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88 DESIGN007 MAGAZINE I OCTOBER 2019 reality was that well-meaning engineers would go down that path, spend weeks designing their products with flex in mind, only to get to the end of the design cycle, request a quote for the design, and unleash the heartbreak of sticker shock on their entire project manage- ment team. But we've seen that pricing, capability and supplier availability are all changing; the more flex is being required as a design necessity, the more PCB shops have stepped in to supply flex- ible circuits. More materials are now being or- dered to fill these needs. This has created com- petition, and the cost of incorporating flex into design has dropped significantly. Shaughnessy: What would you say are the most important considerations when design- ing with flex? Dack: Many of the DFM rules considered in flex design for print and etch, creating blind and bur- ied vias, and determining if the current-carrying capacity of conductors are similar to the ones used for rigid boards. The IPC design guidelines found in the IPC-2221 and IPC-2222 standards are a great place to start. But IPC has a sectional standard, IPC-2223, meant specifically to help the designer get a lead on some important addi - tional considerations for flex design. For any design specifics, my advice will al- ways be to go to the stakeholder responsible for doing the work. Just like you would receive DFM information from your bare board suppli- er in a rigid world, it can be best to go to your onshore and offshore flex suppliers who are happy to provide you with design guidelines in book form. These guidelines cover things like bend radii, material availability, thermal requirements, and design tips that most any flex fabricator will happily provide to a design- er who asks. Again, I recently returned home from walk- ing the aisles of PCB West in Santa Clara. I must have met a dozen companies specializing in manufacturing flex circuitry. When learning anything, a picture is worth a thousand words, but I like to say that a part in hand is worth a thousand pictures. Trade shows are great cation. We call this a "bendable" or "bend-to- fit" application in flex design. Shaughnessy: Do you see more rigid board cus- tomers being forced into flex? We hear about companies that never did flex before that are having to quickly learn all about flex. Dack: I think what's happening is that, with the advent of 3D design coming into play, prod- uct enclosure shapes and contours are becom- ing more and more complex. We see designs being compressed because of limited space in the next generation of smaller products. As al- ways, from a manufacturing standpoint, pow- er and high-density interconnect requirements for an entire shrunken product can be handled much better in volume with a flex circuit solu- tion than with discrete wires. Shaughnessy: When I started writing about this industry in the '90s, flex was kind of a side note; it was really cool, but it was costly, and few companies used it. Dack: My experience in the '90s and up until the last decade was that flex has always been intriguing from a design standpoint. The tragic Kelly Dack

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