Sichuan Jinzhongde Science and Technology Research Institute: Market Status and Research Progress of Electronic Grade Polyimide Films

1. Market status of electronic grade polyimide films

1.1 Downstream market demand
1.1.1 Flexible Printed Circuit Board (FPC)
FPC is a flexible printed circuit board made of flexible copper-clad laminate (FCCL) as the substrate, widely used in electronic products such as mobile phones, laptops, navigation devices, aerospace equipment, etc. Among them, flexible copper-clad laminate (FCCL) accounts for 40% of the entire raw material, and PI film can be used to make flexible copper-clad laminate (FCCL) substrates and cover films, achieving the flexibility of FPC. In 2018, the global output value of FPC reached 12.65 billion US dollars, a year-on-year increase of 1.4%. With the increasing demand for miniaturization of electronic products, it is expected that the global output value of FPC will reach around 14.9 billion US dollars in 2022, which will drive the demand for raw material PI film.

1.1.2 OLED flexible display

OLED is an organic light-emitting diode, and flexible display has become the mainstream trend in the current OLED industry. PI film is the key to achieving flexible display of OLED in smartphones. In 2019, the production capacity of flexible substrate OLED was 11.48 million square meters, accounting for 62.0% of the OLED industry, surpassing rigid substrate OLED. With the continuous updates of smartphones, it is expected that the production capacity of flexible display substrate OLED panels will increase to 19.69 million square meters in 2023. In recent years, the demand for flexible substrates has grown rapidly, driving the expansion of the PI slurry market. In 2019, the global market size of PI substrate materials was about 39.81 million US dollars, and it is expected to reach 85.38 million US dollars by 2022.

1.1.3 5G Communication

In recent years, the development of electronic devices has shifted towards miniaturization, thinning, and integration, resulting in a sharp increase in heat generated per unit volume during operation. Especially in 5G high-frequency communication, higher requirements have been placed on PI insulation thermal conductive films. The popularization of electronic products in the 5G era has driven the demand for PI thermal conductive films. Among various consumer electronics, smartphones have the highest demand for heat dissipation materials. With the promotion of 5G technology, tablet computers have won more market opportunities due to their convenient portability and good display effects. The trend of ultra-thin development is expected to expand the demand for PI thermal conductive films. With the continuous improvement of personal computer performance, power consumption and heat generation will significantly increase, and the required heat dissipation film area for a single unit will expand. In the future, the PI thermal conductive film required for PCs is also expected to increase.

1.2 Supply side market

Due to the high price, difficult research and development, and extremely high technical barriers of PI films, the high-end PI film market is mainly monopolized by countries such as the United States, Japan, and South Korea. According to SKCKOLONPI data, SKCKOLONPI, Zhongyuan Chemical, Dongli DuPont, and DuPont respectively account for 23.0%, 20.0%, 10.0%, and 8.0% of the global market share of electronic grade PI films. These companies have a high concentration and production capacity of over 2000 tons. In China, the market for PI films of electronic grade and above is mainly divided among overseas companies. There are about 20 electronic grade PI film manufacturing factories in China, with most companies supplying cover films with lower performance requirements for electronic products, and a few companies being able to produce high-performance electronic grade polyimide films; The more high-end ultra-thin transparent PI film has not yet been commercially produced by domestic enterprises. At present, domestic enterprises with initial scale production capacity for electronic grade PI films include Times New Materials, Danbang Technology, Ruihuatai, etc.

2.1 PI film for flexible copper-clad laminates

The PI base film and cover film used in FCCL not only require good heat resistance and mechanical properties, but also excellent flexibility, dimensional stability, and dielectric insulation properties.

2.1.1 High dimensional stability PI film

In the FCCL field, the low coefficient of thermal expansion (CTE) is used to describe the high dimensional stability of PI films. The low expansion coefficient of FCCL requires the CTE of PI film to be as close as possible to that of copper, that is, the thermal expansion coefficient of polyimide film is in the range of 15-18 ppm/℃, which can reduce the interface stress caused by the large difference in CTE between the two. At present, the main methods to reduce the CTE of PI thin films are through molecular structure design and improved film formation processes.

Aromatic polyimides introduce hydrogen bonds, rigidity, and planar structural units in the design of molecular structures. The molecular chains are straight and have low steric hindrance. During imidization, the polymer molecular chains form tightly packed and highly oriented ordered arrangements within the plane. Therefore, the CTE of PI films is significantly reduced. However, if rigid groups are introduced into the PI molecular structure, the polymer rigidity is too strong, the molecular chains are too rigid, and the molecular chains will not curl or entangle with each other, resulting in low toughness and brittleness of the cured PI film and no practical value. The CTE of rigid polyimide PMDA/PPD synthesized by Hasegawa from Toho University in Japan is as low as 2.8 ppm/℃, but the film is very brittle and has lost its application value. Jilin University has invented a high adhesion and low linear expansion coefficient polyimide film material and its preparation method. It synthesizes amino monomers containing rigid structures and introducing cyanide groups. The synthesized amino monomers are then condensed with other diamine monomers containing rigid structures and anhydride monomers to form polyimide films. Both adhesion and coefficient of linear expansion have achieved optimal performance, with a CTE that can be reduced from 21.42ppm/℃ to 13.27ppm/℃. It can be applied in the field of high adhesion materials and provides a low coefficient of linear expansion. Hitachi, Japan has released a polyimide film containing bismaleimide that can be addition cured. By introducing rigid chain links, the resulting polyimide film has a low CTE value of 4ppm/℃ and can be used as a copper-clad laminate and flexible printed substrate.

In addition to designing at the molecular structure level to reduce CTE, this goal can also be achieved through improvements and innovations in polyimide film formation processes. Factors affecting the aggregation structure and thermal expansion coefficient of polyimide include the solvent used, coating method, gel process, imidization method and process, drafting condition and annealing condition [4]. Many manufacturers have achieved low CTE and isotropy of PI films through biaxial stretching technology and reasonable control of the stretching ratio TD/MD. The drainability of polyamide acid gel film depends on the solvent content. Only when the solvent content is between 30% and 50%, the polyimide film can be drawn in both TD and MD directions. In order to improve production efficiency, biaxial stretching and imidization can be carried out when the solvent content is high. Dehydrating agents and catalysts can be added to the polyamide acid solution, and chemical imidization reactions can be used to obtain polyimide solutions that can be stretched at higher rates, ultimately resulting in PI films with low CTE and good mechanical properties.

2.1.2 Low dielectric loss PI film

The dielectric constant (Dk) of polymer dielectric materials can be expressed by the following formula:

2.2 PI film for OLED flexible display

In recent years, OLED flexible display technology has been developing towards foldable and rollable directions, and the key material for achieving flexibility is polyimide film. The units that require the use of polyimide film for flexible display include display substrate, display packaging substrate, touch screen substrate, touch screen cover plate, display screen cover plate, etc. The polyimide film used in OLED displays needs to have both high temperature resistance and colorless transparency properties.

In the processing of flexible OLED devices, the processing temperature of low-temperature polycrystalline silicon thin film transistors (LTPS-TFT) is not less than 450 ℃. Therefore, polyimide films as flexible substrates also require extremely high heat resistance (Tg>450 ℃). In addition, polyimide films are required to have an ultra-low thermal expansion coefficient (CTE) of less than 4ppm/℃ within the temperature range of room temperature to 400 ℃, to ensure dimensional stability in high-temperature processes. Compared to the low CTE of polyimide films in FCCL, flexible display substrates require a lower CTE<4ppm/℃. In terms of structural design, polyimide used in flexible display substrates can adopt structural units such as rigid rod like structures, intermolecular hydrogen bonds, or chemical cross-linking groups to achieve ultra-high heat resistance and ultra-low thermal expansion coefficient.

Traditional PI transparent films are usually yellow or brown in color, as they are usually produced by the condensation of aromatic dianhydride and diamine. The presence of conjugated aromatic ring structures on their main chain makes it easy to form intramolecular and intermolecular charge transfer complexes CTC between electron donating diamine and electron withdrawing dianhydride, thereby reducing the transparency of PI in the visible light region and limiting its application in flexible optoelectronic devices. To improve the transparency of PI films, many researchers have introduced large substituents, fluorinated groups, or cyclic structures such as dianhydride or diamine in the design of PI molecular structures, effectively suppressing the formation of CTCs in PI molecular chains and obtaining colorless and transparent PI films. The comprehensive performance of PI films prepared by the three methods has its own advantages and disadvantages. The introduction of large substituents will significantly increase the intermolecular distance, thereby hindering the formation of CTC. However, this method has a complicated preparation process, high cost, and low yield; The introduction of fluorinated groups can reduce the electron donating ability of diamines and improve their transmittance in the UV visible region. However, the addition of fluorine atoms can cause a decrease in thermal stability and mechanical properties. These two methods face significant resistance in the industrialization process and are not suitable for industrial promotion; PI films with introduced lipid ring structures are divided into full fat cyclic PI and semi aromatic PI. The former does not contain conjugated aromatic ring structures and has lower molecular packing density and polarizability, which limits the formation of intra -/intermolecular CTCs. This type of film has low dielectric constant and high optical transparency, but its temperature resistance and rigidity are poor, and its comprehensive performance is greatly reduced, resulting in poor practicality. The latter, due to its aromatic structure, has better thermal performance than fully aliphatic or cycloaliphatic compounds, and the presence of cycloaliphatic compounds can increase transparency, making it an effective solution to balance heat resistance and transparency at present.

2.3 Thermal Conductive PI Film for 5G Communication

With the rapid development of electronic devices, integration, miniaturization, slimness, and high-frequency brought by 5G communication have become new trends in the development of electronic devices. The resulting thermal accumulation phenomenon is becoming increasingly evident, seriously affecting the signal transmission and energy consumption of circuits. The reliability and lifespan of electronic devices are being rigorously tested. The polyimide insulation film used in electronic devices is therefore facing increasingly high thermal conductivity requirements. The traditional thermal conductivity of polyimide film is below 0.2 W/(m · K), which cannot meet the rapid heat dissipation requirements of electronic devices. In recent years, researchers at home and abroad have made some progress in improving the thermal conductivity of polyimide film by blending thermal conductive fillers with polyimide resin.

The selection of thermal conductive fillers mainly considers the heat transfer mode and mechanism. Ceramic fillers mainly based on phonon heat transfer have good thermal conductivity and insulation properties, making them the preferred material for preparing thermal conductive insulation films. Ceramic fillers include boron nitride, aluminum nitride, silicon nitride, etc. Among them, boron nitride (BN) has high thermal conductivity (about 300 W/(m · K)), low dielectric constant and thermal expansion coefficient, excellent chemical stability, and relatively low density, making it an ideal filler for preparing high thermal conductivity and insulating composite materials. BN has four crystal forms: hexagonal, cubic, rhombohedral, and wurtzite, among which hexagonal boron nitride (h-BN) has the most outstanding comprehensive performance.

The size and amount of thermal conductive filler, as well as the interaction between the filler and the matrix interface, have a significant impact on the thermal conductivity of composite materials. In general, when the amount of thermal conductive filler added is the same, the larger the filler size, the more beneficial it is to reduce the contact area between the polymer matrix and the filler, lower the interfacial thermal resistance, and improve the thermal conductivity of the composite material. The amount of thermal conductive filler added is usually directly proportional to the thermal conductivity of the composite film, that is, the larger the amount added, the higher the thermal conductivity of the film. When the amount of thermal conductive filler is small, the filler is easily covered by the polyimide matrix, and the fillers cannot be in good contact with each other, making it difficult to form an effective thermal conductive path. Heat will only accumulate in the material or can only dissipate a small amount of heat, thus failing to achieve good thermal conductivity. In addition to the above two points, the interface compatibility between the thermal conductive filler and the matrix is a key factor affecting the thermal conductivity of composite materials. When inorganic filler BN is added to organic polymer matrix, due to poor interfacial compatibility, it is usually difficult to achieve uniform and effective dispersion. The filler is prone to agglomerate and form large aggregates, causing obvious voids and defects, seriously damaging the mechanical properties of the material. By modifying the surface function of the thermal conductive filler, the interface compatibility and dispersibility between the filler and the polyimide matrix can be effectively improved, which not only enhances the thermal conductivity but also improves the mechanical properties of the composite film, meeting the requirements of practical applications.