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Connect the Dots: Proactive Controlled Impedance

From data centers to smartphones, designers know that the ohms have it. Getting impedance right ensures all-important signal integrity and delivers high-performing boards. Our designers understand the importance of controlled impedance, but not everyone addresses it in their designs. The most common and important controlled impedance types we see include microstrip, stripline, embedded microstrip, and differential pairs.

• Microstrip: The most used form of controlled impedance because it is easy to design and cost-effective. It creates a transmission line using a signal trace on one side of a dielectric layer and a ground plane on the other. Designers can adjust the width and thickness of the trace and the thickness of the dielectric layer to achieve a specific controlled impedance. 
• Stripline: A PCB trace with a signal path routed on an internal layer and propagated between two ground planes. To achieve controlled impedance throughout the entire length of the trace, designers need to calculate the width of the signal trace, thickness of the material between layers, and the distance between the signal trace and ground planes.
• Embedded microstrip: Transmission lines have a signal trace inside the PCB’s dielectric, placed between two layers. To achieve controlled impedance, designers must calculate the trace width, spacing, and dielectric thicknesses of the surrounding material.
• Differential pairs: Two traces on a board that carry equal but opposite signals. Trace width, spacing, copper thickness, and other factors influence impedance. We recommend designers specify controlled impedance signals for differential pairs in the schematic.

Designers need to perform impedance calculations—specifying parameters for controlled impedance in the schematic—and include impedance-related notes with their PCB designs. Every PCB manufacturer’s process is slightly different, so calling out the impedance requirements in the schematic allows the manufacturer to confidently tweak trace width, spacing, material thickness, and type as needed.

If you know your impedance math and do the calculations necessary to create a precise design, the result will be a smoother manufacturing process. PCB trace impedance is determined by its inductive and capacitive reactance, resistance, and conductance (usually ranging from 25 to 125 ohms). Factors dictating impedance include:

• Distance from other copper features
• Width and thickness of the copper signal trace
• The thickness of the material on either side of the copper trace
• The dielectric constant of board material

You can save time, money, and effort if you are aware of the impedance math when you design your board by using one of the many quality impedance calculators. This allows you to build the right tolerances into your design. Impedance testing becomes a double check of your work instead of the tool you rely on to tell you if your documentation is correct.

While documenting impedance requirements properly may seem simple—state your target impedance, trace requirements, and material tolerances—PCB documentation is a details game that often leaves knowledge gaps for your manufacturer. For example, picture a design for a four-layer board with two signal layers, two planes, and a seemingly complete set of drawing notes. What if the documentation doesn’t specify whether both signal layers and trace widths require impedance control? The board manufacturer must then make assumptions, cross their fingers, and move to production or kick the design back to the designer for clarification. One scenario slows you down; the other risks manufacturing unusable boards.

Proactive design methodology, not reliance on testing, is the best way to control impedance and pave the way for efficient production of quality boards. PCB impedance call-outs are helpful, but not as foolproof as crafting a design with the right distance to the reference plane, trace widths, and materials tolerances.

Incomplete or incorrect impedance-related notes are common and can directly impact both board cost and performance. Delays occur when notes do not match the design, there are two trace widths for the same impedance on the same layer, or each signal layer does not have its own impedance requirements. Sometimes the adjustments required are not possible, because they cause interference with other features.

Lack of specificity in the notes can result in extra effort when transitioning from design to manufacture. The documentation typically defines the impedance, not the trace size, or gives a trace width that covers the entire board. Determining the trace size in this case falls to your manufacturer. They can vary trace width, height, and thickness to ensure the correct impedance, but they cannot read minds. Often, manufacturers will be uncertain as to what type of product the board will be used in or if there are underlying reasons why trace size is as critical as the defined impedance. Speaking on behalf of manufacturers, we prefer it when the design shows us what the trace sizes should be, versus working backward from the impedance-related notes.

When you design for controlled impedance, the documentation no longer acts as tea leaves for the manufacturer to guess at. They know you made the effort to get in range, and they have a clear target. If you design your board to hit the exact impedance number that would otherwise be called out in the documentation, you should be within the manufacturer’s tolerance range. 

We recognize that testing is important, such as when there are high-performance or special materials are required. Tests are far more enjoyable when you already know the answers to the questions. When you integrate impedance math into your design process, you get quality boards faster and more efficiently.

This column originally appeared in the May 2025 issue of Design007 Magazine.