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26 DESIGN007 MAGAZINE I JULY 2019 Design methodologies change over time, par- ticularly in the ways to simulate electromag- netic fields and return current paths. In my previous column series on stackup planning, I described the traditional stackup structures that use a combination of signal and power/ ground planes. But to achieve the next level in stackup design, one needs to not only consid- er the placement of signal and plane layers in the stackup but also visualize the electromag- netic fields that propagate the signals through the substrate. The four-part stackup planning column se- ries was published over several months in 2015 in The PCB Design Magazine: • In Part 1 [1] , I looked at how the stackup is built, the materials used in construction, and the lamination process. And I set out some basic rules to follow for high-speed design. It is important to keep return paths, crosstalk, and EMI in mind during the design process. • Part 2 [2] followed with definitions of basic stackups, starting with four and six layers. Of course, this methodology can be used for higher layer count boards (36 layers, 72 layers, and beyond). The virtual mate- rials were replaced with items stocked by the PCB fabricator. • Part 3 [3] examined higher layer count stackups as the four- and six-layer config- urations are not the best choice for high- speed design. In particular, each signal layer should be adjacent to, and close- ly coupled to, an uninterrupted reference plane, which creates a clear return path and eliminates broadside crosstalk. As the layer count increases, these rules become easier to implement but decisions regard- ing return current paths become more challenging. More rules for HSD and EMI were also defined. • In Part 4 [4] , I elaborated on 10+ lay- er counts. The methodology I set out in previous columns can be used to con- Stackup Planning, Part 5 Beyond Design by Barry Olney, IN-CIRCUIT DESIGN PTY LTD / AUSTRALIA

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