Issue link: https://iconnect007.uberflip.com/i/1073397
70 FLEX007 MAGAZINE I JANUARY 2019 There is a rather complex formula to deter- mine an acceptable bend radius but as a rough rule of thumb: • Single-sided and double-sided flex appli- cations: The minimum bend radius should be six times the overall thickness; as an example, if the overall thickness of the flex circuit is 0.012", the minimum bend radius should be 0.072" • Multilayer flex and rigid-flex with bonded inner layers: The minimum bend radius should be 12 times the overall thick- ness; as an example, if the overall thickness of the flex circuit is 0.030", the minimum bend radius should be 0.360" Multiple Flex Layers on a Rigid-flex For improved bending, inner layers should not be bonded together in the flexing area; this is typically referred to as "loose leaf" con- struction. Figure 5 illustrates a 14-layer rigid- flex where the inner layers were not bonded together to improve bend radius. Crosshatched Copper Shielding Using a crosshatch pattern rather than a solid copper shield can greatly increase the flexibility of a flexible cir- cuit. This can be customized by removing more or remov- ing less material based on design needs. Crosshatch- ing can also be applied in just the critical bend areas leaving sections that are not exposed to bending and flex- ing as full copper layers. Conclusion As mentioned earlier, this is certainly not an exhaustive list of all the options avail- able to improve the flexibil- ity of a design but is a good list to review when working on a design and a place to start discussions with your fabricator. Most everyone can tell their tale of the "flex that didn't flex." This short list will help decrease the odds of that occurring. We strongly encour- age early and clear communication about flex- ing and bending requirements with your fab- ricator who can provide suggestions for your specific application. FLEX007 Tara Dunn is the president of Omni PCB, a manufacturer's rep firm specializing in the printed circuit board industry. To read past columns or contact Dunn, click here. Anaya Vardya is president and CEO of American Standard Circuits. To read past columns or contact Vardya, click here. Visit I-007eBooks to download your copies of Ameri- can Standard Circuits' books today: The Printed Circuit Designer's Guide to… Fundamentals of RF/ Microwave PCBs and Flex and Rigid-Flex. Figure 5: Completed PCB flex belts separated.