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26 FLEX007 MAGAZINE I OCTOBER 2018 is applied to the staggering of the length of cir- cuit layers through the bend within the design to provide greater ease of flexing in multilayer and rigid-flex designs. The technique is accom- plished by adding slightly to the length of each succeeding flex layer, moving away from the bend radius. A common rule of thumb is to add length equal to roughly 1.5x the individual layer thickness, but the value can vary based on the tightness of the bend and the number of layers. Therefore, it is recommended that some modeling be carried out in advance of commit- ting to manufacture. A "paper doll" mock-up can be very instruc- tive as a quick check. The extra length with each succeeding layer helps defeat whatever tensor strain might have otherwise built up in the outer metal layers of the multilayer flex, and it prevents buckling of the center of bend layers. If there are questions about the prac - tice, the designer is advised to check with the vendor in the early stages for guidance and assure better first-pass yield. It is important to note that bookbinder requirements are extremely costly to manufacture. It requires a lot of additional up front tooling and fixturing costs (Figure 1). 3. Copper layers greater than 75 mm (2 oz. per square foot): Thicker copper foils are useful in many flex circuit applications where they can serve to address high-current/high- power requirements for both discrete traces and power and ground planes. They can also help hold the shape of a flex circuit which is formed to fit an application. The challenge of thicker copper is that it is more difficult to pro - cess the thicker copper and at the same time hold feature size accurately. This is because etching, which is the most common method of defining circuit features, is an isotropic pro - cess which works in all directions while at the same time undercutting etch resist and leav- ing traces narrower on top and wider on the bottom. A potentially effective alternative is to begin processing with a thinner copper base and then pattern-plate additional copper in the areas where it is needed. That said, thick copper circuits are not necessarily to be avoided, but they should be approached with a full under- standing of the issues. It is recommended that the designer consult with the fabricator about any issues and options before committing to thick copper. 4. Anchoring pads: When the flex layer is an outer layer in flex circuits and rigid-flex cir- cuits, there is a weak point at the conductor/ pad interface. Any stress to this area can cause a fracture of the conductor at this point. The best way to prevent this is to provide specific reinforcements to the isolated pads on flex lay- ers. Anchoring the pads will assist with this type of design. There are a number of ways to potentially anchor pads and some of the popu- lar ones are illustrated in Figure 2. Figure 1: Bookbinder construction. Figure 2: Anchoring pads.

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