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article an introduction to rigid-flex design best practices continues Figure 3: In this commercially produced design, the designers obviously did not apply bookbinder construction, resulting in additional stress on the separated layers (red arrow). • Do not abruptly change trace widths. When traces enter a pad, often in alignment, an abrupt change in trace width creates a weak spot. As a good design practice, use a teardrop pattern to gradually change the width of traces connecting to pads and vias in the flex circuits. • Use hatched polygons. A normal rectangular pour still retains heavily biased stresses in 0°, 45°, and 90° directions. A hexagon, by comparison, statistically provides a more optimal hatch pattern. • Add pad support. Compared to FR-4, copper on a flexible PI substrate is more likely to detach due to repeated stresses involved in bending plus lower adhesion. Consider surface mount pads and non-plated through-holes to be unsupported. Many fabricators recommend additional through-hole plating and recommend additional SMT pad support such as anchoring stubs and reduced coverlay openings. • Stagger double-sided flex traces. Running traces over each other in the same direction distributes tensions between the copper 32 The PCB Design Magazine • January 2014 layers unevenly. Staggering the traces reduces or eliminates the problem. In harmony with the clear trend of increasing rigid-flex PCB manufacturing, updated PCB CAD tools now include the necessary features needed to design rigid-flex circuits. These include multiple layer stack management, components mounted on "inner" flex-circuit layers, and 3D visualization and simulation of the flex circuit portions. Figure 4 demonstrates these capabilities. In addition to the layer stack management and 3D visualization, clearance checking of the components on the flex substrate is also possible. The design rule check engine provides early warning for rigid-flex designs whenever the final bend radii results in mechanical interferences. See Figure 5. Electronic design is becoming a 3D world. And as more rigid PCB designers face technological pressure, many are turning to rigid-flex circuits to help them meet these requirements. Fortunately, EDA companies are incorporating

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