Flex-Rigid PCB Design

Rigid Flex PCB technology can be traced back about 50 years because it requires replacing wiring harness in spacecraft. The first commercial mobile computer (a little more than 25 pounds!) Rigid and flexible technology is used.Today, laptops, wearable technology, medical devices, test equipment and satellites are some applications that rely on flex-rigid pcb.

What is flex-rigid pcb?

Using flex-rigid pcb, flexible circuit board and rigid circuit board are laminated together. flex-rigid pcb crosses the boundary of traditional rigid pcb, and the unique characteristic of flexible circuit is to use flexible circuit which photo etch annealed copper conductor with high conductivity onto flexible insulation film.

Different design rules are suitable for flex-rigid pcb design

Different challenges offset the versatility and flexibility that enable you to build 3D designs and products. The traditional rigid Flex PCB design allows you to mount the components, connectors and chassis of the product to the rigid parts in the components. Similarly, as far as traditional designs are concerned, flexible circuits are used only for interconnection, while reducing mass and improving vibration resistance.

New product design and improved flexible circuit technology introduce new design rules for flex-rigid pcb. Your design team is now free to place components on flexible circuit areas. Combining this degree of freedom with multi-layer method for rigid and flexible design can enable you and your team to build more circuits in the design. However, obtaining this degree of freedom adds some challenges in terms of wiring and holes.

Design considerations

The following is a summary of the key design areas that must be considered when designing flex-rigid pcbs:

Conductor Routing – it is important to choose a corner style for routing moving over flexible areas, avoid sharp corners, and use curves to relieve pressure.
Shape and area of pad – using rounded corners (tears), the rabbit ear (fixed spurs) is used for one-sided bending in order to capture some of the pad’s shape with the overlay.
Through holes – try to avoid through holes in curved areas, especially in dynamic applications.
Cladding – avoid stress rise (exposure of incoming lines) and reduce openings in the cladding 250um.
Plane – cross hatches if possible.
Staggered length – to avoid binding (bending when bending), stagger the length of staggered layers by about 1.5 times the layer thickness.
Service cycle – make the bending area a little longer to aid assembly / disassembly and allow product size changes (excess length is called service cycle).
Copper saving – considering how to make flexible circuit into a board, it is better to adjust the design to ensure the best material use.
Jigsaw – adjusts the flexibility area to fit the particles of the material (bending along the particles).
Tear resistance – bend corners, drill at corners, drill in slits, leave metal at corners.
Wiring – stagger the wiring on the 2-layer board to avoid light beam formation and widen the wiring through the bending area (which is particularly important for permanent bending).
Static bending rate – the ratio of bending radius to circuit thickness. Ideally, the bending ratio of multilayer circuits should be at least 15:1. For double-sided circuits, the minimum ratio should be at least 10:1. For single-layer circuits, the minimum ratio should also be at least 5:1. For dynamic applications, the target bending ratio should be 20-40:1.
Rolled annealed copper is more malleable, and the best choice for the inflexible area of the coating.