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Multilayer Flex PCB

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Multilayer Flexible Circuit Boards

You may be interested in understanding the concept of a multilayer flex PCB or a flex circuit and how the layers are incorporated into the PCB design. In this article, we will explore the terminology and construction techniques involved in multilayer flex PCBs. By gaining a clear understanding of these aspects, you will be able to make informed decisions while selecting the most suitable multilayer flex PCB for your product. Moreover, you will be equipped to avoid common issues associated with this type of PCB. Let’s delve into the details to gain a comprehensive understanding of multilayer flex PCBs.

Definition of Multilayer Flex PCB

Multilayer FPC involves the lamination of three or more layers of single-sided or double-sided flexible circuits. This process incorporates metallized holes created through drilling and electroplating to establish conductive paths between different layers. As a result, the need for complex soldering processes is eliminated. Multilayer flexible circuits offer significant advantages such as enhanced reliability, improved thermal conductivity, and simplified assembly performance.

These multilayer flexible circuit boards possess key characteristics including thinness, flexibility, high wiring density, lightweight construction, and exceptional flexibility. They find wide-ranging applications in various products such as mobile phones, computers, digital cameras, household appliances, communications equipment, new energy vehicles, and aerospace technologies.

In multilayer flex circuit designs, shielding techniques and surface mount techniques are often utilized to meet specific requirements. These techniques contribute to the overall functionality and performance of the multilayer designs.

Materials Used To Manufacture Multilayer Flexible PCBs

The main raw materials involved in the production of multilayer flexible PCBs include base materials, cover films, reinforcing materials, and other auxiliary materials.

Base Materials:

Base materials for multilayer flexible PCBs can be categorized into adhesive and adhesive-free substrates. Adhesive substrates consist of copper foil, adhesive, and PI. They can be further classified as single-sided adhesive substrates or double-sided adhesive substrates.

Adhesive-free substrates, on the other hand, do not contain an adhesive layer. These substrates offer advantages such as reduced thickness, superior dimensional stability, increased heat resistance, enhanced bending resistance, and improved chemical resistance. Hence, adhesive-free substrates are widely utilized in current applications.

Cover Film:

The cover film consists of three components: release paper, adhesive, and PI. During the production process, the release paper is peeled off, leaving behind only the adhesive and PI on the flexible circuit board.

Reinforcing Materials:

Reinforcing materials play a crucial role in strengthening specific areas of the flexible PCB. They compensate for the inherent flexibility of FPC. Commonly used reinforcing materials include FR4, steel sheet, PI, and aluminum sheet.

Other Auxiliary Materials:

Various other auxiliary materials are employed in the manufacturing of multilayer flexible PCBs. These include adhesives (pure glue used for bonding soft and rigid boards, as well as bonding FR-4 or steel sheet reinforcement materials), electromagnetic protective film, and pure copper foil (primarily employed in the production of hollow flexible PCBs).

Common Constructions of Single, Double and Multilayer Flex PCB

Flexible Printed Circuit Boards (Flex PCBs) find extensive application in electronic devices that necessitate flexibility and compactness. They provide numerous benefits, including space efficiency, lightweight construction, and the capacity to conform to irregular shapes. Single-layer, double-layer, and multilayer flex PCBs are constructed in several common configurations to meet specific requirements and design constraints.

Constructions of Flex PCB

Single-Layer Flex PCB:

A single-layer flex PCB comprises a single conductive layer, usually copper, positioned between two flexible polymer layers, commonly made of polyimide. The conductive layer forms the necessary traces and pads for component attachment. This type of flex PCB is suitable for simpler designs with relatively low complexity, or when bending in a single direction is required.

Double-Layer Flex PCB:

Double-layer flex PCBs consist of two conductive layers separated by an insulating layer. Usually composed of copper, the conductive layers are connected through plated through-holes (PTHs). The insulating layer can be a flexible material like polyimide or a rigid material such as FR-4. The top and bottom layers may have separate circuitry or be interconnected through vias. Double-layer flex PCBs offer increased design flexibility, allowing for more intricate circuit layouts and the option to accommodate components on both sides.

Multilayer Flex PCB:

Multilayer flex PCBs incorporate three or more conductive layers, with insulating layers in between. The conductive layers are interconnected using a combination of PTHs and vias. The insulating layers can consist of flexible and rigid materials, chosen based on design requirements. Multilayer flex PCBs provide the highest degree of complexity and functionality, enabling dense circuitry, multiple component mounting options, and advanced signal routing. These PCBs are commonly used in space-constrained applications like smartphones, tablets, and wearable devices.

It’s important to recognize that the specific construction of flex PCBs can vary depending on factors such as the manufacturer, design specifications, and application. The number of layers, materials utilized, and fabrication techniques employed may differ. Nonetheless, the aforementioned principles offer a general understanding of the typical constructions for single-layer, double-layer, and multilayer flex PCBs.

Multilayer Flex PCB Stackup

Here is stack up of a normal 4-layer flexible circuit:

4 Layer Flex PCB Stackup

The proposed design is a 6-layer flexible circuit that incorporates an FR4 material stiffener in its stackup. The Hole Wall Copper thickness measures between 15-20 micrometers. The customer’s specified impedance value is approximately 90 Ohms with a tolerance of +/-10%.

6 Layer Flex PCB Stackup

Types of Multilayer Flex PCBs

Flexible Copper Clad Laminate (FCCL) is a crucial component used in the fabrication of flexible PCBs, including rigid-flex PCBs. It consists of alternating layers of copper foil and polyimide (PI) or polyester (PET). The choice of copper foil and PI directly affects key attributes of multilayer flex PCBs, including thickness and flexibility.

There are two main categories of FCCL based on the PI used: glued and adhesive-free multilayer flex PCBs. Additionally, different types of copper foils, namely Rolled Annealed (RA) copper and Electrodeposited (ED) copper, are used in FCCL for multilayer flex PCBs.

Glued multilayer flex PCBs:

Glued multilayer flex PCBs are manufactured by bonding copper foil and adhesive-coated PI films. However, the adhesive employed in this process exhibits limitations in terms of heat resistance, dimensional stability, and chemical resistance. These PCBs cannot endure temperatures above 200°C, and their tear strength significantly decreases at temperatures of 120°C or higher.

Consequently, glued FCCL is unsuitable for rigid-flex PCBs, which often undergo lamination temperatures exceeding 200°C. These adhesive-based FCCLs are primarily used in flexible PCBs operating at temperatures below 200°C.

Adhesive-free multilayer flex PCBs:

Adhesive-free FCCL can be produced in two ways. The first method involves using a PI film as the carrier and coating copper foil onto its surface. The second technique utilizes copper foil as the carrier, with liquid PI applied to its surface and subsequently cured. Adhesive-free multilayer flex PCBs exhibit minimal dimensional changes even at high temperatures. At 300°C, the dimensional change rate remains within 0.1%. The exceptional dimensional stability of adhesive-free multilayer flex PCBs enables fine lines to be achieved.

This is of particular importance in high-end electronic products like LCDs, PDPs, and COF boards that require fine lines, high density, dimensional stability, and high-temperature resistance. Consequently, adhesive-free multilayer flex PCBs are increasingly preferred. For example, JarnisTech offers adhesive-free multilayer flex PCBs with a minimum trace width/space of 1.6mil/1.6mil.

Copper foil options in FCCL: FCCL for multilayer flex PCBs employs two types of copper foil: Rolled Annealed (RA) copper and Electrodeposited (ED) copper.

RA copper multilayer flex PCBs:

RA copper is obtained by heat and rolling processes to achieve the required thickness on FCCL. RA copper exhibits excellent resistance to bending, making it suitable for rigid-flex PCBs and flexible PCBs subjected to frequent movement. The price of RA copper multilayer flex PCBs increases as the copper thickness decreases (below 0.5oz). The surface of RA copper foil is smooth, typically allowing circuit generation through laser technology. JarnisTech offers RA copper flex PCBs with a minimum copper thickness of 25μm.

ED copper multilayer flex PCBs:

ED copper is applied to FCCL through physical sputtering or electroplating processes, based on the desired thickness. The price of ED copper FCCL increases with higher copper thickness. JarnisTech provides ED copper multilayer flex PCBs with a maximum copper thickness of 3oz. Although ED copper is less resistant to bending, it facilitates easier etching. Consequently, ED copper multilayer flex PCBs are primarily used in static applications, such as connecting circuits and transmitting signals in lithium battery system covers, LED light PCB beauty masks, and various automotive lights (e.g., walling lights, door lights, position lights, marker lights).

Comparing Multi-layer Flexible PCB and Multi-layer Rigid PCB

Multi-layer Flexible PCBs and Multi-layer Rigid PCBs are both types of printed circuit boards used in electronic devices. Here’s a comparison between the two:

Flexibility: Multi-layer Flexible PCBs offer the advantage of flexibility, allowing them to bend and conform to different shapes or fit into tight spaces. Rigid PCBs, on the other hand, are inflexible and cannot be bent.

Design: Flexible PCBs are well-suited for designs where space constraints or complex shapes are a concern. They can be folded, twisted, or rolled to fit specific form factors. Rigid PCBs are better for designs that require a solid and stable structure.

Weight and Size: Flexible PCBs are generally lighter and thinner compared to their rigid counterparts. Their reduced weight and size make them ideal for lightweight and portable devices.

Durability: Rigid PCBs tend to be more durable and resistant to damage, as they are not susceptible to bending or flexing. Flexible PCBs can withstand some degree of bending and twisting but may be more prone to wear and tear.

Cost: In general, rigid PCBs are more cost-effective due to their simpler manufacturing processes and widespread use. Flexible PCBs involve additional manufacturing steps and specialized materials, making them relatively more expensive.

Assembly: Rigid PCBs are typically easier to assemble due to their rigid structure, which provides stability during the soldering process. Flexible PCBs may require specialized assembly techniques to ensure proper alignment and connection of components.

Both types of PCBs have their own advantages and are suitable for specific applications. The choice between the two depends on the specific requirements of the electronic device, such as size limitations, flexibility needs, and cost considerations.

Design Considerations for Multilayer Flex PCBs

Designing a multilayer flexible PCB necessitates careful consideration of various factors to achieve optimal performance and reliability. Several key design considerations to bear in mind are:

Layer Stackup:

The layer stackup is a critical aspect of multilayer flex PCB design. It determines the number of layers, layer thickness, and material selection. The objective is to strike a balance between flexibility, durability, and functionality. The stackup must be optimized to minimize stress on the flex circuit during bending and ensure proper signal integrity.

Trace Routing:

Precise trace routing is vital to maintain signal integrity and prevent crosstalk in a multilayer flex PCB. Close attention should be paid to trace width, spacing, and impedance to ensure reliable signal transmission. It is important to avoid sharp bends or acute angles in trace routing since this can induce stress on the flex circuit and lead to failures.

Component Placement:

Thoughtful component placement is crucial when designing a multilayer flex PCB to accommodate the circuit’s bending and folding requirements. Components should be situated in areas where they won’t impede the movement of the flex circuit. Secure mounting of components is imperative to prevent damage during flexing.

Material Selection:

Choosing suitable materials for a multilayer flex PCB is critical for achieving optimal performance and reliability. Materials must be selected based on their ability to withstand bending and flexing without cracking or breaking. Thermal and electrical properties should also be considered to ensure proper signal transmission and effective heat dissipation.

Manufacturing Difficulties of Multi-layer Flexible PCB

Flexible Printed Circuit Boards (Flex PCBs) are becoming increasingly popular due to their adaptability and space-saving features. However, the manufacturing process of multi-layer Flexible PCBs is complex and can present several challenges. Here are some of the common difficulties encountered in the manufacturing of multi-layer Flexible PCBs:

1. Material Handling: Flexible PCB materials like polyimide are extremely delicate and can easily be damaged during the manufacturing process. This requires special handling and storage procedures to prevent any harm.

2. Alignment: Multi-layer PCBs require precise alignment of each layer. This is more challenging with flexible PCBs because they can bend and distort, which can lead to misalignment.

3. Lamination: The lamination process requires high temperatures and pressure, both of which can potentially damage the flexible materials. It is crucial to carefully control these conditions to prevent deformation or delamination.

4. Drilling and Plating: Drilling holes for vias on flexible PCBs can be difficult due to the material’s softness. The plating process can also be challenging because of the need to ensure a uniform copper layer.

5. Soldering: Flexible PCBs are more susceptible to heat damage during the soldering process. Overheating can easily cause the materials to warp or distort, thereby affecting the overall performance of the circuit board.

6. Circuit Design: Flexible PCBs are often used in applications that require complex, 3D circuit designs. This can make the design and manufacturing process more challenging.

7. Cost: The materials used for flexible PCBs, such as polyimide, are more expensive than those used for rigid PCBs. The more complex manufacturing process also increases the overall cost.

8. Testing and Inspection: Testing flexible PCBs can be challenging due to the potential for damage during handling. The flexibility of the PCB can also make certain types of testing, such as automated optical inspection, more difficult.

To overcome these challenges, manufacturers often need to use specialized equipment and techniques. They must also invest in training their staff to handle flexible PCB materials properly. Despite these difficulties, the advantages of flexible PCBs, such as their lightness, flexibility, and ability to fit into small or awkward spaces, make them an attractive option for many applications.

When to Use Multilayer Flexible PCB?

The Multilayer Flexible PCB board finds its suitability in various applications due to its distinct advantages and capabilities. Here are some specific areas where this type of PCB is often utilized:

Ground and Power Plane Applications:

Multilayer Flexible PCBs are well-suited for incorporating ground and power planes, which provide stable and efficient power distribution throughout the circuit. The flexibility of the board enables it to adapt to different shapes and fit into confined spaces, making it an excellent choice for these applications.

Shielding Applications:

The flexibility of Multilayer Flexible PCBs allows them to be effectively utilized for shielding applications. They can be designed to provide shielding and EMI/RFI protection, ensuring proper isolation and minimizing electromagnetic interference.

High Circuit Density:

When the circuit density and layout requirements exceed the limitations of a single-layer PCB, Multilayer Flexible PCBs become an ideal solution. The additional layers allow for increased routing options and interconnected complexity, accommodating intricate designs with high component density.

Controlled Impedance with Shielding:

Multilayer Flexible PCBs can be designed with controlled impedance characteristics while incorporating shielding layers. This combination enables reliable signal transmission while minimizing signal distortion and external interference.

Increased Circuit Density:

Applications that demand compact form factors and increased circuit density can benefit from Multilayer Flexible PCBs. Their ability to stack multiple layers of circuitry in a flexible format allows for space-efficient designs without compromising functionality.

Crosstalk Elimination:

Multilayer Flexible PCBs offer the advantage of reducing crosstalk between traces. With careful design considerations, such as controlled spacing and proper layer arrangement, they can minimize signal coupling and ensure signal integrity.

These applications highlight the versatility of Multilayer Flexible PCBs, but their potential usage extends beyond these examples. For a comprehensive understanding of how this board can meet your specific requirements, we recommend contacting us to explore further possibilities and specifications.

How To Choose A Multi-layer Flexible PCBs Manufacturer?

Choosing a reliable and suitable manufacturer for multilayer flexible PCBs is crucial to ensure the quality, performance, and timely delivery of your electronic products. Here are some key factors to consider when selecting a multilayer flexible PCB manufacturer:

Experience and Expertise:

Look for a manufacturer with a proven track record and extensive experience in producing multilayer flexible PCBs. Check their history, the number of years they have been in business, and their expertise in manufacturing flexible PCBs specifically.

Quality Standards and Certifications:

Ensure that the manufacturer follows strict quality control measures and holds relevant certifications, such as ISO 9001 or ISO 13485. These certifications indicate their commitment to maintaining high-quality manufacturing processes.

Manufacturing Capabilities:

Assess the manufacturer’s manufacturing capabilities, including their equipment, facilities, and production capacity. Make sure they have the necessary resources to handle the complexity and volume of your multilayer flex PCB requirements.

Design and Engineering Support:

A reputable manufacturer should offer design and engineering support to assist you in optimizing your multilayer flexible PCB design. They should be able to review your design files, provide design for manufacturability (DFM) feedback, and offer technical expertise throughout the manufacturing process.

Material Selection:

Verify that the manufacturer uses high-quality materials suitable for multilayer flexible PCBs. The choice of materials, including the flexible substrates and adhesives, can impact the performance and reliability of the finished PCBs.

Production Lead Time and Flexibility:

Consider the manufacturer’s production lead time and their ability to accommodate your production schedule. Flexibility in terms of order quantity and quick turnaround times can be advantageous, especially if you require rapid prototyping or have changing production requirements.

Quality Assurance and Testing:

Inquire about the manufacturer’s quality assurance processes, including the testing and inspection methods they employ. They should conduct rigorous testing to ensure the reliability and functionality of the multilayer flexible PCBs.

Cost and Pricing:

While cost should not be the sole factor, it’s important to consider the manufacturer’s pricing structure and whether it aligns with your budget. Request detailed quotations, including any additional costs for tooling, testing, or customized requirements.

Customer Reviews and References:

Seek out customer reviews, testimonials, or references from other clients who have worked with the manufacturer. This can provide insights into their reputation, customer satisfaction, and overall performance.

Communication and Support:

Evaluate the manufacturer’s communication channels and responsiveness. Clear and open communication, along with responsive customer support, is crucial for addressing any concerns, providing updates, and resolving issues throughout the manufacturing process.

By considering these factors and conducting thorough research, you can identify a multilayer flexible PCB manufacturer that meets your specific requirements and can deliver high-quality products within your timeline.


Multilayer flexible printed circuit boards (PCBs) offer distinct advantages that make them highly suitable for a wide array of electronic applications. In comparison to traditional rigid PCBs, they provide superior flexibility, durability, and space efficiency.

The growing demand for smaller and more intricate devices has significantly contributed to the increasing popularity of multilayer flex PCBs. These boards find ideal application in industries such as medical devices, aerospace, and military equipment, where space constraints are prevalent, and reliability is paramount.

To ensure an optimal design tailored to specific application requirements, it is crucial to collaborate with an experienced manufacturer like JarnisTech. Our expertise ensures that factors like the number of layers, choice of materials, and overall board size and shape are meticulously considered during the design process.

In conclusion, multilayer flex PCBs are a compelling choice for creating electronic devices that prioritize high quality and reliability. Given their myriad advantages and growing prominence, they are poised to remain a vital component in the electronics industry for years to come.

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