PCB Impedance Calculator

Calculate the characteristic impedance of PCB traces based on dimensions, materials, and layout.

Calculate Your PCB Impedance Calculator

Understanding PCB Impedance

PCB impedance is a critical parameter in high-speed digital and RF circuit design. It represents the opposition that a trace offers to the flow of alternating current, combining both resistance and reactance properties. Controlling impedance is essential for signal integrity, minimizing reflections, and ensuring proper circuit function.

Why PCB Impedance Matters

As signal frequencies increase, traces on PCBs start to behave as transmission lines rather than simple connections. When the signal propagation time along a trace becomes comparable to the rise/fall time of the signal, impedance matching becomes crucial to prevent reflections and maintain signal integrity.

Key Benefits of Controlled Impedance

Minimizes signal reflections that cause ringing and overshooting
Reduces electromagnetic interference (EMI)
Ensures proper signal timing and prevents data errors
Enables higher data rates and frequencies
Ensures compatibility with standard interfaces (USB, HDMI, PCIe, etc.)

Types of PCB Transmission Lines

Microstrip Line

A microstrip is a trace on the outer layer of a PCB with a ground plane beneath it. One side of the trace is exposed to air (or solder mask), while the other side faces the dielectric material.

    ┌───────┐
    │ Trace │  ← W (width)
┌───┴───────┴───┐  ↑
│    PCB        │  │ H (height)
└───────────────┘  ↓
│/////////////│    ← Ground Plane

Microstrip Impedance Formula (Approximate):
Z₀ = (87 / √(εᵣ + 1.41)) × ln(5.98 × H / (0.8 × W + T))

Stripline

A stripline is a trace embedded between two ground planes inside the PCB. The trace is completely surrounded by dielectric material, providing better shielding and isolation.

|/////////////|    ← Top Ground Plane
└───────────────┘  ↑
│    PCB        │  │ H (height)
┌───┬───────┬───┐  │
│   │ Trace │   │  ↓
└───┴───────┴───┘  
|/////////////|    ← Bottom Ground Plane

Stripline Impedance Formula (Approximate):
Z₀ = (60 / √εᵣ) × ln((2 × H) / (0.8 × W + T))

Key Parameters Affecting PCB Impedance

Trace Width (W): Wider traces have lower impedance. This is typically the parameter adjusted to achieve target impedance.
Trace Thickness (T): Standard copper weights (1 oz = 35 μm) affect impedance. Thicker copper lowers impedance slightly.
Dielectric Height (H): The distance between the trace and ground plane. Greater height increases impedance.
Dielectric Constant (εᵣ): Material property that affects impedance. Higher dielectric constants lower impedance. Typical FR-4 values range from 3.8 to 4.6.
Trace Geometry: Edge coupling, surface roughness, and etch factor can all affect the actual impedance of a trace.

Common Standard Impedances

50Ω

Standard for single-ended RF circuits, test equipment, and many high-speed digital interfaces. Optimized for power transfer in RF applications.

75Ω

Used for video applications, cable TV connections, and some RF applications. Optimized for voltage transfer with minimal loss.

100Ω (Differential)

Standard for differential pairs in USB, Ethernet, PCIe, HDMI, and other high-speed interfaces. Each trace in the pair is typically designed for 50Ω single-ended impedance.

PCB Impedance Control Best Practices

Always specify impedance requirements to your PCB manufacturer
Include test coupons on production panels for impedance verification
Maintain consistent trace widths throughout controlled impedance paths
Avoid sharp corners (use 45° angles or curves) on high-frequency traces
Keep traces away from board edges and other sources of impedance discontinuity
Consider using specialized RF-grade PCB materials for critical applications
Account for manufacturing tolerances in your design calculations

Important Note

While this calculator provides good estimates of PCB impedance based on standard formulas, actual impedance can be affected by many factors including manufacturing tolerances, material variations, and trace geometries. For critical applications, verify designs with electromagnetic field solvers and prototype testing.

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Frequently Asked Questions

PCB trace impedance is the opposition to the flow of electrical signals through PCB traces. It's determined by the geometry of the trace (width, thickness), the dielectric properties of the PCB material, and the trace's position relative to ground planes.

Controlling PCB impedance is crucial for high-speed or RF designs to ensure signal integrity, minimize reflections, reduce cross-talk, and maintain proper signal timing. Uncontrolled impedance can lead to signal degradation, EMI issues, and circuit malfunction.

Common standard impedances include 50Ω for single-ended RF and high-speed digital signals, 75Ω for video applications, and 100Ω for differential pairs like USB, HDMI, and Ethernet. The specific standard depends on the application requirements.

PCB impedance can be measured using a Time Domain Reflectometer (TDR) or Vector Network Analyzer (VNA). Many PCB manufacturers also offer impedance testing services where they measure test coupons included on the panel with your PCB.

PCB impedance is affected by trace width, copper thickness, distance to ground/power planes (dielectric thickness), dielectric constant (Er) of the substrate material, trace geometry (microstrip vs. stripline), and surrounding materials.

A microstrip is a trace on the outer layer of a PCB with a ground plane beneath it, exposed to air on one side. A stripline is a trace embedded between two ground planes inside the PCB. Striplines offer better isolation and less radiation but are harder to access for probing.

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