Comprehensive Guide to Multilayer Flexible Circuits

If you are seeking FPC solutions, FS Technology can assist you in the entire process from concept to finished product. We can implement flexible PCB manufacturing services from 1 to 12 layers and meet customers’ needs for assembly and rigid-flex integration. As the project progresses, customers are more concerned about price than you might imagine, and flexible PCB are even more expensive when presented in multi-layer form. For this reason, FS Technology in this article will discuss the knowledge related to multi-layer flexible PCB to help you understand its necessity.

Structure of Multilayer Flexible PCB

Multilayer Flexible PCB, as the name suggests, is a PCB that combines the concepts of flexibility and multiple layers. This means that this type of PCB can form multiple layers internally through a lamination process, adapting to complex three-dimensional spatial requirements and meeting the device’s demands for circuit complexity. Building a multilayer flexible circuit is considered a challenging task, and as the number of layers increases, the cost also rises accordingly. Therefore, the most common multilayer flexible PCB are typically 4-layer structures.

As the number of layers increases, although the circuit density of the inner layers improves, achieving superior electrical performance also leads to a reduction in flexibility. The basic structure of a multilayer flexible PCB is illustrated in the diagram below: it is composed of multiple “circuit layers + adhesive + cover layers,” pressed together through a lamination process, followed by drilling and electroplating to achieve interlayer connections.

Multi-layer flexible PCB stack

Advantages and Disadvantages of Multilayer Flexible PCB


Size Optimization

Due to its multilayer structure, Multilayer flexible PCB is no longer confined to arranging all circuits and connections on a single plane but can be arranged vertically and connected through vias. Ultimately, this allows PCB to integrate more circuits and components within a relatively small physical space, thereby shrinking device size.

High Performance

An essential consideration for PCB performance is signal transmission. Signals need a certain time to traverse the conductors in the circuit during transmission. Based on the speed of electromagnetic waves propagating in conductors, the signal propagation speed in the circuit is usually a small fraction of the speed of light. When signal lines are longer, propagation delay increases. The shorter signal transmission paths of Multilayer flexible PCB result in relatively smaller signal propagation delays, enabling it to offer higher signal frequencies and bandwidth, making it highly effective for high-speed applications.

EMI Resistance

In the Multilayer flexible PCB design, shielding layers can be introduced between different layers to surround sensitive signal lines or analog circuits, achieving isolation in an electromagnetic shielding environment. Additionally, the multilayer structure makes it easier to implement the layout of differential signals, where two opposing signal lines exist as a pair, carrying equal but opposite signals, effectively resisting external electromagnetic radiation.


High Cost

When comparing the costs of PCB under the same circumstances, you’ll find that Rigid-Flex > Flexible > Rigid. This means that even without considering the multilayer structure, the structural cost of Flexible PCB is relatively high. In most applications, a simple Flexible PCB structure is generally suitable, while the purpose of multilayer is to meet highly customized special applications. Additionally, the construction of Multilayer flexible PCB is more challenging, and when you decide to use it, you’ll initially incur higher costs.

High Difficulty

The manufacturing difficulty of Flexible PCB is much higher than that of Rigid PCB, involving processes such as drilling, lamination, assembly, etc., all of which can lead to damage. This phenomenon becomes even more pronounced when dealing with multilayers, requiring manufacturers to put in additional effort. Flexible PCB manufacturers must invest in more advanced equipment to address the issue of low yield. As the number of layers increases, the defect issues of Multilayer flexible PCB become more significant, and manufacturers need to possess extensive rework knowledge.

Applications of Multilayer Flexible PCB

Due to the higher cost and manufacturing complexity of multilayer flexible PCB, their application areas are generally focused on high-end devices.


The flexibility and lightweight nature of multilayer flexible PCB make them ideal for internal components of wearable medical devices. This design helps reduce the burden on patients and improves the comfort of device wear. Additionally, some medical devices are directly used on the human body, requiring a very small volume of medical PCB. Multilayer flexible PCB address this challenge, providing enough space to integrate various sensors and microelectronic components. Some examples include electrocardiographs, ultrasound imaging probes, MRI coils, neurostimulators, and insulin pumps.


Due to the extreme sensitivity to equipment weight in the aerospace field, multilayer flexible PCB are the preferred choice for their lightweight and high-performance characteristics. Additionally, flexible PCB manufacturing materials are typically made of polyimide, a high-performance material advantageous for extreme external environments. Some examples include inertial navigation systems (INS), radar systems, communication antennas, and power management systems.


With the rapid development of autonomous driving technology, the high-performance requirements for automotive electronic systems have driven the widespread use of multilayer flexible PCB in the automotive field. They can withstand the inherent vibrations and physical impacts of automobiles, providing sufficient reliability for internal components. Some examples include infotainment systems, electronic control units (ECU), sensing systems, airbag controllers, and battery management systems (BMS).

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