Heat dissipation structure

Abstract

This heat dissipation structure uses liquid metal for a heat-dissipating material for causing heat to move from a heat-generating surface of a heat generator to a heat-absorbing surface of a heat dissipator, wherein: in each of the heat-generating surface and the heat-absorbing surface facing one another across a gap, a second region is provided so as to surround the peripheral margin of a first region, and the first regions are made to face one another; the wettability with respect to the liquid metal is relatively higher in the first regions than in the second regions; and the heat-dissipating material is packed in so as to fill up an accommodation space between the first regions facing one another across the gap. The heat-dissipating material deforms reversibly between the accommodation space and a space between the second regions outside the accommodation space.

Description

Heat dissipation structure

[0001] The present invention relates to a heat dissipation structure that uses a liquid metal as a thermally conductive material for transferring heat from a heat generating surface of a heat generating body to a heat absorbing surface of a heat dissipating body.

[0002] In recent years, heat dissipation structures using thermal interface materials (TIMs) made of liquid metal have been proposed for power semiconductor elements such as MOSFETs (Metal-Oxide-Semiconductor Field Effect Transistors). Compared to conventional silicone greases and gels, liquid metals are more fluid and can absorb thermal stress in elements, particularly in semiconductor elements that are thin and therefore have poor mechanical strength. Furthermore, because liquid metals have high thermal conductivity, they can also suppress the occurrence of thermal stress in elements.

[0003] For example, Patent Document 1 discloses a heat dissipation structure that uses, as a thermally conductive material, Ga (gallium), a low-melting-point metal, or a metal material that is Ga-based with added In (indium), Sn (tin), etc. The thermally conductive material is a liquid metal at room temperature, or at least becomes a liquid metal and acquires fluidity when a semiconductor element (chip) is in operation.

[0004] Japanese Patent Application Laid-Open No. 2022-138526

[0005] In a heat dissipation structure, a thermally conductive material, including a liquid metal, is held in the gap between a semiconductor device and a heat dissipation material, such as a heat sink, by its surface tension. Here, the thermally conductive material may expand or contract due to heat, generating voids (air bubbles) inside the material, or may be pushed out of the gap, a phenomenon known as "pump-out," which reduces heat dissipation performance. It is desirable to prevent pump-out in thermally conductive materials and maintain high heat dissipation performance.

[0006] The present invention has been made in consideration of the above-mentioned circumstances, and one of its objects is to provide a heat dissipation structure that can maintain high heat dissipation performance in a heat dissipation structure that uses liquid metal as a thermally conductive material to transfer heat from the heat-generating surface of a heat-generating body to the heat-absorbing surface of a heat dissipating body.

[0007] The present inventors have considered that in order to improve the thermal contact stability between a liquid metal, which is a thermally conductive material (hereinafter also referred to as a heat dissipation material), and a heat-generating surface and a heat-absorbing surface, and to ensure high heat dissipation performance inherent to the liquid metal, it is necessary to control the deformation behavior of the liquid metal caused by deformation in the thickness direction, such as pump-out. As a result, the present inventors have come up with the idea of ​​making the liquid metal reversibly deform and restore in the thickness direction, that is, causing elastic deformation behavior.

[0008] In other words, the heat dissipation structure according to the present invention is a heat dissipation structure that uses liquid metal as a heat dissipation material to transfer heat from the heat generating surface of a heat generating element to the heat absorbing surface of a heat dissipating element, and on each of the heat generating surface and heat absorbing surface that face each other with a gap between them, a second region is provided to surround the periphery of a first region, making the first regions face each other, the wettability of the first region to the liquid metal is relatively higher than that of the second region, and the heat dissipation material is filled so as to fill the storage space between the first regions that face each other with the gap between them.

[0009] According to this feature, the liquid metal between the heat generating surface and the heat absorbing surface can be given elastic deformation behavior, and the high heat dissipation performance of the liquid metal can be well maintained.

[0010] In the above configuration, the heat dissipation material may be reversibly deformed between the accommodation space and the space between the second regions on the outside thereof. This feature prevents dissipation of the liquid metal due to deformation in the thickness direction, such as pumping out, and allows high heat dissipation performance to be maintained.

[0011] In the above configuration, the contact angles of the liquid metal with the first region and the second region are respectively set to θ １ and θ ２ When this is done, cosθ １ -cosθ ２ ≧0.19. According to this feature, elastic deformation behavior can be reliably imparted to the liquid metal between the heat-generating surface and the heat-absorbing surface, and the high heat dissipation performance of the liquid metal can be well maintained.

[0012] In the above configuration, the surface roughness of the first region and the second region may be 0.20 μm or more and 0.07 μm or less, respectively, in terms of Rq value. The second region may be provided with a surface coating, which may be composed of a deposit of ceramic powder. Furthermore, the surface coating may be provided to a thickness of 50 μm or less. According to this feature, elastic deformation behavior can be reliably imparted to the liquid metal between the heat-generating surface and the heat-absorbing surface, and the high heat dissipation performance of the liquid metal can be well maintained.

[0013] In the above configuration, the liquid metal may have a viscosity of 100,000 mPa·s or less at 25°C. The liquid metal may also be a Ga-based alloy or a composite material obtained by mixing particles for adjusting viscosity and improving thermal conductivity with a Ga-based alloy. This feature ensures that elastic deformation behavior can be imparted to liquid metal, which generally exhibits viscous deformation behavior, and the high heat dissipation performance of the liquid metal can be well maintained.

[0014] According to one embodiment of the present invention, a heat dissipation structure that can maintain high heat dissipation performance can be provided in a heat dissipation structure that uses liquid metal as a thermally conductive material that transfers heat from the heat-generating surface of a heat-generating body to the heat-absorbing surface of a heat dissipating body.

[0015] 1 is a side cross-sectional view of a heat dissipation structure according to one embodiment of the present invention; FIG. 2 is a top view of a heating element in the heat dissipation structure; FIG. 3 is a side cross-sectional view showing a state in which liquid metal has been applied to a heating element; FIG. 4 is a side cross-sectional view of a heat dissipation structure showing a state in which the liquid metal has overflowed until it contacts a non-wetted portion; FIG. 5 is a side cross-sectional view of another embodiment of a heat dissipation structure according to one embodiment of the present invention; FIG. 6 is a side cross-sectional view of another embodiment of a heat dissipation structure according to one embodiment of the present invention; FIG. 7 is a side cross-sectional view of another embodiment of a heat dissipation structure according to one embodiment of the present invention; FIG. 8 is an electron microscope photograph of aluminic powder, a raw material, in an aerosol deposition method; FIG. 9 is a microscope photograph of aluminum hydroxide powder, a raw material; FIG. 10 is an external photograph of a substrate (left) and after film formation (right); FIG. 11 is an electron microscope photograph of the surface of a film formed; FIG. 12 is a photograph showing liquid metal being dropped onto silicon for contact angle measurement; FIG. 13 is a photograph showing liquid metal being dropped onto copper for contact angle measurement; FIG. 14 is a photograph showing liquid metal being dropped onto a porous film for contact angle measurement; FIG. 15 is a photograph showing liquid metal being dropped onto glass for contact angle measurement; FIG. 16 is a photograph showing liquid metal being dropped onto stainless steel for contact angle measurement. 1 is a table listing the surface roughness of each material and the contact angle of liquid alloy for each pattern formed on the surface of silicon.

[0034] FIG. 1 is a side view showing a pump-out test using a compression testing machine.

[0035] FIG. 2 is a perspective view of a glass plate with a non-wetting portion formed thereon.

[0036] FIG. 3 is a graph showing the relationship between load and crosshead position in a repeated compression test conducted without forming a non-wetting portion.

[0037] FIG. 4 is a side cross-sectional view showing the change in heat dissipation structure in a repeated compression test conducted without forming a non-wetting portion.

[0038] FIG. 5 is a side cross-sectional view showing the change in heat dissipation structure in a repeated compression test conducted without forming a non-wetting portion.

[0039] FIG. 6 is a side cross-sectional view showing the change in heat dissipation structure in a repeated compression test conducted without forming a non-wetting portion.

[0039] FIG. 7 is a side view of a sample used to measure thermal resistance.

[0039] FIG. 8 is a graph listing calculated thermal resistance values.

[0039] FIG. 9 is a graph showing the temperature and thermal resistance of a sample with nothing sandwiched between the Si plates. 10 is a graph showing the temperature and thermal resistance of a sample in which a carbon sheet is sandwiched between Si plates.1 is a graph showing the temperature and thermal resistance of a sample in which liquid metal is sandwiched between Si plates. FIG. 2 is a graph showing the relationship between the interface between the Si plate and the liquid metal and the liquid metal at the thermal resistance of the liquid metal. FIG. 3 is a perspective view showing the positions of the non-wetting portions of the Si plate and the Cu plate. FIG. 4 is a perspective view of a heat dissipation test device. FIG. 5 is a side view showing the stacking order from the heating plate to the Cu plate. FIG. 6 is a graph showing the time and temperature in a heat dissipation test of thermal grease. FIG. 7 is a graph showing the time, temperature, and load in a heat dissipation test of liquid metal conducted without forming a non-wetting portion. FIG. 8 is a photograph of the appearance of the Cu plate after the test. FIG. 9 is a graph showing the time, temperature, and load in a heat dissipation test of liquid metal conducted with a non-wetting portion formed. FIG. 10 is a side view of a steady-state thermal resistance measuring device. FIG. 11 is a perspective view of a measuring rod with a non-wetting portion formed. FIG. 12 is a graph showing thermal resistance measured by a steady-state thermal resistance measuring device.

[0016] A heat dissipation structure 1 according to one embodiment of the present invention will be described with reference to FIGS.

[0017] FIG. 1 is a side cross-sectional view of a heat dissipation structure 1 according to one embodiment of the present invention. As shown in FIG. 1, the heat dissipation structure 1 includes a heat-generating element 2, such as a semiconductor element, a heat dissipation element 3, such as a heat sink, and a heat dissipation material 6 sandwiched between the heat-generating element 2 and the heat dissipation element 3. The semiconductor element refers to, for example, a semiconductor chip such as a CPU, GPU, or memory. The heat dissipation element 3 is a so-called vapor chamber. The heat-generating element 2 has a heat-generating surface 4, and the heat dissipation element 3 has a heat-absorbing surface 5. The heat-generating surface 4 of the heat-generating element 2 and the heat-absorbing surface 5 of the heat dissipation element 3 face each other with a gap d therebetween, and a heat dissipation material 6 containing a liquid metal is sandwiched and held between the heat-generating surface 4 and the heat-absorbing surface 5. In other words, the heat dissipation structure 1 has a structure in which heat is transferred from the heat-generating surface 4 of the heat-generating element 2 through the heat dissipation material 6 to the heat-absorbing surface 5 of the heat dissipation element 3, and then dissipated from the heat dissipation element 3 to the outside. Here, the heat dissipation material 6 includes a liquid metal, and includes both a material that is liquid at room temperature and a material that is solid at room temperature and becomes liquid when heated for heat dissipation. However, for simplicity of explanation, in the embodiment, the heat dissipation material 6 is always in a liquid state.

[0018] FIG. 2 is a top view of the heat-generating element 2 of the heat dissipation structure 1. Referring to FIGS. 1 and 2, the heat-generating surface 4 is provided with a centrally located wetted portion 4a and a non-wetted portion 4b surrounding the periphery of the wetted portion 4a. The wetted portion 4a is a region that has relatively higher wettability with the liquid metal, which is the heat-dissipating material 6, than the non-wetted portion 4b. In other words, the non-wetted portion 4b is a region that has lower wettability with the liquid metal than the wetted portion 4a. The non-wetted portion 4b is arranged to surround the periphery of the wetted portion 4a. In FIG. 2, the strip-shaped regions along each of the four outer edges of the approximately rectangular heat-generating surface 4 are designated as the non-wetted portion 4b, and the approximately rectangular central region excluding this region is designated as the wetted portion 4a.

[0019] Like the heat-generating surface 4, the heat-absorbing surface 5 also has a wet portion 5a and a non-wetting portion 5b surrounding the periphery of the wet portion 5a. The arrangement of the wet portion 5a and the non-wetting portion 5b when the heat-absorbing surface 5 is viewed in plan is the same as that of the top view of the heat-generating element 2 shown in FIG. 2. The wet portion 5a is a region that has relatively higher wettability with respect to the heat-dissipating material 6 than the non-wetting portion 5b. Although not shown, the wet portion 5a and the non-wetting portion 5b are arranged to face the wet portion 4a and the non-wetting portion 4b, respectively. In other words, the boundary between the wet portion 5a and the non-wetting portion 5b is arranged to face the boundary between the wet portion 4a and the non-wetting portion 4b. The heat-dissipating material 6 is filled in the gap (also called the storage space) between the mutually opposing wet portions 4a and 5a.

[0020] FIG. 3 is a side cross-sectional view showing the state in which the thermally conductive material 6 has been applied to the heat-generating element 2. To fabricate the thermally conductive structure 1, first, a wet portion 4a and a non-wet portion 4b are formed on the heat-generating surface 4 of the heat-generating element 2, and a wet portion 5a and a non-wet portion 5b are formed on the heat-absorbing surface 5 of the heat-dissipating element 3. The method for forming the wet portions 4a, 5a and the non-wet portions 4b, 5b will be described in detail later. Next, as shown in FIG. 3, the thermally conductive material 6 is applied to the wet portion 4a. At this time, the thermally conductive material 6 is arranged so as to contact the entire surface of the wet portion 4a, but not the non-wet portion 4b. In other words, an amount of the thermally conductive material 6 that will result in such an arrangement is applied to the wet portion 4a. In other words, the amount of the thermally conductive material 6 applied to the wet portion 4a is determined depending on the area of ​​the wet portion 4a.

[0021] Referring back to FIG. 1 , when the heat sink 3 is placed on the heat generating element 2 so as to sandwich the heat dissipating material 6, the heat dissipating material 6 is placed in contact with the entire wetted portion 5a of the heat absorbing surface 5, but not in contact with the non-wetted portion 5b. In this case, the volume of the heat dissipating material 6 is the product of the gap d between the heat generating surface 4 and the heat absorbing surface 5 and the area of ​​the wetted portion 4a (or wetted portion 5a). In other words, as described above, it is the volume that fills the gap d between the wetted portion 4a and the wetted portion 5a. Therefore, the amount of liquid metal initially applied to the heat generating element 2 is an amount corresponding to the product of the gap d and the area of ​​the wetted portion 4a (or wetted portion 5a). The gap d, which corresponds to the thickness of the heat dissipating material 6, is, for example, 10 μm to 300 μm, preferably 50 μm to 200 μm. The gap d may be large enough to absorb the deformation of the heat generating element 2 even if the heat generating element 2 deforms due to heat generation.

[0022] Generally, when a heat dissipation structure is formed by sandwiching a fluid heat dissipation material, such as grease or liquid, between a heat-generating surface and a heat-absorbing surface, the heat dissipation structure deforms and the volume of the heat dissipation material changes with temperature changes. Repeated temperature changes can cause pump-out, which is the extrusion of the heat dissipation material outside the area sandwiched between the heat-generating and heat-absorbing surfaces. The extruded heat dissipation material can deteriorate or short-circuit the semiconductor element. Furthermore, the extrusion of the heat dissipation material reduces the thickness of the heat dissipation material itself, preventing it from fully filling the gap between the heat-generating and heat-absorbing surfaces and resulting in voids. This can result in a decrease in the heat dissipation performance of the heat dissipation structure. In contrast, the heat dissipation structure 1 according to one embodiment of the present invention can suppress the occurrence of pump-out and voids by utilizing the "elastic deformation behavior" of the heat dissipation material 6, as described below, thereby maintaining high heat dissipation performance.

[0023] 4 is a side cross-sectional view of the heat dissipation structure 1 showing the heat dissipation material 6 protruding to the point of contacting the non-wetting portions 4b and 5b. For example, FIG. 4 shows a case where the heat dissipation material 6 protrudes from the wetted portions 4a and 5a and comes into contact with the non-wetting portions 4b and 5b due to the reduction of the gap d and the expansion of the heat dissipation material 6 caused by a dimensional change in the heat dissipation structure 1. As described above, the heat dissipation material 6 is filled to fill the gap between the wetted portions 4a and 5a. Therefore, even a slight deformation of the heat dissipation material 6 causes the heat dissipation material 6 to protrude outside the gap between the wetted portions 4a and 5a, and the heat dissipation material 6 to straddle the areas of the wetted portions 4a and 5a and the non-wetting portions 4b and 5b.

[0024] When the dimensions of the gap d and the volume of the heat-dissipating material 6 are restored, the heat-dissipating material 6 returns from the region of the non-wetting portions 4b and non-wetting portions 5b to the region of the wetting portions 4a and wetting portions 5a. In other words, the heat-dissipating material 6 reversibly deforms between the gap between the wetting portions 4a and wetting portions 5a and the gap outside it between the non-wetting portions 4b and non-wetting portions 5b. This is thought to be due to a force based on the difference in wettability and surface tension that directs the liquid metal from the region of the non-wetting portions 4b and non-wetting portions 5b, which has low wettability, to the region of the wetting portions 4a and wetting portions 5a, which has high wettability.

[0025] Furthermore, this force is always generated as long as the thermally conductive material 6 straddles the boundary between the wetted portions 4a and 5a and the non-wetted portions 4b and 5b. Therefore, regardless of how the dimension of the gap d changes, if the thermally conductive material 6 is present outside the gap between the wetted portions 4a and 5a, a force is generated that tries to return the thermally conductive material 6 to the gap between the wetted portions 4a and 5a. This allows the thermally conductive material 6 to exhibit elastic deformation behavior that allows the shape of the thermally conductive material 6 to follow the dimensional changes of the gap d. For example, even if the length of the gap d temporarily decreases and the thermally conductive material 6 becomes thin and spread, when the length of the gap d returns to its original size, the thermally conductive material 6 deforms to follow this and fills the gap d between the wetted portions 4a and 5a. This elastic deformation behavior of the thermally conductive material 6 can be reproduced with good reproducibility even when used repeatedly. Therefore, the generation of voids in the thermally conductive material 6 is suppressed even with repeated use, and the thermally conductive material 6 can be prevented from separating from the heat-generating surface 4 or the heat-absorbing surface 5. This allows the high heat dissipation performance of the heat dissipation material 6 to be maintained well.

[0026] Regarding the arrangement of the wetting portions 4a and 5a and the non-wetting portions 4b and 5b, from the viewpoint of reducing thermal resistance, it is preferable to increase the area of ​​the non-wetting portions 4b and 5b, since this allows for a wider arrangement of the heat-dissipating material 6. For example, in such an arrangement, the total area of ​​the non-wetting portions 4b and 5b is preferably 30% or less of the total area of ​​the heat-generating surface 4 and the heat-absorbing surface 5. However, if the heat-dissipating material 6 spreads beyond the areas of the non-wetting portions 4b and 5b, the heat-dissipating material 6 will leak from the heat-generating body and the heat-dissipating body. Therefore, it is preferable to determine the areas in which the non-wetting portions 4b and 5b are provided so that the wetting portions 4a and 5a are wide enough to accommodate changes in the volume of the heat-dissipating material 6 and changes in the size of the gap d.

[0027] It is preferable to cause the thermal dissipation material 6 to exhibit the above-described elastic deformation behavior at room temperature. Therefore, it is preferable to use a liquid metal that is liquid at room temperature as the thermal dissipation material 6. Furthermore, it is preferable that the liquid metal have a viscosity of 100,000 mPa·s or less at 25°C. Specifically, the viscosity of the liquid metal is preferably 2 mPa·s or more and 350 mPa·s or less at 25°C. As such a liquid metal, for example, one or more low-melting-point metals selected from the group consisting of Ga (melting point: 29.8°C, thermal conductivity: 40.6 W / mK), In (melting point: 156.4°C, thermal conductivity: 81.6 W / mK), and Sn (melting point: 231.97°C, thermal conductivity: 66.6 W / mK), or an alloy containing one or more low-melting-point metals selected from Ga, In, and Sn, can be used. Specific examples of alloys include Ga—In, Ga—In—Sn, In—Ag, Sn—Ag—Cu, and In—Sn—Bi.

[0028] In addition to the low-melting-point metals and alloys containing low-melting-point metals described above, the liquid metal can also be mixed with fine particles made of high-thermal conductivity materials such as copper, silver, gold, tungsten, and diamond as particles for adjusting viscosity and improving thermal conductivity, resulting in a composite material with adjusted viscosity and improved thermal conductivity. The fine particle content is preferably 60% by volume or less, or 30% by volume or less, because if it exceeds 60% by volume, the viscosity increases and the liquid metal will not return to its original shape. The particle size of the fine particles may be approximately several tens of micrometers.

[0029] The thermal conductivity of liquid metal is 15 W / mK to 73 W / mK, and for example, the thermal conductivity of Ga-In-Sn liquid metal is 15 W / mK to 28 W / mK. The thermal conductivity of grease is about 10 W / mK, and that of graphite sheet is about 40 W / mK.

[0030] As described above, the heat-generating surface 4 of the heat-generating element 2 is formed with wetted portions 4a and non-wetted portions 4b, and the heat-absorbing surface 5 of the heat-dissipating element 3 is formed with wetted portions 5a and non-wetted portions 5b. Therefore, it is preferable to use a material for the heat-generating surface 4 of the heat-generating element 2 and the heat-absorbing surface 5 of the heat-dissipating element 3 that can control the wettability with respect to the heat-dissipating material 6 or that can be provided with a member such as a film with controlled wettability, and that has sufficient thermal conductivity as a thermally conductive member for the heat-dissipating structure 1. Examples of materials that can be used for the heat-generating surface 4 of the heat-generating element 2 and the heat-absorbing surface 5 of the heat-dissipating element 3 include copper, nickel, stainless steel, silicon, aluminum, alumina, gold, platinum, titanium, titanium oxide, carbon-based materials, mixtures thereof, and glass. Heat-resistant ceramics in which a metal is mixed with aluminum or titanium oxide to improve thermal conductivity may also be used.

[0031] Examples of combinations of materials for the heat generating surface 4 of the heat generating element 2 and the heat absorbing surface 5 of the heat dissipating element 3 used in a CPU or the like include Si-Cu, Cu-Cu, Si-Al, Al-Al, Al-Cu, Cu-Al, etc. Examples of combinations of the base material of the heat generating element 2 and the base material of the heat dissipating element 3 used in an in-vehicle battery or the like include SUS-Cu, SUS-Al, etc.

[0032] Furthermore, a coating layer may be formed on at least one of the wetted portions 4a, 5a and the non-wetted portions 4b, 5b of the materials listed for the heat generating element 2 and the heat dissipating element 3. The coating layer may be formed of an oxide material (SiO ２ , Al ２ O ３ , TiO ２), polymer materials (e.g., polytetrafluoroethylene (PTFE), polypropylene, polyethylene, natural rubber, polyurethane rubber, acrylonitrile-butadiene rubber, silicone rubber, etc.), metal materials such as Ni, Pt, Au, etc., or mixtures thereof may be used. For example, the non-wetted portions 4b and 5b may be a mixed layer of the above-mentioned oxide material and polymer material, while the wetted portions 4a and 5a may be a mixed layer of the above-mentioned metal material (for improving thermal conductivity and wettability) and oxide material and polymer material (for corrosion prevention). It is preferable to use the above-mentioned material that suppresses corrosion by liquid metal as the coating layer. Even when materials that are easily corroded by liquid metal (e.g., aluminum) are used for the heating element 2 and the heat sink 3, corrosion of the heating element 2 and the heat sink 3 can be suppressed by providing the above-mentioned coating layer. When aluminum is used for at least one of the heating element 2 and the heat sink 3, it is preferable to provide a coating layer not only on the non-wetted portions 4b and 5b but also on the wetted portions 4a and 5a. The thickness of the coating layer is preferably 0.2 μm to 50 μm or less so as not to increase the thermal resistance of the heat dissipation structure 1. The thickness of the coating layer in the wetting portions 4a, 5a is preferably 0.2 μm to 5 μm, both inclusive, because the thickness affects the thermal resistance of the heat dissipation structure 1. The thickness of the coating layer in the non-wetting portions 4b, 5b may be the same as or greater than the thickness of the wetting portions 4a, 5a.

[0033] As described above, the wettability of the wetting portion 4a and the wetting portion 5a and the non-wetting portion 4b and the non-wetting portion 5b with respect to the heat-dissipating material 6 differs. Such a difference in wettability can be expressed using the contact angle with the heat-dissipating material 6. When the wetting portion 4a and the wetting portion 5a are made of the same material and the non-wetting portion 4b and the non-wetting portion 5b are made of the same material, the contact angle between the heat-dissipating material 6 and the wetting portion 4a (or the wetting portion 5a) is expressed as θ １ , the contact angle between the heat dissipation material 6 and the non-wetting portion 4b (or the non-wetting portion 5b) is θ ２ When the angle θ is 1 / 2, it is preferable that the following formula (1) is satisfied: １ -cosθ ２≧0.19 (1) Equation (1) is the contact angle θ １ and θ ２ In other words, it means that the difference in wettability between the wetted portion 4a and the non-wetted portion 4b is made large.

[0034] Furthermore, even when the wet portion 4a and the wet portion 5a are made of different materials and the non-wet portion 4b and the non-wet portion 5b are made of the same material, when the wet portion 4a and the wet portion 5a are made of the same material and the non-wet portion 4b and the non-wet portion 5b are made of different materials, or when the wet portion 4a and the wet portion 5a are made of different materials and the non-wet portion 4b and the non-wet portion 5b are made of different materials, it is sufficient that there is a difference in wettability with the heat-dissipating material 6. In the heat-generating body 2, it is sufficient that the contact angle of the wet portion 4a and the contact angle of the non-wet portion 4b satisfy formula (1). In the heat-dissipating body 3, the contact angle between the heat-dissipating material 6 and the wet portion 5a is θ ３ , the contact angle between the heat dissipation material 6 and the non-wetting portion 5b is θ ４ When the above equation is satisfied, the following equation (2) should be satisfied: ３ -cosθ ４ ≧0.19 (2)

[0035] The difference between the wettability of the wetting portion and the wettability of the non-wetting portion is relative. Therefore, in the heat-generating surface 4, the wettability of the wetting portion 4a may be improved, or the wettability of the non-wetting portion 4b may be reduced, or both may be achieved. Similarly, in the heat-absorbing surface 5, the wettability of the wetting portion 5a may be improved, or the wettability of the non-wetting portion 5b may be reduced, or both may be achieved. The improvement in the wettability of the wetting portion 4a and the wetting portion 5a and the reduction in the wettability of the non-wetting portion 4b and the non-wetting portion 5b can be controlled, for example, by the surface roughness of the heat-generating surface 4 and the heat-absorbing surface 5. The surface roughness of the wetted portions 4 a, 5 a is 0.01 μm or more and 4 μm or less, preferably 0.01 μm or more and 0.07 μm or less, in terms of Rq value (root mean square height, JIS B0601:2013), and the surface roughness of the non-wetted portions 4 b, 5 b is 0.10 μm or more and 40 μm or less, preferably 0.20 μm or more and 4 μm or less, in terms of Rq value.

[0036] When the contact angle of the heat dissipating material 6 on a flat surface is greater than 90°, the material is less likely to be wetted by the heat dissipating material 6, and the greater the surface roughness, the less likely the heat dissipating material 6 is to wet the material. Here, a flat surface refers to a surface roughness of about 0.1 μm in terms of Ra value (arithmetic mean roughness, JIS B0601:2013). Examples of materials that are less likely to be wetted by the heat dissipating material 6 include substrates such as copper, stainless steel, glass, and silicon, oxide materials, polymer materials, and metal materials used in the coating layer described above, and porous materials described below. When the wetting portions 4a, 5a and the non-wetting portions 4b, 5b are made of materials that are less likely to be wetted, the surface roughness of the wetting portions 4a, 5a is preferably smaller than the surface roughness of the non-wetting portions 4b, 5b. In this case, although it depends on the relationship between the material and viscosity of the heat dissipation material 6, it is preferable that the surface roughness of the non-wetting portions 4 b and 5 b is 0.20 μm or more in terms of Rq value (root mean square height, JIS B0601:2013), and the surface roughness of the wetting portions 4 a and 5 a is 0.07 μm or less in terms of Rq value.

[0037] To achieve such a relationship between the wetting portion 4a and the non-wetting portion 4b, and between the wetting portion 5a and the non-wetting portion 5b, the heat-generating surface and the heat-absorbing surface may be processed to achieve the surface roughness described above. To increase the surface roughness of the non-wetting portion 4b or the non-wetting portion 5b, for example, a film with controlled surface roughness is preferably formed on the heat-generating surface 4 or the heat-absorbing surface 5. For example, a porous film with dispersed through-holes having an inner diameter of 10 nm or less can be used as the film (also called a coating layer) formed on the non-wetting portion 4b or the non-wetting portion 5b, so that the porosity is 5% or more. Dispersing the through-holes can create a lotus effect through the numerous openings on the surface of the film, thereby reducing wettability. The wettability of the heat-dissipating material 6 can also be controlled by changing the porosity or the inner diameter of the through-holes. Furthermore, it is preferable to set the thickness of the porous film to 0.2 μm to 50 μm or less to avoid increasing the thermal resistance of the heat-dissipating structure 1.

[0038] The porous film described above can be formed by, for example, the aerosol deposition method (AD method). In the aerosol deposition method, ceramic particles accelerated by a carrier gas are bombarded at high speed onto a substrate placed in a vacuum to form a porous film on the substrate. As described above, the porosity and the like can be controlled by the particle size of the raw ceramic particles, the bombardment speed, and the like. A porous film formed by the AD method is called a porous AD film. Alumina (Al ２ O ３ ), aluminum hydroxide (Al(OH) ３ ), zirconia (ZrO ２ ), aluminum nitride (AlN), silicon carbide (SiC), silicon nitride (Si ３ N ４ The particle size of the ceramic particles is preferably 0.2 μm or more and 1 μm or less.

[0039] The surfaces of the non-wetting portions 4b and 5b may be treated by a method for increasing the surface roughness of the non-wetting portions 4b and 5b. The regions of the heat-generating and heat-absorbing surfaces where the non-wetting portions 4b and 5b are to be formed may be patterned by laser patterning, nanoimprinting patterning, plasma etching, chemical etching, or the like. For example, linear convex, cylindrical, or circular hole-shaped patterns may be formed on the heat-generating and heat-absorbing surfaces where the non-wetting portions 4b and 5b are to be formed. When linear convex patterns are formed, they preferably have a width of 1 μm to 50 μm, and the pattern spacing is preferably 1 μm to 50 μm. When cylindrical patterns are formed, they preferably have a diameter of 1 μm to 50 μm, and the pattern spacing is preferably 1 μm to 50 μm. Furthermore, when forming a circular hole pattern, it is preferable that the hole has an inner diameter of 1 μm to 50 μm, and the pattern spacing is preferably 1 μm to 50 μm. Furthermore, the regions of the heat-generating and heat-absorbing surfaces where the non-wetting portions 4b and 5b are to be formed may be roughened by sandblasting or scraping the surface with a file, or by attaching sandpaper. This allows the surface roughness of the non-wetting portions 4b and 5b to be 0.20 μm or more in terms of Rq value. This allows the wettability of the non-wetting portions 4b and 5b to be lower than that of the wettability of the wetting portions 4a and 5a.

[0040] Alternatively, instead of increasing the surface roughness of the non-wetting portion 4 b or the non-wetting portion 5 b, a method of chemically reducing wettability may be used. For example, a fluorine-based functional group with low wettability to liquid metal may be added to the region where the non-wetting portion 4 b or the non-wetting portion 5 b is to be formed, or a hydroxyl group may be introduced using oxygen plasma. This increases the difference between the contact angle of the non-wetting portion 4 b with the heat dissipation material and the contact angle of the non-wetting portion 5 b with the heat dissipation material.

[0041] On the other hand, methods for improving the wettability of the wetting portions 4a and 5a may include adding silane-based functional groups to the wetting portions 4a and 5a or smoothing the surfaces by polishing. Alternatively, a coating layer made of a substance highly wettable with the heat dissipation material 6 may be provided on the wetting portions 4a and 5a. When a Ga-based alloy is used for the heat dissipation material 6, Ga-based materials capable of forming alloys with Ga, such as Ni, gallium, gallium oxide, and gallium nitride, as well as diamond, DLC, graphene, and gold, may be used as the material for the coating layer. Platinum, which is resistant to corrosion by the heat dissipation material 6, may also be used. When a coating layer is provided on the wetting portions 4a and 5a, the thickness of the coating layer is preferably 5 μm or less, so as not to increase the thermal resistance of the heat dissipation structure 1. The thickness of the wetting portions 4a and 5a is preferably the same as the thickness of the non-wetting portions 4b and 5b, or the thickness of the wetting portions 4a and 5a is preferably smaller than the thickness of the non-wetting portions 4b and 5b.

[0042] Depending on the material used as the thermally conductive material 6, the contact angle of the thermally conductive material 6 on a flat surface may be 90° or less. In this case, the greater the surface roughness, the more easily the thermally conductive material 6 wets. Therefore, when a material that causes a contact angle of the thermally conductive material 6 of 90° or less is used in the wetting portions 4a and 5a and a material that causes a contact angle of the thermally conductive material 6 of more than 90° is used in the non-wetting portions 4b and 5b, the surface roughness of the wetting portions 4a and 5a may be greater than the surface roughness of the non-wetting portions 4b and 5b.

[0043] For example, semiconductor elements used in data servers and the like generate a large amount of heat during use. To efficiently transfer heat from the semiconductor element to the heat sink, it is preferable for the heat dissipation material provided between the semiconductor element and the heat sink to have high thermal conductivity, and it is also preferable for the thermal resistance at the interface between the semiconductor element and the heat dissipation material and at the interface between the heat sink and the heat dissipation material to be low. Conventionally used grease has a lower thermal conductivity than liquid metal. Carbon sheets have a higher thermal conductivity than grease. However, due to the roughness of the carbon sheet's surface, air pockets are formed at the interface with the semiconductor element and the interface with the heat sink. This increases the contact resistance at the interface, resulting in an increase in thermal resistivity.

[0044] When a liquid metal is used as the heat dissipation material 6, by increasing the wettability of the region of the heat-generating surface 4 where the heat dissipation material 6 is provided and the region of the heat-absorbing surface 5 where the heat dissipation material 6 is provided, it is possible to increase the contact area between the heat-generating surface 4 and the heat dissipation material 6 and the heat-absorbing surface 5. This makes it possible to reduce the thermal resistance at the interface between the heat-generating surface 4 and the interface between the heat-dissipating material 6 and the heat-absorbing surface 5. Therefore, it is possible to improve the heat dissipation performance of the heat dissipation structure 1.

[0045] In this embodiment, a case has been described in which a treatment or film formation is performed to increase the wettability of the wetting portions 4 a, 5 a and a treatment or film formation is performed to decrease the wettability of the non-wetting portions 4 b, 5 b on the heat-generating surface 4 and the heat-absorbing surface 5, but one embodiment of the present invention is not limited to this. As long as the formulas (1) and (2) that represent the difference in contact angle with the heat-dissipating material 6 are satisfied, only the treatment or film formation to increase the wettability of the wetting portions 4 a, 5 a may be performed, or only the treatment or film formation to decrease the wettability of the non-wetting portions 4 b, 5 b may be performed.

[0046] Even when the non-wetting portions 4b, 5b are formed with controlled surface roughness, the surface roughness may change over time if they are corroded by the heat dissipation material 6. This increases the wettability of the non-wetting portions 4b, 5b, making it impossible for the heat dissipation material 6 that has spread to the non-wetting portions 4b, 5b to return to the wetting portions 4a, 5a, which may result in pump-out or voids. By coating at least the non-wetting portions 4b, 5b with a material that is resistant to corrosion by the heat dissipation material 6, it is possible to prevent the surface roughness of the non-wetting portions 4b, 5b from changing over time. This prevents pump-out and voids from occurring in the heat dissipation structure 1, improving reliability.

[0047] Specifically, the non-wetting portion 4b is made of SiO ２ , Al ２ O ３ , and TiO ２ oxide materials including polytetrafluoroethylene (PTFE), polypropylene, polyethylene, natural rubber, polyurethane rubber, acrylonitrile butadiene rubber, and silicone rubber; metal materials including Ni, Pt, and Au; and Al ２ O ３ , Al(OH) ３ , ZrO ２ , AlN, SiC, Si ３ N ４ Alternatively, a coating layer made of a material selected from the group consisting of porous materials including the following may be provided: The wet portion 4a may also be provided with a coating layer made of the same material as the non-wet portion 4b. When the wet portion 4a and the non-wet portion 4b are provided with coating layers made of the same material, the contact angles of the wet portion 4a and the non-wet portion 4b may be controlled by performing a process to control the surface roughness of at least one of the wet portion 4a and the non-wet portion 4b so that the wet portion 4a and the non-wet portion 4b satisfy formula (1) or formula (2).

[0048] The non-wetted portion 5b also contains SiO ２ , Al ２ O ３ , and TiO ２oxide materials including polytetrafluoroethylene (PTFE), polypropylene, polyethylene, natural rubber, polyurethane rubber, acrylonitrile butadiene rubber, and silicone rubber; metal materials including Ni, Pt, and Au; and Al ２ O ３ , Al(OH) ３ , ZrO ２ , AlN, SiC, Si ３ N ４ Alternatively, a coating layer made of a material selected from the group consisting of porous materials including the following may be provided: The wet portion 5a may also be provided with a coating layer made of the same material as the non-wet portion 5b. When the wet portion 5a and the non-wet portion 5b are provided with coating layers made of the same material, the contact angles of the wet portion 5a and the non-wet portion 5b may be controlled by performing a process to control the surface roughness of at least one of the wet portion 5a and the non-wet portion 5b so that the wet portion 5a and the non-wet portion 5b satisfy formula (1) or formula (2).

[0049] The non-wetting portion 4b and the non-wetting portion 5b are made of SiO ２ , Al ２ O ３ , and TiO ２ oxide materials including polytetrafluoroethylene (PTFE), polypropylene, polyethylene, natural rubber, polyurethane rubber, acrylonitrile butadiene rubber, and silicone rubber; metal materials including Ni, Pt, and Au; and Al ２ O ３ , Al(OH) ３ , ZrO ２ , AlN, SiC, Si ３ N ４Alternatively, a coating layer using a material selected from the group consisting of porous materials including: (a) a porous material selected from the group consisting of (b) a porous material selected from the group consisting of (c) a porous material selected from the group consisting of (d) a porous material selected from the group consisting of (e) a porous material selected from the group consisting of (f ...

[0050] The shapes of the heat generating element 2 and the heat dissipating element 3 may be other than flat as long as they do not impede the elastic deformation behavior of the heat dissipating material 6. Fig. 5 is a side cross-sectional view showing another embodiment of the heat dissipating structure. Fig. 5 shows a heat dissipating structure 10 in which a recess 14c and a recess 15c are provided in positions facing each other in the heat generating element 12 and the heat dissipating element 13, respectively.

[0051] Specifically, as shown in FIG. 5A , a recess 14c is formed on the heat-generating surface 14 of the heat-generating element 12, and a recess 15c is formed on the heat-absorbing surface 15 of the heat-dissipating element 13. The heat-dissipating material 6 is applied to the recess 14c of the heat-generating element 12, and the recess 14c and the recess 15c are placed opposite each other, thereby sandwiching the heat-dissipating material 6 between the recess 14c and the recess 15c. A wetted portion 14a is formed near the center of the recess 14c, and a non-wetted portion 14b is formed to surround the wetted portion 14a. The non-wetted portion 14b is provided over the entire heat-generating surface 14, excluding the wetted portion 14a, within the range that the heat-dissipating material 6 can come into contact with. For example, the non-wetted portion 14b includes the bottom surface of the recess 14c, the rising portion on the periphery of the recess 14c, and surfaces that can come into contact with the heat-absorbing surface 15. Similarly, a wetted portion 15a is formed near the center of the recess 15c, and a non-wetted portion 15b is formed to surround the wetted portion 15a. The non-wetting portion 15b is provided over the entire area of ​​the heat-absorbing surface 15, excluding the wetting portion 15a, within the range that can be contacted by the heat-dissipating material 6. For example, the non-wetting portion 15b also includes the rising portion on the periphery of the recess 15c and the surface that can be in contact with the heat-generating surface 14. At this time, the heat-dissipating material 6 is filled so as to fill the gap between the wetting portion 14a and the wetting portion 15a that face each other.

[0052] 5B , assume that the distance between the heat generating element 12 and the heat sink 13 is shortened due to deformation of the heat dissipation structure 10. At this time, the heat dissipation material 6 maintains its volume and spreads to contact the non-wetting portions 14b and 15b within the recesses 14c and 15c. The heat dissipation material 6 may be filled in an amount that fills the storage space formed by the recesses 14c of the heat generating element 12 and the recesses 15c of the heat sink 13 when the heat generating element 12 and the heat sink 13 come into contact. The gap d between the heat generating element 12 and the heat sink 13 may be 500 μm or less.

[0053] Even in this case, a force acts on the heat dissipation material 6, from the non-wetting portions 14b and 15b, to return to the wetting portions 14a and 15a. As shown in Figure 5A, when the deformation of the heat dissipation structure 10 returns to its original state, the heat dissipation material 6, due to its elastic deformation behavior, returns from the region where the non-wetting portions 14b and 15b face each other to the region where the wetting portions 14a and 15a face each other. It is preferable that the non-wetting portion 14b is formed on the bottom surface of the recess 14c so as to surround the wetting portion 14a, and that the non-wetting portion 14b is formed on the bottom surface of the recess 15c so as to surround the wetting portion 15a, but the non-wetting portion 14b and the non-wetting portion 15b do not have to be provided on the bottom surface of the recess 14c.

[0054] 1 to 4 regarding the materials used for the heat generating element 12 and the heat dissipating element 13 and the control of the contact angles on the heat generating surface 14 and the heat absorbing surface 15. According to the heat dissipation structure 10, when a heavy load is applied such that the shape of the liquid metal cannot be controlled even by controlling the wettability between the wetted and non-wetted portions in the heat dissipation structure 10, it is possible to prevent the liquid metal 16 from leaking out of the heat dissipation structure 10.

[0055] In this example, various tests will be described that were carried out on a heat dissipation structure 1 according to an embodiment of the present invention that was actually manufactured.

[0056] <Verification of Surface Roughness and Contact Angle> A non-wetted film was produced on a substrate by aerosol deposition. A stainless steel (SUS (registered trademark) 304 steel) substrate was used. In the aerosol deposition method, ceramic powder (a mixture of alumina particles and aluminum hydroxide particles) accelerated using dry air at a flow rate of 6 L / min under a vacuum of 400 Pa at room temperature (approximately 300 K) was released from a nozzle with an opening size of 4 mm and collided with a 40 mm x 40 mm substrate. The nozzle moved at a speed of 10 mm / sec along the substrate, spraying the ceramic particles onto the entire main surface of the substrate to obtain a film. In the following description, the obtained film is also referred to as a porous AD film. The surface roughness of the film was measured over a 1 cm length using a surface roughness measuring device (SURFTEST SV-3100, Mitutoyo Corp.), and the film was formed to have an Rq value of 0.26 μm, a thickness of 3 μm, and numerous through-holes with inner diameters of 10 nm or less. The porosity was measured using a mercury porosimeter (BELPORE, Microtrack), and the porosity of the film was 5%.

[0057] Figures 6A and 6B show electron microscope photographs of the ceramic powder used as raw material. In Figure 6C, a stainless steel substrate is shown on the left, and a film formed on the substrate is shown on the right. Figure 6D is an electron microscope photograph of a magnified portion of the film shown in Figure 6C. As shown in Figure 6D, it can be seen that the film formed on the substrate is porous.

[0058] Next, the results of measuring the surface roughness and contact angle for various substrates will be described. Silicon (single crystal), copper, and glass, as well as the porous AD film and stainless steel described in FIG. 6, were used as substrates. First, the surface roughness of the various substrates was measured. Next, Ga-In eutectic liquid metal, which is used as the heat dissipation material 6, was dropped onto each of the various substrates, and the contact angle was measured.

[0059] Figure 7A is a photograph showing the state of liquid metal being dropped onto silicon. Figure 7B is a photograph showing the state of liquid metal being dropped onto copper. Figure 7C is a photograph showing the state of liquid metal being dropped onto a porous AD film. Figure 7D is a photograph showing the state of liquid metal being dropped onto glass. Figure 7E is a photograph showing the state of liquid metal being dropped onto stainless steel. It was also confirmed that liquid metal did not enter the through-holes of the porous AD film.

[0060] Figure 8 shows a table listing the surface roughness of each material and the corresponding contact angle θ of the liquid alloy. As shown in Figure 8, the contact angle of the liquid metal on the porous AD film was 169°, which was larger than the contact angles of the other materials. It is believed that the lotus effect was achieved with the porous AD film.

[0061] For example, when glass is used as the heating element 2, a porous AD film is formed on a part of the heating surface 4 to form a non-wetted portion 4b, and the exposed glass portion is formed as a wetted portion 4a. The contact angle of glass is 133° and the contact angle of the porous AD film is 169°, so in the above-mentioned formula (1), θ １ = 133°, θ ２ = 169°. Therefore, cos θ on the left side of equation (1) １ -cosθ ２ The value of is −0.682 − (−0.982) = 0.300, which satisfies formula (1). Note that copper, stainless steel, glass, and silicon all have a surface roughness Rq value of 0.07 μm or less, and can be used as the wetted portion 4 a in the same way as glass.

[0062] Furthermore, as shown in FIG. 9 , the contact angle of liquid metal was measured on silicon surfaces on which various 10 μm linear, cylindrical, and circular hole patterns were formed. The liquid metal was the same Ga-In eutectic as above. Patterns were formed with varying spacing between the ends of the convex or concave portions of each shape: linear convex (10 μm width), cylindrical (10 μm diameter), and circular hole (10 μm inner diameter), and these were used to measure the contact angle. As shown in FIG. 9 , in the case of linear and circular hole patterns, the contact angle tended to increase as the spacing between the patterns increased. Furthermore, in the case of cylindrical patterns, a large contact angle of 150° or more was maintained regardless of the spacing between the patterns.

[0063] The results shown in FIG. 9 indicate that the contact angle with the liquid metal can be controlled depending on the pattern shape and the pattern spacing.

[0064] <Regarding Pump-Out> Next, the results of a pump-out test performed on the heat dissipation structure will be described.

[0065] FIG. 10 is a side view showing the pump-out test using a compression testing machine. Using a compression testing machine 20 (manufactured by A&D Co., Ltd., a single-column material testing machine; STB-1225L) as shown in FIG. 10, the pump-out situation was investigated by changing the distance between the glass plates sandwiching the liquid metal. The glass plates were 1 mm thick. 50 μL of Ga-In eutectic liquid metal was applied to a lower glass plate 24 placed on a lower compression plate 22. An upper glass plate 25 placed on the underside of an upper compression plate 23 was lowered with a crosshead 21 to compress the liquid metal. The position of the crosshead 21 when the lowered upper glass plate 25 contacted the liquid metal applied to the lower glass plate 24 was defined as the reference position. The direction of compression of the liquid metal from the reference position (downward) was defined as the positive direction, and the direction of moving the upper glass plate 25 away from the lower glass plate 24 (upward) was defined as the negative direction.

[0066] First, a simple unidirectional compression test was performed on a glass plate with no non-wetted area and a glass plate with a non-wetted area. The glass plate had a rectangular main surface measuring 3.5 cm x 2.5 cm.

[0067] 11 is a perspective view of a lower glass plate 24 having a non-wetting portion 24b formed thereon and an upper glass plate 25 having a non-wetting portion 25b formed thereon. As shown in FIG. 11 , the non-wetting portions were formed by forming the porous AD film described above as the non-wetting portion 24b on the lower glass plate 24 and the non-wetting portion 25b on the upper glass plate 25. The glass plates also had rectangular main surfaces measuring 3.5 cm × 2.5 cm, and the non-wetting portions 24b and 25b were formed around the peripheries of the lower glass plate 24 and the upper glass plate 25 to be 2.5 cm × 2.0 cm.

[0068] When liquid metal was compressed on a uniform glass plate at a compression speed of 1 mm / s, the liquid metal spilled from the periphery of the glass plate under a load of 0.5 N. In contrast, when the glass plate had a non-wetted portion formed, the liquid metal spilled under a load of 6.0 N. In other words, it was found that by forming a wetted portion, the glass plate could withstand a load of 10 times or more without pumping out.

[0069] Next, the liquid metal was compressed and unloaded five times using a uniform glass plate. A maximum load of 0.3 N was applied to the liquid metal, which was the load that did not cause pumpout, and the crosshead position was 0.2 mm to 0.25 mm. When unloading, the crosshead movement was stopped when the load reached zero, and the liquid metal was compressed.

[0070] FIG. 12 is a graph showing the relationship between load and crosshead position in a repeated compression test using a uniform glass plate without forming a non-wetting portion. FIGS. 13A to 13C are side cross-sectional views showing the change in the heat dissipation structure during a repeated compression test conducted without forming a non-wetting portion. Referring to FIGS. 12 and 13 together, the liquid metal that was pressurized and spread from the initial state (see FIG. 13A) in the first cycle (see FIG. 13B) was unloaded while still spread without returning to its original thickness (crosshead position was zero) (see FIG. 13C), and further compression and unloading were repeated. Furthermore, from the second cycle onwards, it was found that the crosshead position moved only about 0.05 mm between compression and unloading, and furthermore, the crosshead position during unloading also gradually advanced. This indicates the general behavior of liquid metal as a viscous fluid that spreads when pressurized.

[0071] Next, the liquid metal was repeatedly compressed and unloaded using the glass plate with the non-wetting portions 24b and 25b formed. Figure 14 is a graph showing the relationship between load and crosshead position in a repeated compression test conducted with the non-wetting portions formed. The horizontal axis represents the crosshead position (mm), and the vertical axis represents the load (N). Unlike the case shown in Figure 12 where no non-wetting portions were provided, the crosshead position returned to almost the initial position (zero) upon unloading. Furthermore, during pressurization, a maximum load of approximately 4 N was applied, and the crosshead position at this time was 0.07 mm to 0.08 mm. In other words, compared to the case where no non-wetting portions were provided, the crosshead displacement was small even when a large load was applied. This confirmed that the elastic behavior of the liquid metal also generates a force that pushes back the load in the thickness direction. In other words, it was shown that wetting and spreading of the liquid metal when pressurized was also suppressed.

[0072] 15 is a side cross-sectional view showing the change in the heat dissipation structure during a repeated compression test in which a non-wetted portion was formed. Referring to both Fig. 14 and Fig. 15, the liquid metal, which was pressurized from the initial state (see Fig. 15A) to the non-wetted portions 24b and 25b (see Fig. 15B), returned to its original thickness upon unloading, returning from the non-wetted portions 24b and 25b to the wetted portions 24a and 25a (see Fig. 15A). This remained the same even after five cycles.

[0073] As explained above, it was found that the formation of the non-wetting portions 24b and 25b in the heat dissipation body and the heat generation body causes the liquid metal to exhibit elastic deformation behavior. In particular, it was confirmed that the liquid metal generates a force that tries to push back the load in the thickness direction, thereby suppressing the wetting and spreading when pressure is applied, and can also follow dimensional changes in the gap between the heat generation surface and the heat absorption surface.

[0074] <Thermal Resistance> Next, the results of measuring the thermal resistance of the heat dissipation structure will be described.

[0075] Figure 16 is a side view of the sample used to measure thermal resistance. A heat dissipation structure was fabricated as a sample, in which liquid metal was sandwiched between two Si plates 31. The thickness of each Si plate 31 was 525 μm, and the thickness of the liquid metal was 50 μm. The liquid metal was Ga-In eutectic, as described above. For comparison, a heat dissipation structure with only Si plates and no liquid metal, and a heat dissipation structure in which a carbon sheet (VB-200, manufactured by Zeon Corporation) was sandwiched between the Si plates 31 instead of liquid metal, were also fabricated.

[0076] The thermal resistance of each of the heat dissipation structures described above was measured. First, the thermal diffusivity in the thickness direction was measured at room temperature (25°C) without pressure using a xenon flash analyzer (HyperFlash) manufactured by NETZSCH. The thermal conductivity was calculated by multiplying the thermal diffusivity by the specific heat and density obtained from literature values, and the thermal resistance in the thickness direction (cm) was calculated from the thickness of the Si plate and the sample sandwiched between the Si plates. ２ The thermal resistance was measured at three points for each heat dissipation structure fabricated.

[0077] Fig. 17 is a graph showing the calculated thermal resistance values. The thermal resistance shown in Fig. 17 is the thickness direction of the entire sample including the Si plate. As shown in Fig. 17, in the heat dissipation structure, the thermal resistance of only the Si plate without liquid metal is 1.37 cm ２ K / W~1.38cm ２ The thermal resistance of the heat dissipation structure with a carbon sheet sandwiched between silicon plates was 3.55 cm ２ K / W~3.90cm ２ The thermal resistance of the heat dissipation structure with liquid metal sandwiched between the Si plates was 0.11 cm ２ The heat dissipation structure using liquid metal as the sample showed the lowest value.

[0078] The thermal resistance of each of the heat dissipation structures described above was measured when the temperature was repeatedly changed from -20°C to 120°C. Thermal resistance was measured at three points for each heat dissipation structure fabricated, and the average of the three points was calculated. Figure 18 is a graph showing the temperature and thermal resistance of a sample with nothing sandwiched between the Si plates. Figure 19 is a graph showing the temperature and thermal resistance of a sample with a carbon sheet sandwiched between the Si plates. Figure 20 is a graph showing the temperature and thermal resistance of a sample with liquid metal sandwiched between the Si plates. As shown in Figures 18 to 20, the heat dissipation structure with liquid metal sandwiched between the Si plates 31 had the lowest thermal resistance at all temperatures. Furthermore, the liquid metal used in this example solidified at 15°C. As shown in Figure 21, even when the liquid metal solidified, the thermal resistance remained low compared to Figures 18 and 19, demonstrating that fluctuations in thermal resistance were suppressed.

[0079] Next, the thermal resistance of samples in the heat dissipation structure was measured with the thickness of the liquid metal changed to 10 μm, 40 μm, and 100 μm. The thermal resistance was measured at 25°C. Figure 21 is a graph showing the relationship between the interface between the Si plate and the liquid metal and the thermal resistance of the liquid metal. The thermal resistance shown in Figure 21 is the combined thermal resistance of the liquid metal and the interface. As shown in Figure 21, it can be seen that the thermal resistance decreases as the thickness of the liquid metal decreases. Furthermore, when the thickness of the liquid metal is 10 μm, the thermal resistance is 0.001 cm ２ When the thickness of the liquid metal is 40 μm, the thermal resistance is 0.048 cm ２ When the thickness of the liquid metal is 100 μm, the thermal resistance is 0.078 cm ２ It was also confirmed that no voids were generated even when the thickness of the liquid metal was set to 10 μm.

[0080] <Stability of Heat Dissipation Performance> Next, the results of a heat dissipation test conducted to investigate the stability of heat dissipation performance in a situation simulating an actual use environment of the heat dissipation structure will be described.

[0081] FIG. 22 is a perspective view showing the positions of the non-wetted portions of the Si plate and Cu plate in the heat dissipation structure. A Si plate was used as the heating element, and a Cu plate was used as the heat dissipation element. Each of the Si plate and Cu plate was a square, measuring 3 cm on each side. For both the Si plate and Cu plate, a 2.5 mm width from the periphery was left as the non-wetted portion so that the wetted portion was 2.5 cm on each side. A 3 μm thick porous film (porous AD film) was formed as the non-wetted portion. After applying 0.5 g of liquid metal (Ga-In eutectic) as the heat dissipation material 6 to the wetted portion 35 a, the Si plate 32 was placed on top of the Cu plate 33 to create the heat dissipation structure.

[0082] Next, a heat dissipation test was performed on the heat dissipation structure. FIG. 23A is a perspective view of the heat dissipation test apparatus, and FIG. 23B is a side view showing the stacking order from the Si plate to the Cu plate. As shown in FIG. 23A, in the heat dissipation test apparatus 40, a Cu heating plate 42 is attached to a load cell 41 fixed to an automatic stage, and the heating plate 42 can be moved up and down together with the Si plate 32 attached to its underside. One end of the heating plate 42 is heated by a 20 W heater 43, and the temperature of the opposite end can be measured by a thermocouple thermometer 44. Meanwhile, the Cu plate 33 is fixed to a cool plate 45 maintained at 15°C below the Si plate 32 so as to face it. The heating plate 42 is lowered together with the load cell 41, and the point at which the Si plate 32 contacts the heat dissipation material 6 is set as the zero point. The automatic stage repeatedly applies a displacement of ±200 μm (width 400 μm) up and down from the zero point to the Si plate 32, with one cycle occurring every 2 seconds. This allows us to simulate the repeated thermal stress and thermal strain of power semiconductors.

[0083] First, as a comparative test, a similar configuration to that shown in FIG. 23B was used, but without forming non-wetting portions on the Si plate 32 and Cu plate 33. Thermal grease (CW7250, manufactured by Chemtronics) was used as the heat dissipation material, and temperature measurements were performed while repeatedly applying displacement. FIG. 24 is a graph showing the time and temperature of the thermal grease heat dissipation test. As shown in FIG. 24, the temperature continued to rise immediately after the start of the test, and a sudden temperature increase occurred after several hundred cycles. It further reached 100°C after approximately 1,200 seconds (approximately 600 cycles), and the test was terminated by the activation of the safety device. The sudden temperature increase is believed to be due to poor thermal contact.

[0084] Next, as a comparative test, temperature measurements were performed on the configuration shown in Figure 23B and a similar configuration in which liquid metal was used as the heat dissipation material without forming non-wetting areas on the Si plate 32 and Cu plate 33. The load change of the load cell was also measured. Figure 25 is a graph showing the time, temperature, and load of a heat dissipation test using liquid metal without forming non-wetting areas. A temperature rise of approximately 2°C occurred approximately 100 seconds after the start of the test, and a temperature rise of approximately 4°C occurred at approximately 7,000 seconds. This temperature rise is thought to be due to the deterioration of thermal contact due to the generation of voids. The temperature rise is lower than that of thermal grease, likely due to the liquid metal's thermal conductivity being more than 10 times higher and its high fluidity. Furthermore, the load slightly decreased toward the end of the test compared to immediately after the start of the test. This also indicates the generation of voids.

[0085] Fig. 26 is a photograph of the appearance of the Cu plate after the heat dissipation test. As shown in Fig. 26, it was confirmed that there were patches of no liquid metal on the surface of the Cu plate 33 after the test. In other words, the occurrence of voids was confirmed.

[0086] Finally, temperature measurements were performed on a heat dissipation structure in which non-wetted portions were formed on the Si plate 32 and the Cu plate 33 and liquid metal was used as the heat dissipation material. The load change of the load cell was also measured. Figure 27 is a graph showing the time, temperature, and load of a heat dissipation test using liquid metal with non-wetted portions formed. As shown in Figure 27, a temperature rise of about 2°C occurred after the start of the test, similar to the case without wetted portions, but the temperature remained almost constant for approximately 7,000 seconds. The temperature change during this period was within ±1°C. In other words, there was almost no change in the thermal contact state, and it was found that the thermal contact state could be maintained stably over a long period of time. Furthermore, the fact that there was almost no change in the load suggests that the amount of liquid metal was maintained constant.

[0087] Figure 28 is a photograph of the appearance of the Cu plate after the heat dissipation test. As shown in Figure 28, after the test, it was observed that the liquid metal spread evenly over the wetted surface of the Cu plate 33, with almost no leakage to the non-wetted surface. This indicates that the occurrence of voids and pump-out was sufficiently suppressed.

[0088] As described above, it was confirmed that the heat dissipation structure of this embodiment can maintain the high heat dissipation performance provided by the liquid metal.

[0089] <Thermal Resistance of Heat Dissipating Material> Next, the results of measuring the thermal resistance of the heat dissipating material will be described.

[0090] Figure 29 is a side view of the steady-state thermal resistance measuring device. As shown in Figure 29, the heat dissipation material 60 is sandwiched between measuring rods 53 and 54. Measuring rod 53 includes a thermocouple 56, and measuring rod 54 includes a thermocouple 52. Figure 30 is a perspective view of measuring rod 53. The surface of measuring rod 53 was coated with gold to form a wetted portion 53a, and sandpaper (TRUSCO sheet paper, grain size #400, manufactured by Trusco Nakayama Corporation) was attached to form a non-wetted portion 53b, thereby controlling wettability. Although not shown, measuring rod 54 also had wetted and non-wetted portions formed, similar to measuring rod 53. Thermally conductive grease (thermal dissipation oil compound G746, manufactured by Shin-Etsu Chemical Co., Ltd.) and Ga-In eutectic liquid metal were used as the heat dissipation material 60.

[0091] In a steady-state thermal resistance measuring device, the upper measuring rod 54 was heated to 100°C using a heating plate, and the lower measuring rod 53 was cooled to 20°C using a cooling plate. A liquid metal was used as the heat-dissipating material 60, with a thickness of 200 μm, and the thermal resistance was measured in accordance with ASTM D5470. The upper measuring rod 54 was then displaced up and down by ±30 μm 100 times using a mechanical testing machine 59, repeatedly deforming the heat-dissipating material 60. The thermal resistance of the heat-dissipating material 60 was then measured in accordance with ASTM D5470 at a thickness of 200 μm, thereby evaluating the change in thermal resistance of the heat-dissipating material 60 when repeatedly deformed under actual use. Furthermore, a thermally conductive grease was used as the heat-dissipating material 60, with a thickness of 200 μm, and the thermal resistance was measured in the same manner as when a liquid metal was used. Furthermore, the heat-dissipating material 60 was sandwiched between the measuring rods 53 and 54 without controlling the wettability of the wetted and non-wetted portions on their surfaces. A liquid metal was used as the heat dissipation material 60, and the thickness was set to 200 μm. The thermal resistance was measured in the same manner as in the case of using the liquid metal sandwiched between the measuring rods 53 and 54 with controlled wettability.

[0092] 31 shows the results of measuring the thermal resistance of thermal grease, liquid metal sandwiched between measuring rods 53 and 54 with an uncontrolled wettability difference, and liquid metal sandwiched between measuring rods 53 and 54 with a controlled wettability difference. It was found that the thermal resistance of both the thermal grease and the liquid metal sandwiched between measuring rods 53 and 54 with an uncontrolled wettability difference increased after 100 displacements. In contrast, it was found that the thermal resistance of the liquid metal sandwiched between measuring rods 53 and 54 with a controlled wettability difference remained unchanged even after 100 displacements, and remained the same as in the initial state.

[0093] This example demonstrates that the heat dissipation structure according to one embodiment of the present invention has low thermal resistance and can suppress fluctuations in thermal resistance. As a result, even when a heat-generating element such as a semiconductor element generates heat, the heat can be efficiently dissipated. Therefore, the reliability of a device in which a semiconductor element is mounted can be improved.

[0094] Although the embodiments of the present invention and modifications based thereon have been described above, the present invention is not necessarily limited to these examples. Furthermore, those skilled in the art will be able to find various alternative embodiments and modifications without departing from the spirit of the present invention or the scope of the appended claims.

[0095] 1: heat dissipation structure, 2: heat generating element, 3: heat dissipation element, 4: heat generating surface, 4a: wetted portion, 4b: non-wetted portion, 5: heat absorbing surface, 5a: wetted portion, 5b: non-wetted portion, 6: heat dissipation material, 10: heat dissipation structure, 12: heat generating element, 13: heat dissipation element, 14: heat generating surface, 14a: wetted portion, 14b: non-wetted portion, 14c: recess, 15: heat absorbing surface, 15a: wetted portion, 15b: non-wetted portion, 15c: recess, 20: compression tester, 21: crosshead, 22: compression Plate, 23: compression plate, 24: lower glass plate, 24a: wetted portion, 24b: non-wetted portion, 25: upper glass plate, 25a: wetted portion, 25b: non-wetted portion, 31: plate, 32: plate, 33: plate, 40: heat dissipation test device, 41: load cell, 42: heating plate, 43: heater, 44: thermometer, 45: cool plate, 52: thermocouple, 53: measuring rod, 54: measuring rod, 56: thermocouple, 59: mechanical testing machine, 60: heat dissipation material

Claims

1. A heat dissipation structure that uses liquid metal as a heat dissipation material to transfer heat from the heat generating surface of a heat generating body to the heat absorbing surface of a heat dissipating body, wherein a second region is provided on each of the heat generating surface and the heat absorbing surface that face each other with a gap between them, surrounding the periphery of a first region, and the first regions are opposed to each other, the wettability of the first region to the liquid metal is relatively higher than that of the second region, and the heat dissipation material is filled to fill the storage space between the first regions that face each other with the gap between them.

2. The heat dissipation structure according to claim 1, wherein the heat dissipation material reversibly deforms in the accommodation space and in the space between the second regions on the outside thereof.

3. The contact angles of the liquid metal with the first region and the second region are each θ １ and θ ２ Then, cosθ １ -cosθ ２ The heat dissipation structure according to claim 1 or 2, wherein the ratio of the radiative stress to the thermal conductivity is ≧0.

19.

4. The heat dissipation structure according to claim 3, wherein the surface roughness of the first region is 0.07 μm or less in Rq value, and the surface roughness of the second region is 0.20 μm or more in Rq value.

5. The heat dissipation structure of claim 4, wherein said second region is provided with a surface coating, said surface coating comprising a deposit of ceramic powder.

6. The heat dissipation structure according to claim 5, wherein the surface coating is provided to a thickness of 50 μm or less.

7. The heat dissipation structure according to claim 1 or 2, wherein the liquid metal has a viscosity of 100,000 mPa·s or less at 25°C.

8. The heat dissipation structure according to claim 7, wherein the liquid metal is a Ga-based alloy or a composite material in which particles for adjusting viscosity and improving thermal conductivity are mixed into a Ga-based alloy.

9. A heat dissipation structure comprising: a heating element having a heat generating surface; a heat dissipation element having a heat absorption surface opposite the heat generating surface; and a heat dissipation material sandwiched between the heating element and the heat dissipation element and containing a liquid metal for transferring heat from the heat generating surface to the heat absorption surface, wherein the heating element has, on the heat generating surface, a first region where the heat dissipation material is disposed, and a second region surrounding the first region and having a lower wettability to the liquid metal than the first region, and the heat dissipation element has, on the heat absorption surface, a third region where the heat dissipation material is disposed, and a fourth region surrounding the third region and having a lower wettability to the liquid metal than the third region, and the heat dissipation material is provided so as to fill a gap between the second region and the fourth region.

10. The heat dissipation structure according to claim 9, wherein the heat dissipation material is reversibly deformable between the gap between the second region and the fourth region and the gap between the first region and the third region.

11. The contact angle between the heat dissipation material and the first region is θ １ The contact angle between the heat dissipation material and the second region is θ ２ Then, cosθ １ -cosθ ２ The heat dissipation structure according to claim 9 , wherein the ratio of the thermal conductivity to the thermal conductivity is ≧0.

19.

12. The heat dissipation structure according to claim 9, wherein the surface roughness of the first region is 0.07 μm or less in terms of Rq value, and the surface roughness of the second region is 0.20 μm or more in terms of Rq value.

13. The contact angle between the heat dissipation material and the third region is θ ３ The contact angle between the heat dissipation material and the fourth region is θ ４ Then, cosθ ３ -cosθ ４ The heat dissipation structure according to claim 9 , wherein the ratio of the thermal expansion coefficient to the thermal expansion coefficient of the heat dissipation structure satisfies ≧0.

20.

14. The heat dissipation structure according to claim 9, wherein the surface roughness of the third region is 0.07 μm or less in terms of Rq value, and the surface roughness of the fourth region is 0.20 μm or more in terms of Rq value.

15. In the second region, SiO ２ , Al ２ O ３ , and TiO ２ oxide materials including polytetrafluoroethylene (PTFE), polypropylene, polyethylene, natural rubber, polyurethane rubber, acrylonitrile butadiene rubber, and silicone rubber; and metal materials including Ni, Pt, and Au; and Al. ２ O ３ , Al(OH) ３ , ZrO ２ , AlN, SiC, Si ３ N ４ The heat dissipation structure according to claim 9 , further comprising a coating layer using a material selected from the group consisting of porous materials including 16. In the fourth region, SiO ２ , Al ２ O ３ , and TiO ２ oxide materials including polytetrafluoroethylene (PTFE), polypropylene, polyethylene, natural rubber, polyurethane rubber, acrylonitrile butadiene rubber, and silicone rubber; metal materials including Ni, Pt, and Au; and Al ２ O ３ , Al(OH) ３ , ZrO ２ , AlN, SiC, Si ３ N ４ The heat dissipation structure according to claim 9 , further comprising a coating layer using a material selected from the group consisting of porous materials including 17. In the second region and the fourth region, SiO ２ , Al ２ O ３ , and TiO ２ oxide materials including polytetrafluoroethylene (PTFE), polypropylene, polyethylene, natural rubber, polyurethane rubber, acrylonitrile butadiene rubber, and silicone rubber; metal materials including Ni, Pt, and Au; and Al ２ O ３ , Al(OH) ３ , ZrO ２ , AlN, SiC, Si ３ N ４ The heat dissipation structure according to claim 9 , further comprising a coating layer using a material selected from the group consisting of porous materials including 18. The heat dissipation structure according to claim 9, wherein the heat dissipation material has a viscosity of 2 mPa·s to 350 mPa·s at 25°C.

19. The heat dissipation structure according to claim 9, wherein the heat dissipation material is a Ga-based alloy or a composite material in which particles for adjusting viscosity and improving thermal conductivity are mixed into a Ga-based alloy.

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