Advances in Research on Metal-based Thermal Interface Materials

With the continuous development of electronic technology, the integration, miniaturization and high power density of chips have become its main development direction. This puts higher demands on thermal management technology. The thermal management system of the chip is more complicated. In addition to devices such as heat sinks with high thermal conductivity and heat sinks with high heat dissipation efficiency, reducing the contact thermal resistance between electronic components and heat sinks is also an issue that needs to be focused on in chip thermal management systems.

When electronic components and heat sinks are in contact with each other, air gaps exist at the solid-solid contact interface. The actual contact area is about 10% of the macroscopic contact area, with the bulk of the voids being filled with air. Air is a poor conductor of heat, and the thermal conductivity of air at room temperature is only 0. 026 W/(m·K). The presence of air hinders the heat transfer between the interfaces, which leads to an increase in the interface thermal resistance between the chip and the heat sink. Therefore, it greatly reduces system heat dissipation efficiency and reduces chip lifespan. In order to ensure the normal operation of the heating element, materials that can conduct heat quickly and effectively are filled between the heating electronic element and the heat sink. This material is called Thermal Interface Materials (TIM). This uses high thermal conductivity and high ductility materials to fill the gap between the two to improve heat transport capacity, effectively reduce interface thermal resistance and improve the efficiency of the heat sink. In this way, the efficient operation of the chip is further ensured and the service life thereof is improved.

An ideal TIM should have characteristics such as low thickness, high thermal conductivity, and low contact thermal resistance. In the actual selection and design of TIM, in addition to the total interface thermal resistance, other factors should be considered comprehensively, such as electrical insulation, mechanical strength, etc. With the continuous development of TIM, many kinds of commercial products have emerged in the market. This mainly includes thermal grease, thermal adhesive, thermal gel, thermal phase change material and thermal pad. Traditional polymer-based thermal interface materials account for nearly 90 percent of all TIM products. As the demand for heat dissipation of electronic components increases year by year, metal-based thermal interface materials have become a research hot topic due to their high thermal conductivity, and their market share has also increased year by year. Many scholars have summarized the current status of the TIM industry and analyzed the market conditions of different types of TIM. However, there is a lack of systematic elaboration on metal-based thermal interface materials.

This paper systematically introduces the research progress of metal-based TIMs. Metal-based TIMs are summarized from the aspects of material type and performance characteristics. The future development of TIMs is prospected to provide reference for thermal management technology research.

TIMs are an important part of the heat dissipation structure of electronic components. The common chip heat dissipation structure and heat dissipation process are shown in Figure 1.

It can be seen from Figure 1 that TIMs are placed between the chip and the vapor chamber, and between the vapor chamber and the radiator. The heat generated by the chip is transferred to the environment through TIMs1, vapor chamber, TIMs2 and heat sink. Figure 2 is a microscopic schematic diagram of the device interface contact before and after filling TIMs.

Figure 2(a) is the actual situation where the electronic components are in direct contact with the heat sink. It can be seen from the figure that there are few actual contact points and the contact is incomplete. Figure 2(b) is the actual situation of filling TIMs between electronic components and heat sinks. The TIMs shown are filled with air gaps to the maximum extent, which allows for tight device connections and maximum heat dissipation. Since TIMs cannot fully contact with electronic components and heat sinks. The existing interface thermal resistance makes the temperature difference corresponding to each interface larger. ΔT in the figure is the temperature difference between the cooling plate and electronic components. ΔT_contact is the temperature difference between the thermal interface material and the heat sink. ΔT_TIM is the temperature difference between the upper and lower surfaces of the thermal interface material. The bonding line thickness in the figure refers to the thickness of TIMs. The bond line thickness is an important parameter to study the thermal conductivity of TIMs and calculate the interfacial thermal resistance.

Due to the variety of commercially available TIMs, each product has its own advantages and disadvantages. The current commercial TIMs are mainly divided into the following categories.

(1) Thermal grease

Thermally conductive silicone grease is usually a paste material made of a high thermally conductive solid and a liquid with good fluidity and a certain viscosity through a defoaming method. It is widely used in industry and belongs to high temperature resistant organic materials. The thermal conductive silicone grease has better adhesion to the contact surface, and the thickness can be controlled to be very thin. At the same time, it's cheap. But its biggest disadvantage is that it will stain the base material during use. Since the thermal grease is a liquid paste, it exhibits a serious pump-out effect. If it is mobile and used for a long time, it will gradually fail, which reduces the reliability of the system.

(2) Thermal gasket

Thermally conductive gasket is a kind of soft and elastic thermally conductive interface layer material formed by heating and curing with high molecular polymer material or other materials as the matrix, adding high thermally conductive fillers and additives. It can not only fill the uneven gap between the electronic components and the heat sink, effectively transfer heat, but also play the role of sealing, shock absorption and insulation. However, due to the high content of heat-conducting particles in some products, it increases the contradiction between the rigidity, softness and filling rate of the material. Therefore, this limits the overall performance of the composite thermal interface material. In addition, thermal pads are sensitive to temperature. If the temperature of electronic components and thermal pads increases, the pads will experience stress relaxation. The filling area is reduced, and the heat conduction effect becomes worse.

(3) Phase change thermal interface materials

Phase-change thermal interface materials refer to a class of materials that can undergo solid-liquid or solid-solid phase transitions with changes in temperature. It has a certain thermal conductivity, which can reduce the interface thermal resistance and realize heat transfer. Phase change thermal interface materials combine the dual advantages of thermal pads and thermal grease. When the temperature of electronic components increases during operation, the material undergoes a phase change to a liquid state, effectively wetting the thermal interface. It has the same filling ability as thermal grease, which can fill the interface gap to the maximum extent. This reduces the interfacial thermal resistance. In addition, phase change thermal interface materials absorb and release latent heat during the phase change process. It has the effect of energy buffering, which can prevent the working temperature of electronic components from changing too fast. This prolongs the use time of the electronic components. However, the thermal conductivity of the phase change thermal interface material is average, and the thickness is difficult to control.

In addition to the above three types of TIMs, commercially available TIMs also include thermally conductive gels and metal sheets. Typical thermal interface materials and their heat transfer properties are shown in Table 1.

(4) Metal-based thermal interface materials

Metal-based thermal interface materials include low-melting point metals and metal-matrix composite materials that use low-melting point metals as the matrix and add high thermal conductivity enhancement phases. Due to the high thermal conductivity of metal itself, the inherent thermal conductivity of prepared TIMs far exceeds that of polymer TIMs. The reported thermal conductivity of metal-based thermal interface materials is between 10 and 40 W/(m K), which is 2 orders of magnitude higher than that of traditional organic or inorganic materials. Moreover, low-melting-point metals and their composite materials can be melted within the temperature range that the chip can bear. This fully fills the interface gap and greatly reduces the interface thermal resistance, which can ensure efficient and stable heat dissipation of the chip. Therefore, in recent years, low-melting point metals and their composites have rapidly become a research hot topic in the field of TIMs and have received extensive attention.

Metal-based thermal interface materials are favored in high power density semiconductors due to their excellent thermal conductivity. It is mainly low melting point metals and metal matrix composites. Low melting point metals mainly include Ga, Sn, In, Bi and alloys composed of them as main components. This type of material has many advantages such as high thermal conductivity, good fluidity, low interface thermal resistance, and easy realization of solid-liquid phase transition. At present, it has been applied in many fields such as thermal control and energy, additive manufacturing (3D printing), biomedicine, and flexible intelligent machines. This has been a hot topic in both academia and industry in recent years. Scholars have used numerical simulation methods to study the heat dissipation of liquid metals, which has promoted the further development of this type of materials. Metal matrix composites as TIMs mainly use low-melting point metals as the matrix. The reinforcing phase can be inorganic non-metal, such as ceramics, carbon, graphite, etc., or metal particles, such as Cu, Zn, etc.

Low-melting-point metals refer to metals and their alloys with a melting point below 300 °C, and are considered as potential phase-change thermal interface materials. The common disadvantage of many potential phase change materials is low thermal conductivity, for example, the thermal conductivity of organic materials is 0. 15 ~ 0． 3 W /( m·K), the thermal conductivity of salt water compound is 0. 4 ~ 0． 7W/(m·K). Its low thermal conductivity will lead to poor heat exchange between the heat transfer fluid and the surface of the electronic component, resulting in a large interfacial thermal resistance. Low-melting-point metals have many advantages, such as tens of times higher thermal conductivity than traditional TIMs, relatively stable physical and chemical properties, high boiling point, and non-corrosiveness. Metals with low melting points can also achieve solid-liquid phase transitions, absorbing and releasing heat quickly. It has obvious advantages in thermal management technology. Table 2 lists typical thermophysical properties of several low melting point metals or alloys. The superscripts on the values in the table indicate the test temperature. a is 25°C, b is 200°C, c is 160°C, d is 100°C, n is 50°C, and m is the melting point of the metal.

Low melting point metals have high thermal conductivity, strong fluidity and wide liquid phase working area. It can be used as better TIMs for high-power chip heat dissipation, but too strong fluidity will cause leakage, which may cause chip short circuit.

Metal matrix composites are composite materials made of metal as the matrix and combined with one or several reinforcements. Most of its reinforcing phase materials are inorganic nonmetals, and metal wires, particles, etc. can also be used. Together with polymer matrix composites and ceramic matrix composites, it constitutes a modern composite material system. Metal matrix composites have good comprehensive mechanical properties such as shear strength, toughness and fatigue. At the same time, it also has the advantages of thermal conductivity, electrical conductivity, wear resistance, small thermal expansion coefficient, no aging and no pollution.

When TIMs are prepared by adding high thermal conductivity ceramics or carbon materials to the low melting point metal matrix, the thermal conductivity difference between TIMs and chips and heat sinks can be improved while improving the thermal conductivity of the material.

When using metal matrix composites to prepare TIMs, the addition of high thermal conductivity particles can greatly increase the thermal conductivity of the material and improve the performance of TIMs. When the service temperature is higher than the melting point of the matrix alloy, the added reinforcing phase can effectively increase the viscosity of the material, reduce the fluidity of the material, and effectively improve the problem of chip short circuit caused by material flow. It is an ideal TIMs. However, there are still many problems in the wettability of the reinforcement phase and the matrix in metal matrix composites. How to improve the interface between the two and further improve the thermal conductivity and strong plasticity of the material is the key to the development of a new generation of TIMs.

Metal-based thermal interface materials have broad application prospects in high-power semiconductor thermal management systems due to their high thermal conductivity. In this paper, the low-melting point metals and their composites used in TIMs are systematically summarized from the aspects of material composition, preparation process and material properties. On this basis, the following suggestions are put forward for the design and development of metal-based thermal interface materials in the future.

(1) Low-melting-point metals can fully fill the interface due to their excellent fluidity, but there is also the problem of leakage leading to chip short circuits. This requires research into ways to better limit their mobility. At the same time, the oxidation of metal materials during long-term service and the etching of materials on both sides of the interface also need to be paid attention to.

(2) For low-melting point metal matrix composites, future research should focus on improving the interfacial bonding between the reinforcing phase and the matrix. In order to further improve the performance of the material, it is necessary to focus on the surface modification and composite form of the reinforcing phase.

(3) In order to provide a solid theoretical basis for the design of TIMs, it is necessary to strengthen the research on the thermal conduction mechanism of TIMs to select an appropriate thermal conductivity model.