A printed circuit board (PCB) is a type of board used to support mechanically and electrically connect electrical components using conductive tracks, pads, and other features etched from copper sheets laminated onto a non-conductive substrate. PCBs are commonly used in manufacturing various electronic devices and products, such as computers, smartphones, and other electronics. They are also used to construct multiple types of machinery and industrial equipment. Many different types of printed circuit boards are used in various worldwide applications, and one of the most common ones is polyimide PCBs.

What is Polyimide PCB?

Polyimide, or polyamide, refers to polymers made up of progressive imide monomers. These polymers are manufactured usually using a synthetic process, and this manufacturing process is called polymerization. Polyimides PCB fabrication involves many chemicals bonded with amides to produce synthetic polyimides. The strong synthetic polymer bond makes them highly heat resistant. The printed circuit boards manufactured with these polymers are called polyimide PCBs. The chemical composition of Polyimide PCB provides it with the ability to be used in various industrial applications.

What is the difference between Polyimide and FR-4 PCBs?

FR-4 (Flame Retardant Level-4) material is one of the most common substrates for fabricating circuit boards. Level 4 represents the grade of the material, which is highly flame and temperature resistant. They can manufacture PCBs with one or more layers and can handle situations requiring continual flexing. Following are the differences mentioned between FR-4 and polyimide PCBs.

• Flame Retardant Level-4 printed circuit boards are made of laminated copper and epoxy glass resin, making them more rigid than polyimide PCBs. 
• Polyimide PCBs have three times more tensile strength than FR-4 PCBs.
• FR-4 circuit boards are less chemical and heat resistant as compared to polyimide PCBs.
• Polyimide PCBs are slightly more expensive than FR-4 PCBs, but their admirable quality of extreme durability and flexibility balances the overpricing.
• The thermal cycling facility of polyimide PCBs is quite impressive compared to FR-4 PCBs.

Types of Polyimide PCBs

Polyimide PCBs are classified by the type of material used in the synthetization of polyimides used in circuit boards. As a result, highly flexible or rigid polyimide circuit boards can be formed. Different types of polyimide PCBs include:

1. Second Generation Polyimides

Second Generation Polyimides are also known as pure polyimides, which are one of the simplest polyimide PCBs. They do not contain flame retardant like bromine, making them more thermally stable and temperature resistant. 

Pure polyimide PCBs are more flexible, making them suitable for electrical and communication devices.

• Third Generation Polyimides

The very next generation of pure polyimides is third-generation polyimides PCBs. They are different from the second-generation polyimides since they have additional flame retardants that make them less thermally stable. Production time and cost for third-generation polyimide PCBs are low, which makes them superior to second-generation polyimide PCBs.

• Filled Polyimides

Filled polyimide PCBs are more like multilayer PCBs. They contain additional fillers that restrain resin shrinkage and increase the reliability of circuit boards. Shrinkage causes cracks on the circuit boards while doing challenging operations like drilling.

• Low-Flow Polyimides

Low-Flow Polyimides PCBs can be used to place ordinary printed circuit boards because they are more resistant to harsh conditions. They are referred to as rigid polyimide PCBs due to the involvement of many resins and flow restrictions, making them less flexible. 

Popular polyimide laminates

1. Kapton HN film

Kapton HN film is a polyimide film that acts as an insulation material with excellent thermal, mechanical, chemical, and electrical properties. It is recommended for applications that require a film with stable properties that cover a wide range of temperatures. HN is the standardized type of Kapton film; other types include FN, CR, FCR, and HPP-ST. Kapton is a polymer of polyimide that can tolerate extremely high temperatures. It is produced from the condensation of pyromellitic dianhydride and other aniline derivatives. 

Kapton HN film has excellent thermal properties and can withstand a temperature range of -269 degrees to +400 degrees Celsius. It has good mechanical, electrical, and chemical properties. Its applications include use in electric motors, generators, transformers, electrical equipment and apparatus, and fiber optic cables. 

• Isola P95 material

Isola p95 is a laminate material with a high Tg value of 260 degree Celsius which is why this material can withstand high temperatures. It has a low dissipation factor and very high decomposition temperature allowing the laminate to function in extreme environments. It has low moisture absorption and low dielectric constant which means it is a good insulator that hardly absorbs water, making it safe to use in harsh weather conditions.

P95 has a very high thermal reliability and performance. The material complies to industrial standards and is a professional’s favorite for fabricating PCBs. It also complies with environmental regulations as it does not contain toxic substances like halogen and lead. P95 is widely used in the aerospace industry and military applications as it provides all the necessary properties for flight control systems and inventory usage. It is also a popular component in the automotive industry, where vehicles use it inside the engine. P95’s thermal reliability is crucial in this scenario as it offers thermal endurance, not allowing parts to expand. It is also a very cost-effective option for PCBs in such industries.

Benefits of Polyimide PCBs

Polyimide PCBs are a specific type of printed circuit board; they are mainly used when you need any of the following characteristics in your circuit boards:

1. Utmost Durability

Polyimide PCBs can maintain their shapes as they are highly resistant to physical stresses. These printed circuit boards are perfect for use in harsh conditions and environments like aerospace industry and military.

• Thermal stability

Unlike other printed circuit boards, Polyimide PCBs can survive and remain stable for a wide range of temperatures. They can work perfectly for up to 260 degrees Celsius. In general, polyamide is a non-flammable material with excellent physical and chemical properties. This thermal conductivity property saves polyimide PCBs from any thermal damage during the fabrication or repair.

• Flexibility

The combined quality of extreme stability and flexibility makes polyimide PCBs superior to ordinary printed circuit boards. Polyimide material is mainly used to produce rigid-flex and flexible printed circuit boards.

• Improved Tensile Strength

Polyimide PCBs are strong and highly resistant to warping. They can be stretched to maximum capacity without any fracture. They can even be twisted, except for low-flow polyimide PCBs. Polyimide PCBs are significantly used in conditions where the physical durability of circuit boards is paramount.

• Highly Chemical Resistant

Exposing the polyimide PCBs to different corrosive chemicals would not cause any harm to them as they are chemically stable. This property of polyimide PCBs makes them more suitable for use in several chemical industries.

• Good Electrical Properties

Having good electrical properties is an essential feature and requirement to avoid signal integrity. Polyamide PCBs have a variable dielectric constant varying between 2.78 and 3.48 and a lower dielectric loss that ranges between 0.01 and 0.03. It is resistant to radiation and exhibits this property in microwave applications where they are transparent.

Generally, polyimide PCBs are superior to many other types of printed circuit boards in many aspects, but in many qualities, they lack, and other circuit boards are used in placement.

Applications of Polyimide PCBs

Polymers manufactured either by a natural or synthetic process can be used in various applications, mainly in the fabrication of printed circuit boards. They are designed as thermoplastics or thermosets and are used as laminating resins, insulating coatings, and plastics. Many rigid or flexible printed circuit boards are made with polyimide material and used in respective industries as per the material involved. Some typical applications of polyimide PCBs are:

• Medical Industry 

Polyimide PCBs are more efficient than other printed circuit boards. Medical treatments and applications require high efficiency and flexible equipment such as ultrasound, advanced imaging, and implants. Polyimide PCBs are, therefore, widely used in the medical industry.

• Electronics

Electronics are evolving to become more compact. The flexibility and durability of these polyimide printed circuit boards make them essential to be used in modern-day electronics. Electronic devices in different firms face the most challenging conditions, and continuous heavy-duty usage leads to the extreme thermal rise of the components. Therefore, the thermal stability of polyimide PCBs is the aspect that they are used in modern electronics and telecommunication equipment.

• Aerospace and Military Industries

Polyimide PCBs are highly durable and stable as they can bear maximum physical stresses and retain their shape. Aerospace and military industries use advanced electronics to work under crucial and turbulent environments, and polyimide PCBs are used in these industries for their stability and reliability.

Conclusion

In conclusion, polyimide PCBs are a type of printed circuit board that uses polyimide as the base material instead of traditional FR-4 glass-reinforced epoxy. Polyimide offers several advantages, including a higher operating temperature range, excellent thermal stability, and low dielectric constant. These properties make polyimide PCBs well-suited for high-reliability applications where other printed circuit boards may not be suitable.