In the design of modern electronic products, flex and rigid-flex circuit boards (PCBs) are widely used because of their lightweight and flexible characteristics. However, flex circuit boards are easily deformed or damaged by mechanical stress during welding, assembly and long-term use. Therefore, an element that can improve the structural strength and stability of the circuit board is needed, and thus PCB stiffener appears. This time, this paper will carry on an in-depth discussion of PCB stiffeners, including the definition of PCB stiffeners, main functions, applications, advantages, and design points.

What is a PCB Stiffener?

PCB Stiffener is a material used to increase the structural rigidity and support strength of flex PCBs and rigid-flex PCBs, which not only helps to improve the mechanical properties of circuit boards, but also enhances the assembly efficiency, making the boards more stable when carrying heavier components. Especially in flex PCBs, it can provide additional mechanical support to avoid deformation or damage to the PCBs during operation, welding and use. Overall, PCB stiffener plays an important role in improving the stability and durability of the circuit boards.

Why Do Flex PCBs Need Stiffeners?

A flex PCB requires a stiffener primarily to enhance mechanical strength and stability. Since the material of a flex PCB is very thin and can be bent and folded, certain key areas often need additional support and protection to maintain integrity during use. It is commonly used in welding areas, connector ports, heavy component areas, and other areas where strength needs to be increased. For example:

1. Welding area:

Flex PCB will be deformed during welding, resulting in weak welding points and affecting the stability of electrical connections. At this time, it is necessary to add a PCB stiffener board in the welding area, especially the welding point of the connector or component, to prevent the circuit board from warping or damage due to welding heat and operating force, and improve the reliability of welding.

2. Connector area:

The connector area of the board will be inserted and removed repeatedly, and the stress generated at this time will cause the flex board to bend or tear, and even cause the connector to fall off. However, adding a PCB stiffener in these areas can effectively prevent circuit board damage caused by repeated insertion or use of the connector.

3. Heavy components area:

For heavier components, if there is no PCB stiffener support, the flex area will not be able to withstand the weight of the component, resulting in bending or even damage. Therefore, for areas where heavier components or components need to be installed, a PCB stiffener needs to be added to provide additional support and disperse the pressure of heavy objects, preventing flex PCB deformation.

After understanding why flex PCBs need stiffeners, let's take a look at parameters of PCB stiffeners.

Parameters of PCB Stiffeners

Common Flex PCB Stiffener Materials

PCB Stiffener materials are rich, common materials such as FR4, polyimide (PI) and metal materials.

In some specific applications, PCB stiffener busbar is also used, which not only enhances the rigidity of the circuit board, but also can be used to conduct current.

Stiffener Thicknesses

The thickness of PCB stiffener varies with different materials, and the appropriate thickness needs to be selected according to the specific application and support requirements.

Stiffener Performance Comparison

Stiffeners of different materials have great differences in performance; please see the following table. This form will help you choose the most suitable stiffeners material.

Multi-layer PCB Stiffeners

Some designs require multi-layer PCB stiffeners or a combination of multiple materials, in which case you need to consider several points to ensure the reinforcement effect and the performance of the overall board:

Material Compatibility:

Different materials have different coefficient of thermal expansion (CTE) and will expand or contract at different rates under temperature changes. Therefore, when combining a variety of materials, we should choose materials with similar thermal expansion coefficients, so as to avoid delamination, warping or cracking caused by temperature changes.

Interlayer Bonding:

The bonding between layers is very important for the multilayer stiffeners. Good interlayer bonding ensures that the reinforced plate remains intact under mechanical stress. Therefore, we recommend the use of a special high-strength adhesive between the multi-layer stiffeners (the heat resistance and anti-aging of the adhesive should meet the requirements of the use of the circuit board).

Thickness:

The number of PCB reinforcement boards is large, and the board will inevitably be thicker. Therefore, for the multi-layer PCB stiffener, we must control the thickness of each layer to ensure that it meets the thickness limit and assembly requirements of the circuit board and can provide sufficient support without affecting the overall flexibility of the flex PCB.

Weight:

In flex PCB or rigid-flex PCB, weight can affect the bending ability and flexibility of the board. Therefore, in the application of multi-layer stiffeners or to use the board for lightweight applications, you should choose lightweight materials, such as polyimide (PI) or thin FR4.

Placement of Stiffeners

In a Flex PCB, the position of the stiffener has a significant impact on the overall performance and reliability of the board. Common placement locations include:

Top or bottom layer: The top layer of reinforcement can increase the tensile strength when the board is bent, while the bottom layer of reinforcement can increase the compressive strength. For flex circuit boards that need to be bent frequently, single-sided or double-sided reinforcement plate placement can effectively prevent cracks or deformation.

Designated areas: like connector areas, welding areas or areas where heavy components need to be installed, these areas are subjected to greater mechanical stress, reinforcement plates are needed to improve the durability and stability of the board.

Design Principle of PCB Stiffeners

Follow the following guidelines to ensure the effective integration and functionality of the reinforcement board:

Layer position: The reinforcement board should be as close as possible to the core layer of the flex circuit board to ensure maximum support. (For multi-layer stiffeners, we should consider the overall rigidity and flexibility of the balance.)

Layer bonding: The reliability of layer bonding is directly related to the long-term stability of the board, so it is recommended to use high-strength, heat-resistant adhesives to avoid separation or displacement between layers.

Outline design: The outline of the reinforcement board should match the shape of the circuit board to avoid sharp corners or irregular edges, so as to avoid stress concentration leading to damage to the reinforcement board or circuit board. (The smooth contour design can better adapt to the bending needs of the circuit board and extend the service life.)

Installation characteristics: We can design the reinforcement board to be easy to install and remove according to the application requirements. (For some areas that need to be welded, the reinforcement plate can be designed to be temporarily disassembled for easy repair and replacement.)

Flex transition: When designing the transition between the reinforcement plate and the flex area, it should be ensured that the transition area has appropriate flexibility. (Avoid cracks or breaks due to rigidity differences.)

Conclusion

In summary, the PCB stiffener plays a crucial role in the flex and rigid-flex PCB, which not only enhances the mechanical strength of the circuit board, but also effectively improves its stability and durability. I hope this article can provide you with a clear design reference so that you can better choose and configure the reinforcement board in practical applications. If you have any questions, please feel free to contact us!