Why do flexible circuit boards exhibit lower signal loss in high-frequency signal transmission?
Publish Time: 2025-09-16
With the rapid development of 5G communications, high-speed data transmission, millimeter-wave radar, wearable devices, and IoT technologies, electronic systems are increasingly demanding higher signal transmission frequencies, with operating frequencies now generally entering the GHz and even tens of GHz range. In high-frequency signal transmission, signal loss becomes a key bottleneck affecting system performance, potentially leading to data errors, reduced transmission distance, and increased power consumption. Traditional rigid printed circuit boards are often limited in high-frequency applications by significant dielectric loss, conductor loss, and radiation loss. In contrast, flexible circuit boards, thanks to their unique material system, structural design, and manufacturing processes, exhibit lower signal loss in high-frequency signal transmission, making them an ideal choice for high-speed electronic systems. Their low loss characteristics are primarily due to the following core factors.1. The Advantages of Substrates with Low Dielectric Constant and Low Dissipation FactorWhen a signal propagates through a transmission line, its speed and loss are directly influenced by the dielectric properties of the substrate material. The dielectric constant determines the signal propagation speed, while the dissipation factor reflects the material's ability to convert electromagnetic energy into heat in an alternating electric field. The lower the Df value, the lower the dielectric loss. Flexible circuit boards typically use polyimide or higher-end liquid crystal polymers as their substrate.2. Precisely Controllable Impedance Matching and Uniform Transmission Line StructureHigh-frequency signals are extremely sensitive to the impedance continuity of the transmission path. Any impedance change will cause signal reflection and loss. Flexible circuit boards are manufactured using photolithography, enabling micron-level control of line width, spacing, and dielectric thickness—far exceeding the machining accuracy of traditional rigid boards. This high-precision manufacturing capability enables FPCs to achieve highly consistent characteristic impedance, ensuring impedance continuity during signal transmission and reducing reflection loss. Furthermore, flexible boards often utilize optimized transmission line structures such as coplanar waveguides or stripline circuits. By precisely designing the spacing between the ground plane and signal lines, electromagnetic field distribution is effectively controlled, reducing crosstalk and radiation loss.3. Low-Roughness Copper Foil and Optimized Conductor StructureConductor loss is another major source of high-frequency signal attenuation, primarily due to the skin effect—the higher the frequency, the more current is concentrated on the conductor surface. Therefore, conductor surface roughness significantly affects loss: the rougher the surface, the longer the current path, the greater the resistance, and the higher the loss. Flexible circuit boards commonly utilize rolled annealed copper or ultra-low-profile electrolytic copper foil, whose surface roughness can be controlled below 0.5μm, significantly lower than the electrolytic copper commonly used in traditional rigid boards. The smooth copper foil surface significantly reduces conductor loss at high frequencies. Furthermore, FPCs can further reduce resistance and improve transmission efficiency by increasing conductor thickness or adopting wide trace designs.4. Compact Integration and Short Transmission Path DesignThe bendable and foldable nature of flexible circuit boards enables three-dimensional wiring, directly connecting high-frequency components along the shortest possible paths, significantly shortening signal transmission distances. According to transmission line theory, signal loss is proportional to transmission length, and shortening the path directly reduces overall loss. In compact devices such as smartphones and TWS earbuds, FPCs are often used to connect the motherboard to millimeter-wave antenna modules, enabling near-end chip-antenna integration and avoiding the additional losses and impedance mismatch associated with traditional cable connections.5. Excellent High-Frequency Electromagnetic Shielding PerformanceTo reduce electromagnetic interference (EMI) and radiation losses, flexible circuit boards can incorporate multi-layer shielding structures. For example, in double-sided FPCs, continuous ground planes can be provided on both sides of the signal layer, creating a shielded environment similar to a microstrip line. In multi-layer FPCs, dedicated shielding layers or a conductive coating can also be added. These designs effectively confine electromagnetic fields to the transmission lines, reducing outward radiated energy losses while improving anti-interference capabilities and ensuring high-frequency signal integrity.Flexible circuit boards exhibit lower signal loss in high-frequency signal transmission due to the combined effects of their low-loss substrate, high-precision manufacturing process, low-roughness copper foil, optimized transmission line structure, and 3D integration capabilities. These attributes offer irreplaceable advantages in applications such as 5G RF modules, high-speed communication interfaces, automotive radar, and medical electronics.
