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Design007-Jan2021

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14 DESIGN007 MAGAZINE I JANUARY 2021 design rules that are slightly larger than they planned when they don't understand all the trade-offs and tolerances that we have to con- sider in our fabrication processes. Shaughnessy: It's where the rubber meets the road. Ellis: It's totally where the rubber meets the road. It's better to plan carefully before you start putting the rubber to the road. Shaughnessy: That's one of the things that we hear: Some designers are doing stackups as they go. Ellis: Yes, and if they don't have all the fabrica- tor's input, which they would not necessarily know from their experience and point of view, they can accidentally start a design with extreme features and end up with a finished product that can be manufactured in small prototype quanti- ties, but not in production volumes. Shaughnessy: I've heard some designers say that if they can get the stackup right, that's 80% of the design, basically. Ellis: It could be, especially if their fabricator also provides all the minimum design guide- lines, before they start routing their circuits. Shaughnessy: Give us a quick overview of how you work with a customer who has a potential problem with the stackup. Ellis: With my regular customers, if they have a new product and it's got special requirements, they call me before they even start routing their boards. For rigid designs, we begin by discussing project requirements to determine the subgroup, which establishes which fab sites should be considered, by the complexity of the stackup. Then we review the specific design attri- butes shown below, including BGA pitches and geometries, to provide enough information to build the initial stackup: • Stackup • Material • Overall thickness (tol +/-10%) • Number of lamination cycles (adds cost) • Layers – Number – Cu thickness of each layer Figure 1: Rigid PCB conventional and HDI subgroups. Pick the simplest subgroup that your design can conform to. (Figure 1)

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