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SEPTEMBER 2023 I DESIGN007 MAGAZINE 27 cuit outline and edges of interior located holes and cutouts. In addition, cutouts should always maintain a minimum radius of 0.75 mm in each corner, typical of that illustrated in Figure 4. Circuit Conductor Density Determining conductor density and the rout- ing area requirement on the flexible portion of the circuit will depend on the number and width established for signal conductors. For both single and two metal flex, the overall width of the base material must also provide adequate surface area for cover-layer film bonding. e general rule for a conductor to the dielectric material's outer edge is a minimum of 0.70 mm (~0.030") but, where possible, the copper-to- edge distance should be maximized. Addition- ally, the minimum distance between the exte- rior edge of the flex circuit and the edges of interior located holes and cutouts should not be less than 1.30 mm (~0.050"). But, typical of the conductor-to-edge spacing noted above, this provides a greater distance where possible. e examples in Figure 5 furnish general guid- ance for providing clearance between key fea- tures of the flexible circuit. When the conductor path on the flexible circuit must change direction, the designer must avoid sharp, 90-degree turns. Ideally, these conductor paths should be routed with a nested chamfer or generous radii. Multiple conductor paths routed in parallel should also maintain uniform spacing with progressively larger radii from the center axis. Bend Area Consideration e flexible circuit should not be bent or formed in areas where there is a discontinu- ity in the cover-layer, termination of plating, or any other stress-concentrating feature. e acceptability of this condition is determined by the thickness of materials, the radius of the bend, and the severity of the operating environment. Flexible circuits designed for dynamic applications may be more susceptible to conductor failure than the bend-to-install or mechanically formed application. Circuit fabricators recommend that design- ers avoid unnecessary circuit thickness in the bend zone because it can hinder flexible capa- bilities. If a portion of the flex circuit needs Figure 3: Slot and slit fortification. Figure 4: Maintaining distance between circuit edge to holes and cutouts. Figure 5: General physical feature clearance guide- lines for flexible circuits.