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4 Layer Flex PCB

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4 Layer Flex PCB

Single or double-layer flex PCBs have limitations when it comes to meeting the power and speed requirements of larger and more complex projects. These boards are not suitable for demanding applications. In such cases, 4-layer flex PCBs offer a crucial solution. Compared to single or double-layer PCBs, 4-layer printed circuit boards provide numerous advantages.

To avoid costly mistakes in the future, it is essential to have a comprehensive understanding of 4-layer flex PCB materials, design considerations, applications, benefits, and layout. This article aims to provide detailed and insightful information on each aspect of 4-layer rigid-flex PCBs. Are you ready to explore? Let’s delve into it!

What is 4 Layer Flex PCB?

Flexible printed circuit boards (PCBs) are composed of electronic circuits mounted on flexible plastic substrates, allowing for versatile and adaptable designs. It is possible to create flexible circuit boards by screen-printing silver circuits on polyester materials.

When it comes to 4-layer flex PCBs, they comprise four conductive layers and are typically constructed using a material called Polyimide. Although FR4 can be used, Polyimide offers several advantages that make it the preferred choice. Polyimide is known for its exceptional thermal stability, enabling 4-layer flex PCBs to withstand higher temperatures. Consequently, these PCBs find extensive use in high-temperature applications.

Furthermore, Polyimide provides significant flexibility, facilitating the mounting of electronic devices that would be challenging to assemble on rigid boards. This flexibility streamlines the design process and allows for intricate designs that would be impossible to achieve with rigid circuit boards. Additionally, the use of Polyimide reduces the overall weight of printed circuit boards.

Materials Used in Making 4 Layer Flex PCB

The materials used in 4-layer flex PCBs play a crucial role in their longevity and effectiveness. These materials enable the PCBs to meet the demands of various applications. The materials for 4-layer flex PCBs can be categorized into three main types.

Base Material:

In flexible circuit boards, the base material acts as the foundation. In the case of rigid-flex PCBs, a flexible polymer film serves as the base material. The substrate material is typically made of polyimide, which offers excellent flexibility and can withstand high-temperature applications. Polyimide provides the necessary characteristics for rigid-flex PCBs, reducing board thickness compared to rigid boards. Its flexibility helps prevent breakage, which is common in rigid boards. Another advantage of polyimide is that it does not require adhesive for copper attachment.


Adhesive materials are used to form bonds in the PCB. When polyimide is used as the base material in rigid-flex PCBs, the choice of adhesive is critical as it can affect the laminate’s performance. Using polyimide adhesives can present certain challenges, so alternative adhesive materials from different polymer families are often used. The thickness of the adhesive in flex circuits varies depending on the specific application requirements.

Conductive Material:

The conductive material is a key component of 4-layer flex PCBs, particularly in industrial control equipment and industrial applications. Copper is commonly used as the conductive material in these PCBs. Copper offers excellent electrical properties, is available in various thicknesses, and is cost-effective, making it highly desirable for 4-layer flex circuit boards. Copper foils come in two main types: electrodeposited and rolled/wrought. Additionally, there is a demand for thinner copper foils in modern applications.

By carefully selecting and utilizing these materials, 4-layer flex PCBs can be optimized for performance and durability in a wide range of applications.

4 Layer Flex PCB Stack-up and Configurations

In adherence to regulations, the minimum total thickness of the thinnest variant of a 4-layer PCB measures at 0.282 mm with a tolerance of +/- 0.03 mm. However, it is worth noting that there are alternative options available, with the maximum thickness reaching 0.45 mm. The thickness of copper employed in the PCB holds significance as it directly impacts the trace width and current carrying capacity.


4 Layer Flex PCB Stackup

The composition of the multilayer flex board’s stack-up consists of the following layers:

1. Coverlay: This layer comprises polyimide material combined with adhesive.
2. Copper trace: A layer featuring copper traces, accompanied by an adhesive layer.
3. Another layer of copper trace: Similar to the previous layer, this one also includes copper traces and an adhesive layer.
4. Bottom layer of coverlay: This layer is composed of coverlay, serving as the bottom-most component of the stack-up.

By following this stack-up arrangement, the PCB achieves the desired structure and functionality.


Symmetrical Arrangement:

In the symmetrical arrangement, the PCB structure consists of a coverlayer, followed by a signal layer (Layer 1), a ground plane (Layer 2), a flexible core dielectric, another signal layer (Layer 3), a ground plane (Layer 4), and finally, another coverlayer. This configuration offers effective electromagnetic interference (EMI) control by employing ground plane shielding and maintaining similar layer structures on both sides of the core. This design helps minimize EMI emissions and provides improved signal integrity.

Asymmetrical Arrangement:

In the asymmetrical arrangement, the PCB structure includes a coverlayer, a signal layer (Layer 1), a ground plane (Layer 1), a power layer, a flexible core dielectric, another ground plane (Layer 2), a signal layer (Layer 3), and finally, another coverlayer. This configuration is designed to provide isolation between signals by distributing power and ground planes on each side of the core dielectric. By separating power and ground planes, this arrangement helps reduce crosstalk and interference between different signal paths, enhancing overall signal integrity.

High-Speed Arrangement:

In the high-speed arrangement, the PCB structure comprises a coverlayer, a signal layer (Layer 1), a ground plane (Layer 1), another signal layer (Layer 2), a flexible core dielectric, yet another signal layer (Layer 3), another ground plane (Layer 2), and finally, another coverlayer. This configuration is specifically tailored for fast signals that require impedance control. The outer layers are dedicated to carrying these fast signals, while power and ground planes are strategically placed to serve as constant reference planes. This arrangement helps maintain signal integrity, minimize signal loss, and control impedance, ensuring reliable high-speed signal transmission.

Please note that the specific arrangement and layer configuration of a PCB can vary depending on the design requirements and the intended application. These examples illustrate common approaches used in PCB designs to achieve specific goals related to EMI control, signal isolation, and high-speed signal transmission.

Benefts of 4 Layer Flex PCBs

While rigid printed circuit boards are widely used globally, flexible printed circuit boards offer distinct advantages. Here are some of the key benefits:

Compatibility: 4-layer flex PCBs are highly compatible with a wide range of connectors and components, surpassing the compatibility of rigid PCBs.

Stability and Reliability: Flexible circuit boards exhibit greater stability and reliability compared to rigid PCBs. With fewer interconnects and solder joints, the chances of faults are reduced, resulting in increased overall reliability.

Weight and Space Efficiency: Flex PCBs are lightweight and occupy significantly less space than traditional PCBs. They require only around ten percent of the space and weight, leading to reduced transportation costs, greater installation flexibility, and easier packaging processes.

Higher Circuit Density: 4-layer flex PCBs offer higher circuit density, allowing them to meet the requirements of dynamic flexing applications.

Wide Temperature Range: Flex PCBs have a wider temperature range, enabling them to operate under higher currents and enhancing the lifespan of electronic components.

Improved Signal Quality and Impedance Control: Four-layer flex PCBs provide enhanced signal quality, precise impedance control, and robust operation, giving them an advantage over other alternatives available in the market.

These advantages make 4-layer flex PCBs a favorable choice in various industries, offering improved performance, reliability, and efficiency for electronic systems and devices.

Challenges in 4 Layer Flex PCB Fabrication

The production of 4-layer flex PCBs presents inherent challenges despite their significant advantages. These difficulties include:

Achieving fine lines and spaces on multiple flexible layers: Ensuring precise and narrow traces on multiple flexible layers is a complex task.

Holding tight layer-to-layer registration through all photolithography stages: Maintaining accurate alignment between different layers throughout the photolithography process is crucial but can be challenging.

Producing a high density of buried and blind microvias with thin dielectric: Creating numerous small-diameter microvias that are buried or blind, while maintaining a thin dielectric layer, poses a manufacturing challenge.

Preventing lifted lands or cracked barrel plating on plated through-holes (PTHs) during flexing: The flexing action can potentially cause lifted lands or cracked barrel plating on PTHs, necessitating careful design and material selection.

Maintaining reliable coverlayer and stiffener adhesion through dynamic flexing: Ensuring consistent adhesion between the coverlayer, stiffener, and flexible layers during dynamic flexing is critical for the PCB’s long-term reliability.

Controlling characteristic impedances across multiple flex layers: Achieving consistent characteristic impedances across different flex layers is a complex task due to variations in materials and geometries.

Managing thermomechanical stresses and fatigue-prone areas: Flex PCBs are subjected to thermomechanical stresses, which can lead to fatigue and failure in certain areas. Managing these stresses is essential for ensuring long-term reliability.

Qualification testing of impedance shifts under long-term flex cycling: Conducting thorough qualification testing to assess impedance shifts under prolonged flex cycling is necessary to validate the PCB’s performance and reliability.

Ensuring flex fold and bend cycle life reaches millions of cycles: Flex PCBs must be designed and manufactured to withstand millions of cycles of flexing and bending without degradation or failure.

Addressing these challenges requires expertise in design, material selection, manufacturing processes, and thorough testing to ensure the production of high-quality and reliable 4-layer flex PCBs.

4 Layer Flex Design Recommendations

When designing 4-layer flex PCBs, it is important to follow key guidelines to ensure optimal performance and reliability. These guidelines include:

Plan Stack-up Arrangement:

Carefully arrange the stackup to isolate critical signals using ground planes, allowing for better signal integrity and minimizing interference.

Impedance Matched Traces and Shielding:

Use impedance matched traces for high-speed signals and provide adequate shielding to minimize signal degradation and electromagnetic interference.

Model Performance under Dynamic Bending Conditions:

Consider the effects of bending on signal integrity and impedance, and simulate the performance under various bending scenarios to ensure reliable operation.

Adjust trace dimensions in flexing areas:

Increase the trace and space dimensions in areas that undergo flexing to prevent mechanical stress and ensure durability compared to rigid sections.

Incorporate strain relief structures:

Include strain relief structures at the interfaces between rigid and flex sections to minimize stress concentration and enhance mechanical reliability.

Utilize thermal reliefs:

Implement thermal reliefs for solder joints to reduce stress on the flex PCB, especially during temperature variations, minimizing the risk of solder joint failure.

Add stiffeners or covers:

Integrate stiffeners or covers at connection points or areas requiring extra rigidity to enhance mechanical stability and prevent damage during handling or assembly.

Component placement:

Maximize the distance of components from high flex zones to avoid potential damage or strain on sensitive components during bending or flexing.

Controlled bend radii:

Specify multiple controlled bend radii rather than arbitrary flexing to ensure consistent and reliable performance during repeated flexing.

Check for acid traps:

Review layouts for susceptibility to acid traps during fabrication, ensuring proper copper etching and avoiding issues related to acid entrapment.

Microvia capture pads:

Specify microvia capture pads with a 1:1 aspect ratio to ensure reliable connections and minimize the risk of via failures.

Consider shielding, insulation, and abrasion resistance:

Take into account the need for shielding, insulation, and abrasion resistance in the stackup design, depending on the application requirements and environmental conditions.

By following these guidelines, designers can enhance the performance, reliability, and manufacturability of 4-layer flex PCBs, meeting the specific needs of their applications.

Applications of 4 Layer Flex PCB

Flex PCBs offer several advantages such as reduced weight and a low bend radius, making them highly suitable for implementation in medical devices and industrial control equipment. In the automotive industry, these flexible PCBs find effective utilization in vehicle interior control equipment, ABS systems, dashboards, and other related applications.

Furthermore, flex boards provide a comprehensive solution for the consumer electronics industry, serving as a versatile component in various devices such as calculators, motion monitors, cameras, keyboards, tablets, and more.

Within the medical sector, flex PCBs are employed for interconnections involving resistors, circuits, and capacitors, enabling seamless integration and functionality. The impressive power, thinness, and stiffener capabilities of flex PCBs make them an ideal choice, particularly in aeronautical and marine equipment applications.

In the mobile phone market, layer flex PCBs hold a central position and remain a popular choice. Their assembly process, along with the soothing green color on one or both sides, adds visual appeal. In compact devices like cell phones, these flex PCBs facilitate multiple interconnections, enhancing overall efficiency and performance.


The utilization of 4-layer flex PCB technology offers the opportunity to create innovative products that exhibit high routing density while maintaining dynamic flexing capabilities. However, realizing the full potential of this technology necessitates careful consideration of stackup configurations, meticulous process control, and proficient flex design techniques. Collaborating with an experienced flex PCB manufacturer-JarnisTech, renowned for their expertise in 4-layer technology, empowers designers to maximize the benefits of 4-layer flex PCBs and cultivate state-of-the-art solutions for the integration of flexible electronics across diverse industries.

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