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SEPTEMBER 2023 I DESIGN007 MAGAZINE 55 Bend Radius In most cases, PCB vendors never have to worry about bending a rigid board. With flex, bending is the most urgent issue. At the design and layout stage it's all about the bend, including material choices, bend loca- tion and degree of severity, and component placement. IPC-2223 has a good section on bend radius calculations. It is usually recommended to calculate bend radius as 12x the thickness of the flex. e stiffness of a flex board is primarily driven by the copper weight, but polyimide thickness is critical. Layout Design Considerations ere are a variety of design considerations for flex that are much different than PCBs. Filleting: It is better to add filleting (tear drop) for plated through-holes in the flex area as it helps to distribute stress over a broader area and it improves reliability. Via location in rigid-flex: If the vias are placed in the flex area of rigid-flex, they should be at least 0.050" away from the rigid/flex junc- tion and 0.050" away from the start of the bend area. On the rigid side of the rigid-flex, they should follow the same 0.050" keep away rule. Routing traces: e preferred practice is to route conductors perpendicular, if possible, to bend and fold areas in a single metal layer. Routing traces in multi flex layers: Traces should not be on top of each other layer-to- layer so as to avoid creating an I-beam effect. HDI: PCB vendors are actively supplying blind, buried, staggered, and stacked vias. e PCB industry shines in high layer counts and HDI. While the features and vias on a flex cir- cuit are very small, the ability to do blind, bur- ied, and staggered vias is more difficult. How- ever, the flex market is moving forward with technology to better support flex. Figure 2: Bend radius is usually determined by using a formula that is 12x the thickness of the flex. Figure 3: Examples of recommended and non-recommended constructions.

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