1. Core Classification of Flexible Circuits for PCBA Boards

Flexible circuits for PCBA boards are primarily classified by the number of conductive layers, structural forms and application scenarios, with three mainstream categories available to meet diverse application requirements:

(1) Classification by the number of conductive layers

Classified based on the number of conductive copper layers, this classification determines the signal transmission capability and integration level of the circuit, and is suitable for electronic devices with different complexity levels.

• Single-layer flexible circuit: Equipped with a single conductive copper layer, it features a simple structure and low cost. The copper layer is fabricated by Electro-Deposition (ED) or Rolled Annealed (RA) process, with excellent thinness and flexibility (minimum thickness of 0.05mm). It is applicable to simple signal transmission scenarios such as hard disk drive cables and small sensor connecting wires.

• Double-layer flexible circuit: Two copper layers are arranged on both sides of the flexible substrate and electrically connected through plated-through holes (PTH). It offers higher integration and flexible wiring, which can effectively reduce space occupation. It is suitable for core circuit connections of small consumer electronic products such as smart watches and Bluetooth earphones.

• Multi-layer flexible circuit: Three or more copper layers are stacked and interconnected via blind vias and buried vias, and is usually combined with shielding technology and surface mount technology to enhance performance. It features high integration, strong anti-interference capability and a wide temperature resistance range (-269℃~260℃), and is applicable to high-end scenarios such as foldable smartphones and industrial robots.

(2) Classification by structural forms

This classification focuses on the physical structural characteristics of flexible circuits, combining the advantages of rigid and flexible circuits to adapt to different installation scenarios, among which the rigid-flex combined type is experiencing the fastest growth in the industry.

• Pure flexible circuit: Manufactured with flexible insulating substrates (mainly Polyimide (PI) and Polyethylene Terephthalate (PET)) without rigid areas, it can be folded and wound 360° (minimum bending radius of 0.1mm for the PI+rolled copper combination) and fit curved and irregular surfaces. It is typically applied in medical health patches, hinge circuits of foldable screens and other products.

• Rigid-flex combined circuit: Integrating rigid printed circuit boards with flexible circuits and realizing electrical interconnection through plated-through holes, it has both the stable support of rigid circuits and the bendability of flexible circuits. Heavy electronic components can be mounted on rigid areas, while complex spatial wiring can be achieved on flexible areas. It is widely used in automobile central control systems, medical equipment, unmanned aerial vehicles (UAVs) and other products.

(3) Special types of flexible circuits

In addition to the above mainstream categories, two special types of flexible circuits are optimally designed for specific scenario requirements, further expanding the application boundary of flexible circuits:

• Engraved flexible circuit: The thickness of the conductive layer can be flexibly adjusted as required, enabling precise impedance control and improved flexibility in specific areas. It is suitable for high-frequency application scenarios with high requirements for signal integrity.

• Component-integrated flexible circuit: Electronic components are directly embedded in the flexible substrate to achieve ultra-thin and compact packaging design, which can further reduce product volume and adapt to the extreme miniaturization requirements of micro medical devices and wearable devices.

2. Core Application Advantages of Flexible Circuits for PCBA Boards

Compared with traditional rigid PCBA boards, the advantages of flexible circuits are not only reflected in physical forms, but also can be converted into the core competitiveness of products, providing multi-dimensional support for enterprises in space utilization, reliability and cost control. Its core advantages are summarized in the following five aspects:

(1) Superior physical properties, adaptable to complex scenarios

The most prominent advantage of flexible circuits is their thinness, bendability and strong adaptability, which perfectly solves the problem that rigid PCBs are "fixed in form and unable to adapt to complex spaces". With a thickness of 0.05~0.3mm (only 1/20~1/3 of that of rigid PCBs (1.0~2.0mm)) and 60% lighter weight for the same area (PI-based FPC vs FR-4 rigid PCB), it can effectively reduce the overall weight of products and improve portability. It can withstand more than one million bends (depending on the substrate and copper foil); for example, the FPC at the hinge of a foldable screen can withstand more than 180,000 folds with a bending radius of 1.5mm, meeting the long-term use requirements of products. Meanwhile, it can fit curved and irregular surfaces to avoid measurement errors caused by the poor fit of rigid PCBs.

(2) Improved space utilization, facilitating product miniaturization

With the increasing demand for miniaturization in consumer electronics and automotive electronics, flexible circuits greatly improve space utilization through folded wiring and wire harness replacement. Adopting Z-type and U-type folded wiring to utilize vertical space, it reduces space occupation by 30%~50% compared with the planar wiring of rigid PCBs; for example, a Z-type folded double-layer FPC applied in a TWS earphone charging case can save 45% of space. It can integrate multiple signal channels (power, data, control) to replace traditional messy copper wire harnesses, reducing signal interference while greatly reducing product volume and weight.

(3) Outstanding reliability, extending product service life

Flexible circuits have much higher reliability than rigid PCBs in scenarios such as vibration, frequent bending and extreme environments. The copper foil (especially rolled copper) has good ductility, which can absorb vibration energy and avoid circuit breakage, enabling continuous operation for 2 years without failure under 5g acceleration vibration environment (while the solder joints of rigid PCBs are prone to falling off with a failure on average every 3 months). In addition, PI-based flexible circuits have a wide temperature resistance range, suitable for extreme temperature environments such as automobile engine compartments and industrial high-temperature equipment. At the same time, flexible circuits reduce fault-prone points such as connectors and wire harnesses (such faults account for 40% of total faults in traditional wiring), greatly lowering the product failure rate and maintenance costs.

(4) Simplifying assembly process, reducing comprehensive costs

Although the unit price of flexible circuits is higher than that of rigid PCBs, from the perspective of the product life cycle, it can reduce comprehensive costs by simplifying assembly and reducing the number of components. It can integrate multiple signal channels, reducing the use of connectors and wire harnesses and simplifying assembly procedures; for example, a single FPC applied in a smart bracelet can replace multiple rigid PCBs and connectors, greatly shortening assembly time and saving labor costs. When the output exceeds 100,000 pieces, the mold cost of flexible circuits can be effectively amortized, making the total cost close to or even lower than that of rigid PCBs.

(5) High design freedom, empowering product innovation

The wiring of flexible circuits can be flexibly designed according to product shape and installation space without being limited by the planar layout of rigid PCBs, providing greater design freedom for enterprises to create differentiated products and adapt to the design requirements of wearable devices, foldable screens, industrial robots and other products. In addition, flexible circuits support High-Density Interconnection (HDI) design, which can achieve more compact circuit layout, thinner layer thickness and micro vias, adapting to the high integration requirements of high-end electronic products and driving the development of products towards thinner, lighter and more intelligent directions.