Introduction:

A Flexible Printed Circuit Board (FPCB) is a type of PCB made from flexible, high-folding-grade polyester film or polyimide substrate, which undergoes a series of manufacturing processes. It is designed to house components and circuits that provide electrical functions with high reliability, excellent flexibility, and light weight. Also known as a flexible plate or soft board, its abbreviation is FPCB. Initially used primarily in aerospace and military applications, FPCBs are now widely used in consumer electronics. Compared to traditional rigid PCBs, flexible PCBs offer several advantages, including:

• Compact size, lightweight, and thin profile
• Flexible and adaptable design
• High reliability
• Cost-effective
• Excellent dielectric and heat resistance

Thanks to its light weight, thinness, and flexibility, flexible PCBs contribute to the development of compact and lightweight electronic devices. This trend is likely to continue as future electronic products evolve. Therefore, the growth potential of flexible PCBs is expected to surpass that of traditional printed circuit boards. Like rigid PCBs, flexible PCBs use three types of conductive vias for electrical interconnections: through-holes, blind holes, and buried holes. Figure 1 illustrates the interconnection method used in flexible PCBs. Electroplating technology is the most widely used method for creating these electrical connections, owing to its ability to provide uniform plating and effective conductivity.

Figure 1

Flexible PCBs can be classified based on the number of layers, similar to traditional rigid PCBs. These categories include:

• Single-sided Flex-PCB
• Double-sided Flex-PCB
• Multi-layer Flex-PCB

A single-sided Flex-PCB has a single conductive layer created by chemical etching, while a double-sided Flex-PCB has conductive layers on both sides. A metal layer is deposited onto the insulating material to form conductive paths. Multi-layer Flex-PCBs are typically made by laminating several single-sided or double-sided Flex-PCBs and using Plated Through Holes (PTH) to form conductive channels between layers. Figure 2 shows the classification of flexible PCBs.

Figure 2

Manufacturing Process Overview:

The manufacturing process for flexible printed circuit boards closely resembles that of traditional rigid PCBs. It can be broadly divided into two main methods: additive and subtractive. The subtractive method involves transferring circuit patterns onto the copper-clad surface of the PCB through electroplating or photochemical methods, followed by chemical etching to remove the unwanted copper and leave the desired circuit patterns. The main processes involved are photochemical etching, screen printing, and electroplating. However, this method has limitations in line width and spacing, which may not meet the demands of modern electronics.

The additive method, in contrast, involves selectively depositing conductive metal onto an insulating substrate to form the circuit pattern. This method includes processes such as the semi-additive process, total additive process, photoforming, and multi-step routing. The semi-additive process, in particular, combines electroplating with rapid etching to form fine circuit patterns. Figure 3 illustrates the process flow of the semi-additive method.

Figure 3

In the semi-additive process, an insulating laminate is first coated with a thin layer of copper using a chemical copper plating process to act as a seed layer. The area for the fine lines is then defined through film lamination, exposure, and development. The pattern is electroplated with additional metal (e.g., Cu, Ni, Pb) to thicken the copper in the desired areas. Finally, the dry film is removed, and the non-patterned copper is etched away, leaving the final circuit pattern. This method helps address challenges such as product line instability and difficulty in producing thick copper circuits.

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