Investigation on Thermal Conductivity Silver-coated Copper/GaInSn Composite Thermal Interface Materials

To enhance the heat transfer performance of GaInSn liquid metal, a new type of composite thermal interface material with GaInSn as thermal conductive matrix and silver-coated copper particles as thermal conductive fillers was prepared in this article. The morphology of the composites were characterized by scanning electron microscopy. The thermal conductivity of silver-coated copper/GaInSn composites with different silver-coated copper particle contents were measured by laser flash method. The interfacial bonding of the composites were investigated by X-ray diffraction. The results show that when the volume ratio of silver-coated copper particles is 15%, thermal conductivity of the silver-coated copper/GaInSn composite reaches the largest value of 25.01 W·m−1·K−1. The silver-coated copper/GaInSn composite material and commercially available thermal grease were put into the chip and copper plate to simulate the actual application scenarios for comparison. The heat dissipation capacity of silver-coated copper/GaInSn composite material is obviously better than that of the two commercially available thermal grease, and the chip temperature can be reduced by about 5°C.


Introduction
With the continuous increase of computing power of electronic devices, as well as the continuous development of multi-function, integration and miniaturization, the heat load is becoming increasingly high [1][2][3] .In order to reduce the temperature of the heat producing element, a high thermal conductivity heat dissipation element is placed on its surface to conduct heat.However, the contact surface between the heat generating element and the heat dissipation element cannot fully adhere at the micro level, resulting in high interface contact thermal resistance and poor heat conduction effect [4] .Therefore, the interface gap can be filled with thermal interface materials to increase the area of the contact surface between the heat producing element and the heat dissipating element.This will reduce the interface contact thermal resistance and improve thermal conductivity efficiency [5][6][7] .
Most of the traditional thermal interface materials are organic thermal interface materials, which are usually made by adding high thermal conductive fillers to the matrix of silicone oil, polymer, resin, etc., but the poor thermal conductivity of the matrix itself limits its upper limit [8][9] .Thermal graphite sheets are also common thermal interface materials, but the thermal conductivity (~15 Wm -1 K -1 ) is also relatively low [10] .Metals have natural advantages when used as thermal interface materials because of their intrinsic high thermal conductivity (for example, the thermal conductivity of indium sheet can reach 86 Wm -1 K -1 [11] ).In particular, liquid metals not only have high thermal conductivity, but also have strong fluidity and low interfacial thermal resistance due to their special morphology.Roy et al. [12] developed a liquid metal composed of Ga, In, Bi and Sn that can provide interfacial thermal resistance as low as 0.5 mm 2 K/W.However, the good fluidity of liquid metal makes it easy to overflow from the interface, and there is the risk of short-circuiting electronic components.
In order to overcome the problem of liquid metal thermal interface material overflow, in recent years, it is widely used to add thermal conductive fillers to liquid metal to improve the viscosity.At the same time, filling with high thermal conductive fillers can also improve the thermal conductivity of thermal interface materials.At present, the main thermal conductive fillers for particle reinforcement are Ag [13][14] , Cu [15][16] , Fe [17] , Ni [18] , Al2O3 [19] , diamond [20] , carbon fiber [21] , graphene [22] and so on, among which the thermal conductivity of liquid metal is high when Cu and Ag are added.Tang et al. [15] added Cu powder to GaIn alloy (which would become intermetallic compound CuGa2 after dispersion), and the thermal conductivity of the composite increased with the increase of Cu powder content, up to a maximum of 50 Wm -1 K -1 .But Ji [16] conducted research on the aging of Cu/GaInSn composite materials, and the thermal conductivity of the composite materials plummeted from 38 Wm -1 K -1 to 9 Wm -1 K -1 within 200 hours.Lin et al. [13] added 3wt% Ag powder into pure Ga to obtain a composite thermal interface material with a thermal conductivity of 46 Wm -1 K -1 , and tested its heat dissipation performance on smart phones, which is superior to commercial thermal grease.Kong et al. [14] tried to add Ag powder into eutectic GaInSn, and obtained thermal paste with good solid liquid contact.
In this study, a series of silver-coated copper/GaInSn composites were prepared by using silvercoated copper powder as thermal filler and GaInSn liquid metal as thermal matrix considering thermal conductivity, stability and price.The morphology of silver-coated copper powder and composite was characterized by scanning electron microscope.The interface reaction of composite was investigated by X-ray diffraction, and the thermal conductivity was measured by laser thermal conductivity testing instrument.Finally, the heat dissipation performance of composite was compared with that of commercial thermal grease by self-built chip pseudo-true temperature measurement system.

Materials used in experiments
Purity of gallium, indium, tin >99.99%;The silver content of silver-coated copper powder is 20 wt%, the particle size is 11 μm; The liquid metal Ga67In20.5Sn12.5 (wt%, EGaInSn) is a ternary eutectic alloy with a melting point of 10.7℃, and the thermal conductivity of Ga67In20.5Sn12.5 is 13 Wm -1 K -1 .
Commercially available products for thermal conductivity comparison are Kafuter K-5211 and OMEGA THERM "201".

Sample preparation
The preparation process of Ga67In20.5Sn12.5 liquid metal is as follows: First, according to the required mass ratio, the corresponding mass of gallium, indium and tin is weighed with a high-precision electronic balance and put into the beaker; then the metal is melted and mixed evenly in an argon atmosphere; finally, the evenly mixed liquid metal is naturally cooled to room temperature and ready for use.
Weigh the liquid metal and the corresponding silver-coated copper powder according to a certain volume fraction; then put it into the teflon beaker, after that stir it with a mechanical agitator at 300r/min for 1h, and obtain the silver-coated copper/GaInSn composite material.

Analysis and testing
The microstructure of spherical silver-coated copper powder and silver-coated copper/GaInSn composites were characterized by scanning electron microscopy.The specific heat capacity, thermal diffusivity and thermal conductivity of the composites were measured by laser thermal conductivity testing instrument (NETZSCH LFA457).

Results and discussion
3.1.Microstructure analysis of silver-coated copper powder and silver-coated copper/GaInSn composites Figure 1(a) is a scanning electron microscope photograph of silver-coated copper powder.It can be seen that the silver-coated copper powder is nearly spherical and well-dispersed, with a particle size of about 11 μm. Figure 1(b) shows the electron microscope photo of silver-coated copper/GaInSn composite material.Obviously, the morphology of silver-coated copper/GaInSn composite material is not a simple structure of silver-coated copper powder wrapped in GaInSn liquid metal.There are two different shapes of particles in GaInSn liquid metal, lamellar and ellipsoidal, which are denoted as A and B. Figure 1(cg) shows the results of surface scanning in figure 1(b) by EDS on SEM, corresponding to Ga, In, Sn, Ag and Cu elements respectively.According to figure 1(b), it was found that there was enrichment of In and Ag in the location of the lamellar particles, but the main components of the ellipsoidal particles could not be analyzed through figure 1(c-g).Therefore, point scanning was performed on A and B to measure the proportion of each element, as shown in figure 1(h).It can be seen that there is enrichment of In and Ag in place A, that is, at the location of the lamellar particles, which is consistent with the EDS results in figure 1(c-g).There is Cu and Ga enrichment at B, that is, the location of the ellipsoidal particle.It is suggested that the ellipsoidal particle may be a compound formed by the reaction of Cu and Ga elements.Silver-coated copper powder is a composite powder made by plating silver on the surface of copper powder.Therefore, Ag on the surface of silver-coated copper powder reacts with In in GaInSn liquid metal in the early stage after silver-coated copper powder is added to GaInSn liquid metal to form AgIn2 intermetallic compound, i.e., lamellar particle A in figure 1(b).With the gradual depletion of Ag coating on the surface of silver-coated copper powder, Cu gradually emerged and reacted with Ga in GaInSn liquid metal to form CuGa2 intermetallic compounds.

Analysis of thermal conductivity of silver-coated copper/GaInSn composites
Thermal conductivity is a key parameter when a material is used as a thermal interface material.After understanding the chemical composition of silver-coated copper/GaInSn composites, we tested the thermal diffusion coefficients of silver-coated copper/GaInSn composites with different volume fractions of silver-coated copper powder, which are listed in Table 1. Figure 3 shows the relationship between the thermal conductivity of silver-coated copper/GaInSn composite material and the volume fraction of silver-coated copper powder.The dashed line is the thermal conductivity of Ga67In20.5Sn12.5 liquid metal (13 Wm -1 K -1 ).It can be seen that the thermal conductivity of silver-coated copper/GaInSn composite is higher than that of Ga67In20.5Sn12.5 liquid metal, and increases with the increase of the volume fraction of silver-coated copper.However, when the volume fraction of silver-coated copper is greater than or equal to 20%, the composite is ash like, leading to an increase in contact thermal resistance, which has been unable to be used as a thermal interface material.Even so, when the volume fraction of silver-coated copper powder is 15% in paste form, the thermal conductivity of the composite material is already 1.9 times that of Ga67In20.5Sn12.5 liquid metal, greatly improving thermal conductivity while reducing the risk of liquid metal overflow.Table 1.Thermal properties of silver-coated copper/GaInSn composites with different volume fractions.

Volume fraction
Density / (g• cm -3 ) Thermal conductivity / (Wm -1 K -1 ) 5% In order to better compare the heat dissipation capacity of silver-coated copper/GaInSn composite material with that of commercially available thermal grease, they were respectively put into the chip pseudo-real temperature measurement system shown in figure 4 to simulate the actual application scenario and observe the temperature change of the chip.The chip pseudo-real temperature measurement system in figure 4 is composed of the chip and the copper plate below to simulate the heat source and heat sink in the actual scene.The copper plate can be cooled by water to adjust the temperature, and the chip temperature is provided by the power supply.Additionally, an infrared thermal imager can measure the temperature of the chip surface and display it on the monitor.The thermal conductivity of each thermal interface material is listed in table 2 for comparison, where No.1 is the blank control group of materials without thermal interface.To be close to the actual use state of TIMs, we controlled the thickness of the silver-coated copper/GaInSn TIM to 0.2 mm through screen printing.Figure 5 (a) shows the temperature change of the chip surface when different thermal interface materials fill the contact interface between the chip and the copper plate at the input power of 22W, and (b) shows the corresponding infrared thermal phase diagram.The temperature of chips 1-4 increases with the increase of time, and the temperature of chips with thermal interface materials is lower than that without interface materials.Although the nominal thermal conductivity of the two commercially available thermal grease is very different, the actual chip temperature is similar.The heat dissipation capacity of the silver-coated copper/GaInSn composite material prepared in this research is obviously better than that of the two commercial thermal grease, and the chip temperature can be reduced by about 5℃.Therefore, in some situations with high heat transfer requirements, liquid metal thermal paste has a good application prospect.

Conclusion
By adding silver-coated copper powder to Ga67In20.5Sn12.5 liquid metal to reduce its fluidity, liquid metal thermal paste was successfully prepared.The thermal paste is composed of liquid phase and CuGa2, AgIn2 intermetallic compound generated.Compared with Ga67In20.5Sn12.5 liquid metal, the thermal conductivity has been significantly improved.When the volume content of silver-coated copper powder is 15%, the thermal conductivity is 25.01 W/m• K, which is equivalent to 1.9 times of Ga67In20.5Sn12.5 liquid metal.The copper/GaInSn composite and the commercial thermal grease were put into the chip and the copper plate to simulate the actual application scenario.The heat dissipation capacity of the silver-coated copper/GaInSn composite prepared in this research is obviously better than that of the two commercial thermal grease, and the chip temperature can be reduced by about 5℃.

Figure 1 .
Figure 1.(a) SEM photograph of silver-coated copper powder; (b) SEM photograph of silver-coated copper/GaInSn composites; (c-g) EDS distribution of Ga, In, Sn, Ag and Cu corresponding to Figure 1(b); (h) The proportion of elements scanned by points A and B in Figure 1(b).

Figure 2 .
Figure 2. XRD of silver-coated copper/GaInSn composite.Silver-coated copper powder is a composite powder made by plating silver on the surface of copper powder.Therefore, Ag on the surface of silver-coated copper powder reacts with In in GaInSn liquid metal in the early stage after silver-coated copper powder is added to GaInSn liquid metal to form AgIn2 intermetallic compound, i.e., lamellar particle A in figure1(b).With the gradual depletion of Ag coating on the surface of silver-coated copper powder, Cu gradually emerged and reacted with Ga in GaInSn liquid metal to form CuGa2 intermetallic compounds.

Figure 3 .
Figure 3. Relation between thermal conductivity of silver-coated copper/GaInSn composites and the volume fraction of silver-coated copper powder.

Figure 4 .
Figure 4. Schematic diagram of chip pseudo-real temperature measurement system.

Figure 5 .
Figure 5. (a) Infrared thermal phase diagram and (b)temperature change of each TIM.

Table 2 .
Thermal conductivity of each thermal interface materials.