Rigid-Flex designs are the bringing together of Rigid PCB's and flexible polyimide inner layers.  Sometimes there are multiple rigid sections interconnected through one or more flexible layers.  The advantage of these designs is the elimination of connectors and improved signal integrity.  Connectors are a reliability issue and create a discontinuity in critical signal flow, so eliminating them and having a continuous uninterrupted signal path is a big bonus.  Flex thickness & width and amount of copper will affect flexibility and bend radius.   If multiple flex layers are used and it requires a tight bend radius, "loose Leaf" construction may be required.

rigid-flex designs

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