Research progress of high thermal conductivity PI polyimide dielectric film

In the era of highly thin, multifunctional, and integrated electronic devices, it is inevitable to accumulate heat inside composite materials, which seriously affects the stable operation and service life of equipment. How to achieve fast and efficient thermal conductivity and heat dissipation of dielectric materials has become a key issue affecting the development of electronic devices. The traditional polyimide has a low intrinsic thermal conductivity, which limits its application in fields such as electrical equipment and smart grids. The development of new high thermal conductivity polyimide dielectric film materials has become a research focus both domestically and internationally. This article introduces the thermal conduction mechanism of composite materials, summarizes the research progress and development status of thermal conductive polyimide films in recent years, and focuses on the influence of thermal conductive fillers, interface compatibility, and molding processes on the thermal conductivity of materials. Finally, combined with the future development needs of thermal conductive polyimide composite dielectric materials, some key scientific and technological issues in research are summarized and discussed.

01 Introduction

Polymer materials are widely used in fields such as electronics, communications, military equipment manufacturing, aerospace, etc. due to their excellent electrical insulation, chemical corrosion resistance, lightweight, and low density. Polyimide (PI) is an aromatic heterocyclic polymer compound constructed from chain links containing imide groups [- C (O) - N (R) - C (O) -]. It has excellent electrical insulation, radiation resistance, mechanical properties, and is known as a "problem-solving expert". PI, as a structural or functional material, has great development prospects, especially for PI film materials, which are known as the "golden film". It is one of the earliest polyimide products developed and applied, widely used in printed circuit boards, electronic packaging, interlayer media, display panels and other fields (see Figure 1). The application of polyimide film materials in modern electronic devices, industrial devices represented by chips, hybrid electric vehicles, and light-emitting diodes has led to a gradual reduction in product size due to high integration and high power. The problem of doubled heat generation has become increasingly prominent, seriously affecting the operational performance and service life of products. The efficient thermal conductivity and heat dissipation of thermal management systems have attracted widespread attention.

Related studies have shown that for every 2 ℃ increase in temperature of electronic devices, reliability decreases by 10%; The temperature increase of 8-12 ℃ reduces the service life by half, and the thermal conductivity of the material has become an important parameter affecting the normal operation of the equipment. Polymer materials have shown great potential in solving thermal conductivity and heat dissipation problems, but the intrinsic thermal conductivity of polyimide materials is relatively low, usually below 0.2 W/(m · K), far lower than materials such as metals, carbon, ceramics, etc., greatly limiting the application of PI films in high-tech fields. It is of great significance to seek appropriate methods to improve the thermal conductivity of polyimide materials in order to ensure the normal operation and safety of equipment. In order to solve the thermal conductivity and heat dissipation problems of polyimide materials, researchers mainly carry out work from two aspects. One is to modify the PI matrix body, starting from the perspective of molecular structure design, based on PI's 1-3 level structure design and construction of ordered structures; Inducing the formation of ordered structures through mechanical stretching, shearing, centrifugation, spinning, and other methods; Based on intermolecular interactions, especially the advantage of hydrogen bonding, interpenetrating and entanglement structures are formed between molecular chains, as well as hydrogen bonding interactions between side groups. The strategy to improve the intrinsic thermal conductivity of polyimide is to change the morphology of the matrix chain structure, so that the curled molecular chains present a stretched state, improve the orderliness of chain segment aggregation, and create a pathway for phonon transmission, thereby improving the intrinsic thermal conductivity of the matrix. The second is to use PI as the matrix and add high thermal conductivity fillers in the matrix, which is also an effective strategy to improve thermal conductivity. Currently, theoretical research and industrial production of high thermal conductivity polyimide composites at home and abroad mainly focus on filled PI composites. Thermal conductive fillers are interconnected in the PI matrix to form ordered thermal pathways, reducing scattering during phonon transfer and achieving rapid heat transfer. The thermal conductivity of composite materials is determined by factors such as the structure of the PI matrix and the properties of the fillers, the arrangement of fillers in the matrix, and the interaction between the matrix and fillers. At the same time, the influence of the construction of thermal conductivity pathways and preparation processes on the thermal conductivity of materials should also be considered.
02 Heat conduction mechanism

Heat is the energy generated by the movement, rotation, and vibration of microscopic particles such as molecules, atoms, and electrons inside a material. The thermal conductivity mechanism of a material is closely related to the collision and transfer of microscopic particles inside it. The carriers of heat conduction include molecules, electrons, phonons (energy quanta of lattice vibrations), and photons. Heat is transferred from the high-temperature part of the material to the low-temperature part, and in essence, it can be considered as molecules and atoms with larger amplitudes driving molecules and atoms with smaller amplitudes to vibrate.

The thermal conductivity of particle collisions in materials varies depending on the role of the thermal carrier in the material. There are a large number of freely moving electrons inside the metal, which transfer heat through interactions or collisions. Metals are also crystals, and the process of heat conduction is completed through the vibration of the lattice, that is, phonon conduction still exists. However, the heat transfer efficiency of free electrons is much higher than that of phonon heat transfer. Therefore, the heat conduction carrier of metals is mainly electrons. In non-conductive crystals, molecules or atoms are orderly distributed on the lattice, and the thermal conduction mode is mainly phonon conduction. Its thermal conductivity mainly depends on the degree of crystallization and orientation of the material, and is believed to depend on the scattering degree of phonons from a mechanistic perspective. The main reasons for phonon scattering are: high entanglement of molecular chains, voids in molecular structures, interface and structural defects, and weak interactions between molecular chains. The static scattering of phonons is caused by various defects, while the dynamic scattering is caused by the non harmonic vibration of molecular chains. The rotation of molecular chains and the entanglement between them can intensify the non harmonic vibration. At the same time, the various conformations generated by the rotation within the chain segment can also cause phonon scattering. Most polymers are saturated systems where there are no free moving electrons or intense collisions between electrons, and heat is mainly transferred through phonons. Molecular chains vibrate when heated, and thermal conduction mainly relies on the thermal vibrations around molecules or atoms at fixed positions, transferring heat to adjacent molecules or atoms in sequence. The thermal conduction of polymers is shown in Figure 3. The thermal conductivity mechanism of polymer in Figure 3 is characterized by complex molecular chains, easy entanglement, polydispersity of molecular weight, and high molecular weight. The crystallinity is not very high