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Estimated reading time: 8 minutes
Flex Time: Pointers for Your First Rigid-Flex Design
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 rigidflex design.
Layers
Gerber files, artwork, and other aspects of communicating 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 realize 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 highshock, or high-vibration, environments.
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 similar to a hardboard design. The Gerber files, drill files, and design guidelines and recommendations will be very much alike. Where they differ is in controlled impedance requirements, material layups, detailed fabrication drawings, and special design rules around the rigid-to-flex transition areas.
Thus, a typical rigid-flex fabrication package will look comparable to a hardboard design package. Impedance Considerations Controlled impedance traces have become more prolific in digital and high-speed circuit 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 provide 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 software 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 values.
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 product 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 modeling 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 impedance value that you want to be modeled, 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 rigidflex 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 sections of the board. As a designer, this will require you to neck down the circuits to their correct geometries and spacing. The neckdown 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 transition 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 sections of the board. Because of this and the fact that each value needs to be tested in the impedance coupons 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 fabricator can fit on the panel, which ultimately 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.
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More Columns from Flex Time
Flex Time: Alternative Constructions in Rigid-flex DesignsWhy is Rigid-Flex So Expensive?