High-performance Thermal Interface Materials With Diphase Continuous Structure Reinforced By Liquid Metal

With the trend towards miniaturization and high performance of microelectronic devices,how to effectively reduce the thermal resistance between chips and heat dissipation units has become more and more important.Especially in the thermal management of some high-power electronic products,the selection of thermal interface materials and supporting assembly processes have become one of the most critical technologies.Gallium-based liquid metal has the characteristics of good fluidity,stable chemical properties,high thermal conductivity,non-toxic and harmless,which make it an excellent alternative for thermal interface materials.However,gallium-based liquid metal is facing the following problems that need to be solved urgently:(1)liquid metal may be extruded out from the gap between the chip and heat sink,causing the short-circuit of electronic components;(2)the surface tension of gallium and gallium alloy is as high as 0.5-0.72 N/m,which make it have poor wettability with substrates;(3)the annual output of gallium is less than 300 tons,so the use of gallium needs to be minimized.In this study,a high-performance thermal interface material with diphase continuous structure reinforced by liquid metal is designed.The strengthening fillers of diamond particles are connected by gallium to construct a three-dimensional(3D)skeleton leading to significantly enhanced thermal conductivity,and then silicone resin is infiltrated into the continuous holes of skeleton to form gap pad materials.By analyzing the microstructure of thermal interface materials,it is demonstrated that the formation of continuous holes in 3D skeleton originates from the self-gap of the diamond particles and the bridging effect during powder compaction.It is also confirmed that the infiltration and filling process of liquid polymers in porous media should be attribute to the combined effects of capillary induced force and resistances such as liquid gravity,frictional resistance and non-linear forces caused by turbulence.To improve the interface combination status between inorganic fillers and liquid metal,chromium transition layer is prepared on the surfaces of diamond particles by magnetron sputtering method,obtaining a multilayer interface structure of "C-CrxCy-Cr".The detection of CrxCy transition layer confirms that this strong carbide forming element could react with the diamond substrate at the beginning of deposition,which can improve the phonon mismatch between heterogeneous materials,avoiding serious carrier scattering and thermal conductivity degradation.Liquid metal gallium has good wettability on chromium coating,and the coating is also an excellent diffusion barrier layer,which consumes relatively slowly with liquid metal at 120?.Thermal conductivity of gallium-based composite materials filled with chromium-coated diamond particles is measured by laser flash analysis using a three-layer structure sample.The interfacial thermal conductivity between diamond and liquid metal is calculated to be 15×106 W/(m2·K)using the differential effective medium(DEM)model.The effects of preparation parameters,such as power-compacting stress and the ratio of liquid metal to diamond,on the thermal conductivity of thermal interface materials are discussed.The ratio of liquid metal to diamond mainly affects the amount of liquid metal coating on the surfaces of diamond particles,while the power-compacting stress mainly affects the average surface space of diamond particles and the continuous holes in 3D skeleton formed by the bridging effect.Both of them control the range of contact points and contact area between diamond particles,which is closely related to the heat-transfer capability.The maximum thermal conductivity of the thermal interface materials could reach 29 W/(m·K)for proper preparation parameters.Otherwise,the thermal conductivity of this composite is more sensitive to the change of heat-transfer rate through liquid metal and the interfacial thermal resistance between liquid metal and diamond particles,compared with thermal conductivity of diamond particles.The effects of preparation parameters,such as powder-compacting stress,the ratio of diamond to liquid metal,and polymer matrix strength,on the compression properties of thermal interface materials are studied.When a relatively low powder-compacting stress is adopted,the 3D skeleton composed of diamond particles and liquid metal is loose and unstable.Conversely when a higher pressure is applied,the skeleton structure will be denser and more stable,causing strong frictional force and mechanical interaction between diamond particles,and therefore the compression deformation of composite material becomes very difficult.In addition,the compression behavior of thermal interface materials will be seriously affected by the strength of the silicone rubber,seeing that the displacement of diamond particles is mainly limited by the polymer matrix around them.Besides,since the self-gap size of diamond stacking structure is much smaller than the holes formed by the bridging effect,the self-gap will be preferentially filled by the liquid metal as the proportion of liquid metal to diamond gradually increases,and meanwhile the number and distribution of the large holes formed by the bridging effect will not change significantly.Thus,the compression deformation behavior of composite is almost not affected by the ratio of diamond to liquid metal to diamond.The total thermal resistance,thermal conductivity and interface contact thermal resistance of a selected composite are measured using the method of steady-state heat flow close to the actual application condition.The thermal conductivity is 20.4 W/(m·K)and the interface contact thermal resistance is only 0.206 K·mm2/W,which is better than most of research achievements reported.Moreover,when the thermal interface materials are applied on metal substrates,liquid metal will exude from the surfaces of this composites under compacting pressure,causing a wetting reaction with the clean metal substrates to form an interface combination similar to soldering at normal temperature,thereby obtaining extremely low interface contact thermal resistance.A new approach is developed to fabricate single-crystal Sn solder joints with a line-type structure.The primary purpose is to investigate the diffusion characteristics of Ni in single-crystal Sn with four different grain orientations during electromigration.An interesting new experimental phenomenon that Ni3Sn4 forms on the surfaces of single-crystal Sn with regularity occurs,which should be attributed to the extremely anisotropic diffusion property of Ni in single-crystal Sn.Besides,the diffusion velocity of Ni in single-crystal Sn during electromigration is ranked as followed:(001)>(101)>(301)>(100).Experimental observations are in good agreement with kinetic analysis.The results can be used to clarify the relationship between electromigration-induced failure modes and Sn grain orientation,explain why the preferential accumulation of intermetallic compounds often appears at certain locations of the anode interface,and illuminate the phenomenon that intermetallic compounds selectively form in specific grain boundaries or on certain grain surfaces.