How to Calculate PCB Impedance Control for Your Circuit Board

April 2, 2020 , in Blog

Science and mathematics are indeed based on precision, or the ability to assign an accurate measurable value to items, actions, and events. Even so, imponderables still remain and must be addressed when analyzing, designing, and building physical systems. A common one that permeates through most scientific, mathematic, and engineering work is the Heisenberg uncertainty principle, which states that the position and speed of an object (including subatomic particles) cannot both be measured simultaneously. And we are still trying to finalize an exact solution for π.

Using Smith Chart to calculate PCB impedance

Calculating PCB Impedance Control using Smith Chart

When designing PCBAs, in particular, high speed board design, determining the best impedance value is very important, as it provides control for trace parameter definition and board material selection. Explicitly determining or calculating impedances on a circuit board, however, poses a significant challenge. This was true when designers primarily used Smith charts and continues to be so today. Yet, the importance of board impedance control mandates that we employ a reliable method that does yield acceptable results. Therefore, let’s take a look at some of the options available and see if we can hone in on a PCB impedance control calculation method that is both accurate and practical.

PCB Impedance Calculation Methods

Signal propagation in PCB design is basically transmission line design, right? So why is it such a challenging task? The answer to that question speaks directly to the challenge. Today, circuit boards are complex, small and multifunctional. This means that you are often confronted with multiple signal types (e.g. digital, RF and DC) and different types of traces (e.g. microstrip and stripline) that are of varying length and orientation. Additionally, there are constraints to contend with such as creeping and clearance, board edge, footprint pad layout, and trace width and spacing distances. Additional concerns are matching input and output connection impedances, not to mention material properties that impact impedance (e.g. dielectric constant).

Obviously, managing all of these board parameters can quickly become cumbersome. Not to mention, with the objectives of maximizing signal integrity, eliminating reflections and minimizing noise or EMI. Even in the face of these daunting challenges, PCB impedance control is an important parameter that requires a determination method. To simplify calculations, it is almost essential to make assumptions as a comprehensive closed-form solution is much too complex. This, coupled with the uniqueness of each design, means there is an almost inexhaustible number of possibilities for calculating board impedance control, and your results will likely vary to some degree for different methods. Nevertheless, the following methods are commonly applied.

Impedance Calculation Methods

  • Smith chart

The Smith chart has been around since long before computers and PCB design software was available. It is used to graphically determine the reflection coefficient relationship between source and load impedances. A series of mathematical manipulations can then be used to yield the impedance value for the line or trace.

For more information on using Smith charts, see Impedance Matching And The Smith Chart Tutorial.

  • Computer simulation

Today, the more advanced PCB design software packages include control impedance calculation functionality. The advantage here is that your other design parameters are available and can be automatically modified to perform various simulations from which you can select the best design for manufacture. The disadvantage is that this type of functionality can cause the design software to be quite expensive.

  • Online Calculator

Much more common than design packages with PCB impedance control simulation functionality are online calculators where you can find either controlled impedance or trace parameters given the desired impedance. Although not as comprehensive as simulation programs, these can put you in the ballpark where only minor tweaking by your contract manufacturer (CM) may be necessary for manufacturability. Most of these calculators are based on calculations given in IPC 2141A Design Guide for High-Speed Controlled Impedance Circuit Boards.

Each of the methods listed above can be utilized to determine the PCB impedance control for your design. Whether or not you opt for one of these specific methods, it is probably a good idea for you to include a simple and reliable method in your design toolbox as well.

A Quick Practical Method to Calculate PCB Impedance Control

To make the best use of your impedance control, it should be performed early during your board layout as opposed to at the end. This is so that you can incorporate the effects on your spacing between surface elements as well as stackup parameters, such as dielectric height before finalizing your routing. By following the procedure below, you will quickly arrive at a reasonably accurate controlled impedance value.

PCB Impedance Control Calculation Process

1. Calculate rise time for your trace.

2. Calculate the maximum trace length before transmission effects become an issue.

3. Calculate the characteristic impedance for your trace based on trace type and geometry.

Example for a simple microstrip:

NOTE: H, W, and T are defined by based on your design’s PCB layout for traces criterion. This includes current level requirements, spacing constraints, and surface availability.

For other equations, please refer to IPC-2141A. These calculations should be done for the trace that will carry the highest frequency signals to determine the control impedance. This impedance can then be used to calculate trace parameters for other paths.

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Regardless of your technique for determining the PCB impedance control, it is advisable that you work in conjunction with your CM, as DFM rules and guidelines, as well as material availability, will impact the manufacturing of your board. At Tempo, the industry’s fastest PCB prototype and low volume manufacturer, we have the capabilities to ensure that your design intent is met and your boards are buildable.

And to help you get started on the best path, we furnish information for your DFM checks and enable you to easily view and download DRC files. If you’re an Altium Designer or Cadence Allegro user, you can simply add these files to your PCB design software. For Mentor Pads or other design packages, we furnish DRC information in other CAD formats and Excel.

If you are ready to have your design manufactured, try our quote tool to upload your CAD and BOM files. If you want more information on calculating PCB Impedance Control for your high-speed design, contact us.

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