What is polyimide?
Polyimide (PI) is a type of high-performance resin that has high heat resistance, chemical resistance, mechanical strength, and electrical insulation. It is characterized by the inclusion of imide groups (-CO-N-CO-) in its structure, and exhibits excellent performance even in harsh environments.
- Extremely high heat resistance
The continuous use temperature is high at 250 to 400°C, and some have a thermal decomposition temperature of over 500°C. - High mechanical strength
Maintains high strength and durability even in the form of thin films or fibers. - High chemical resistance
It has excellent resistance to acids and organic solvents. - Excellent electrical insulation properties
It has high dielectric strength and is widely used as an insulating material in electronic devices. - Good dimensional stability
The thermal expansion coefficient is low and there is little change in shape.
Applications of polyimide include: Polyimide is an essential high-performance material in a variety of cutting-edge technological fields, including electronic devices and the aerospace industry.
＜Electronics and Electrical Field＞
・Flexible Printed Circuit board（FPC）
・Semiconductor chip protection film
・Insulated wire coating
＜Aviation and space field＞
・High temperature components and lightweight construction materials
・Insulation for satellites and aircraft
＜Industrial use＞
・High heat resistant filter
・Heat resistant bearing
・Composite material
＜Medical field＞
・Devices that take advantage of biocompatibility
Polyimide design
The design of polyimide (PI) is considered mainly from three perspectives: molecular design, synthesis process, and processing method. Depending on the application and required properties, it is important to select appropriate monomers and control the structure.
Molecular design of polyimide
Polyimide is synthesized by polycondensation reaction of diamine (-NH₂) and acid anhydride (-COOCO-). The following factors are taken into consideration in molecular design.
Main chain structure design
- Highly rigid aromatic polyimide
It has high heat resistance and high strength, but its low solubility makes it difficult to process.
Example: Kapton - Flexible polyimide
To improve solubility and processability, ether groups (-O-) or alkyl groups are introduced into the main chain.
Example: Soluble polyimide - High heat resistant polyimide
Strengthening the aromatic structure and improving heat resistance.
Example: Polyimide with benzoxazole groups
Introduction of functional groups
- Fluorinated polyimide (low dielectric constant, improved chemical resistance)
- Triazine-containing polyimide (improved heat resistance)
- Imide ring cleavage type polyimide (improved solubility)
Representative monomer
| Monomer | Amine/Acid | Features |
| p-Phenylenediamine (PDA) | Diamine | Improved rigidity and heat resistance |
| Oxydianiline (ODA) | Diamine | Improved flexibility and workability |
| Pyromellitic anhydride (PMDA) | Acid anhydride | Highly heat-resistant aromatic skeleton |
| Bisphenol A Diamine | Diamine | For flexible polyimide |
Polyimide synthesis process
Polyimides are mainly synthesized by two-step and one-step methods.
Two-step method (polyamic acid method)
- Polyamic acid (PAA) synthesis
・Diamine and acid anhydride are reacted in an organic solvent (NMP, DMF).
R-NH2 + O=C-O-C=O → R–CO–NH–Ar–NH–CO–R（Polyamic Acid）
R is an aromatic ring (such as a benzene ring), and the reaction proceeds easily at room temperature.
Solvents that can be used include NMP (N-methyl-2-pyrrolidone), DMF, and DMAc. - Thermal or chemical dehydration
By heating or chemically dehydrating the polyamic acid, imide rings are formed, resulting in the polyimide.
Polyamic Acid →（Heat or dehydrating agents）→ Polyimide + H2O
Thermal imidization at 200-350°C converts it into a stable aromatic imide structure.
Chemical imidization using a dehydrating agent (e.g., acetic anhydride + pyridine) is also an option.
One-step method (direct imidization method)
・Diamine and acid anhydride are directly cyclically polymerized.
Because the reaction is carried out at high temperatures (>200°C), it is not suitable for film formation, but is suitable for powder molding and injection molding.
Diamine + acid anhydride are directly condensed at high temperature (>180℃) → polyimide
Below are examples of synthetic designs for typical applications.
| Application | Point | Synthesis method |
| FPC board | Film-like, heat-resistant and insulating | 2-step method + ODA/PMDA |
| Molded parts | Mechanical strength and formability | 1-step method + PDA/PMDA |
| Optical applications | Transparent and low refractive index | Introducing fluorinated diamine |
Photosensitive Polyimide
Photosensitive polyimide is a polyimide resin that can form patterns in response to light. Original polyimide has high heat resistance and chemical stability, so it is widely used in electronic materials and the aerospace industry, but regular polyimide cannot be used in photolithography processes (the process of drawing patterns with ultraviolet light).
Main applications include semiconductor packaging (chip protection film, insulating layer for level wiring), flexible printed circuit boards (FPC), MEMS (microelectromechanical systems), and displays (OLED, TFT), etc.
So, by giving polyimide light sensitivity, a highly functional material called “photosensitive polyimide” can be patterned like a photoresist. There are two main types of photosensitive polyimide.
-Negative type: The parts exposed to light harden and remain.
-Positive type: The parts exposed to light become more soluble and are removed.
Its features include:
・High heat resistance: Can withstand heat of around 400°C
・High electrical insulation: Ideal for electronic circuit applications
・High mechanical strength
・Chemical stability: Resistant to acids and alkalis
・Patterns can be formed with light: Suitable for forming microstructures
| Supplier | Product Name (Series) | Remarks |
| JSR | PI series | For semiconductors, good heat resistance |
| HD MicroSystems | PI-2771, HD-8820 | Joint Venture with DuPont |
| Toray | Photoneece® series | Often used for FPC and LCD |
| UBE | UPILEX® VT-P series | High heat resistance, for fine patterns |
| FUJIFILM | Durimide® series | Negative type, good insulation |
・Processing process (example: negative type)
The following is a typical process flow for negative type photosensitive polyimide.
- Spin Coating
PSPI is applied uniformly onto the wafer. The thickness is about a few μm to 50 μm. - Pre-bake (soft bake)
Dry at 80-120°C. Remove the solvent and stabilize. - Exposure (ultraviolet light irradiation)
UV exposure using a mask. A chemical change occurs in the exposed area. - Development
Remove the non-exposed areas with a developer (exposed areas remain in the case of negative type). - Post-bake (polyimidization = imidization reaction)
Heate at 300-400°C to completely polyimidize (maximize heat resistance and insulation performance).