Advancements in Flexible PCB Materials and Manufacturing

In recent years, the world of flexible PCBs (printed circuit boards) has experienced significant growth and technological advancements. These developments have led to an increase in market share and expanded applications due to the inherent benefits of flexibility, thinness, and lightweight design. In this comprehensive article, we will explore the primary materials used in flexible PCBs, their performance improvements, and the evolving landscape of flexible PCB manufacturing technologies.

Enhancing Performance through Substrate Materials

The performance of flexible PCBs depends largely on the properties of the substrate materials used. Substrates play a critical role as they serve as both conductor carriers and insulating mediums between circuits. Furthermore, they must exhibit the ability to bend and curl, which is essential for flexible PCBs.

Commonly used substrate materials for flexible PCBs include Polyimide (PI) film and Polyester (PET) film, with other polymer films like Polyethylene Naphthalate (PEN), Polytetrafluoroethylene (PTFE), and Aramid also finding applications. The choice of substrate material should be based on a careful evaluation of their performance and cost-effectiveness.

Polyimide (PI) Film

Polyimide film, a thermosetting resin, is a leading substrate material for flexible copper-clad laminates (FCCL). Unlike most thermosetting resins, PI does not soften or flow at high temperatures, making it uniquely suitable for flexible PCBs. PI boasts high thermal resistance and excellent electrical characteristics. However, it has a drawback in terms of humidity absorption and tear strength. Upgraded PI films have significantly lower humidity absorption at 0.7%, compared to the typical rate of 1.6%. They also exhibit improved dimensional stability, with tolerances reduced from ±0.04% to ±0.02%.

Polyester (PET) Film

PET resin offers favorable mechanical and electrical performance. Its main limitation is its poor heat resistance, rendering it unsuitable for direct soldering and assembly processes. However, the performance of Polyethylene Naphthalate (PEN) is superior to that of PET but falls short of the qualities of PI, making it a suitable alternative in some applications.

Liquid Crystal Polymer (LCP)

To address the limitations of Polyimide substrates, Liquid Crystal Polymer (LCP) has emerged as a promising alternative. LCP is a thermoplastic material that can be processed into thin films suitable for PCBs. It exhibits low humidity absorption (0.04%) and a dielectric constant of 2.85 (1GHz), making it suitable for high-frequency digital circuits. LCP’s properties, including high-frequency compatibility, thermal dimensional stability, and low humidity absorption, have led to its adoption in flexible PCB manufacturing.

Halogen-Free Substrate Materials

The electronics industry has witnessed a shift towards environmental responsibility. Regulations such as RoHS (Restriction of Hazardous Substances) and WEEE (Waste Electrical and Electronic Equipment) have prohibited the use of hazardous substances in electronic devices. As a result, halogen-free substrate materials compatible with environmental requirements have been developed and are now widely used in both rigid and flexible PCBs.

Evolving Substrate Materials

As the demand for flexible PCBs continues to grow, research and development efforts are ongoing to explore new substrate materials that can further enhance the performance of flexible circuits. With over 2000 types of applicable plastic films available worldwide, there is undoubtedly room for the discovery of materials that can push the boundaries of flexible PCB design.

The Role of Adhesive in Flexible PCBs

Adhesive is a critical component in flexible PCBs, as it is responsible for bonding the copper foil and substrate material film. Adhesives are classified into various categories, including PI resin, PET resin, modified epoxy resin, and acrylic resin. Among these, modified epoxy resin and acrylic resin are more commonly used due to their high adhesive strength.

Two-Layer PI Substrate Material

Traditional flexible copper-clad laminate (FCCL) typically consists of three layers: polyimide, adhesive, and copper foil. However, the adhesive layer can sometimes negatively impact the performance of flexible PCBs, particularly in terms of electrical performance and dimensional stability. To address this issue, two-layer flexible CCL (2L-FCCL) without adhesive has been developed. This advancement not only improves the environmental compatibility of flexible PCBs but also meets the requirements of lead-free soldering by raising the temperature threshold from 220°C to 260-300°C. A comparison between 2L-FCCL without adhesive and 3L-FCCL with adhesive is provided in Table 1 below.

Three methods are typically used for manufacturing 2L-FCCL:

1. Electroplating
2. Film coating
3. Lamination

Each method has its advantages and is chosen based on specific requirements. Electroplating, for instance, is suitable for rolling production and thinner substrate materials, offering a cost-effective solution. Film coating is ideal for mass-volume production, offering cost-efficiency. Lamination, on the other hand, excels in double-sided board fabrication.

Liquid Crystal Polymer (LCP) Substrate Material

Liquid Crystal Polymer (LCP) is a relatively new substrate material that seeks to address the limitations of polyimide substrates. LCP film is coated with copper foil and undergoes hot pressing to create single-sided or double-sided copper-clad laminates (CCL). LCP-based CCLs offer exceptional properties, including a low water absorption rate of only 0.04% and a dielectric constant compatible with high-frequency digital circuits (2.85 at 1GHz). LCP’s properties make it an ideal choice for high-frequency applications in flexible PCBs.

Halogen-Free Flexible Substrate Materials

Environmental regulations have compelled the electronics industry to transition to halogen-free substrate materials. Since 2003, the EU has implemented RoHS and WEEE regulations, restricting the use of six hazardous substances and regulating the processing of waste electronic and electric equipment. These regulations have impacted the materials used in PCBs, including flexible PCBs. As a result, various components of flexible PCBs, such as FCCL, coverlay, prepreg, solder mask, and reinforcement boards, must meet fire resistance and halogen-free requirements.

Advancements in Copper Foil

Conductivity is a critical property in flexible PCBs, and copper foil serves as the primary conducting material. In addition to copper, alloy materials like aluminum, nickel, gold, and silver are sometimes used. The choice of copper foil depends on the specific application and manufacturing method.

Electro Deposit (ED) Copper Foil

ED copper foil is one of the most commonly used conducting materials in flexible PCBs. It is characterized by a fish-scale crystal structure, resulting in a smooth copper foil with good toughness. ED copper foil is well-suited for dynamically flexible PCBs that require high flexibility.

Rolled and Annealed (RA) Copper Foil

RA copper foil, on the other hand, features a columnar crystal structure, resulting in an even and flat structure. This type of copper foil is ideal for roughening and etching processes. RA copper foil is often used in high-density flexible PCBs.

Emerging Demands in Copper Foil

As the demand for high-density flexible PCBs with finer pitches ranging from 40μm to 50μm increases, new requirements are placed on copper foil manufacturing. These requirements include low surface roughness and ultra-thin copper foils to meet the needs of mass-volume PCB production.

Conductive Silver Paste

Flexible PCB fabrication involves the use of conductive ink, which is printed on insulating film to create wire or shielding layers. Conductive silver paste is the primary material used for this purpose. It is essential that the printed conductive layer exhibits low resistance, ensures a solid connection, and retains flexibility. Additionally, the printing process should be easily implementable, and curing should be rapid.

Modern conductive silver paste meets these demands, enabling the formation of conductive patterns on thermosetting or thermoplastic polymer films, fabrics, and paper. This technology also extends to creating graphics used in RFID products. Final products featuring conductive silver paste are rigorously tested for high-temperature storage, humidity resistance, and high and low-temperature cycling performance. Conductive silver paste aligns with environmental protection requirements and cost-effectiveness.

Photosensitive Polyimide (PI) Coverlay

Traditional PI/adhesive coverlays have faced limitations in meeting the requirements of high-density, dimensionally stable, and environmentally friendly flexible PCBs. In response, Photo-Imageable Coverlay (PIC) has been developed. PIC, depending on modified epoxy or acrylic resins, has gained popularity due to its high resolution, excellent bonding strength, and flexibility. However, these PICs have limitations, including low dimensional stability on high-density PCBs, low glass transition temperature (Tg), and limited thermal resistance.

Conclusion

Flexible PCBs have come a long way in terms of materials and manufacturing technologies. Advances in substrate materials, copper foils, conductive pastes, and coverlays have opened up new possibilities for high-density, high-frequency, and environmentally friendly flexible PCBs. As the demand for flexible electronics continues to grow, it is clear that ongoing research and development efforts will lead to further innovations in materials and processes.

For efficient and reliable flexible PCB manufacturing, consider reaching out to Highleap Electronic. With over a decade of experience in flexible PCB manufacturing, we are equipped to meet your demands and provide high-quality solutions. Contact us today to discuss your flexible circuit board fabrication and assembly requirements or request a flexible PCB quote. The future of flexible electronics is bright, and we are here to help you bring your innovative ideas to life.