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34 FLEX007 MAGAZINE I OCTOBER 2018 ware that is designed to predict how each of the dielectrics, reference planes, and circuits relate to one another. Trying to do this in a rigid-flex design using free online impedance calculators will almost always return false val- ues. There are just too many interactions to use single-value calculators. Additionally, some of the material suppliers give global DK values when they can be different at different signal speeds in reality. The best path forward is to purchase the software, which is available as a standalone prod- uct for an annual license fee and is often included and built into some of the more popular CAD PWB layout tools. The software is not cheap, but it is far more accurate than the free online tools. Whether you decide to purchase the software or not, it is always wise to involve your fabricator at the start of your design to either predict the impedances you desire, or to double check your work if you used your own software. Your fabricator does impedance mod- eling dozens of times a day and has modeled impedance circuits for many years. They will have a material library with Dk values for the different material constructions and will know all of the impedance values for every thickness of every dielectric they use. For each imped- ance value that you want to be mod- eled, tell your fabricator the value you desire, the type or characteristic of differential, on what layer(s), what speed the signal is at, what layer(s) the reference plane(s) are on, and any mechanical considerations (e.g., the board cannot be thicker than 0.062", etc.). There are two important differences to keep in mind with impedance modeling of rigid- flex circuit boards. The first is that the values, trace widths, and spacings will be different in the flex sections than in the rigid sections. Your fabricator should provide you with a model showing both calculations whenever you have impedance-controlled circuits in both the flexible sections and the rigid sec - tions of the board. As a designer, this will require you to neck down the circuits to their correct geometries and spacing. The neck- down should occur 0.050" or more into the rigid board to prevent stress on the circuits at the neckdown area and the flex-to-rigid tran- sition area. You should also note that the number of impedance values can quickly multiply with rigid-flex designs. Each value often needs modeling and testing in the flex and rigid sec- tions of the board. Because of this and the fact that each value needs to be tested in the impedance cou- pons that are built into your part's production panels, the size of the coupons can grow very large, very quickly. The larger the impedance coupons on the panel, the fewer number of parts that your fabrica- tor can fit on the panel, which ulti- mately increases your cost. Thus, it is wise to specify only those circuits that you truly need to be tested on your print. You and your fabricator can model all the values you want through the whole PWB, but just put on the print the values you truly need. Material Layup If you modeled the impedances as previously described, you are approximately 90% of the way towards a material layup. The report will give you the material cross-section with copper thick- nesses, dielectric thicknesses, Dk values, etc. If you don't have any impedance values to model, or you just need a straightforward material layup, ask your fabricator for their recommendations. Here is what they will want from you to get started: Figure 1: Six-layer rigid-flex board with controlled impedance circuitry.

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