Anyone in the hardware industry knows that FPCs are like the “nerve pathways” of electronic devices.

From the flexible cables in smartwatches to the hinge connectors in foldable screens and the moving joints of robots, all of these rely on FPCs for support.

But do you know what determines the performance of an FPC?

The answer lies in its core “skeleton”—the FCCL substrate.

FCCL substrates are divided into “adhesive-coated” and “adhesive-free” types.

Today, we’ll take an in-depth look at exactly what the differences are between the two.

Fig 1

Structures of Adhesive-Based and Adhesive-Free Substrates

Adhesive-Backed Substrates

These consist primarily of a three-layer structure—PI film, adhesive (AD), and copper foil—formed through a process of coating, pressing, and baking.

The adhesive is typically an epoxy resin or acrylic adhesive.

Adhesive-Free Substrates

These consist of a PI film and copper foil. There are two manufacturing methods: the coating method, in which liquid PI is applied to the copper foil; and the hot-pressing method, in which the PI film and copper foil are bonded together under heat and pressure.

The PI used in this method requires special treatment and is also known as TPI.

The core objective of both methods is to achieve a strong bond between polyimide (PI) and copper foil without the use of traditional adhesives (AD), thereby delivering superior performance.

Detailed descriptions of these two mainstream processes are as follows:

Coating Method

Process:

In this method, a layer of liquid polyacrylamide (PAA) precursor solution is applied directly to the surface of the copper foil.

Following high-temperature baking and an imidization reaction, a solid PI film is formed in situ on the copper foil.

Features:

The process is relatively simple and is particularly suitable for manufacturing single-sided flexible circuits.

Lamination Method

Process:

This method uses specially treated thermoplastic polyimide (TPI) film.

Under high-temperature and high-pressure conditions, the TPI film melts and flows, filling the microscopic gaps on the surface of the copper foil.

Upon cooling, it mechanically interlocks with the copper foil, thereby achieving a strong bond.

Features:

This is currently a widely used technology on the market; many leading FPC manufacturers employ this process to produce high-quality adhesive-free substrates.

Fig 2

Difference Between Adhesive-Coated and Adhesive-Free Substrates

Although the only structural difference between adhesive-coated and adhesive-free substrates is the presence of a single layer of adhesive, their performance characteristics are worlds apart.

Adhesive-Based Substrates

Advantages:

Simple manufacturing process, low cost, suitable for fast-moving consumer electronics with minimal reliability requirements

Disadvantages:

1. Poor stability: The coefficient of thermal expansion (CTE) of the adhesive layer does not match that of PI and copper foil.

This makes the board highly prone to internal stress during temperature changes, which can easily lead to unstable dimensional shrinkage or warping.

2. Prone to aging: The adhesive ages easily at high temperatures and is prone to microcracks after repeated bending, severely affecting product lifespan.

3. Poor heat resistance: The adhesive layer has extremely poor heat resistance; it softens easily in high-temperature environments and may even release harmful gases, and it can cause pads to detach during soldering.

4. Poor processability: During FPC production, the presence of adhesive can lead to unstable expansion and contraction, as well as issues such as adhesive pull-out during drilling.

Adhesive-Free Substrates:

Advantages:

1. Excellent dimensional stability: The CTE of adhesive-free substrates closely matches that of copper foil, enabling them to maintain extremely high dimensional accuracy even under extreme temperature fluctuations.

2. Higher heat resistance: They can withstand temperatures of up to 150°C for extended periods and up to 280°C for short periods, making them well-suited for high-temperature environments such as automotive electronics and industrial control systems.

3. Longer service life: Adhesive-free substrates have a simpler structure and are approximately 10–25 μm thinner than adhesive-based substrates, allowing for a smaller bending radius and significantly improved dynamic bending endurance.

4. Superior electrical performance: The dielectric constant (Dk) and dielectric loss (Df) of adhesive-free substrates are more stable.

In high-speed signal transmission, they provide a purer dielectric environment, ensuring the continuity of characteristic impedance and reducing signal loss, making them ideal substrates for 5G and high-speed signal transmission.

Disadvantages:

The manufacturing process is relatively complex, and the cost is significantly higher than that of adhesive-based substrates.

Fig 3 Difference between adhesive-backed and non-adhesive-backed FPC substrates

Conclusion

FCCL substrates play a critical role in determining the performance, reliability, and application range of flexible printed circuits (FPCs).

Although adhesive-based substrates remain widely used due to their simple manufacturing process and lower cost, their limitations in dimensional stability, heat resistance, and long-term reliability make them unsuitable for many advanced applications.

In contrast, adhesive-free FCCL substrates offer superior thermal performance, electrical characteristics, mechanical flexibility, and dimensional accuracy by eliminating the traditional adhesive layer.

These advantages make them an ideal choice for high-performance applications such as automotive electronics, 5G communication devices, foldable displays, wearable electronics, and intelligent robotics.

As electronic products continue to become thinner, faster, and more flexible, the demand for high-quality adhesive-free FCCL substrates will continue to grow, driving further innovation in FPC technology and next-generation electronic manufacturing.