Issue link: https://iconnect007.uberflip.com/i/1073397
66 FLEX007 MAGAZINE I JANUARY 2019 One of the primary advantages of moving to a flexible circuit design from a rigid board is the ability to package the flex in three dimensions, bending or folding into imaginative configu- rations and saving precious space in the final package. While flexible materials are robust and can withstand many flex cycles, nearly everyone can share a war story about the flex that didn't originally perform as expected with copper cracking after installation. I [Tara] remember an example from my early days in flex fabrication. We had built a fairly simple, double-sided flex with FR-4 stiffeners on both ends. After installation, the customer contacted us because the copper was crack- ing while it was being bent. In that case—and most cases even today—fabricators often have only a 2D view of the design. After some inves- tigation of how the flex was being used, we made several recommendations to improve the performance of the circuit. Materials were adjusted, traces were re-routed to keep all of the traces on one layer in the critical bend area, polyimide stiffeners were added to guide the bend exactly where it needed to be in the unit. Rather than plating electrodeposited (ED) copper onto the more flexible rolled-annealed (RA) copper, we button-plated and only plated ED copper on the pads and plated through- holes. The end result? Success! No more crack- ing. Stories like this are not uncommon. Fabrica- tors have quite an arsenal of tips and tricks to help improve flex life and avoid damage to flex materials during installation and use, yet are often building the circuits without knowledge of how it is going to be used in the final appli- cation. While our intention is to share some of the common methods of improving flexibility, we also want to strongly encourage everyone to communicate the flex and bend areas in the fabrication drawings or have discussions with your fabricator prior to release to take advan- tage of the knowledge they have to draw from and improve the performance of your new design. Electrodeposited (ED) Copper Foil ED copper is formed by electrolytic depo- sition onto a slowly rotating polished drum from a copper-sulfate solution. When an elec- tric field is applied, copper is deposited on the drum as it rotates at a very slow pace; the slower the pace, the thicker the copper. The Don't Build Flex That Doesn't Flex Flex Talk by Tara Dunn, OMNI PCB Co-written with Anaya Vardya, AMERICAN STANDARD CIRCUITS