Microstrip Impedance Calculator

Zachariah Peterson
|  Created: February 9, 2022  |  Updated: April 19, 2022
Microstrip impedance calculator

Microstrips are one common routing style used in mid-range frequency applications like sub-1 GHz radios, as well as higher speed buses with signal-ended signals (e.g., DDR). If you need to find and use an efficient and accurate microstrip impedance calculator that provides lossless impedance values, you can use our free tool below. This impedance calculator gives you a good start for estimating the trace width and PCB dielectric thickness that will produce a target impedance value. Make sure you use an accurate tool like this before you start building your stackup as this can help you identify the best laminate materials to use for your target impedance and trace width.

How to Use a Microstrip Impedance Calculator

Most of the impedance calculators you’ll find online use a similar process: enter your microstrip geometry and Dk value for your PCB substrate materials, and the tool will return the impedance for your microstrip trace. As you adjust the width of the line, you can eventually get the calculator to converge on the desired impedance value, and you have finished designing the microstrip trace.

Note that some online calculators will determine the width for a given target impedance, copper weight, and substrate thickness. These other calculators will just use the IPC-2141 formulas that are solved for the trace width. The results are known to be inaccurate, especially when one examines the effective dielectric constant predicted with these calculators. However, the calculator provided here uses a more accurate, but more complex, set of equations for trace impedance.

 
 
 
 
 
 
 
 
Microstrip impedance geometry

 

Results

 

The calculator shown above uses Wadell’s equations for microstrip impedance, which can be found in the seminal textbook Transmission Line Design Handbook. These equations are relatively simple compared to other transmission line geometries, comprising only 2 interrelated equations for the microstrip impedance and effective dielectric constant. The resulting impedance value is known to be highly accurate over a broad range of frequencies spanning well beyond 1 GHz.

For differential pairs of microstrips, the formulas involved are much more complicated because coupling between the traces in the differential pair must be included in the impedance calculation. This makes the problem of determining impedance much more complex, and building an initial lossless model requires a larger set of formulas from Wadell’s textbook or an electromagnetic field solver. If you need to use differential microstrips, we have another differential impedance calculator that can give you a highly accurate estimate of differential microstrip impedance.

 

Microstrip Impedance Calculators Don’t Include Some Data

When you need to get an initial estimate of trace width required for a target impedance, then trace calculator tools are very useful can they can help your understand how your substrate materials will affect the impedance of a trace. However, there are many important pieces of information that aren’t included in online microstrip impedance calculators:

  • Losses - Almost every online PCB impedance calculator will completely ignore losses, or they simply produce a geometry-independent value for attenuation based on loss tangent, which is incorrect. The trace geometry and roughness of copper foils used in PCB layer stacks are also important. Learn More.
  • Dispersion - Geometric dispersion is included in the formulas, but not dielectric dispersion due to variations in the dielectric constant with frequency. Learn More.
  • Solder mask - While losses in the solder mask can be included in a PCB transmission line impedance calculation, the resulting formulas are very complex and are difficult to use in a calculator application. Learn More.
  • No etch factor - Etching causes traces to be somewhat trapezoidal on the finished board, which will cause the real trace to have a different impedance. Etch compensation is normally applied during front-end engineering to help correct this imbalance. Learn More.
  • No propagation delay - The propagation delay value for a PCB transmission line will always contain some loss, but this is not captured in simple online calculators, or if they do it’s estimated from an incorrect formula. Learn More.

Use a Layer Stack Tool With a Built-in Field Solver

Obviously, there is a lot that goes into estimating microstrip impedance. Improving the results and producing more accurate models requires manually modifying the lossless impedance model by adding back in the skin effect/roughness impedance, or you must solve Maxwell’s equations directly. While the latter approach is more accurate in an arbitrary geometry, this requires a field solver or a numerical model that is extracted from field solver results.

The advanced PCB Layer Stack Manager in Altium Designer includes a field solver that calculates impedance for common geometries, including microstrips, striplines, coplanar lines, and differential lines. These features are accessed in the Layer Stack Manager utility inside the PCB Editor in Altium Designer. Simply click the Impedance tab and create a new calculation, and then create a design rule based on the impedance calculation results. The only thing you need to know to get started is the dielectric constant of your substrate materials.

Differential microstrip impedance calculator
Altium Designer includes an integrated field solver that provides highly accurate microstrip impedance calculations.

The stackup design features in Altium Designer® can help you quickly get an accurate microstrip impedance calculation from a library of materials, or from totally custom materials with known material constant values. When you’re ready to release these files to your collaborators for more advanced simulations, the Altium 365™ platform makes it easy to collaborate and share your projects. Everything you need to design and produce advanced electronics can be found in one software package.

We have only scratched the surface of what is possible to do with Altium Designer on Altium 365. Start your free trial of Altium Designer + Altium 365 today.

About Author

About Author

Zachariah Peterson has an extensive technical background in academia and industry. He currently provides research, design, and marketing services to companies in the electronics industry. Prior to working in the PCB industry, he taught at Portland State University and conducted research on random laser theory, materials, and stability. His background in scientific research spans topics in nanoparticle lasers, electronic and optoelectronic semiconductor devices, environmental sensors, and stochastics. His work has been published in over a dozen peer-reviewed journals and conference proceedings, and he has written 1000+ technical blogs on PCB design for a number of companies. He is a member of IEEE Photonics Society, IEEE Electronics Packaging Society, American Physical Society, and the Printed Circuit Engineering Association (PCEA), and he previously served on the INCITS Quantum Computing Technical Advisory Committee.

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