Issue link: https://iconnect007.uberflip.com/i/1007258
JULY 2018 I FLEX007 MAGAZINE 37 • 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. • If any portion of any flex requires a part such as a flex connector, plug, or jack, use stiffeners. • For the flex bend region (the area of the flex that will bend or twist), which has its own set of fundamental best practices and guidelines, the designer can consider these best practices maximize reliability: – Never change the width of the traces within the region. – Be sure to distribute traces evenly. – Route traces perpendicular to the bend direction because any lack of symmetry will increase the chance of stress buildup. – Vias are not permitted within the region. – Hatched power/ground planes should be parallel with the region. Cross- hatched power/ground planes are pre- ferred, however the cross-hatch pattern should be at a 45-degree angle in relation to the bend line. Using an ECAD tool that can calculate the cross-hatch angle in relation to the bend line (when creating the plane fill) can save time, particularly for designs with odd angle bend lines. – It's critical to note that the bend radius calculation is likely the greatest challenge associated with the bend region, thus requiring close collabora- tion with the fabricator. Based on the application, the bend-radius require- ments will vary. 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 radius than dynamic flexes. The bend radius is critical in order to avoid compression (area inside of the bend) or tension reliability issues. Figure 2: Rigid-flex bend area DRCs can help ensure process compliance.