That’s actually a quite good question. A lot of folks use flex circuits when a simple thin FR-4 circuit would do the job nicely. Stiffness increases as cube function of thickness, so making an FR-4 board thin means it can be quite flexible but it is still fragile. This brings to mind something I recall seeing some years back.

It was a rigid flex circuit that was made entirely from FR-4. The center area of the circuit was milled down to the very thin layer of insulation over or under the circuits, depending on one’s point of view. It was not meant for dynamic flexing, but then, most flex circuits are not purposefully meant for dynamic applications. Dynamic flexing is truly the domain of flexible substrates and applications where very tight bends are required. If one wants to explore making circuits in the manner spoken here, it would be very important to provide a fillet of material on the thick-to-thin transition to protect the circuit from damage. The devil is always in the details.

Cheers,

Joe