The loss of power in the operation of electronic devices is mainly converted into thermal energy, resulting in an increase in the temperature of electronic equipment and thermal stress, seriously affecting the reliability and service life of electronic devices, so the need for these residual heat energy as soon as possible out. In this heat dissipation process, thermal interface materials play a crucial role. Thermal interface materials are mainly used to fill the electronic devices and heat sink contact micro-gaps and surface unevenness of the holes, reducing the thermal resistance of heat transfer.
In recent years, with the rapid development of electronic technology, the feature size of electronic devices has been dramatically reduced from the micron scale to the nanometer scale, while the integration degree is increasing at a high rate of 40% to 50% per year. With the arrival of the 5G era featuring high frequency and high speed and the maturity of 5G technology, various types of wireless mobile terminal devices, such as smart wear, driverless cars, VR/AR, etc., are being vigorously developed, and upgrading of hardware parts and components has occurred. Compared with 4G wireless mobile terminal equipment, the chip processing capacity of 5G wireless mobile terminal equipment has been greatly increased to 4~5 times that of 4G, thus power consumption is greatly increased, and the heat generated is also significantly increased; the number of antennas of 5G wireless mobile terminals has also reached 5~10 times that of 4G wireless mobile terminals; 5G wireless mobile terminals also adopt new materials such as ceramics and glass shells, which do not have a shielding effect on the 5G signals, but these materials are not suitable for 5G wireless terminals. and other new materials, but the heat dissipation performance of these materials is weaker than that of metals, so materials with better thermal conductivity are needed. At the same time, the construction of 5G communication base stations also requires a large number of thermal interface materials for rapid heat dissipation. Therefore, on the one hand, the latest development of electronic technology has opened up a new field of application for thermal interface materials, making the role of thermal interface materials more and more important in all kinds of electronic products, and becoming an important material in the electronic heat dissipation engineering, and the use of which will continue to increase significantly in the future; on the other hand, the continuous updating and upgrading of electronic products has put forward brand new performance requirements and technological challenges for the thermal interface materials related to the industrial chain.
At present, the common thermal interface materials on the market include thermally conductive silicone gel, thermally conductive spacer, thermally conductive silicone grease, thermally conductive adhesive, thermally conductive adhesive tape, phase change materials, welding materials and carbon-based thermal interface materials. The main features, advantages and disadvantages of different thermal interface materials are shown in Table 1.
Table 1. Characteristics, advantages and disadvantages of typical thermal interface materials.
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01 Composition and characteristics of thermally conductive silicone gel
Silicone gel is a kind of liquid and solid together called "solid-liquid coexisting materials" special silicone rubber, to polymer compounds constitute a network structure, with unique properties. Before curing is generally divided into A, B two-component, in the catalytic effect of platinum metal compounds, silicone resin matrix on the vinyl or propylene-based and crosslinking agent molecules on the reaction of the silica-hydrogen group. The whole reaction vulcanisation is an addition reaction, which does not produce by-products and therefore does not produce shrinkage. Silicone rubber is a straight-chain polyorganosiloxane with a high molar mass (generally above 148 000 g/mol), and its structural formula, similar to that of silicone oil, is shown in Figure 1.
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Fig. 1. Schematic structure of silicone rubber.
From the figure, it can be seen that: R is usually methyl, but in order to improve or enhance certain properties, other groups such as ethyl, vinyl, phenyl and trifluoroaldehyde groups can be introduced, R′ is hydroxyl or alkyl, n represents the number of links. The main chain in the polysiloxane molecule is composed of Si-O-Si bonds, and its main properties are:
(1) Physical and chemical properties are stable, basically independent of temperature, can be used in the temperature range of 50 ~ 250 ℃, electrical insulation properties and resistance to high and low temperatures (-50 ~ 250 ℃) are excellent.
(2) Without primer or surface treatment agent, it can be physically adhered to the surface of most common electronic devices or other materials, and there is no by-products or shrinkage during the curing process.
(3) The system is colourless and transparent, so it is easy to observe the internal structure of the potting component when used as a potting material. After curing is semi-solidified state, many of the adherent has good adhesion and sealing performance, with excellent resistance to hot and cold alternating properties.
(4) Longer operable time, no rapid gelation after two-component mixing. Heating will promote curing, curing time can be flexibly controlled by adjusting the curing temperature. It has good self-levelling property, which is convenient to flow into the minutiae between micro-components in the circuit.
(5) For different application scenarios, the hardness, fluidity, curing time and other properties of the gel can be flexibly adjusted, and functional fillers can also be added to prepare silicone gels with flame retardancy, electrical conductivity or thermal conductivity.
(6) Good self-repairing ability, after cracking by external forces, it has the ability of automatic healing, with the
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