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Connect the Dots: A Proactive Approach to Controlled Impedance

Most can agree that controlling impedance is critical to signal integrity and board performance in devices powering high-speed digital applications, telecommunications, or RF communications. How to do so is another matter. It is common practice to include impedance-related notes with a PCB design and rely on the manufacturer to determine the proper trace parameters. This inherently passive methodology often leads to delays, cost overruns, and even batches of useless boards.

PCB trace impedance is determined by its inductive and capacitive reactance, resistance, and conductance, usually ranging from 25–125 ohms. Factors dictating impedance include:

• The distance from other copper features
• The width and thickness of the copper signal trace
• The thickness of 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 sit down to design your board. Gain this awareness by using a good impedance calculator, and you can 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. Documenting impedance requirements properly is more onerous than most people realize. Though it seems simple (e.g., 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 and two planes and a seemingly complete set of drawing notes. Now, let’s say the documentation doesn’t specify if both signal layers and trace widths require impedance control. In this case, the board manufacturer makes assumptions and heads for production or kicks it back to the designer for clarification. One scenario slows you down, and the other risks manufacture of boards that may not work properly.

To read this entire column, which appeared in the September 2019 issue of Design007 Magazine, click here.