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80 DESIGN007 MAGAZINE I FEBRUARY 2022 out in the field during use. e minimum bend radius is something to consider, as well, where tighter bend radiuses require more careful attention. ere are some important differences rela- tive to rigid PCBs that you should consider when designing a rigid-flex project. You can connect flex cores using a loose-leaf or bonded approach (Figure 7.3). With a loose-leaf design, you have air gaps between individual flex cores, and you get a cheaper and more flexible design. Bonded designs require an additional sublamination with bondply and prepreg, resulting in a stiffer design. Although at least 20 times thicker than the loose-leaf design, it improves stripline impedance control because you have tightly controlled current return paths. Figure 7.4 is example rigid-flex stackup with a four-layer rigid substack on the le and the right connected by a two-layer flex substack. e polyimide flex core runs end-to-end through the middle. e outside of the flex board has adhesive and coverlay, and the rigid substack has low-flow prepreg layers adjacent to the coverlay and coverlay adhesive. Dynamic Flexing and Flex-to-Install Recommendations With flex substacks, thinner overall thickness will outlast thicker constructions. e con- struction should be balanced in the Z-direction to avoid concentration of stresses. Unbalanc- ing of materials, including copper, dielectric, and adhesive thickness, can decrease flexure life because it causes stress to be distributed nonuniformly. Copper thickness can affect flex life too, depending on the type of flexure. For maxi- mum flexibility, use rolled annealed copper because of its greater ductility and elongation. e copper grain direction should be paral- lel to the conductors. Manufacturers that do rigid-flex know how to handle that for you. e bending and flexing action should be perpendicular to the conductors wherever possible; use radiused corners where perpen- dicularity is not possible. If you have to do some type of angled routing or changing direc- tion in the flex portion, radius the corners. And no plated through-holes should be used in the flexing area. For dynamic flex and flex-to- Figure 7.3: Loose-leaf designs are most flexible for 90- and 180-degree bends. They are the simplest to manufacture but not recommended for impedance control. Bonded is less flexible, thus requiring a more generous bend radius.

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