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 What Is Polyimide? Overview Its Properties and Applications

What Is Polyimide? Overview Its Properties and Applications

Introduction:

Polyimide is widely used in the design and manufacture of FPC. Then why is the competitively priced FR4 not chosen? As a manufacturer of flexible and rigid-flexible boards. we will provide a comprehensive overview of what is polyimide in this article. In addition to the differences between FR4 and polyimide materials for rigid and FPC. Customers who use a flexible circuit design for the first time will find this extremely useful to understand polyimide.

What Is Polyimide?

Bismaleimide + Maleic Anhydride → Polyimide + Byproducts

Polyimides, also known as polyamides, are polymers composed of imide monomers. This diverse group of polymers includes a variety of natural and synthetic materials. For example, natural polyamides include silk and wool.

What Is Polyimide For FPC ?

what is polyimide

Polyimide refers to a class of polymers containing an imide ring (-CO-NR-CO-) on the main chain. That is one of the best overall performance of organic polymer materials. Its high-temperature resistance of more than 400℃. long-term use of the temperature range -200 ~ 300℃. part of the melting point is not obvious, high insulation properties. 10 3 Hz dielectric constant 4.0, dielectric loss of only 0.004 ~ 0.007, is F to H level insulation.

Polyimide is a type of substrate material used in the manufacturing of FPC. Polyimide is a high-performance polymer known for its excellent thermal stability. mechanical strength, and electrical insulation properties. It is often used in applications where the PCB needs to withstand extreme temperatures and mechanical stress.

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Type of polyimide PCB

The polyamides used as board bases in the PCB manufacturing process are mass-produced by synthesis. Synthetic polyimides produced by polymerizing a variety of chemicals containing an imide structure. Most commonly, this method uses bismaleimide and maleic anhydride. The different chemicals and additives used in the process can produce different results. resulting in various types of polyimides with various advantages.

Flexible Printed Circuits (FPCs) typically use polyimide materials as the substrate. due to their excellent flexibility, thermal stability, and electrical insulation properties. The most common type of polyimide used in FPCs is aromatic polyimide. Here are some details about the polyimide materials commonly used in FPCs:

※ Aromatic Polyimide (e.g., Kapton):

Aromatic polyimides, with Kapton being one of the most recognized brands in FPCs. That well known for their exceptional thermal stability. electrical insulation properties, and mechanical strength.

※ Adhesiveless Polyimide Films:

Adhesiveless polyimide films are polyimide substrates. That do not require an additional adhesive layer between the polyimide and copper layers. This reduces the overall thickness of the FPC and enhances flexibility.

※ Thermoplastic Polyimides:

Some FPCs may use thermoplastic polyimides. which melted and reprocessed, allowing for easier assembly and repair of flexible circuits.

※ Polyimide/Polyester Hybrids:

In certain applications where a balance between cost and performance required. polyimide/polyester hybrid materials used. These combine the flexibility of polyester with the thermal and properties of polyimide.

The choice of polyimide material for an FPC depends on various factors. including the specific requirements of the application, the desired level of flexibility. the operating temperature range, and the need for electrical insulation. Manufacturers select the most suitable polyimide material. To meet the design and performance standards of the FPC while considering factors. Such as cost-effectiveness and ease of fabrication.

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Features of polyimide PCB in PFC

High-Temperature Resistance: 

Polyimide PCBs can withstand elevated temperatures without degrading or losing their electrical properties. This is particularly important in PFC applications. where components may generate heat during operation.

FPC material polyimide

Excellent Flexibility:

Polyimide PCBs are flexible, which allows them to bend and conform to irregular shapes or fit into tight spaces. This flexibility is advantageous when designing compact PFC modules. or integrating circuits into curved or non-standard form factors.

Outstanding Electrical Insulation:

Polyimide is an excellent electrical insulator. In PFC applications, where voltage levels can be high, maintaining electrical isolation. between components and traces is crucial to prevent electrical breakdown or arcing.

Chemical Resistance:

Polyimide is resistant to many chemicals, which can be beneficial in PFC environments. where exposure to various substances is possible.

Dimensional Stability:

Polyimide materials tend to maintain their shape and dimensional stability under thermal stress. ensuring that the PCB’s mechanical integrity preserved even in high-temperature environments.

Thin and Lightweight:

Polyimide PCBs are often thinner and lighter than traditional rigid PCBs. making them suitable for weight-sensitive or space-constrained PFC applications.

Compatibility with High-Frequency Signals:

Polyimide PCBs can support high-frequency signals. which can be essential in PFC circuits that operate at high switching frequencies.

Resistance to Moisture and Humidity:

Some polyimide materials offer good resistance to moisture and humidity. which is advantageous in environments with variable humidity levels.

Durability:

Polyimide PCBs known for their durability and long service life. even in challenging conditions, making them a reliable choice for PFC applications.

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Polyimide Material VS. FR4 in FPC

Distinctive structural features

Liquid polyimide poured onto a copper layer. and the material is then cured to a solid state, a solid plastic that is highly ductile. FR4-grade materials are typically laminated glass fibers. These materials produced by melting the raw glass material. and extruding it into fiber yarn filaments. The fiber yarns are then woven together and coated with resins and coupling agents to improve adhesion.

There is also a hybrid rigid material that uses a polyimide-based resin. combined with a matte layer of fiberglass to withstand extremely high temperatures. The key difference is the brittleness of the cured epoxy. and the woven fiberglass matt in rigid materials that provide a high degree of stiffness.

Differences in material properties

The key property differences between FR4 and polyimide flexibles are dielectric constant (DK). maximum continuous operating temperature and moisture absorption.

Both temperature ratings and DK values derive from the resin system used. epoxy-based resins for FR4 limit maximum operating temperatures to the 150°C range. compared to 300°C for polyimides. rigid materials based on blended polyimides have a higher temperature rating of approximately 220°C . due to the use of polyimide-based resins. Polyimide Flex materials also have better dielectric constant (DK) values in the range of 3.2 – 3.4. Standard FR4 has a DK of 4.0 – 4.4. Actual values depend on the signal frequency.

Polyimide materials, as a natural property, absorb up to 2% moisture by weight, compared to 0.1% or less for FR4. The increased moisture absorption of polyimide does not change. or negatively affect its mechanical or electrical properties. However, it does need to be oven-dried before assembly. to prevent trapped moisture from flashing. to vapor and potentially causing delamination of layers, covers or reinforcements.

Material Thicknesses & Copper Types

Polyimide flexible materials are thinner than FR4 materials. Due to the lack of reinforcement and ductility of the curing resin. This is due to the lack of reinforcement and ductility of the cured resin. the epoxy-based resin of FR4 cures to a hard and somewhat brittle state,. while the polyimide maintains very good ductility. The minimum thickness of FR4 is also limited by the glass fibers. The standard thickness range for flexible materials is ½ to 3 mils. while the standard thickness range for FR4 is 2 to 125 mils. Copper thicknesses are similar, with a standard range of 1/3 OZ to 2 OZ. Both materials used with thicker and thinner copper if desired.

 

FR4 PCB

The main difference is the type of copper available in the polyimide flex. FR4 materials use electrolytic (ED) copper with a vertical grain structure. This type of copper used in flexible materials, but the more common type rolled annealed (RA) copper. Standard ED copper converted through a rolled annealing process. to form a horizontal and elongated copper grain structure. This allows for a higher degree of ductility. which is essential to ensure mechanical reliability. in tight static bending and dynamic bending applications. The use of RA copper is essential for dynamic applications. Due to the elongated grain structure, the grain direction of RA copper. needs to oriented along the length of the bending area to maximize the benefits.

Impact On Impedance Controlled Circuits

When used in impedance-controlled designs, polyimide flex has two inherent advantages over FR4 . The lower DK allows the use of thinner cores, which when combined with thinner wire widths and spacings. reduces the thickness of the flexible design and allows for tighter minimum bend radii. The flexible material is also completely homogeneous. without the small discontinuities caused by matte glass fibers in the rigid material. High-end rigid materials minimize this by changing the configuration of the glass matte but at an increased cost.

Polyimide Flex PCB and Laminate Materials

Polyimide Flexible PCBs are the most common type of FPC. and consist of an overlay layer and a Flexible Copper Clad Laminate (FCCL). That accounts for the largest percentage of the material properties of PI PCBs. the FPC overlay consists of a PI film and an adhesive layer, and the FCCL consists of PI, copper, and adhesive. The color of the external PI film of a flexible polyimide PCB can be yellow, white or black.

Rigid-flexible polyimide PCBs are flexible polyimide PCBs. the inner layer is FPC and the outer rigid layer is the FR4 PCB layer.

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Polyimide VS. FR4 for Application

Polyamide flex and rigid-flex boards are widely used in the PCB industry. Some examples include:

Polyimide Application

Computer Electronics:

Computer environments, especially laptops, require a certain level of flexibility. and durability to withstand the physical stresses of everyday use. In addition, the environment in which computer PCBs operate can become unusually hot. Polyamide flexible and rigid-flexible PCBs are well suited to these environments. thanks to their flexibility, durability and thermal stability.

Automotive electronics:

Modern automobiles rely heavily on electronics and often use flexible PCBs. to handle the vibrations and heat that often occur inside the vehicle.

Consumer electronics:

Due to the pressures of everyday use, consumer electronics. including smartphones and tablets, are often made from flexible or rigid-flexible PCBs. Flexible PCBs are being further developed for potential future consumer electronics. such as flexible tablets and smartphones.

Electronics for the medical industry:

Many medical applications require a degree of flexibility in the electronics used. especially in the case of implants, prosthetics. and imaging technologies, which often require more mobility.

Military and aerospace electronics:

Aerospace and military applications often prefer polyamide materials for PCBs. because of their reliability, thermal stability, and flexibility. This is due to the physical stresses involved in these industries. and the frequent need for field repairs for which other materials are less suitable.

FR4 Application

DC board applications:

The vast majority of DC boards done in FR4 because of its low cost, high performance and ease of use.

Low Frequency Applications:

FR4 boards are commonly used in low frequency applications. due to their low cost and comparability to other materials. While FR4 is not suitable for high frequency applications above 2 GHz. it is a good choice for low frequency applications. that require functionality at a modest cost.

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Compared to FR4, Why Choose with Polyimide?

Choosing between polyimide and FR-4 for your PCB or FPC . depends on the specific requirements and constraints of your application. Each material has its advantages and disadvantages. and the choice based on factors such as flexibility. temperature resistance, electrical performance, and application needs. Here are some reasons to choose polyimide over FR-4:

Flexibility:

Polyimide is inherently flexible and can bend and conform. to irregular shapes without breaking or cracking. It is ideal for applications where the PCB needs to be flexible or bent. such as in wearable devices, automotive sensors, and medical equipment.

High-Temperature Resistance:

Polyimide has excellent thermal stability and can withstand high temperatures. without losing its mechanical or electrical properties. This makes it suitable for applications where components generate heat. or where the PCB exposed to elevated temperatures.

Thin and Lightweight:

Polyimide PCBs are thinner and lighter than FR-4 PCBs. making them suitable for weight-sensitive applications, especially in aerospace and portable electronics.

Excellent Electrical Insulation:

Polyimide has exceptional electrical insulation properties. with low dielectric constant and low dissipation factor. It is well-suited for high-frequency and high-speed circuits.

Chemical Resistance:

Polyimide is resistant to many chemicals, making it suitable for applications. where exposure to harsh chemicals or solvents is a concern.

Custom Shapes and Sizes:

Polyimide can be easily cut and shaped to fit specific design requirements. This flexibility in design is advantageous for complex or unique-shaped PCBs.

High-Speed Applications:

Polyimide’s excellent dielectric properties make it suitable for high-speed signal transmission. in applications like data communications and high-frequency RF devices.

Harsh Environments:

Polyimide is a good choice for applications in harsh or demanding environments. such as automotive under-the-hood electronics or aerospace components.

However, it essential to note that FR-4 still has its place in the electronics industry. and is often chosen for traditional rigid PCBs and cost-sensitive applications. The choice between polyimide and FR-4 based on the specific needs of your project. considering factors like flexibility, temperature requirements, electrical performance, and budget constraints. In some cases, a combination of both materials (rigid-flex PCBs) may be the best solution. to meet various design requirements within a single circuit board. 

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