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We can all agree that controlling impedance is critical to signal integrity and board performance in devices powering high speed digital applications, telecom, or RF communication. 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 delay, cost overrun, and even batches of useless boards.
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:
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 — 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, the other risks manufacture of boards that may not work properly.
We believe that proactive design method, not reliance on testing is the best method for controlling impedance and, just as important, setting the stage for an efficient production of quality boards. PCB impedance callouts are helpful, but they are not as foolproof as simply 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 result when notes do not match 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 you transition from design to manufacture. The documentation typically defines the impedance, not the trace size, or gives a trace width that covers the whole 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. More often than not manufacturers will not know for sure what type of product the board will be used in or if there are underlying reasons why trace size is just as critical as the defined impedance. Speaking on behalf of manufacturers, we very much prefer it when the design shows us what the trace sizes should be versus working backwards from the impedance-related notes.
The math is the math when it comes to controlled impedance and it has to be done at some point. Why not do it during the design phase rather than after it? If you know the right trace width, draw it in. It doesn’t have to be hard work. There are a variety of free online controlled impedance calculators available to make the math easy. No point in solving a quadratic equation (if it were only that easy) with pencil and paper if there’s a button on your calculator that will do it in less than a second.
If you feel like breaking out the papyrus and doing the math yourself, here is one of the simpler equations:
Or you can just use a calculator…
Your documentation should not be a confusing ambiguity. When you design for controlled impedance, the documentation instead becomes a double check that can help speed your design through the manufacturing process. If you design your board to hit the exact impedance number that would otherwise be called out in the documentation, you should be well within the manufacturer’s tolerance range.
We recognize that testing is important in many instances, such as when there are high performance or special materials requirements. In any case, tests are far more enjoyable when you know the answers to the questions before hand. When you integrate impedance math into your design process, you get quality boards faster and more efficiently.
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