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Design007-Sept2019

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70 DESIGN007 MAGAZINE I SEPTEMBER 2019 ness 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 proper- ly is more onerous than most people realize. Though it seems simple (e.g., state your tar- get impedance, trace requirements, and mate- rial tolerances), PCB documentation is a de- tails game that often leaves knowledge gaps for your manufacturer. For example, picture a design for a four-lay- er board with two signal layers and two planes and a seemingly complete set of drawing notes. Now, let's say the documentation doesn't spec- ify if both signal layers and trace widths re- quire 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 (Figure 1). At Sunstone, we believe that a proactive de- sign method, not reliance on testing, is the best way to control impedance and set the stage for an efficient production of quality boards, which is just as important. PCB impedance callouts are helpful, but they are not as fool- proof as simply crafting a design with the right distance to the reference plane, trace widths, and materials tolerances. Incomplete or incor- rect impedance-related notes are common and can directly impact both board cost and per- formance. 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 re- quired are not possible because they cause in- terference with other features. Lack of specificity in the notes can result in extra effort when you transition from design to manufacture. The documenta- tion typically defines the imped- ance, 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 rea- sons why the trace size is just as critical as the defined impedance. At Sunstone, we much prefer it when the design shows us what the trace sizes should be versus working backward from the impedance-related notes. The math is the math when it comes to con- trolled 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. There's no point in solv- ing a quadratic equation (if it were only that easy) with a pencil and paper if there's a but- ton on your calculator that will do it in less than a second. Figure 1: Assumptions can slow you down. Figure 2: Example controlled impedance equation.

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