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20 FLEX007 MAGAZINE I OCTOBER 2018 have a plated through-hole connecting all the layers, while the FR-4 stiffener is used only for component support. The density of component areas is often the driving factor toward rigid- flex. 2. Stackup Your fabricator can help ensure that you are meeting thickness and impedance require- ments for the design. They will also provide guidance on materials that are in stock and materials that may need to be special ordered so that material lead-time can be factored into the project plan. Further, your fabricator can also discuss tradeoffs of various materials, so you can be sure you are designing with the most cost-effective construction. 3. Array design and panel utilization Typically, panel utilization or the "number up" is the biggest cost driver for flexible cir- cuit designs. As with rigid designs, fabricators price by production panel, with the piece price being the panel price divided by the number of parts per panel. It is important to under- stand your fabricator's preferred panel size. Common panels sizes are 12" x 18" and 18" x 24". Fabricators commonly use the outside one-inch border of the manufacturing panel for coupons and tooling holes. Effectively, when designing, optimizing the useable space of 16" x 22" and 10" x 16" with individual pieces or arrays will result in the lowest cost option. Rigid-flex often takes on unusual shapes that are not necessarily the standard square or rectangle we see with rigid boards. Standard panelization software may not consider this. If the design can be reverse-nested to increase the number of parts per panel, this can sig- nificantly impact price and is worth time for review when setting up the array configura- tion. 4. Dynamically flexing Clearly communicating areas that your flex circuit will be dynamically flexing will greatly benefit your design. Your fabricator will be able to review the design to ensure you are following best practices. Further, when setting the tooling for manufacturing, they will be able to orient the circuits on the production panel properly. Copper grain structure now becomes critical. The orientation with the grain struc- ture could impact the material utilization and piece price. 5. Blind and buried via structures It is always recommended to interact with your fabricator when developing blind and buried via structures with flex and rigid-flex. As you develop these structures, you are add- ing base copper on various layers. This can impact the smallest lines and spaces possible on those layers. A Case Study The following is a case study that illustrates why it is important to work with your PCB fabricator during the design phase. We once encountered a telecommunication applica- tion that had a 50% failure during installation due to cracking of the copper in the flex area. When the customer came to us, we reviewed the stackup and redesigned it by: • Converting stackup to adhesiveless materials • Decreasing flex thickness from 11.8 mils to 8.4 mils (29% decrease) The extra thickness was adding rigidity to the flex area and causing cracking. Summary There are many things to think about when considering a rigid-flex design to solve a pack- Typically, panel utilization or the "number up" is the biggest cost driver for flexible circuit designs.

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