1. Rigid-flex designs typically transition from rigid to flexible and back to rigid again. Rigid areas typically have more layers than flexible areas, and the material transitions from FR-4 to polyimide at the transition area. When intersecting, the overlap of rigid and flexible materials requires holes to be positioned away from the transition area to maintain integrity. Furthermore, many rigid-flex designs incorporate reinforcing materials such as stainless steel or aluminum to provide additional support for connectors and components.
2. Flex circuits have curved conductors, which can affect wiring. Due to potential material stress, placing components or vias near the bend line is not an option. Even with proper component placement, flex circuits can cause repeated mechanical stress on surface-mount pads and vias. These stresses can be mitigated by using through-hole plating and supporting the board with additional cover material.
3. When designing rigid-flex boards, it's important to consider the electromechanical factors affecting both flex and rigid circuit boards, paying attention to the bend radius to thickness ratio. For flex circuits, narrowing or increasing the thickness of the bend area increases the likelihood of failure. In this case, it is recommended to maintain a bend radius of at least ten times the thickness of the flex circuit material.
4. Avoid bending the flex circuit along its exterior or compressing it along its interior. Increasing the bend angle beyond 90° increases tension at one point and pressure at another.
5. A key issue in rigid-flex reliability is the thickness and type of conductors in the flex area. Using heavy copper, gold, or nickel plating reduces bending flexibility and can lead to mechanical stress and fracture. Thickness and mechanical stress can be reduced by reducing the amount of plating on the conductors and using only pads
