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32 FLEX007 MAGAZINE I OCTOBER 2018 If you are new to rigid-flex designs—or have never done a rigid-flex PWB layout—you might wonder how it is similar to and different from hardboard design. In previous columns, I've discussed cost drivers in rigid-flex boards and applications where rigid-flex designs will outperform all other packaging methods, which more than justifies the increased cost. In this column, I'll address critical items you need to know to successfully create a stable and robust rigid- flex design. Layers Gerber files, artwork, and other aspects of communicat- ing your rigid-flex design to a fabricator are very similar to what you would provide your fabricator for a hard board. Some folks don't real- ize that the flex layers extend all the way through the rigid section on a rigid-flex board. This is why rigid-flex boards provide such a high degree of reliability in high- shock, or high-vibration, envi- ronments. The flex layers are integrated right into the board, just like a layer of 0.004" core in a rigid board. A typical fabrication package will look simi- lar to a hardboard design. The Gerber files, drill files, and design guidelines and recommenda- tions will be very much alike. Where they dif- fer is in controlled impedance requirements, material layups, detailed fabri- cation drawings, and special design rules around the rigid-to-flex transition areas. Thus, a typical rigid-flex fabrication package will look com- parable to a hardboard design package. Impedance Considerations Controlled impedance traces have become more pro- lific in digital and high-speed cir- cuit designs. The software and test vehicles to provide this level of control have progressed in step with the demand as well. The dielectric materials in the flexible sections are different than hardboards and generally pro- vide better electrical performance. The coverlayers and bondplys also offer better electrical performance. These materials have varying dielectric constant (Dk) values and should be modeled in soft- Flex Time by Bob Burns, PRINTED CIRCUITS Pointers for Your First Rigid-Flex Design

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