• Trace on an inner layer between two ground planes
• Lower propagation speed (~50% of light speed)
• Better shielding — lower EMI
• Used for high-speed inner-layer routing

Impedance Calculation

Key Formula (Microstrip, Approximate)

Z0 ≈ (87 / sqrt(Er + 1.41)) x ln(5.98H / (0.8W + T))

Where:

• Er = dielectric constant
• H = dielectric height (mils)
• W = trace width (mils)
• T = trace thickness (mils)

Practical Examples (FR-4, Dk=4.2, 1oz copper)

50 ohm single-ended microstrip:

100 ohm differential pair:

Use Field Solvers

Approximate formulas are useful for estimation, but production impedance control requires a 2D field solver (e.g., Polar Si9000, Saturn PCB Toolkit, or your EDA tool’s built-in solver) that accounts for:

• Exact trace geometry (trapezoidal cross-section from etching)
• Solder mask effects
• Adjacent trace coupling
• Copper roughness

Impedance Testing with TDR

Time Domain Reflectometry (TDR) is the standard method for verifying controlled impedance:

1. A fast-rise-time pulse is sent down the trace
2. Reflections from impedance discontinuities are measured
3. The impedance profile along the trace length is plotted
4. Typical test coupon includes dedicated impedance traces on the panel edge

TDR Test Coupons

• Fabricators include test coupons on each production panel
• Coupons replicate the exact stackup and trace geometries of the production board
• TDR measurement is performed on every panel (or sample-based for high volume)
• Results are documented in an impedance test report

Stackup Impact on Impedance

The PCB stackup directly determines impedance. Key considerations:

1. Prepreg/core selection: Different prepreg styles (1080, 2116, 7628) have different thicknesses and Dk values
2. Dk tolerance: FR-4 Dk can vary +/-5-10%. Use tighter Dk-tolerance materials for strict impedance control
3. Copper roughness: Rougher copper (standard ED) has slightly higher effective Dk than smooth copper (RTF/VLP)
4. Symmetric stackup: Always use a symmetric layer arrangement to prevent board warpage and ensure consistent impedance

Conclusion

Impedance control is essential for any design with high-speed digital or RF signals. Work closely with your PCB manufacturer to define the stackup, specify impedance targets and tolerances, and ensure proper test coupons are included. Most manufacturers provide free impedance calculation support — take advantage of this service early in your design process to avoid costly redesigns.