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26 DESIGN007 MAGAZINE I SEPTEMBER 2023 ning the flexible circuit design include several primary elements: • Quantity of signal traces required across the flexible area • Conductor widths required for current- carrying capacity • Spacing required for voltage isolation • EMI shielding and impedance control requirement • Determine the surface area required for routing the circuit Flexible circuit complexity can have a signif- icant impact on manufacturing lead-time and unit cost. Designers commonly create rigid boards with a more uniform rectangle outline, but flexible circuits or the flexible portions of the rigid-flex circuit outline are more likely to be profiled to meet specific application. But before finalizing the outline for the circuit, the designer is advised to prepare a physical model to confirm the dimensions initially established meets expectation. e example shown in Fig- ure 1 represents a flexible circuit outline with holes, slots and openings, and circuit exten- sions for connectivity with other assemblies. If the circuit will include surface mount com- ponents and/or interface connectors, these areas must be defined on the model as well. To ensure the finished part will hold up to the various stages of assembly and use, the designer must include attributes that will mini- mize physical damage: inside corners, nar- row slots and slits, or in areas accommodating connectors or areas reserved for component mounting. Corner and Slot Feature Development Avoid sharp corners in all recesses of the flexible circuit. e designer must specify a radius at these locations in order to lessen the potential for cracking or tearing of the flexible material. Providing a radius on the inside cor- ner (Figure 2a) will resist the propagation of tearing. When the inside corner must provide an unobstructed corner profile, the designer can position the radius beyond the corner (Figure 2b). Manufacturers recommend providing a large radius on all inside corners. is can contrib- ute to increasing part reliability, one that will be more resistant to tearing when twisted. Regarding "slit" and "slot" features, flex- ible circuit fabricators recommend that these features be furnished with a hole or full radii where they terminate. For the "slit" features, a 1.5 mm (~0.06") diameter hole at the end of the "slit" typical of the example furnished in Figure 3a, will provide a barrier to minimize tearing when the flexible circuit is physically stressed. When a wider slot feature resembling that shown in Figure 3b is included in the flexi- ble circuit, fabricators recommend a full radius at its end. To further minimize tear and cracking of the material at the inside corner areas, a narrow pat- tern of copper foil can be retained that follows the profile of the radii. If the corner or slot is subjected to excessive strain during use, include an additional thickness of the base dielectric in the area to further reinforce the site. Holes and cutouts in the flexible circuit should maintain a minimum distance of 1.3 mm between the exterior edge of the flex cir- Figure 1: Flexible circuit outline example. Figure 2: Inside corner tear restraint.