Design Recommendations for PCB Impedance Control

October 12, 2021 Cadence PCB Solutions

Key Takeaways

  • Defining impedance control

  • Circuit board layer stackups

  • Setting up your PCB design tools for effective impedance control trace routing

Differential pair routing for PCB impedance control

A placed and routed circuit board

As they say, there’s a price to be paid for everything. We all enjoy the massive amount of computing power that smartphones put in the palm of our hands, but the price of this functionality is an enormous increase in design complexity. High frequencies, faster device switching speeds, and elevated amounts of noise and interference have made PCB layouts more complicated as engineers combat signal integrity problems in their designs.

We can no longer consider circuit board trace routing as simple point-to-point connections. It is essential that layout designers understand critical net requirements in their designs and layout the circuitry accordingly. These traces must be treated as transmission lines and configured and routed for optimum impedance control to ensure they provide the clearest signal quality and full data integrity. To do this, here are more details on PCB impedance control in circuit board layout.

The Need for PCB Impedance Control

The combination of capacitance and inductance creating opposition to current flow is what is referred to as impedance. All wires and traces in a PCB design will generate some impedance, which is measured in ohms. Due to the differing environments spread across a circuit board, the impedance value can change or be considered “uncontrolled.” And while uncontrolled impedance may not be an issue for many circuits on a PCB, it can greatly affect high-frequency signals.

At higher frequencies, signals behave more like high-speed transmission lines instead of regular point-to-point connections. The success of these signals depends on their clear transmission and reception between the source and the load. However, with varying impedance values throughout the routed trace, the signal can be reflected and travel in the opposite direction of the original signal. This reflection will superimpose itself on the initial signal, causing a distortion that results in a changed signal profile between the transmitter and the receiver. This distortion will degrade the signal’s integrity, perhaps to the point of it failing to fulfill its intended function.

To prevent signal integrity problems like these, high-frequency transmission line circuits must travel through traces with impedance values controlled evenly throughout the line. This is done by controlling both the geometry of the conducting trace and the environment the trace is routed through. The trace is controlled by specifying its routing width and copper weight, while the environment is controlled through careful selection of the dielectric material containing the trace for the proper Dk (dielectric constant) value. It is also important to configure the board layer stackup with dedicated routing layers for controlled impedance lines that are next to, or sandwiched between, two ground planes. This is commonly referred to as a microstrip or stripline configuration. Lastly, the controlled impedance environment will regulate the dielectric spacing between the traces and the planes as well as the spacing to other traces.

Next, we will look more in-depth at the board layer stackup for PCB impedance control routing.

A circuit board with differential pair routing on the exterior layer

Differential pair routing on a circuit board

Setting Up Your PCB Layer Stackup for Impedance Controlled Routing

As we have seen, controlling the impedance for routing sensitive traces depends on the following factors:

  • Trace geometry: The cross-section volume of copper in the trace must be carefully chosen, as both the trace width and its thickness (copper weight) have to be factored into impedance calculations.

  • Dielectric material: The dielectric constant (Dk) value of the non-conductive PCB material used to support and insulate the trace from the ground planes is also part of the impedance calculation.

  • Spacing: The final part of the calculation is the vertical spacing between the signal trace and the adjacent ground planes and the spacing to traces of other signals.

To successfully control the impedance of transmission line routing, dielectric board materials must be matched to the traces geometry. This may require specific material types for PCB fabrication and adjustments to the trace geometry and spacing values. The Dk value of the dielectric material will vary with frequency, so the impedance calculation for each board will be different depending on the frequency of the circuitry. Designers can use online calculators to determine the board layer stackup, plus most PCB design systems include their own impedance calculators. Additionally, the circuit board manufacturer can often help with these calculations.

PCB manufacturers understand the relationships between board materials and circuitry needs due to their experience building multiple high-speed designs. They can help with your board layer stackup configuration as long as they have the following data from you:

  • The target impedance for single-ended and differential pair controlled impedance routing on your board.
  • The desired board layers for controlled impedance routing.
  • The target board materials.

With this information, the circuit board manufacturer can often recommend board layer stackup configurations that will be the most efficient for your design. Many manufacturers can also exchange this PCB layer data directly with your CAD system to cut down on the potential for error and increase your design time.

Let’s look at how you can set up your PCB design system for the most efficient routing of transmission lines for PCB impedance control.

Cadence Allegro’s Constraint Manager setting up the design rules for diff pair routing

The Constraint Manager from Cadence Allegro being used to set up design rules for diff pairs

Using the Full Extent of the Design Features in Your Layout Tools

The material choices, layer widths, and configurations of the layers in a PCB stackup are all important for controlling impedance. However, in addition to the stackup, other parameters of a design should be set up to help with controlled impedance routing. Some of these parameters are directly related, such as trace widths and spacings, while others will simplify your layout job.

Schematic

Take the time to ensure that the schematic is fully prepared before you start the layout. Obviously, the schematic will continue to be changed for design enhancements and alterations, but it should be ready to synchronize with the PCB layout database. This includes ensuring that its components are approved and updated, the controlled impedance signals are classified as differential pairs or single-ended nets, and some of the basic rules and classes are entered into the database.

Design Parameters

Many designers leave the parameters at the default settings, but modifying these to fit your personal preferences can make your job easier during layout. Selected net colors, fill patterns, and highlights are just a few of the changes that can enhance your efficiency with the tools. Others include units of measurement, grids, text display, and even how the toolbars and commands are configured.

Design Constraints

Your PCB CAD tools should have a comprehensive system of design rules and constraints that will allow you to set up rules for your controlled impedance trace routing. For instance, in Cadence Allegro’s PCB Editor, you can set up net classes and rules for a variety of signal requirements in your PCB design. One of these, as shown in the picture above, is the differential pairs in the design. Allegro allows you to set up all of the pertinent routing rules, including trace widths and spacings, to satisfy their electrical requirements.

Routing Commands

There are many trace routing features available in PCB layout systems. You can route traces to specified lengths, add curves, slide them, or route them in specific patterns or topologies. For controlled impedance routing, you need the ability to route traces at a specific width, spacing, and on designated layers. The routing features in your layout tools can easily accommodate this in conjunction with the design rules and constraints already set up. Your layout tools will also have routing functionality to automatically route differential pairs as pairs, as you can see in the picture below. These capabilities will save you the time and difficulty of trying to route them together manually while maintaining the required spacing to each other.

This only touches on a few of the many design capabilities within PCB layout tools today. As CAD systems continually improve, we will constantly see enhancements and upgrades to ease the workflow for designers working with PCB impedance control. For instance, here are a couple of advanced features that can help. 

Diff pair routing in Cadence Allegro’s PCB Editor

An example of differential pair routing made possible by automated trace routing features

Helpful Advanced Functionalities in PCB Design Tools 

To help designers solve signal integrity problems while they are laying out their PCB designs, Cadence has provided another useful tool in their arsenal of engineering software. The Impedance Analysis Workflow shown in the video at the top of this article allows designers to analyze their critical nets to find potential SI problems. Once corrected, a second run of the analysis will show how the changes have improved the signal integrity of the board.

We also mentioned earlier the importance of communicating with your PCB manufacturer to exchange critical board layer stackup information. Cadence has also helped in this area, with features that import and export board layer stackup information in the IPC-2581 format. This allows you to partner with a similarly equipped manufacturer and pull their data directly into your design. Instead of relying on verbal or written communication, you can pull in design-ready data for your board layer stackup, conductive and dielectric materials definitions, coatings, and other important physical PCB characteristics.

To learn more about board layer stackup strategies, please look at our E-books on stackup cost adders and DFM considerations.

If you’re looking to learn more about how Cadence has the solution for you, talk to our team of experts

About the Author

Cadence PCB solutions is a complete front to back design tool to enable fast and efficient product creation. Cadence enables users accurately shorten design cycles to hand off to manufacturing through modern, IPC-2581 industry standard.

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