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4 Layer Rigid-flex PCB

Jarnistech > Rigid-flex PCB > 4 Layer Rigid-flex PCB

Rigid-flex printed circuit boards (PCBs) have gained considerable attention in recent years due to their unique combination of flexibility and rigidity, offering numerous advantages in various electronic applications. Among these, the 4 Layer Rigid-Flex PCB stands out as a versatile solution that provides enhanced design flexibility, improved reliability, and space-saving capabilities. This article delves into the intricacies of 4 Layer Rigid-Flex PCBs, exploring their construction, manufacturing process, and highlighting the advantages they offer in modern electronics.

What is 4 Layer Rigid-Flex PCB?

A 4 Layer Rigid-Flex PCB, also known as a four-layer rigid-flexible printed circuit board, is a type of PCB that combines the benefits of both rigid and flexible circuitry in a single board. This design integrates rigid PCB sections with flexible PCB sections, enabling the board to bend or flex while maintaining electrical connectivity.

The “4 layer” refers to the number of conductive layers in the PCB stack-up. In this case, the PCB consists of four layers of conductive traces and planes sandwiched between insulating layers. These conductive layers provide pathways for electrical signals to travel across the board, while the insulating layers provide mechanical support and separation between the conductive layers.

The rigid sections of the PCB are made from traditional rigid PCB materials like FR4, while the flexible sections usually employ a polyimide or similar flexible substrate. The rigid and flexible sections are bonded together to create a cohesive board structure.

Understanding Rigid-Flex PCB Construction

Understanding the construction of rigid-flex PCBs is essential to grasp their unique characteristics and benefits. Here is an overview of rigid-flex PCB construction:

Layer Arrangement and Stack-Up:

Rigid Layers: The rigid sections of the PCB are made from standard rigid PCB materials like FR4. These layers provide mechanical support and house components.

Flexible Layers: The flexible sections use flexible substrates such as polyimide. These layers allow the PCB to bend and flex without breaking or compromising electrical connectivity.

Adhesive Layers: Adhesive layers are used to bond the rigid and flexible layers together, creating a cohesive structure. These layers must provide both electrical isolation and mechanical stability.

Conductive Traces and Planes:

Rigid Layers: Conductive traces and planes on the rigid layers are typically created using copper, etching processes, and patterned plating techniques. These traces and planes provide electrical connectivity between components and connections.

Flexible Layers: Similar to rigid layers, the flexible layers also have conductive traces, but these traces are designed to flex and withstand repetitive bending without cracking or delamination. The traces may use thinner copper foils to enhance flexibility.

Via Technology:

Through-hole Vias: Through-hole vias are used to establish electrical connections between different layers of the PCB. These vias pass through both the rigid and flexible layers, providing continuity throughout the board.

Blind/Buried Vias: In some cases, blind or buried vias may be employed to make connections between specific layers without traversing the entire board’s thickness. They allow for more compact and efficient designs.

Coverlay and Solder Mask:

Coverlay: Coverlay is a protective layer of flexible material applied over the flexible sections, encapsulating and protecting the conductive traces. It provides insulation, moisture resistance, and protection against environmental factors.

Solder Mask: The solder mask is applied to both rigid and flexible sections to protect the copper traces during soldering processes. It also enhances electrical insulation and prevents solder bridging.

Rigid-flex PCB construction requires careful planning and design considerations to ensure the proper alignment of rigid and flexible sections, appropriate bending areas, and controlled impedance characteristics. Understanding these construction aspects is crucial for achieving reliable and durable rigid-flex PCBs in various applications.

4 Layer Rigid-Flex PCB Stack-up Tips

When considering the layer stack-up for a robust rigid-flex PCB, it is crucial to account for the stresses that the flex board may experience due to bends and curves.

Rigid-Flex PCB 4L Stack-up

4 Layer Rigid-flex PCB Stack-up

To establish an optimized stack-up, adhere to the following guidelines:

Position the flexible section symmetrically at the center of the rigid PCB layers. By maintaining symmetry, you ensure balanced mechanical properties and mitigate the risk of excessive stress concentration in specific areas.

Ensure that traces routed on the flexible sections of the PCB run perpendicular to any bending lines. This orientation prevents premature fatigue and wear of the copper. Perpendicular alignment helps distribute the stress evenly, enhancing the longevity of the flexible circuitry.

Employ curved traces instead of employing 45 or 90-degree straight traces commonly used in rigid PCBs. In the case of flex PCBs, bending the copper in an arc formation minimizes the risk of fatigue. Curved traces accommodate the flexing motion without subjecting the copper to excessive strain.

When dealing with a two-layer flex portion, stagger the traces to avoid their alignment directly on top of each other. This arrangement prevents thickness build-up, where one layer’s copper intersects with another layer’s copper. By staggering the traces, you maintain a consistent thickness profile, optimizing the structural integrity of the PCB.

By following these recommendations, a professional PCB design engineer can develop a robust layer stack-up for a rigid-flex PCB, effectively addressing the stresses associated with bends and curves.

Manufacturing Process for 4 Layer Rigid-Flex PCBs

The manufacturing process for 4 Layer Rigid-Flex PCBs involves several steps to fabricate a high-quality board. Here is an overview of the typical manufacturing process:

Design and Engineering:

PCB Design: The design phase involves creating the PCB layout using specialized software, considering the placement of components, routing of traces, and differentiation between rigid and flexible sections.

Design Verification: The design is thoroughly reviewed and verified to ensure it meets the required specifications, manufacturing capabilities, and reliability standards. Any necessary adjustments are made during this stage.

Material Preparation:

Material Selection: The appropriate materials are selected for rigid and flexible layers based on the specific requirements of the design, such as FR4 for rigid layers and polyimide for flexible layers.

Material Cutting: The selected materials are cut into the desired panel sizes to accommodate multiple PCBs during the manufacturing process.

Inner Layer Processing:

Inner Layer Imaging: The inner layers of the PCB stack-up undergo imaging, where a photosensitive material is applied and then exposed to ultraviolet light using the design artwork to define the trace patterns.

Etching and Cleaning: Unwanted copper is etched away using a chemical solution, leaving behind the desired copper traces. The board is thoroughly cleaned to remove any residue.

Lamination and Layer Alignment:

Layer Stacking: The inner layers, along with insulation layers and adhesive layers, are stacked according to the pre-defined stack-up configuration. Alignment pins or registration holes ensure precise layer registration.

Lamination: The stacked layers are subjected to heat and pressure in a lamination press, bonding them together into a single solid panel.

Drilling and Plating:

Drilling: Holes for component mounting and electrical connections are drilled into the panel using precise CNC drilling machines. These holes are typically plated through to establish electrical continuity between layers.

Plating: Copper plating is performed to create conductive pathways within the drilled holes, ensuring electrical connectivity between the layers. The panel is then plated with a thin layer of copper to form the outer conductive traces.

Outer Layer Processing:

Outer Layer Imaging: Photosensitive material is applied to the outer layers, and the design artwork is used to expose and define the trace patterns.

Etching and Cleaning: Unwanted copper is etched away, leaving behind the desired copper traces on the outer layers. The board is thoroughly cleaned to remove any residues.

Solder Mask and Silkscreen:

Solder Mask Application: A solder mask is applied to the entire board, protecting the copper traces and ensuring proper soldering during assembly. Openings for component pads and vias are created.

Silkscreen Printing: Component and reference designators, logos, or other markings are printed on the board using a silkscreen process, aiding assembly and identification.

Surface Finishing:

Surface Finish Application: The exposed copper surfaces are coated with a surface finish to protect them from oxidation and ensure good solderability. Common options include immersion gold, HASL (Hot Air Solder Leveling), or OSP (Organic Solderability Preservative).

Routing and Profiling:

Trimming and Routing: The panel is cut into individual PCBs using routing machines, separating them from the larger panel. Additional routing may be performed to create unique board shapes or features.

Profiling: The PCBs are profiled to their final dimensions, removing excess material and creating smooth board edges.

Testing and Inspection:

Electrical Testing: Each PCB undergoes electrical testing to verify its functionality and ensure all connections are intact.

Visual Inspection: A thorough visual inspection is performed to check for any manufacturing defects, such as misaligned traces, solder mask issues, or other irregularities.

Assembly and Soldering:

Components are mounted on the PCB using automated or manual assembly processes. Soldering techniques, such as reflow soldering or wave soldering, are applied to establish electrical connections between components and the PCB.

Final Testing and Quality Control:

Functional Testing: The fully assembled PCB is subjected to functional testing to verify its proper operation and adherence to specifications.

Quality Control Measures: Strict quality control measures are implemented throughout the manufacturing process to ensure the final product meets the required standards.

The overall manufacturing process for 4 Layer Rigid-Flex PCBs involves precision, control, and adherence to quality standards to produce reliable and high-performance boards.

Advantages of 4 Layer Rigid-Flex PCB

4 Layer Rigid-Flex PCBs offer several advantages over traditional rigid PCBs or flexible PCBs. Here are some key advantages:

Space Saving and Compact Design:

Rigid-flex PCBs allow for three-dimensional designs, enabling the integration of multiple PCBs or components into a single compact unit. This can significantly reduce the overall size and weight of the electronic device, making it ideal for applications with limited space.

Increased Reliability:

The combination of rigid and flexible sections in a single board eliminates the need for connectors, cables, and solder joints, which are common points of failure in traditional PCB assemblies. Rigid-flex PCBs provide improved reliability, as they have fewer mechanical interconnections and reduced susceptibility to vibrations, shocks, and thermal cycling.

Enhanced Durability:

The flexible sections of a rigid-flex PCB can bend and flex without breaking or causing damage to the board, making them suitable for applications that involve repeated bending or flexing. This durability ensures longevity, making them ideal for devices exposed to harsh environments or subject to constant movement or vibration.

Improved Signal Integrity:

Rigid-flex PCBs minimize signal degradation by reducing the number of interconnects and impedance mismatches. The controlled impedance characteristics of the PCB can be accurately maintained, resulting in better signal transmission, reduced EMI/RFI interference, and improved overall performance.

Simplified Assembly and Reduced Manufacturing Steps:

Rigid-flex PCBs eliminate the need for additional connectors and cables, simplifying the assembly process. Fewer components and interconnections reduce the manufacturing steps, thereby decreasing assembly time and overall manufacturing costs.

Design Flexibility:

Rigid-flex PCBs offer greater design flexibility and customization options compared to traditional rigid PCBs or flexible PCBs. The combination of rigid and flexible sections provides designers with more freedom to create complex shapes, conform to odd form factors, and integrate components in unique ways, enabling innovative product designs.

Weight Reduction:

Rigid-flex PCBs are lighter in weight compared to traditional PCB assemblies, as they eliminate the need for bulky connectors, cables, and interconnects. This advantage is crucial in applications where weight reduction is a priority, such as aerospace, automotive, and handheld electronic devices.

Improved Thermal Management:

Rigid-flex PCBs can effectively dissipate heat due to the design integration of rigid PCB sections. The heat generated by components mounted on the rigid sections can be conducted and dissipated throughout the board, enhancing thermal management and reducing the risk of overheating.

These advantages make 4 Layer Rigid-Flex PCBs a preferred choice for various applications, including medical devices, aerospace systems, automotive electronics, wearable devices, and other compact electronic systems that require reliability, durability, and space optimization.

Why Choose JarnisTech

A Reliable 4L Rigid-flex PCB Supplier in China

JarnisTech is a leading manufacturer and designer of high-quality rigid-flex printed Circuit Boards (PCB), catering to both standard and custom specifications. Our UL certified production facility is equipped with state-of-the-art machinery, enabling us to meet diverse requirements and deliver quick turnaround times.

We take pride in offering a comprehensive range of 4-layer rigid-flex PCBs that conform to international quality standards, ensuring our products remain competitive in the market. With our wealth of industry experience and a skilled team of professionals, we have gained recognition as one of China’s preferred manufacturers of flexible printed circuit boards.

As a trusted partner, we rely on our proven standard processes to produce 4-layer rigid-flex PCBs. These boards are constructed using FR4+Polyimide materials, with width/space of 8/8mil, finish thickness of 0.2mm (flex) and 1.0mm for the rigid portions. The board thickness ranges from 0.5mm to 3.0mm (0.02″ to 0.12″), while copper thickness options include 0.5 OZ, 1.0 OZ, 2.0 OZ, 3.0 OZ, and up to 6 OZ. Our customers have the flexibility to select solder mask colors from a range including white, black, blue, green, and red, while we also offer the Taiyo PSR4000 white solder mask. Additionally, surface finishes such as Immersion Gold, HASL, and OSP can be applied to enhance durability and visual appeal.

4 Layer Rigid-Flex PCB – JarnisTech
Model:Rigid-flex board-4L
Surface treatment:immersion gold
Width/space:8/8 mil
Finish thickness:0.2mm(flex),1.0mm
Min Hot:0.8mm
Board Thickness:0.5mm~3.0mm (0.02″~0.12″)
Copper thickness:0.5 OZ, 1.0 OZ, 2.0 OZ, 3.0 OZ, up to 6 OZ
Outline:Routing, punching, V-Cut
Solder mask:White/Black/Blue/Green/Red, Taiyo PSR4000 white
Legend/Silkscreen Color:Black/White
Surface finishing:Immersion Gold, HASL, OSP
Max Panel size:18″*24″
Packing:Vacuum/Plastic bag

At JarnisTech, we take pride in providing seamless and integrated end-to-end solutions for flexible printed circuits, ensuring total customer satisfaction. We serve clients across various industries and offer not only flexible printed circuit boards but also rigid printed circuit boards. Reach out to us today to discover how we can assist you with your specific requirements.


The 4 Layer Rigid-Flex PCB represents a significant advancement in PCB technology, bridging the gap between traditional rigid PCBs and flexible PCBs. Its unique construction, utilizing both rigid and flexible sections, provides a myriad of advantages in terms of space-saving, reliability, durability, and design flexibility.

As electronic devices continue to evolve and demand more compact and robust solutions, the 4 Layer Rigid-Flex PCB offers a compelling option for various industries, including aerospace, automotive, medical, and consumer electronics. With ongoing advancements in materials, manufacturing techniques, and design methodologies, we can expect even greater possibilities and innovations with 4 Layer Rigid-Flex PCBs in the future

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