AN 958: Board Design Guidelines

ID 683073
Date 1/28/2022
Public
Document Table of Contents

5.1.8.1. Microstrip Impedance

A circuit trace routed on an outside layer of the PCB with a reference plane (i.e., GND or VCC) below it constitutes a microstrip layout. Use the following equation to calculate the impedance of a microstrip trace layout.

Using typical values of W = 8 mil, H = 5 mil, T = 1.4 mil, εr and (FR-4) = 4.1 and solving for microstrip impedance (Z0) yields:

The measurement unit in the equation is mils (i.e., 1 mil = .001 inches). Also, copper (Cu) trace thickness (T) is usually measured in ounces (i.e., 1 oz = 1.4 mil).

Figure 72 shows microstrip trace impedance versus trace width (W) using the values in the equation, keeping dielectric height and trace thickness constant.

Figure 72. Microstrip Trace Impedance with Changing Trace Width

Figure 73 shows microstrip trace impedance versus height (H), using the values in the microstrip trace impedance equation, keeping trace width and trace thickness constant.

Figure 73. Microstrip Trace Impedance with Changing Height

The impedance graphs show that the change in impedance is inversely proportional to trace width and directly proportional to trace height above the ground plane.

Figure 74 plots microstrip trace impedance versus trace thickness (T) using the values in the microstrip trace impedance equation, keeping trace width and dielectric height constant. This figure shows that as trace thickness increases, trace impedance decreases.

Figure 74. Microstrip Trace Impedance with Changing Trace Thickness
  • Z0 = [(87/√(εr + 1.41)) ln [(5.98 ×H)/(0.8W + T)] Ω
  • Z0 = [(87/√(4.1+1.41)) ln (5.98×(5)/0.8(.8)=1.4)] Ω
  • Z0 ~ 50 Ω
 

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