Issue link: https://iconnect007.uberflip.com/i/1449048
82 DESIGN007 MAGAZINE I FEBRUARY 2022 flexibility. When designing a flex hatched plane, your traces should go over the knuckles in the crosshatching. If you have a differential pair and you are concerned about the imped- ance, the differential pair pitch and the diago- nal pitch should be equal. Put your traces right over the knuckles. Figure 7.5 illustrates the hatched-plane parameters to consider. Minimum Bend, Radius, and Failure Modes For flex types 1 and 2 as shown in Figure 7.1, the minimum bend radius should be at least six times the overall thickness of the flex substack. For flex types 3 and 4, loose leaf, the mini- mum bend radius should be at least 12 times the overall thickness, and for flex Types 3 and 4, with bonded flex layers, the minimum bend radius should be at least 20 times the overall thickness. If these minimum radii are not considered, the result can be an open circuit from conduc- tor fracture, distortions in the adhesive and polyimide, or you can have actual delamina- tion just like you could with a rigid design. e "bookbinding" technique in flexible-cir- cuit design incorporates greater length of the flex layers on the outside of the bend, similar to the spine of a book. is is the most expensive approach, so it is something to consider when all other methods to achieve 90- or 180-degree flexure have been exhausted. Artwork length- ening in the flex area depends on whether the bend is 90 or 180 degrees. A 180-degree bend means twice the length, which adds a signifi- cant cost to the manufacturing process. IPC-2223 highlights preferred and undesir- able bend-area routing practices. Some high- level examples follow: • Avoid changing the trace width in the bend region; instead, keep it the same width all the way through the bend area. • Distribute the traces evenly. Lack of symmetry increases the likelihood of stress buildup. • Route traces perpendicular to the bend region. • Do not put vias in the bend area. FLEX007 Bill Hargin is the chief everything officer at Z-zero, developer of the PCB stackup design and mate- rial selection software, Z-planner Enterprise and author of The Printed Circuit Designer's Guide to... Stackups: The Design within the Design. Figure 7.5: In this example, the hatch pitch is 18. The diagonal pitch is 25.5. Hatch angle is 45 degrees, and the hatch offset is 1.1 mils, but ideally would be zero. (Image from Z-planner Enterprise.)