PCB Impedance Calculator

This advanced PCB impedance calculator supports precise calculations for single-ended and differential transmission lines, including Microstrip, Stripline, Edge-Coupled Microstrip, and Edge-Coupled Stripline. Designed with visual trace-type selectors and industry-standard formulas, it enables engineers to model characteristic impedance (

Z_{0}

) and differential impedance (

Z_{d i ff}

) by inputting material properties and trace geometry. The tool adapts dynamically to display relevant parameters (e.g., trace spacing for differential pairs) and provides real-time results for optimal PCB design.

1. Select Trace Type

Microstrip

Stripline

Edge-Coupled Microstrip

Edge-Coupled Stripline

2. Enter Parameters

Dielectric Constant (εr)

Trace Thickness (T)

mils

Trace Width (W)

mils

Substrate Height (H)

mils

Trace Spacing (S)

mils

Microstrip Diagram

Formula

Z_{o} \approx \frac{87}{\sqrt{ε_{r} + 1.41}} \ln (\frac{5.98 H}{0.8 W + T})

Stripline Diagram

Formula

Z_{o} \approx \frac{60}{\sqrt{ε_{r}}} \ln (\frac{1.9 B}{0.8 W + T})

Edge-Coupled Microstrip Diagram

Formula

Z_{diff} \approx 2 \cdot Z_{o} (1 - 0.347 e^{- 2.9 \frac{S}{H}})

Edge-Coupled Stripline Diagram

Formula

Z_{diff} \approx 2 \cdot Z_{o} (1 - 0.748 e^{- 1.5 \frac{S}{B}})

3. Results

Characteristic Impedance (Zo)-- Ω

Disclaimer: These calculations are for estimation purposes only. For final designs, always use professional simulation software.

Usage Guide

Select Trace Type
Choose from four trace configurations with visual icons:
- Microstrip: Single trace on outer layer over a ground plane.
- Stripline: Trace embedded between two ground planes.
- Edge-Coupled Microstrip: Differential pair on outer layer.
- Edge-Coupled Stripline: Differential pair embedded between ground planes.
Enter Parameters
- Dielectric Constant (εr): Material’s electrical permittivity (e.g., 4.4 for FR-4).
- Trace Thickness (T): Copper thickness in mils (1 oz = 1.37 mils).
- Trace Width (W): Conductor width in mils.
- Substrate Height (H)/Plane Separation (B): Distance to ground plane(s).
- Trace Spacing (S): Appears for differential pairs; distance between traces.
View Results
- Characteristic Impedance (Zo): For single-ended traces.
- Differential Impedance (Zdiff): For coupled pairs, displayed automatically for edge-coupled types.

Formula Explanations

Single-Ended Microstrip Impedance

Z 0 = \frac{87}{\sqrt{ε_{r} + 1.41}} • \ln (\frac{5.98 H}{0.8 W + T})

Variables:

Z₀: Characteristic impedance of the microstrip line (Ω)
- Key parameter for single-ended signal integrity
- Typical target: 50Ω for RF, 60-70Ω for digital signals
ε_r: Substrate dielectric constant
- FR-4: 4.2-4.6 @ 1MHz
- Rogers RO3003: 3.0 @ 10GHz
H: Substrate height from trace to ground plane (mils)
- Also known as dielectric height
- Thinner H increases Z₀ for same trace width
W: Trace width (mils)
- Wider traces lower Z₀ linearly
- Minimum width limited by manufacturing (typically ≥4mils)
T: Trace thickness (mils)
- 1oz copper: 1.37mils (35μm)
- 2oz copper: 2.74mils (70μm)

Symmetric Stripline Impedance

Z 0 = \frac{60}{\sqrt{ε_{r}}} • \ln (\frac{1.9 B}{0.8 W + T})

Variables:

Z₀: Characteristic impedance of stripline (Ω)
- Enclosed between two ground planes for better shielding
- Typical target: 50Ω for controlled impedance designs
ε_r: Dielectric constant of core material
- High-frequency materials: ε_r stability critical
- Example: Isola FR408HR: ε_r=3.48 @ 10GHz
B: Total distance between ground planes (mils)
- Also called "plane separation" or "stackup height"
- B = 2H for symmetric stripline with centered trace
W: Trace width (mils)
- Narrower W increases Z₀ in stripline designs
- Width-to-thickness ratio affects field distribution
T: Trace thickness (mils)
- Thicker traces reduce DC resistance but impact Z₀ slightly
- Considered in denominator for geometric correction

Edge-Coupled Microstrip Differential Impedance

Z_{diff} = 2 • Z 0 • (1 - 0.347 • e^{- \frac{2.9 S}{H}})

Variables:

Z_diff: Differential impedance of coupled microstrip (Ω)
- Typical targets: 100Ω (USB), 90Ω (Ethernet)
- Depends on both single-ended Z₀ and coupling factor
Z₀: Single-ended microstrip impedance (Ω)
- Base impedance of each trace in the pair
- Assumes infinite ground plane for isolation
S: Spacing between coupled traces (mils)
- Critical for crosstalk and differential impedance control
- S/H ratio determines exponential coupling factor
- Common rule: S ≥ 2W for minimal crosstalk
H: Substrate height (mils)
- Affects field penetration into substrate
- Lower H increases electromagnetic coupling between traces

Edge-Coupled Stripline Differential Impedance

Z_{diff} = 2 • Z 0 • (1 - 0.748 • e^{- \frac{1.5 S}{B}})

Variables:

Z_diff: Differential impedance of coupled stripline (Ω)
- Preferred for high-speed signals requiring low EMI
- Typical value: 100Ω for DDR4 differential pairs
Z₀: Single-ended stripline impedance (Ω)
- Impedance of each trace when isolated
- Calculated using symmetric stripline formula
S: Spacing between coupled traces (mils)
- Smaller S increases differential impedance due to coupling
- Exponential term: e^-1.5S/B models field overlap
B: Plane separation (mils)
- Total distance between top and bottom ground planes
- B = 2H for centered traces in symmetric stackups
- Larger B reduces coupling effect for same trace spacing

FAQ

What is characteristic impedance (Z₀)?

Characteristic impedance is the resistance a signal "sees" as it travels along a transmission line, determined by trace geometry and material properties. A mismatch in

Z 0

causes signal reflections, degrading integrity. For example, a microstrip with

W = 10 mils

H = 6 mils

, and

ε r = 4.4

has:

Z 0 = 87 \sqrt (ε r + 1.41) \cdot ln (5.98 \cdot H 0.8 \cdot W + T) \approx 50 Ω

What's the difference between microstrip and stripline?

Microstrip: Single trace on the surface with a ground plane below.
- Advantages: Easy to route, suitable for low-frequency designs.
- Disadvantages: Radiates EMI, sensitive to board flexing.
Stripline: Trace sandwiched between two ground planes.
- Advantages: Better EMI shielding, stable at high frequencies.
- Disadvantages: Requires inner layers, more complex to route.

How does trace spacing affect differential impedance?

In edge-coupled pairs, increased spacing

S

reduces electromagnetic coupling, increasing differential impedance

Z diff

. For microstrips:

Z diff = 2 \cdot Z 0 \cdot (1 - 0.347 \cdot e -2.9S/H)

When $S = H$ : $Z diff \approx 2Z 0 \cdot 0.76$
When $S = 3H$ : $Z diff \approx 2Z 0 \cdot 0.97$

What's the difference between single-ended and differential impedance?

Parameter	Single-Ended (Z₀)	Differential (Z_diff)
Definition	Impedance from trace to ground	Impedance between two coupled traces
Typical Values	50Ω (RF), 60-70Ω (digital)	100Ω (USB), 90Ω (Ethernet)
Application	Single-ended signals (e.g., GPIO)	Differential signals (e.g., LVDS, PCIe)
Design Focus	Trace width and ground plane distance	Trace spacing and coupling coefficient

Differential pairs offer better noise immunity because the differential signal cancels common-mode noise. For example, USB 3.0 requires with and on a 6-mil FR-4 substrate.

Why choose Edge-Coupled Microstrip over Stripline for differential pairs?

Microstrip: Easier to route, but radiates EMI and is sensitive to board bend.
Stripline: Better shielding, less crosstalk, and more stable at high frequencies, but requires inner layers.

What role does dielectric constant (εr) play in impedance?

A higher

ε r

increases the effective permittivity of the transmission line, decreasing

Z 0

. For example:

FR-4 ( $ε r = 4.4$ ): $Z 0 \approx 50 Ω$ for $W = 10 mils, H = 6 mils$
Rogers RO3003 ( $ε r = 3.0$ ): $Z 0 \approx 58 Ω$ for the same geometry

Key Dielectric Properties

$ε r$ : Relative permittivity, affects field confinement.
- High-frequency materials: $ε r$ stability is critical
- Example: Isola FR408HR: $ε r = 3.48$ @ 10GHz
Loss Tangent (Df): Energy loss factor, impacts signal attenuation.
- FR-4: Df ≈ 0.02 @ 1MHz
- Rogers RO4350B: Df = 0.004 @ 10GHz

How accurate are these calculations?

Results are based on IPC-standard approximations. Real-world factors like:

Trace roughness (e.g., 2.1μm RMS)
Solder mask thickness (0.5-1.0mils)
Manufacturing tolerances (±10% for trace width)
Dielectric thickness variation (±5%)

PCB Impedance Calculator

1. Select Trace Type

2. Enter Parameters

Microstrip Diagram

Formula

Stripline Diagram

Formula

Edge-Coupled Microstrip Diagram

Formula

Edge-Coupled Stripline Diagram

Formula

3. Results

Usage Guide

Formula Explanations

Single-Ended Microstrip Impedance

Symmetric Stripline Impedance

Edge-Coupled Microstrip Differential Impedance

Edge-Coupled Stripline Differential Impedance

FAQ

Our PCB Services

Resources

Contact

PCB Impedance Calculator

1. Select Trace Type

2. Enter Parameters

Microstrip Diagram

Formula

Stripline Diagram

Formula

Edge-Coupled Microstrip Diagram

Formula

Edge-Coupled Stripline Diagram

Formula

3. Results

Usage Guide

Formula Explanations

Single-Ended Microstrip Impedance

Symmetric Stripline Impedance

Edge-Coupled Microstrip Differential Impedance

Edge-Coupled Stripline Differential Impedance

FAQ

Our PCB Services

Resources

Contact

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