PCB Impedance Calculator

PCB Trace Impedance Formula:

\[ Z_0 = \frac{87}{\sqrt{\varepsilon_r + 1.41}} \times \ln\left(\frac{5.98 \times H}{0.8 \times W + T}\right) \]

Dielectric Constant (εr):

unitless

Height (H):

mil

Width (W):

mil

Thickness (T):

mil

Characteristic Impedance (Z₀):

Unit Converter ▲

Unit Converter ▼

From:	To:

1. What is PCB Trace Impedance?

PCB trace impedance is the characteristic impedance of a transmission line on a printed circuit board. It's a critical parameter in high-speed digital and RF circuit design that affects signal integrity, reflection, and electromagnetic compatibility.

2. How Does the Calculator Work?

The calculator uses the microstrip impedance formula:

\[ Z_0 = \frac{87}{\sqrt{\varepsilon_r + 1.41}} \times \ln\left(\frac{5.98 \times H}{0.8 \times W + T}\right) \]

Where:

\( Z_0 \) — Characteristic impedance (ohms)
\( \varepsilon_r \) — Dielectric constant of the substrate material
\( H \) — Height of the substrate (mil)
\( W \) — Width of the trace (mil)
\( T \) — Thickness of the trace (mil)

Explanation: This formula calculates the characteristic impedance of a microstrip transmission line, which consists of a conducting strip separated from a ground plane by a dielectric layer.

3. Importance of Impedance Matching

Details: Proper impedance matching is essential for minimizing signal reflections, reducing electromagnetic interference, and ensuring reliable data transmission in high-speed circuits. Mismatched impedance can cause signal degradation, data errors, and reduced system performance.

4. Using the Calculator

Tips: Enter the dielectric constant (typically 4.2-4.5 for FR4), substrate height, trace width, and trace thickness. All values must be positive numbers. For best results, use manufacturer-provided dielectric constant values.

5. Frequently Asked Questions (FAQ)

Q1: What is a typical impedance value for PCB traces?
A: Common values are 50Ω for RF circuits and 75Ω for video applications, though specific requirements vary based on the application and signal characteristics.

Q2: How accurate is this formula?
A: This formula provides a good approximation for typical PCB designs, but for precise calculations, specialized electromagnetic simulation software should be used.

Q3: What factors affect PCB trace impedance?
A: Dielectric constant, substrate thickness, trace width, trace thickness, and the presence of solder mask all affect the final impedance.

Q4: Why use mil units instead of millimeters?
A: The PCB industry traditionally uses mils (thousandths of an inch) for measurements, though many modern tools support both metric and imperial units.

Q5: When is impedance control critical?
A: Impedance control is crucial for high-speed digital signals (above 50-100 MHz), RF circuits, differential pairs, and any application where signal integrity is paramount.