Rigid-Flex PCB: Your Comprehensive Guide to the Future of Circuit Technology

Circuit boards display a wide array of characteristics, configurations, and dimensions. They can be categorized by frequency, materials employed, or the number of layers incorporated. Typical PCB types include Rigid, Flex, and Rigid-Flex. Rigid PCBs are more frequently encountered, while flex and rigid-flex versions are less common. Nevertheless, a select group of skilled manufacturers can produce these specialized PCBs upon request. This article provides a brief explanation of rigid-flex PCB, its applications, and various factors to choose from while selecting this PCB.

What is Rigid-Flex PCB?

Rigid-flex printed circuit boards combine the technologies of both flexible and rigid boards. They typically comprise several layers of flexible circuit substrates connected to one or multiple rigid boards, either externally or internally, depending on the application's design requirements. The flexible substrates are intended for constant flexing and are often shaped into a curve during production or installation.

Designing rigid-flex boards is more complex than creating standard rigid boards, as they are developed in a 3D space, providing increased spatial efficiency. This three-dimensional design approach allows designers to bend, twist, and fold the flexible board substrates to achieve the desired form for the final application's packaging.

Rigid-Flex PCB Stack-up

When it comes to flexible PCB or rigid-flex PCB, there are numerous layer stacks to choose from. The most basic option involves a single flex region with two copper layers that are constructed on a flexible substrate material such as polyimide. This type of flex region can be directly integrated into a rigid-flex PCB stackup or utilized on its own. In the case of a rigid-flex PCB, the coverlay film is fused to the prepreg during lamination to allow for the assembly of FR-4 stiffener regions onto the flex ribbon.

Typically, a rigid-flex stack-up comprises at least one flexible layer pair positioned between rigid sections. For instance, double-sided flexible PCBs laminated with rigid sections and layers of pre-preg are the most commonly designed rigid-flex boards, as illustrated in the figure above.

It is worth noting that the stiffener sections utilize prepreg to bond to the flex section in all these arrangements. Nonetheless, the layer stack depicted above is not limited to just one internal copper layer and two rigid FR-4 layers. Multiple rigid layers can be stacked on each side of the flex region, and the FR-4 layers on each end can have varying layer counts since they will be laid up and press laminated individually.

Distinctions between Rigid-Flex PCBs and Traditional PCBs:

The Process of Rigid Flex PCBs

Due to the hybrid nature of rigid-flex PCBs, which combine elements of both FPC (Flexible Printed Circuits) and traditional rigid PCBs, the production of rigid-flex PCBs necessitates facilities equipped for both FPC and PCB manufacturing. Initially, electronic engineers design the circuit and form factor of the flexible part of the board based on specific requirements. Subsequently, these designs are sent to factories capable of producing rigid-flex PCBs. There, CAM (Computer-Aided Manufacturing) engineers process and plan based on the provided files. Following this, the required FPCs are produced on FPC production lines, while PCBs are manufactured on PCB lines. After manufacturing, according to the electronic engineer's specifications, the FPC and PCB components are seamlessly laminated together using a lamination press. This process is followed by several detailed steps to finally produce the rigid-flex PCB. A critical stage in this process, given the complexity and detail-oriented nature of rigid-flex PCBs, involves a thorough inspection before shipment. This is essential due to the high value of these boards, ensuring that no losses are incurred by either the supplier or the customer due to product issues.

Below is a brief production flow diagram for rigid-flex PCBs:

Applications of Rigid-Flex PCBs

Industrial Uses - Industrial applications of rigid-flex PCBs include those in the industrial, military, and medical fields. Most industrial components require precision, safety, and durability. Therefore, the demanded characteristics for rigid-flex boards are high reliability, precision, low impedance loss, complete signal transmission quality, and durability.

Mobile Phones - In mobile phones, the use of rigid-flex PCBs is common in areas such as the folding hinge of flip phones, image modules, buttons, and RF modules. Currently, the use of rigid-flex boards in mobile phones enhances the durability and long-term reliability of the phone's folding mechanism.

Consumer Electronics - In consumer electronics, using rigid-flex PCBs at the same line density can increase the total usable surface area of the PCB, thereby improving the circuit's carrying capacity and reducing the limitations on signal transmission and assembly error rates. On the other hand, because rigid-flex boards are lighter and thinner and can be flexibly wired, they significantly contribute to reducing size and weight.

Automotive - Inside vehicles, rigid-flex PCBs are used for various purposes, including buttons on the steering wheel connected to the motherboard, connections between the car video system screen and the control panel, connections for speakers or functional buttons on side doors, rearview radar imaging systems, sensors, car communication systems, satellite navigation, rear seat control panels, and front-end controller connection boards, and external detection systems, among others.