2. The adoption of FPCBs in consumer electronics is on the rise. Alongside growing market demands for the aesthetic design of electronic products, existing PCB and HDI multilayer circuit board materials often fall short in meeting evolving structural needs. While the circuit density of FPCBs may not reach PCB levels, they have become essential materials in most consumer electronics.
3. FPCB (Flexible Printed Circuit Board) boasts structural flexibility, allowing it to adapt to various bending angles without the risk of the carrier board breaking. This flexibility makes FPCBs crucial to the design trends of electronic products.
4. However, FPCBs cannot flex indefinitely. To prevent excessive bending and stress, copper foil is typically reinforced with patches.
5. FPCBs serve as flexible connectors for multiple functional carrier boards.
6. For specialized structural models requiring significant deflection, FPCBs can be elastically laser-cut, enhancing their flexing capabilities.
**FPCB Material Characteristics**
The characteristics of Flexible Printed Circuit Boards (FPCB) extend beyond their soft materials; they are also notably lightweight and extremely thin. This light structure allows the material to be flexed multiple times without compromising the insulation integrity found in rigid PCBs.
The flexible plastic substrate and wire layout of soft boards limit their ability to handle excessively high conduction currents and voltages. Consequently, soft board designs are rarely utilized in high-power electronic circuits. Conversely, in consumer electronics that operate at lower current and power levels, soft boards see widespread use.
Due to the cost constraints imposed by the key material, polyimide (PI), the unit cost of soft boards remains high. As a result, in product design, soft boards are typically not employed as the primary carrier board. Instead, they are selectively integrated into key design elements requiring “soft” characteristics. For instance, soft boards are used in the electronic zoom lenses of digital cameras and in the electronic circuits of optical disc drive read heads. These applications necessitate mobility and functionality that rigid circuit board materials cannot provide, leading to the adoption of flexible circuit designs.
PI, known as polyimide, can be classified into various structures based on its heat resistance and molecular composition. These include fully aromatic PI and semi-aromatic PI. Fully aromatic PI is linear and can be either infusible or thermoplastic. Infusible materials cannot be injection molded during production, although they can be compressed and sintered; conversely, thermoplastic materials can be produced through injection molding.
Semi-aromatic PI is categorized under polyetherimide, which is generally thermoplastic and amenable to injection molding. Thermosetting PI, on the other hand, can utilize various raw material properties for lamination, compression, or transfer molding.
**FPCB Board Material: High Heat Resistance and Stability**
In the realm of chemical materials, PI is suitable for applications such as gaskets, seals, and flexible multilayer circuit board substrates. Fully aromatic materials represent the highest heat resistance among polymers, with a thermal resistance range of 250 to 360°C. Meanwhile, bismaleimide-type PI, used for flexible circuit boards, has a slightly lower heat resistance of around 200°C.
Bismaleimide PI exhibits excellent mechanical properties, minimal temperature variance, and remarkable stability under high-temperature conditions, with low creep deformation and a reduced thermal expansion rate. Across a temperature range of -200 to +250°C, the material remains largely unchanged. Furthermore, bismaleimide PI is chemically resistant; for example, when immersed in 5% hydrochloric acid at 99°C, it retains a significant level of tensile strength. This material also offers outstanding friction and wear properties, demonstrating a degree of wear resistance in applications prone to abrasion.
In addition to its primary material characteristics, the structural composition of the FPCB substrate is crucial. The FPCB comprises a cover film (upper layer) that serves as an insulating and protective element, combined with an insulating base material, rolled copper foil, and adhesive to form the overall structure. Common insulating materials for FPCB substrates include polyester (PET) and polyimide (PI), each with its unique advantages and disadvantages.
**FPCB Production Materials and Procedures: Enhancing Terminal Flexibility**
FPCB applications are diverse, encompassing wiring, printed circuits, connectors, and multifunctional integrated systems. Based on functionality, FPCBs can be designed for spatial efficiency, shape alteration, folding, and flexural assembly, effectively mitigating electrostatic interference in electronic devices. When product quality is fundamentally linked to the flexible board—regardless of cost—it not only reduces design volume but also significantly minimizes the overall product size.
The FPCB substrate structure is relatively simple, primarily comprising an upper protective layer and a central wiring layer. During mass production, flexible circuit boards can incorporate positioning holes for process alignment and post-processing. The shape of the FPCB can be adjusted to meet spatial requirements or designed to be folded. By integrating anti-EMI and static resistance isolation designs on the outer layer, flexible circuit boards can also effectively address high-efficiency EMI issues.
At the critical circuitry level, the uppermost layer of the FPCB consists of copper, which can be either rolled annealed (RA) or electro deposited (ED). While the production cost of ED copper is lower, it is more prone to fractures or faults. In contrast, RA copper, though more expensive, offers superior flexibility, making it the preferred choice for flexible circuit boards subjected to significant deflection.
For FPCB formation, various layers—including the cover layer, calendered copper, and base material—are bonded using adhesives, typically acrylic or modified epoxy. Epoxy resin, while having lower heat resistance and mainly used for consumer goods, contrasts with acrylic, which offers high heat resistance and bonding strength, albeit with inferior insulation and electrical properties. In FPCB manufacturing, the adhesive typically contributes 20-40μm (micrometers) to the overall thickness.
**Reinforcing Material Performance in Highly Flexible Applications**
In the FPCB manufacturing process, the copper foil and substrate are initially prepared, followed by cutting, perforation, and electroplating. After the FPCB holes are created, a photoresist material is applied, and the coating process is completed. The exposure and development stages process the etched circuits. After these steps, solvent etching is performed to form conductive circuits, followed by surface cleaning to remove solvents. An even layer of adhesive is then applied to the FPCB base and etched copper foil, after which the cover layer is attached.
Following these operations, the FPCB is approximately 80% complete. Attention must then be directed towards the connection points of the FPCB, including enhancing the guide welding process, and finishing the FPCB’s appearance, which may involve laser cutting to achieve specific designs. If the FPCB incorporates both soft and hard materials or requires welding with functional modules, secondary processing or reinforcement board designs may be necessary.
FPCBs have numerous applications, and while they are not overly complex to manufacture, they are limited in their ability to produce intricate, compact circuits. Excessively thin circuits can lead to a reduced cross-sectional area of the copper foil, increasing the likelihood of internal circuit breaks when flexed. Therefore, highly intricate circuits typically utilize HDI (High-Density Interconnect) multilayer boards to meet related circuit requirements, while FPCBs are employed for connecting various functional carrier boards through numerous data transmission interfaces.
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