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58 The PCB Magazine • November 2016 the layer immediately adjacent to it. Each sub- composite length is calculated to allow the sub- composite to rest under it without interference. By virtue of our thickest sub-composite now being 0.3 mm (0.012") we can recalculate our minimum bend radius based on this new value. We now have a recommended minimum bend radius of 3.0 mm (0.12"), one-third of our origi- nal design minimum! Stress Applied in Flex Zone During Form Using the materials above the stress-forces on the book-binder portion of our rigid-flex were calculated. Plane strain allowed for a 2D cross-section. Contact between sub-composites was modeled allowing the effect of sub-composites coming into contact with each other to be observed. As there are no direct modeling parameters for flex materials, assumptions had to be made. The flex layers were all assigned the same modulus with the rigid sections being treated as steel. As seen in Figure 4, the reactive force be- tween the two ends of the rigid-flex was ana- lyzed. Final Rigid-Flex Stack-up and Microsections Figure 5 shows the final stack-up. Figure 6 shows the construction without cutback cover- UNIQUE IMPLEMENTATION OF A RIGID-FLEX CIRCUIT Figure 4: Sub-composite stress model. Figure 5: Final bookbinder rigid-flex stack-up.