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16 The PCB Design Magazine • March 2017 • Do not change the width of the traces within the region. • Distribute traces evenly. • Route traces perpendicular to the bend direction (any lack of symmetry increases the chance of stress buildup). • No vias permitted within the region. • Hatched power/ground planes should be parallel with the region. As previ- ously noted, cross-hatched power/ground planes are preferred, however the cross- hatch pattern should be at a 45° angle in relation to the bend line. An ECAD tool that can calculate the cross-hatch angle in relation to the bend line when creating the plane fill will save time and effort, especially for designs with odd angle bend lines. • Most importantly, the bend radius calcu- lation is perhaps the greatest challenge associated with the bend region, and as such, will require close collaboration with the fabricator. The bend radius require- ments will vary based on the application. Generally speaking, the flex will either be static (bend once during installation) or dynamic (bending can occur numerous times throughout the products lifespan). Static flexes will require a tighter bend ra- dius than dynamic flexes. The bend radius is critical in order to avoid compression (area inside of the bend) or tension reli- ability issues. The above best practices and guidelines are introductory in nature, and as such, are not in- tended to be all inclusive. These are the areas that the first-time rigid-flex designer needs to focus on and become familiar with as quickly as possible. They are all unique to the technol- ogy and almost all will come into play for even the simplest rigid-flex design. For reference, other items that will need to be reviewed with the fabricator include; impedance control, hole to interface distance (where the flex interfaces with the rigid), construction options, laminates and bonding materials, surface finish, and cover layer design. RIGID-FLEX DESIGN TIPS AND BEST PRACTICES Figure 3: Rigid-flex bend area DRCs can help ensure process compliance.