Rigid-Flex PCBs combine rigid circuit boards and flexible circuits into one integrated structure. They include robust rigid areas for component assembly and flexible areas that can bend or fold, with continuous copper layers providing electrical interconnection.
The Rigid-Flex PCB layer stack-up is the core of design and manufacturing, as it directly affects the board structure, electrical performance, and signal integrity.
This article introduces typical 4-layer / 6-layer / 8-layer / 10-layer Rigid-Flex PCB stack-up designs.
Basic Elements of Rigid-Flex PCB Stack-up Design
The rigid sections are based on FR-4 core materials laminated together by prepregs, like glue that bonds the cores during the lamination process. Copper foils are then pressed on each layer to form the circuitry.
The flexible sections use polyimide (PI) base material, clad with copper on both sides, and protected by a coverlay film (not solder mask), making them capable of free bending with no rigid materials inside.
Common Rigid-Flex Layer Stack-up Examples (4L / 6L / 8L / 10L)
4-Layer Rigid-Flex PCB：
- 2 flex layers + 2 rigid layers, 0.5 oz flex copper, 1.6 mm total PCB thickness
- 2 flex layers + 2 rigid layers, 1 oz flex copper, 1.6 mm total PCB thickness
6-Layer Rigid-Flex PCB
- 2 flex layers + 4 rigid layers, 1 oz flex copper, 1.6 mm PCB thickness
- 2 flex layers + 4 rigid layers, 1 oz flex copper, 1.6 mm PCB thickness
8-Layer Rigid-Flex PCB
- 2 flex layers + 6 rigid layers, 0.5 oz flex copper, 1.6 mm PCB thickness
- 2 flex layers + 6 rigid layers, 1 oz flex copper, 1.6 mm PCB thickness
10-Layer Rigid-Flex PCB
- 2 flex layers + 8 rigid layers, 0.5 oz flex copper, 1.6 mm PCB thickness
- 2 flex layers + 8 rigid layers, 1 oz flex copper, 1.6 mm PCB thickness
Key Considerations & Design Recommendations
Rigid Sections
- Proper signal integrity planning: solid power/ground reference layers, impedance control
- Avoid warpage caused by large solid planes + dense vias
- Ensure prepreg and adhesive selection handles thermal stress in reflow
Flexible Sections
- Trace direction must follow bending direction to prevent copper cracks. This is critical for mechanical reliability and bend radius
- Absolutely NO vias (through, blind, or buried) allowed in bending areas
- Impedance control in flexible areas requires recalculation due to different dielectric properties
- Choose coverlay instead of solder mask for durable protection
Flex-Rigid Transition Zones
- Keep stack-up changes smooth to reduce stress concentration
- Use copper relief (cross-hatch or segmented copper) instead of large solid planes
- All routing must enter the transition area perpendicularly to the dividing line
- Match material properties to prevent delamination during bending
Conclusion
Rigid-Flex PCB layer stack-up is essential to achieving both mechanical durability and electrical performance. We recommend referencing our standard stack-up options during your stack-up design phase for optimal manufacturability.
If you require stack-up of different copper thicknesses, different flex layer, different PCB thickness, please contact your sales representative or our online service for further details.
