flex PCBs differ from rigid-flex PCBs
Rigid-flex PCBs replace multiple FR4 PCBs and wire harnesses in devices like cameras, wearables and drones to provide greater reliability. They also eliminate the need for multiple solder joints and the associated risk of failure from vibrations and environmental stresses. These advantages make rigid-flex circuits ideal for a wide range of applications that require the device to be compact and durable in design.
How do flex and rigid-flex pcbs differ from each other? A flex PCB has a flexible core layer with a fixed-width copper clad. This copper cladding is then fixed to a flexible substrate by either a woven epoxy, a laminated polyimide or an adhesive-based substrate. The flex core is also made from a flexible material that can be formed to accommodate an array of shapes.
A rigid-flex PCB is similar to a standard FR4 PCB with the addition of a flex section. This flex section is typically stabilized by stiffeners made from FR-4 fiberglass or polyimide. Stiffeners help the flex sections maintain their shape and prevent the board from folding or otherwise damaging the internal wiring. A flex PCB with stiffeners is classified by IPC as type 1.
How do flex PCBs differ from rigid-flex PCBs?
For a single-layer flex board, the common stackup includes an unreinforced flex substrate of polyimide dielectric film cladded to rolled copper. It also includes a prepreg layer that insulates the copper layers and allows manufacturers to control the amount of copper deposited on each pad/via. To ensure a good etch yield, it’s important to maintain a pad-to-copper distance of at least 8 mil.
The etch process for rigid-flex PCBs can be more complex than that for traditional FR4 boards due to the flexible core. This is because the flex layer absorbs the movement and contraction of the PCB, which affects etch yield. To minimize the impact, the manufacturer may use a pad-only plating technique (button plating) to reduce the thickness of the copper and increase the etch yield. The manufacturer can also choose to utilize adhesiveless PI as the base for the copper layer, which significantly reduces the possibility of via breakage during the production process.
A flex PCB’s manufacturing costs are usually more expensive than that of a rigid-flex PCB for the same number of layers and surface finish. The increased complexity of the lamination process, varying line width and spacing and the requirement for multiple surface finishes can all increase costs. To mitigate these increased costs, it’s critical to optimize the design during the early stages of the design cycle to minimize the number of layers, reduce the thickness of the flex sections and select appropriate materials.
To streamline the design process, it’s best to work with integrated MCAD and ECAD tools that support flex and rigid-flex PCBs. This allows designers to easily create 3D views of the bending region and prepare their design for manufacturing by generating all necessary deliverables, including Gerber files. Using these tools will also reduce the manual routing time needed for even the most complex rigid-flex designs.
