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March 2017 • The PCB Design Magazine 15 always vary, as such the designer needs to manage them efficiently and ensure they are properly conveyed to the fabricator. An ECAD tool that supports region-spe- cific stackups (not ideal) or board-specific stackups (preferred) will help simplify this complex task. • Board outline management: The multiple boards in a rigid-flex design need to be properly configured and managed throughout the design process. Rigid-flex designs are in fact electromechanical proj- ects that require collaboration between the electrical and mechanical domains. The ability to import and automatically create multiple (and potentially complex) board outlines from mechanical CAD data will both save time and reduce the poten- tial for errors. • Fabricator interaction: This aspect of rigid-flex design cannot be stressed enough. Feedback from the fabricator on stackup, material, keepout regions, bend requirements, stiffeners, etc., will help en- sure the design can be properly fabricated with high reliability. • Signal and power integrity analysis: Most signal integrity and power integrity tools assume a single PCB with a uniform stackup. For rigid-flex designs the analysis tools must recognize flex-specific layers and local stackups in order to ensure cor- rect analysis results. • 3D design and verification: The abil- ity to define the bend parameters (bend radius, bend angle, and bend origin), edit the design in 3D, view the design in the context of the enclosure, and perform 3D rigid-flex aware design rule checks (DRC), cannot be overstated. 3D design and veri- fication ensure that the design team is taking advantage of open real estate in all three dimensions while, at the same time, identifying potential design issues. As previously mentioned, since flex design requirements will most likely be new to the first-time rigid-flex designer, some of the fun- damental flex-specific best practices and guide- lines are as follows: RIGID-FLEX DESIGN TIPS AND BEST PRACTICES • Ensure that the trace width and trace spacing are both as large as possible. • Traces should be routed using round cor- ners. Ninety-degree corners should be avoided. Furthermore, the round corners must be true arcs. Segmented arcs will create stress fractures. In most cases, the trace contour should mimic the flex board outline contour. An ECAD tool that allows the trace routing to automatically follow the board outline contour will help save time. • If there is a need to route on more than one layer, stagger the traces for adjacent conductors. • Cross-hatch power/ground planes as per- mitted by electrical requirements (cross- hatching a plane has significant impact on the impedance of any conductor using it as a return path). • Stiffeners should be used if any portion of any flex requires a part such as a flex con- nector, plug, or jack. Finally, the area of the flex that will bend or twist, also known as the flex bend region, has its own set of fundamental best practices and guidelines in order to maximize reliability: Figure 2: Three rigid and two flex PCBs as viewed in their 3D state.