PCB Impedance Calculator
Calculate characteristic impedance for microstrip and stripline PCB traces. Essential for high-speed digital and RF design.
Calculator
1oz copper ≈ 0.035mm (1.4mil)
Distance to reference plane
FR-4 typical: 4.2-4.8
Microstrip
┌─────┐ ← Trace (W×T) ═══╧═════╧═══ ▒▒▒▒▒▒▒▒▒▒▒▒▒ ← Dielectric (H) ═════════════ ← Ground Plane
Stripline
═════════════ ← Top Ground ▒▒▒▒▒▒▒▒▒▒▒▒▒ ▒▒┌─────┐▒▒▒▒ ← Trace (centered) ▒▒▒▒▒▒▒▒▒▒▒▒▒ ═════════════ ← Bottom Ground
How to Use This Calculator
This PCB impedance calculator helps you design controlled-impedance traces for high-speed digital and RF applications using industry-standard formulas.
- Select Trace Type — Choose microstrip (outer layer) or stripline (inner layer)
- Choose Material — Select a preset or enter custom dielectric constant
- Enter Dimensions — Input trace width, thickness, and dielectric height
- Click Calculate — Get impedance and transmission line parameters
For most designs, aim for 50Ω (single-ended) or 90-100Ω (differential). Adjust trace width to achieve your target impedance.
Impedance Theory
Characteristic impedance (Z₀) determines how electromagnetic waves propagate along a PCB trace. Mismatched impedances cause signal reflections, leading to ringing, overshoot, and data errors.
Why Impedance Control Matters
- Signal Integrity: Matched impedances minimize reflections
- High-Speed Design: Critical for signals above ~50 MHz
- RF Circuits: Required for antennas, filters, and matching networks
- Differential Pairs: USB, HDMI, Ethernet require specific impedance
Microstrip Formula (Wheeler/IPC-2141)
Key Parameters
| Trace Width (w) | Wider traces = lower impedance |
| Dielectric Height (h) | Taller dielectric = higher impedance |
| Dielectric Constant (εr) | Higher εr = lower impedance, slower signals |
| Trace Thickness (t) | Thicker traces = slightly lower impedance |
Trace Types Explained
Microstrip
A microstrip is a trace on the outer layer of a PCB with a ground plane below. It's the most common controlled-impedance structure.
- Advantages: Easy to manufacture, accessible for probing
- Disadvantages: More susceptible to EMI, lower effective εr
- Common Use: Single-ended signals, short high-speed traces
Stripline
A stripline is an inner-layer trace sandwiched between two ground planes. It provides better shielding but is harder to access.
- Advantages: Better EMI shielding, consistent impedance
- Disadvantages: Harder to manufacture and debug
- Common Use: Long high-speed traces, sensitive signals
Common Materials
| Material | εr | Application |
|---|---|---|
| FR-4 | 4.2-4.8 | Standard PCBs, up to ~1 GHz |
| Rogers 4350B | 3.48 | RF, microwave, high-speed digital |
| PTFE/Teflon | 2.1 | High-frequency RF, mmWave |
| Isola IS680 | 3.17 | High-speed digital, low loss |
Design Tips
Common Target Impedances
- 50Ω Single-ended: Most common, RF standard
- 75Ω Single-ended: Video signals, cable TV
- 90Ω Differential: USB 2.0/3.0, SATA
- 100Ω Differential: Ethernet, PCIe, HDMI
Manufacturing Tolerances
Typical PCB manufacturers guarantee ±10% impedance tolerance. For tighter tolerances (±5%), specify "controlled impedance" and provide target values.
Best Practices
- Maintain consistent trace width throughout the signal path
- Avoid sharp corners (use 45° or curved bends)
- Keep reference planes solid under high-speed traces
- Account for solder mask effect (can reduce impedance by 2-3Ω)
- Verify with your PCB manufacturer's stackup
Stackup Example (4-Layer)
| Layer 1 | Signal (microstrip) | 1oz copper |
| Prepreg | 7628 (8 mil) | εr ≈ 4.5 |
| Layer 2 | Ground plane | 1oz copper |
| Core | 39 mil | εr ≈ 4.2 |
| Layer 3 | Power plane | 1oz copper |
| Prepreg | 7628 (8 mil) | εr ≈ 4.5 |
| Layer 4 | Signal (microstrip) | 1oz copper |
Frequently Asked Questions
How accurate is this calculator?
This calculator uses industry-standard approximations (Wheeler, IPC-2141). Results are typically within 5% of field solver results for standard geometries. For critical designs, use your PCB manufacturer's impedance calculator or a 2D field solver.
When do I need controlled impedance?
Control impedance when the trace length exceeds 1/10 of the signal wavelength. For digital signals, this typically means: frequencies above 50 MHz, rise times under 1ns, or trace lengths over 2 inches for fast logic.
What's the difference between Z₀ and Zdiff?
Z₀ is single-ended impedance (one trace to ground). Zdiff is differential impedance (between two traces). For loosely coupled pairs: Zdiff ≈ 2 × Z₀. For tightly coupled pairs: Zdiff is lower due to coupling.
Does solder mask affect impedance?
Yes. Solder mask has εr ≈ 3.3-4.0 and typically reduces impedance by 2-3Ω. For critical traces, request solder mask openings or account for it in your calculations.
Why is FR-4 εr variable?
FR-4 dielectric constant varies with: frequency (lower at high freq), resin content, glass style, and manufacturer. Standard FR-4 is 4.2-4.8. Use your manufacturer's specific value for accurate calculations.
Verify Your Component Selections
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