Issue link: https://iconnect007.uberflip.com/i/1507356
28 DESIGN007 MAGAZINE I SEPTEMBER 2023 to be thicker for supporting components or connectors, add a stiffener. Suppliers further note that calculating bend-cycles over the projected life of the product is crucial to your design. When the circuit is flexed beyond the limit ratios established, the copper foil may elongate, become brittle, and generate micro- cracks in the bend zone. Guidelines for mini- mum bending radius for 90-degree "bend-to- install" for fully bonded circuits: • Single sided, 10:1 • Double sided, 10:1 • Multilayer, 20:1 A rule of thumb for estimating minimum bend radii is 10x material thickness for single- metal layer circuits and 12x for double-metal layer circuits. For maximum dynamic flex life and reli- ability, conductors in the bend area should be routed perpendicular to and evenly spaced across the bend area. Furthermore, the designer should not specify additional plating thickness in areas requiring bending or flex- ibility. Likewise, conductors should maintain a uniform width in the bend area and, when possible, the neutral bend axis should be at the center of the conductor pattern. For dynamic flex applications that require repeated bending up to 90-degree from flat, the recommended minimum bending radius (R) is: • For single-sided flex, 100:1 • For double-sided flex, 150:1 • Multilayer dynamic flexing is not recommended A rule of thumb for estimating minimum bend radii is 10x material thickness for single- metal layer circuits and 12x for two-metal layer circuits. In summary, for maximum dynamic flex life and reliability, conductors in the bend area should adhere to the following considerations: • Conductors should be routed perpen- dicular to the bend area • Conductors should be uniformly spaced across the bend area • Do not apply or plate additional metals in the bend area • Conductors should maintain a uniform width in the bend area • Do not place SMT components or plated-through via holes in the bend area Supplier Capability Assessment In preparation for developing the flexible circuit, the designer must gain a general under- standing of the flexible circuit fabrication process variations and review design guide- lines furnished by the potential supplier(s). You will find that the capabilities and process methods can vary significantly from one sup- plier to another. Flexible circuit suppliers, for example, may publish their capability in two classifications: standard and advanced. While their standard level of product will typically use an image, print, and etch process, fabrica- tors with more advanced imaging and pattern- plating capability are able to reduce conduc- tor geometries. For controlling product cost and maximizing process yield, however, the designer may consider the more conservative approach for conductor routing. Appearance Vern Solberg will conduct a half-day tutorial course on "Flexible Circuit Design for Manu- facturing Principles," Oct. 10, at the SMTA International Conference 2023 in Minneapolis, Minnesota. is course addresses the design, fabrication, and assembly criteria for develop- ing and implementing both flexible and rigid- flex circuits. To register and participate in this timely tutorial event, visit smta.org. DESIGN007 Vern Solberg is an independent technical consultant specializing in SMT and microelectronics design and manufacturing tech- nology. To read past columns, click here.