PCB Impedance Calculator
Calculate characteristic impedance of PCB traces for microstrip and stripline configurations. Determine trace impedance for signal integrity design.
Typical: 0.035mm (1 oz copper)
FR-4: ~4.5, Rogers: 2.2-10.2
How to Use This Calculator
- Select trace type: Microstrip (outer layer) or Stripline (inner layer).
- Enter trace width in millimeters.
- Enter trace thickness (copper thickness, typically 0.035mm for 1 oz).
- Enter dielectric thickness (distance to ground plane).
- Enter dielectric constant (εr, typically 4.5 for FR-4).
- The calculator displays the characteristic impedance in ohms.
PCB Impedance Formulas
Impedance depends on trace geometry and dielectric properties:
Where ε_eff = effective dielectric constant, w_eff = effective width
Stripline: Z₀ ≈ (377/√ε_r) × (b/(w + 0.8t))
Where b = total dielectric thickness, w = trace width, t = trace thickness
Example: Microstrip, 0.2mm width, 0.035mm thickness, 0.1mm dielectric, εr=4.5: Z₀ ≈ 50Ω. These are simplified formulas—actual calculations are more complex and account for fringing fields.
Full Description
PCB trace impedance is a critical parameter in high-speed digital and RF circuit design. It determines how signals propagate along traces and affects signal integrity, power transfer, and electromagnetic compatibility. Controlling impedance is essential for preventing reflections, maintaining signal quality, and ensuring reliable operation of high-speed circuits.
Impedance depends on trace geometry (width, thickness), dielectric properties (thickness, dielectric constant), and trace configuration (microstrip or stripline). Microstrip traces are on outer layers with a ground plane below, while stripline traces are between two ground planes on inner layers. Stripline provides better isolation and more consistent impedance but is harder to access. Common impedance values include 50Ω for RF circuits, 75Ω for video, and 100Ω for differential pairs.
This calculator helps you determine PCB trace impedance. Enter trace dimensions and dielectric properties, and it calculates the characteristic impedance. Use it when designing high-speed circuits, planning PCB layouts, understanding signal integrity, or verifying impedance requirements. Remember that actual manufacturing tolerances affect impedance, so verify with measurements and work with your PCB manufacturer for controlled impedance traces.
Frequently Asked Questions
What is PCB trace impedance?
PCB trace impedance is the characteristic impedance of a trace on a printed circuit board. It depends on trace geometry (width, thickness), dielectric properties (thickness, constant), and trace type (microstrip, stripline). Common values: 50Ω (RF), 75Ω (video), 100Ω (differential).
What's the difference between microstrip and stripline?
Microstrip: Trace on outer layer with ground plane below. Exposed to air on one side. Common for surface layers. Stripline: Trace between two ground planes (inner layer). Better isolation, more consistent impedance, but harder to access for testing.
How do I control trace impedance?
Adjust trace width (wider = lower impedance), dielectric thickness (thicker = higher impedance), or dielectric constant (higher εr = lower impedance). Use impedance calculators during design, then verify with TDR (Time Domain Reflectometry) measurements.
Why is impedance matching important?
Impedance matching prevents signal reflections that cause distortion, loss, and interference. Mismatched impedances create standing waves, reduce signal quality, and can damage components. For high-speed signals (>100MHz), impedance control is critical.