PCB Impedance Calculation Guide

Detailed explanation of the PCB circuit board impedance calculation formula

Mainly covers the origin, significance, calculation method and precautions of impedance in practical applications.

1.The origin and significance of impedance：

1. Impedance is derived from the telegraph equation and is an important parameter when a signal propagates in a transmission line.
2. Inconsistent characteristic impedance will cause reflection and affect signal integrity, so impedance matching is crucial in circuit design.

2.Basic Concepts of Impedance Calculation：

• Stackup: The multi-layer structure of a PCB board, including power/ground layers and routing layers.
• Oz (ounce): In PCB manufacturing, it represents the unit of copper thickness. 1Oz is equal to 28.3 grams of copper spread over an area of ​​one square foot.

• Dielectric constant (DK): A parameter that describes the electrical properties of dielectric materials and affects the signal transmission speed and impedance value.

3.Calculation of characteristic impedance of propagation line：

• Propagation line types: microstrip line and stripline. Microstrip line has only one reference ground, while stripline has two reference grounds.
• Impedance calculation requirements: Single-line impedance is usually 55 or 60Ohm, differential line impedance is 70~110Ohm, and thickness is required to be 1~2mm.
• Calculation method: Use a specific formula (such as polar si6000) to calculate and determine the line width (W) and line spacing based on the trace impedance requirements。

4.Characteristic impedance formula：

• Microstrip Line Formula：Z＝{87/[sqrt(εr+1.41)]}ln[5.98H/(0.8W+T)]，Applicable to the cases of 0.1<(W/H)<2.0 and 1<(εr)<15.
• Stripline Formula：Z=[60/sqrt(εr)]ln{4H/[0.67π(0.8W+T)]}，Applicable to situations where W/H<0.35 and T/H<0.25.

5.Calculation Considerations：

• Choose the appropriate calculation method based on the manufacturer's process capabilities。
• The surface routing may be calculated using coated microstrip, taking into account the impact of the manufacturer's green paint。
• Manufacturers may adjust parameters during the actual manufacturing process, so the calculation results do not need to be very accurate.
• When calculating differential impedance, the requirements of differential impedance and single-ended impedance must be met at the same time。

6.Practical considerations：

• Impedance matching is critical to reducing reflections, crosstalk, and other issues。
• When designing a PCB, you need to select the appropriate impedance value and routing method based on the signal transmission requirements and board characteristics.。
• Closely communicate with manufacturers to ensure that the impedance value in actual production meets the design requirements。

This document explains in detail the principles and methods of PCB impedance calculation, including the origin, significance, calculation formula and practical application precautions of impedance. For more information, please contact：  https://www.cmaisz.com/