In the traditional notion that PCB substrate materials are either rigid or flexible, even so-called semi flex PCB only thin specific areas through deep milling techniques to provide flexibility while fundamentally remaining rigid. Is there a material that can break these traditional constraints and meet broader manufacturing needs? The answer lies in Polyimide. This high-performance engineering plastic can virtually meet all the requirements of PCB projects, whether they demand rigidity, flexibility, high temperature tolerance, high frequency, corrosion resistance, or other specifications.
Introduction to Polyimide Material
Polyimide, abbreviated as PI, is a high-molecular-weight compound with an aromatic heterocyclic structure that includes imide functional groups in its molecular chain. The molecular structure comprises two carbonyl (C=O) groups bonded to nitrogen (N), granting it outstanding thermal and electrical properties. Consequently, Polyimide is extensively used in the PCB domain as a substrate material.
Currently, high-molecular-weight materials in the market are categorized as general plastics, engineering plastics, and specialized engineering plastics. Polyimide falls into the category of specialized engineering plastics and is considered the pinnacle of the pyramid due to its superior overall performance.
The development history of Polyimide dates back to the early 1930s, and DuPont’s introduction of Kapton in the 1960s marked the earliest commercial application of Polyimide. Initially developed as a rigid material, Polyimide later evolved into a thin film form, becoming a primary flexible substrate and coverlay. Many industry professionals believe that without Polyimide films, today’s microelectronics technology would not have reached its current state.
Polyimide Material Properties
As a popular choice, many laminate manufacturers have invested in the research and production of Polyimide. DuPont, Shengyi, Panasonic, and Isola are among the most renowned companies in this field. The following table presents the performance data for Polyimide laminates produced by these companies:
|Glass Transition Temp
Note: This table is sourced from the internet, and for accurate information, it is recommended to check the official websites.
From the Polyimide property parameters, we can observe that this material possesses excellent physical, electrical, and thermal properties, making Polyimide PCB laminate suitable for various applications, including military, aerospace, home appliances, and more.
Polyimide challenges the inherent perception of low reliability in flexible PCB. With a tensile strength not less than 5.5 MPa, Polyimide is comparable to any other flexible PCB substrate material, allowing it to withstand mechanical stress caused by vibrations and maintaining the integrity of the circuit structure.
Moreover, Polyimide exhibits a glass transition temperature exceeding 250°C, a moisture absorption rate of 0.02%, and exceptional chemical stability. These properties enable Polyimide PCB to confidently face extreme environments, maintaining excellent electrical performance even during prolonged operation.
One of the crucial advantages of Polyimide is its ease of processing and compatibility with various processes. Unlike some metal materials that have limitations, Polyimide PCB allows designers to construct multi-layer circuits according to project requirements. Combining the advantages of multiple layers with electrical performance, Polyimide PCB can serve high-frequency, RF, and microwave circuits.
Types of Polyimide Material
This refers to pure Polyimide resin, with Arlon 85N being a typical example. It is called “pure” because it does not contain brominated flame retardants or other additives during processing. While reducing the content of harmful substances, it lacks UL flame retardancy. Additionally, due to the high molecular weight of the resin, high melt viscosity and poor flowability may lead to extended lamination times.
This is an updated version of Polyimide developed to meet specific PCB requirements. Compared to the previous generation, this version includes additional flame retardants to enhance the flame resistance of Polyimide PCB, better preventing electrical fires. This innovative improvement allows it to maintain excellent performance at extremely high temperatures, such as a bend strength rate of 44.3% and a bend modulus retention rate of 90.3% at 500℃. Besides the advantages in thermal performance, it effectively shortens the required curing temperature and time, optimizing production efficiency.
As the name suggests, this involves adding additional filling materials, including inorganic materials, metals, nanomaterials, etc., during processing. These additional fillers can alter the mechanical properties, thermal properties, and processing flowability of Polyimide. In Polyimide PCB manufacturing, filled Polyimide significantly enhances the mechanical performance of the substrate, making PCB more convenient to CNC process and less prone to damage. However, it’s important to note that filled Polyimide has higher costs and may increase the weight of PCB to some extent.
In the field of plastics and polymers, flowability usually refers to the flow or meltability of materials during heating and processing. Low-flow Polyimide has minimal flowability during processing, influenced by factors such as molecular structure, cross-linking degree, or additives. Due to its lower flowability, low-flow Polyimide typically lacks the flexibility of standard PCB and is mainly used to construct rigid Polyimide PCB.
Polyimide vs. FR4
In rigid PCB, Polyimide and FR4 are the two most commonly used materials, each with its own advantages and characteristics. Let’s discuss how to choose between them:
The molecular structure comparison between Polyimide and FR4 is shown in the figure above. It’s evident that Polyimide has more benzene ring structures and double bonds, indicating that Polyimide has higher thermal performance than FR4. To illustrate this with a clear example:
If we simultaneously heat FR4 PCB and Polyimide PCB from the ambient temperature of 23°C to 150°C, FR4 PCB can withstand approximately 1000 cycles, while Polyimide PCB can endure five times that. This difference becomes even more pronounced at temperatures exceeding 220°C, where FR4 PCB can withstand only 10 cycles, whereas Polyimide PCB can endure ten times that under extreme conditions.
This example highlights why almost all military PCB use Polyimide instead of FR4. For applications with higher reliability requirements, where failure is not acceptable, or for applications related to human life, it is strongly recommended to use Polyimide as the PCB substrate material. Examples include aircraft controllers, drilling probe control systems, high-precision medical equipment, and more.
Polyimide PCB also surpasses FR4 PCB in terms of electrical performance. This is because FR4 is typically a glass fiber-reinforced epoxy resin composite material. On the one hand, glass fibers themselves have a certain dielectric constant. On the other hand, epoxy resin in FR4 may contain polar groups. Under the action of an electric field, polar molecules can induce the generation of electric dipoles, increasing the dielectric constant. In contrast, the molecules of Polyimide are generally nonpolar and have a more uniform chain structure, reducing the impact of the electric field propagation in the material.
Considering that commercial projects involve mass production, even a one-cent difference in the cost of substrate materials can lead to significant cost differences. When comparing the prices of substrate materials under the same conditions, the ranking from low to high is approximately:
CME-1 → CEM-3 → FR4 → Polyimide → Aluminum → Polyimide Film → Alumina → Rogers
Obviously, FR4 is more cost-effective than Polyimide. Generally, FR4 PCB is considered a universal circuit board. Therefore, when project requirements for PCB performance are not high, it is strongly recommended to use FR4.
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