Polyimide Films

As one of the best-performing film-type insulating materials in the world today, polyimide films are considered on par with carbon fiber and aramid fiber. They are regarded as the three critical high polymer materials to overcome the bottleneck of worldwide high-tech industries development. With the application of new technologies such as copolymer modification, continuous exploration, and improvement of their formulation design and production processes, polyimide films have developed more functional applications, expanding their downstream application fields.

I. Overview of the Polyimide Industry

Polyimide (PI) refers to a polymer containing imide structures in its molecular backbone. High-performance polyimides usually have aromatic and heterocyclic structures as their main components. PI boasts the highest flame retardancy level (UL-94), excellent electrical insulation, mechanical properties, chemical stability, aging resistance, radiation resistance, and dielectric properties (dielectric constant of 4.0 and dielectric loss of only 0.004 to 0.007 at 103 Hz), and classified as F to H-grade insulation. Moreover These properties remain stable over a wide temperature range (-269°C to 400°C). Therefore it has been dubbed “the most promising engineering plastic of the 21st century” and “a problem solver,” it is often said that “without polyimide, there would be no modern microelectronics technology.”. PI stands at the pinnacle of polymer materials in terms of performance.

PI films exhibit excellent mechanical properties, dielectric properties, chemical stability, and high resistance to radiation, corrosion, and extreme temperatures. Known as “golden films,” they are recognized as one of the best super engineering polymer materials globally, alongside carbon fiber and aramid fiber, as a bottleneck material in developing high-tech industries.

II. Polyimide Film Industry Chain

1. Upstream:
Upstream materials include dianhydride PMDA, diamine ODA, and other raw materials. Some specialized PI monomers have already been localized in China. And the raw materials for PI films include PI monomers and PI slurry. PI monomers consist of dianhydride monomers and diamine monomers.

2. Midstream:
PI films, owing to their superior properties, have a wide range of downstream applications. Polyimide products come in various forms, including films, foams, fibers, photosensitive polyimides, and polyimide-based composites, with films, the most important form, accounting for over 70% of the market. Manufacturing PI films involves resin polymerization, casting, stretching, imidization, and post-treatment processes.
Applications of various PI film: include thermal control PI films (precursor for high thermal conductivity graphite films), electronic PI films (substrates and printing), electrical PI films (corona-resistant and class C), and aerospace PI films (polyimide composite aluminum foil MAM).

3. Downstream:
Common materials for FCCL board films include polyimide (PI), polyester (PET), polyethylene naphthalate (PEN), and liquid crystal polymer (LCP).

III. Polyimide Film Production Process

Before imidization, PI films must be formed by various methods such as casting, casting and stretching (biaxial orientation), dipping (aluminum foil coating), spraying, extrusion, and deposition. The forming process significantly impacts the film’s performance and production methods. Currently, casting and casting-stretching methods are most commonly used, with casting-stretching preferred for high-performance films. In China, the casting and dipping methods are mature, although the dipping method, due to poor insulation performance, is gradually being phased out. High-difficulty methods like spraying, extrusion, and deposition are primarily mastered by advanced Japanese companies in 2016.

Imidization is mainly achieved through two methods: thermal imidization and chemical imidization. Thermal imidization involves heating polyamic acid to a certain temperature for dehydration and cyclization. Chemical imidization involves adding a dehydrating agent and catalyst to polyamic acid solution below -5°C, followed by rapid mixing and heating to induce dehydration and cyclization. Although thermal imidization is simpler in process and equipment than chemical imidization, the resulting film’s physical and chemical properties are inferior, unsuitable for producing electronic-grade PI films. Before 2014, most Chinese manufacturers used thermal imidization, while developed countries had largely transitioned to chemical imidization.

IV. Applications of Polyimide Films in Flexible Electronics

Applications of PI films in flexible electronics include:
1.Electronic insulation substrates: Flexible printed circuit boards, automatic welding carriers, chip-on-film substrates.
2.Integrated circuit manufacturing: Interlayer insulation, chip passivation and protection layers, photoresists, alpha particle barriers.
3.Electronic tapes: Self-adhesive tapes, pressure-sensitive tapes, electric heating tapes.
4.Electronic sensors: Capacitive humidity sensors.
5.Liquid crystal displays: LCD circuit packaging, liquid crystal alignment films.
6.Flexible OLED displays: Flexible substrates, film touch panels, transparent cover plates.
7.Optical communication/optical devices: Optical waveguides, optical micro-lenses, thin-film filters.

V. Advantages and Disadvantages of Polyimide Films

1. Advantages:
High Strength and Heat Resistance: PI films possess extreme strength and high-temperature resistance, making them ideal for aerospace, electronics, and electrical applications.
Excellent Insulation: They offer superior insulation properties, effectively preventing current leakage and ensuring the safe operation of electrical equipment.
Chemical Stability: PI films are highly stable against various chemicals, making them suitable for complex environments.
Lightweight and Thin: These films are lightweight and thin, facilitating easy processing and transportation, reducing production costs, and enhancing product performance.
Wide Application Range: PI films are widely used in aerospace, electronics, electrical, new energy, and biomedical fields, with a promising market outlook.

2. Disadvantages:
High Production Costs: The complex production process requires sophisticated equipment and technology, leading to high costs and limiting applications in some fields.
Processing Difficulty: As high strength and resistance properties of polyimide films. It requires strict control of temperature and pressure during processing, increasing the difficulty.
Limited Weather Resistance: Although polyimide films have excellent performance, long-term outdoor use may see performance degradation due to UV exposure and moisture.

As a new material, polyimide films have many advantages but also confront some disadvantages. With continuous technological advancements, PI film performance is expected to improve further, expanding its application fields. Meanwhile researchers are also striving to overcome technical challenges and reduce production costs, fostering broader adoption of polyimide films.

VI. Research Directions for Polyimide Films

Current research on ultra-thin PI films focuses on two main areas:
Thinning of Standard Films: The excellent thermal properties and mechanical properties of the polyimide films ensuring stable performance during thinning. The main technical bottleneck involves optimizing preparation equipment and process parameters. Ultra-thin PI films have extensive application prospects in modern industry. Internationally, significant efforts are made in research and development, with some batch products already available.
Researching and developing of Functional Ultra-thin PI Films: Its performance is closely tied to equipment, processes, molecular design, and new synthesis methods. Maintaining inherent mechanical and thermal properties while ensuring special functionalities presents a significant challenge and primary research focus.

High-performance PI films are crucial for the rapid development of high-tech industries in China. However, certain advantages exist in our development path:
1. Accumulation of R&D and Technical Talent: Decades of experience have resulted in substantial R&D expertise and talent pools among PI film manufacturers, anticipating qualitative breakthroughs.
2. Proximity to Major Downstream Customers: Major markets for OLED, flexible circuit boards, and graphite films are in mainland China, providing local manufacturers with ample opportunities to understand product requirements and venture into high-end markets.
3. Accelerated Industry Upgrades through Southeast-Asia Talent Mobility: The rise of China’s high-end manufacturing has led to increased mobility of technical talent among mainland China, Taiwan, Japan, and Korea, accelerating domestic technological advancements and product upgrades.