HEAT DISSIPATION STRUCTURE

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of International Patent Application No. PCT/JP2024/25925, filed on Jul. 19, 2024, which claims the benefit of priority to Japanese Patent Application No. 2023-120282, filed on Jul. 24, 2023, the entire contents of which are incorporated herein by reference.

FIELD

The present invention relates to a heat dissipation structure using a liquid metal as a thermal conductive material for transferring heat from a heating surface of a heating element to a heat absorbing surface of a heat dissipator.

BACKGROUND

In recent years, there has been proposed a heat dissipation structure using a thermal conductive material (TIM: Thermal Interface material) in a power semiconductor device such as MOSFET (Metal-Oxide-Semiconductor Field Effect Transistor). A liquid metal can absorb thermal stress of an element due to fluidity as compared with conventional silicon grease or gel, and in particular, can suppress thermal stress breakage of a semiconductor element which is inferior in mechanical strength due to thinning. Further, since the liquid metal has high thermal conductivity, the generation of thermal stress of the element itself can be suppressed.

For example, Patent Document 1 (Japanese Laid-Open Patent Publication No. 2022-138526) discloses a heat dissipation structure in which Ga (gallium), which is a low-melting-point metal, and a metal material in which In (indium) or Sn (tin) are added with Ga as the main component are used as the thermal conductive material. The thermal conductive material is a liquid metal at room temperature or becomes a liquid metal at least when the semiconductor device (chip) is operated to obtain fluidity.

SUMMARY

Inventors of the present application have considered that in order to increase thermal contact stability between the liquid metal, which is a thermal conductive material (hereinafter also referred to as a heat dissipator), and a heating surface and heat absorbing surface, and to ensure a high heat dissipation performance inherent to the liquid metal, it is necessary to control a deformation behavior of the liquid metal caused by deformation in a thickness direction, such as pumping out. As a result, the inventors of the present invention have come up with the idea of making it possible to obtain reversible deformation and restoration in the thickness direction, that is, to make the liquid metal exhibit elastic deformation behavior.

In other words, the heat dissipation structure according to the present invention includes a liquid metal as a heat dissipation material to transfer heat from a heat generating surface of a heating element to a heat absorbing surface of a heat dissipator, wherein the heat generating surface and the heat absorbing surface face each other with a gap, and a second region is provided to surround a periphery of a first region in each of the heat generating surface and the heat absorbing surface, such that the first region in the heat generating surface faces the first region in the heat absorbing surface and the second region in the heat generating surface faces the second region in the heat absorbing surface, the liquid metal fills an accommodating space between the first regions facing each other with the gap, a wettability of the liquid metal is relatively higher in the first region than in the second region.

According to this feature, the liquid metal between the heating surface and the heat absorbing surface can be imparted elastic deformation behavior, and the high heat dissipation performance of the liquid metal can be well maintained.

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.

In the above configuration, when contact angles between the liquid metal and the first region and the second region are θ₁ and θ₂, respectively, cos θ₁-cos θ₂≥0.19. According to this feature, it is possible to reliably impart elastic deformation behavior to the liquid metal between the heating surface and the heat absorbing surface, and it is possible to favorably maintain the high heat dissipation performance of the liquid metal.

In the above configuration, a surface roughness of the first region and a surface roughness of the second region may be set to 0.20 μm or more and 0.07 μm or less in terms of an Rq value, respectively. The second region may also be provided with a surface coating, the surface coating comprising a deposit of ceramic powder. Furthermore, the surface coating may be provided to a thickness of 50 μm or less. According to this feature, it is possible to reliably impart elastic deformation behavior to the liquid metal between the heating surface and the heat absorbing surface, and it is possible to favorably maintain the high heat dissipation performance of the liquid metal.

In the above configuration, the liquid metal may have a viscosity of 100,000 mPa·s or less at 25° C. Further, the liquid metal may be a Ga-based alloy or a composite material obtained by mixing particles for adjusting viscosity and improving thermal conductivity into a Ga-based alloy. According to this feature, it is possible to reliably impart elastic deformation behavior to the liquid metal, which generally exhibits viscous deformation behavior, and to favorably maintain the high heat dissipation performance of the liquid metal.

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 a liquid metal as a thermal conductive material that transfers heat from the heating surface of a heating element to the heat absorbing surface of a heat dissipator.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a cross-sectional side view of a heat dissipation structure according to an embodiment of the present invention.

FIG. 2 is a top view of a heating element in the heat dissipation structure.

FIG. 3 is a cross-sectional side view showing a state in which a liquid metal is applied onto a heating element.

FIG. 4 is a cross-sectional side view of the heat dissipation structure showing a state in which the liquid metal protrudes until it comes into contact with a non-wettable region.

FIG. 5A is a cross-sectional side view showing another example of a heat dissipation structure according to an embodiment of the present disclosure.

FIG. 5B is a cross-sectional side view showing another example of a heat dissipation structure according to an embodiment of the present disclosure.

FIG. 6A is an electron-micrograph of an alumic powder of a raw material in an aerosol deposition.

FIG. 6B is a photomicrograph of an aluminum hydroxide powder of a raw material.

FIG. 6C is an external photograph of a substrate (left) and the substrate after film formation (right).

FIG. 6D is an electron-micrograph of a surface of a formed film.

FIG. 7A is a photograph showing a state in which a liquid metal is dropped on silicon for measuring a contact angle.

FIG. 7B is a photograph showing a state in which a liquid metal is dropped on copper for measuring a contact angle.

FIG. 7C is a photograph showing a state in which a liquid metal is dropped on a porous film for measuring a contact angle.

FIG. 7D is a photograph showing a state in which a liquid metal is dropped on a glass for measuring a contact angle.

FIG. 7E is a photograph showing a state in which a liquid metal is dropped on a stainless-steel plate for measuring a contact angle.

FIG. 8 is a list of surface roughness of each material and a contact angle of a liquid alloy with respect thereto.

FIG. 9 is a list of contact angles of a liquid alloy for each pattern formed on a surface of silicon.

FIG. 10 is a side view showing a state of a pump-out test by a compression testing machine.

FIG. 11 is a perspective view of a glass plate having a non-wettable region formed thereon.

FIG. 12 is a graph showing a relationship between a load and a crosshead position in a repeated compression test performed without forming a non-wettable region.

FIG. 13A is a cross-sectional view showing a change in heat dissipation in a repeated compression test performed without forming a non-wettable region.

FIG. 13B is a cross-sectional view showing a change in heat dissipation in a repeated compression test performed without forming a non-wettable region.

FIG. 13C is a cross-sectional view showing a change in heat dissipation in a repeated compression test performed without forming a non-wettable region.

FIG. 14 is a graph showing a relationship between a load and a crosshead position in a repeated compression test performed by forming a non-wettable region.

FIG. 15A is a cross-sectional side view showing a change of a heat dissipation structure during a repeated compression test performed by forming a non-wettable region.

FIG. 15B is a cross-sectional side view showing a change of a heat dissipation structure during a repeated compression test performed by forming a non-wettable region.

FIG. 16 is a side view of a sample used to measure thermal resistance.

FIG. 17 is a graph showing a list of calculated thermal resistance values.

FIG. 18 is a graph showing temperature and thermal resistance of a sample without any sandwiching between Si plates.

FIG. 19 is a graph showing temperature and thermal resistance of a sample in which a carbon sheet is sandwiched between Si plates.

FIG. 20 is a graph showing temperature and thermal resistance of a sample in which a liquid-metal is sandwiched between Si plates.

FIG. 21 is a graph showing a relationship between a thickness of a liquid metal and thermal resistance of a sample.

FIG. 22 is a perspective view showing positions of non-wettable regions of an Si plate and a Cu plate.

FIG. 23A is a perspective view of a heat dissipation test apparatus.

FIG. 23B is a side view showing a stacking order from a heater plate to a Cu plate.

FIG. 24 is a graph showing time and temperature of a thermal grease heat dissipation test.

FIG. 25 is a graph showing time, temperature, and load of a heat dissipation test of a liquid metal performed without forming a non-wettable region.

FIG. 26 is an external photograph of a Cu plate after the test.

FIG. 27 is a graph showing time, temperature, and load of a heat dissipation test of a liquid metal performed by forming a non-wettable region.

FIG. 28 is an external photograph of a Cu plate after the test.

FIG. 29 is a side view of a steady-state thermal resistance measurement apparatus.

FIG. 30 is a perspective view of a measurement rod forming a non-wettable region.

FIG. 31 is a graph showing thermal resistance measured by a steady-state thermal resistance measurement apparatus.

DESCRIPTION OF EMBODIMENTS

In a heat dissipation structure, a thermal conductive material including a liquid metal is held in a gap between a semiconductor device and a heat dissipator such as a heat sink by its surface tension. Here, there are known a phenomenon in which the thermal conductive material expands or contracts due to heat to generate voids (bubbles) therein, and a phenomenon in which a thermal conductive material is extruded from the gap, which degrades heat dissipation performance called “pump-out”. It is desired to prevent pump-out and maintain high heat dissipation performance even in the thermal conductive material.

An object of the present invention is to provide a heat dissipation structure capable of maintaining high heat dissipation performance in a heat dissipation structure using a liquid metal as a heat conductive material for transferring heat from a heating surface of a heating element to a heat absorbing surface of a heat dissipator.

A heat dissipation structure 1 according to an embodiment of the present invention will be described with reference to FIG. 1 to FIG. 5.

FIG. 1 is a cross-sectional side view of a heat dissipation structure 1 according to an embodiment of the present invention. As shown in FIG. 1, the heat dissipation structure 1 includes a heating element 2 such as a semiconductor element that generates heat, a heat dissipator 3 such as a heat sink, and a heat dissipation material 6 sandwiched between the heating element 2 and the heat dissipator 3. Semiconductor devices refer to semiconductor chips such as, for example, a CPU, a GPU, and memories. The heat dissipator 3 is a so-called vapor chamber. The heating element 2 includes a heating surface 4, and the heat dissipator 3 includes a heat absorbing surface 5. The heating surface 4 of the heating element 2 and the heat absorbing surface 5 of the heat dissipator 3 face each other with a gap d therebetween, and hold the heat dissipation material 6 including a liquid metal between the heating surface 4 and the heat absorbing surface 5. That is, the heat dissipation structure 1 has a structure in which heat is transferred from the heating surface 4 of the heating element 2 to the heat absorbing surface 5 of the heat dissipator 3 via the heat dissipation material 6 to further radiate heat to the outside from the heat dissipator 3. Here, the heat dissipation material 6 includes a liquid metal, and includes both a case where the material is a liquid at room temperature and a case where the material is a solid at room temperature and becomes a liquid by heating at the time of heat dissipation. However, for the sake of simplicity, in the embodiment, a case where the heat dissipation material 6 is always a liquid will be described.

FIG. 2 is a top view of the heating element 2 of the heat dissipation structure 1. Referring to FIG. 1 and FIG. 2, the heating surface 4 is provided with a wettable region 4a arranged in the center and a non-wettable region 4b surrounding a periphery of the wettable region 4a. The wettable region 4a is an area having a wettability that is relatively higher than that of the non-wettable region 4b with respect to the liquid metal which is the heat dissipation material 6. That is, the non-wettable region 4b is a region having a wettability lower than a wettability of the wettable region 4a with respect to the liquid metal. Although the non-wettable region 4b is arranged so as to surround the periphery of the wettable region 4a, in FIG. 2, in the substantially rectangular heating surface 4, strip-shaped regions along each of the four sides of the outer periphery are the non-wettable region 4b, and the wettable region 4a is a substantially rectangular region of the center except this region.

Similar to the heating surface 4, the heat absorbing surface 5 also includes a wettable region 5a and a non-wettable region 5b surrounding a periphery of the wettable region 5a. Arrangement of the wettable region 5a and the non-wettable region 5b when the heat absorbing surface 5 is viewed in a plan view is the same as a top view of the heating element 2 shown in FIG. 2. Further, the wettable region 5a is a region having a wettability relatively higher than a wettability of the non-wettable region 5b with respect to the heat dissipation material 6. Although not shown, the wettable region 5a and the non-wettable region 5b are arranged to face the wettable region 4a and the non-wettable region 4b, respectively. That is, a boundary between the wettable region 5a and the non-wettable region 5b is arranged so as to face a boundary between the wettable region 4a and the non-wettable region 4b. The heat dissipation material 6 is filled so as to fill a gap (also referred to as an accommodation space) between the wettable region 4a and the wettable region 5a facing each other.

FIG. 3 is a cross-sectional side view showing a state in which the heat dissipation material 6 is applied on the heating element 2. When manufacturing the heat dissipation structure 1, first, the wettable region 4a and the non-wettable region 4b are formed on the heating surface 4 of the heating element 2, and the wettable region 5a and the non-wettable region 5b are formed on the heat absorbing surface 5 of the heat dissipator 3. Methods for forming the wettable regions 4a and 5a and the non-wettable regions 4b and 5b will be described in detail later. Next, as shown in FIG. 3, the heat dissipation material 6 is applied on the wettable region 4a. At this time, the heat dissipation material 6 is arranged so as to be in contact with an entire surface of the wettable region 4a, and is arranged so as not to be in contact with the non-wettable region 4b. That is, an amount of the heat dissipation material 6 in such an arrangement is applied to the wettable region 4a. In other words, an amount of the heat dissipation material 6 applied to the wettable region 4a is determined according to an area of the wettable region 4a.

Here, referring back to FIG. 1, in the case where the heat dissipator 3 is arranged on the heating element 2 so as to sandwich the heat dissipation material 6, the heat dissipation material 6 is arranged in contact with an entire surface of the wettable region 5a on the heat absorbing surface 5 so as not to be in contact with the non-wettable region 5b. At this time, a volume of the heat dissipation material 6 is a product of an area of the gap d between the heating surface 4 and the heat absorbing surface 5 and the area of the wettable region 4a (or the wettable region 5a). That is, the volume fills the gap d between the wettable region 4a and the wettable region 5a as described above. Therefore, an amount of liquid metal initially applied to the heating element 2 is an amount corresponding to the product of the gap d and the area of the wettable region 4a (or the wettable region 5a). The gap d corresponding to a thickness of the heat dissipation material 6 is, for example, 10 μm or more and 300 μm or less, preferably 50 μm or more and 200 μm or less. The gap d may have any size as long as it can absorb the deformation of the heating element 2 even if the heating element 2 is deformed by heat generation.

In general, in the case where a heat dissipation structure in which a heat dissipation material having fluidity such as grease or a liquid is sandwiched between a heating surface and a heat absorbing surface is used, deformation of the heat dissipation structure and volume change of the heat dissipation material occur with a change in temperature. Then, by repeating the temperature change, there is a case where pump-out is generated in which the heat dissipation material is pushed out to the outside of a portion sandwiched between the heating surface and the heat absorbing surface. The extruded heat dissipation material may cause the semiconductor element to deteriorate or short out. Further, due to the heat dissipation material being extruded, the thickness of the heat dissipation material itself is reduced, and it is not possible to sufficiently fill the gap between the heating surface and the heat absorbing surface resulting in voids. As a result, the heat dissipation performance of the heat dissipation structure may deteriorate. On the other hand, according to the heat dissipation structure 1 according to the embodiment of the present invention, the generation of pump-out and voids can be suppressed by the “elastic deformation behavior” of the heat dissipation material 6 as described later, and high heat dissipation performance can be favorably maintained.

FIG. 4 is a cross-sectional side view of the heat dissipation structure 1, showing the heat dissipation material 6 protruding until it contacts the non-wettable region 4b and the non-wettable region 5b. For example, FIG. 4 shows a situation in which the heat dissipation material 6 protrudes from the wettable region 4a and the wettable region 5a by the reduction of the gap d and the expansion of the heat dissipation material 6 due to a dimensional change of the heat dissipation structure 1, and comes into contact with the non-wettable region 4b and the non-wettable region 5b. As described above, the heat dissipation material 6 is filled so as to fill the gap between the wettable region 4a and the wettable region 5a. Therefore, the heat dissipation material 6 protrudes to the outer side of the gap between the wettable region 4a and the wettable region 5a even with slight deformation, so that the heat dissipation material 6 straddles the respective regions of the wettable region 4a and the wettable region 5a and the non-wettable region 4b and the non-wettable region 5b.

Here, when the size of the gap d and the volume of the heat dissipation material 6 return to their original size and volume, the heat dissipation material 6 returns from the regions of the non-wettable region 4b and the non-wettable region 5b to the regions of the wettable region 4a and the wettable region 5a. That is, the heat dissipation material 6 reversibly deforms between the gap between the wettable region 4a and the wettable region 5a and the gap between the non-wettable region 4b and the non-wettable region 5b on the outer side thereof. It is believed that this is caused by the difference in wettability and force based on surface tension that directs a liquid metal from the regions of the non-wettable region 4b and the non-wettable region 5b to the regions of the wettable region 4a and the wettable region 5a with high wettability.

Also, This Force Occurs Whenever the Heat Dissipation Material 6 Straddles the border between the wettable region 4a and the wettable region 5a and the non-wettable region 4b and the non-wettable region 5b. Therefore, no matter how the dimension of the gap d changes, if the heat dissipation material 6 is outside the gap between the wettable region 4a and the wettable region 5a, a force is generated to try to return the heat dissipation material 6 to the gap between the wettable region 4a and the wettable region 5a. Thus, it is possible to elastically deform the shape of the heat dissipation material 6 to follow the dimensional change of the gap d. For example, even if the length of the gap d temporarily decreases and the heat dissipation material 6 spreads thinly, when the length of the gap d returns to its original size, the heat dissipation material 6 deforms accordingly and fills the gap d between the wettable region 4a and the wettable region 5a. The elastic deformation behavior of the heat dissipation material 6 can be generated with good reproducibility even when repeated many times. Therefore, it is possible to suppress generation of voids in the heat dissipation material 6 even for repeated use, and to suppress separation of the heat dissipation material 6 from the heating surface 4 and the heat absorbing surface 5. Thus, high heat dissipation performance by the heat dissipation material 6 can be favorably maintained.

Regarding the arrangement of the wettable region 4a and the wettable region 5a and the non-wettable region 4b and the non-wettable region 5b, from the viewpoint of reducing thermal resistance, it is preferable to increase the area of the non-wettable regions 4b and 5b, as this allows for a wider arrangement of the heat dissipation material 6. As such an arrangement, for example, a total area of the non-wettable region 4b and the non-wettable region 5b is preferably 30% or less of a total area of the heating surface 4 and the heat absorbing surface 5. However, if the heat dissipation material 6 is wet and spreads to the outer side of the non-wettable region 4b and the non-wettable region 5b, the heat dissipation material 6 will leak from the heat generation element and the heat dissipator. Therefore, it is preferable that a region in which the non-wettable region 4b and the non-wettable region 5b are arranged is determined so as to give the wettable region 4a and the wettable region 5a a width that can correspond to the volume change of the heat dissipation material 6 and the dimensional change of the gap d.

The elastic deformation behavior of the heat-dissipation material 6 as described above is preferably produced at room temperature. Therefore, it is preferable to use a liquid metal which is liquid at room temperature as the heat dissipation material 6. In addition, it is preferable that the liquid metal has a viscosity of 100000 mPa·s or less at 25° C. Specifically, the liquid metal viscosity 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 alloys containing one or more low-melting-point metals of Ga, In, and Sn can be used. Specific examples of alloys include Ga—In, Ga—In—Sn, In—Ag, Sn—Ag—Cu, In—Sn—Bi and the like.

Further, in addition to the low-melting-point metal and the alloy containing the low-melting-point metals described above, the liquid metal can also be a composite material with adjusted viscosity and improved thermal conductivity, by mixing in fine particles made of a material having a high thermal conductivity such as copper, silver, gold, tungsten or diamond as particles to adjust viscosity and improve thermal conductivity. A content of the fine particles is preferably 60% or less by volume or 30% or less by volume because the viscosity increases when the content exceeds 60% by volume and the shape of the liquid metal does not return to the original shape. Also, the particle size of the fine particles may be about several tens of μm.

The thermal conductivity of the liquid metal is 15 W/mK to 73 W/mK, for example, thermal conductivity of a Ga—In—Sn based liquid metal is 15 W/mK to 28 W/mK. The thermal conductivity of the grease is about 10 W/mK, and the graphite sheet is about 40 W/mK.

As described above, the wettable region 4a and the non-wettable region 4b are formed on the heating surface 4 of the heating element 2, and the wettable region 5a and the non-wettable region 5b are formed on the heat absorbing surface 5 of the heat dissipator 3. Therefore, as a material on the heating surface 4 of the heating element 2 and the material on the heat absorbing surface 5 of the heat dissipator 3, a material capable of controlling wettability with respect to the heat dissipation material 6 or a material capable of imparting a member such as a film body in which wettability is controlled, and a material having sufficient thermal conductivity as a heat conductive member of the heat dissipation structure 1 is preferably used. As the material on the heating surface 4 of the heating element 2 and the material on the heat absorbing surface 5 of the heat dissipator 3, for example, copper, nickel, stainless steel, silicon, aluminum, alumina, gold, platinum, titanium, titanium oxide, a carbon-based material, a mixture thereof, glass, or the like is used. Further, a thermal resistant ceramic obtained by mixing aluminum or titanium oxide with a metal for improving the thermal conductivity may be used.

A combination of the material of the heating surface 4 of the heating element 2 and the material of the heat absorbing surface 5 of the heat dissipator 3 applied to a CPU or the like may be Si—Cu, Cu—Cu, Si—Al, Al—Al, Al—Cu, Cu—Al, or the like. In addition, examples of combinations of a base material of the heating element 2 and a base material of the heat dissipator 3 used in automotive batteries or the like include SUS-Cu, SUS-Al or the like.

Further, for the materials listed for the heating element 2 and the heat dissipator 3, a coating layer may be formed on at least one of the wettable regions 4a and 5a and the non-wettable regions 4b and 5b. As the coating layer, an oxide material (SiO₂, Al₂O₃, TiO₂), a polymeric material (for example, polytetrafluoroethylene (PTFE), polypropylene, polyethylene, natural rubber, polyurethane rubber, acrylonitrile-butadiene rubber, silicone rubber, or the like), a metallic material such as Ni, Pt, Au, or a mixed material thereof may be used. For example, the non-wettable regions 4b and 5b may use a mixed layer obtained by mixing the oxide material and polymer material described above, and the wettable regions 4a, 5a may use a mixed layer obtained by mixing the metal material (for improving thermal conductivity and wettability) and the oxide material and polymer material (for preventing corrosion) described above. As the coating layer, it is preferable to use the material described above that suppresses corrosion by the liquid metal. Even if a material (for example, aluminum) that is easily corroded with respect to the liquid metal is used as the heating element 2 and the heat dissipator 3, the coating layer described above can be provided to suppress the corrosion of the heating element 2 and the heat dissipator 3. In the case where aluminum is used for at least one of the heating element 2 and the heat dissipator 3, it is preferable to provide coating layers not only on the non-wettable regions 4b and 5b but also on the wettable regions 4a and 5a. The thickness of the coating layer is preferably 0.2 μm or more and 50 μm or less so as not to increase the thermal resistance of the heat dissipation structure 1. Since the wettable regions 4a and 5a affect the thermal resistance of the heat dissipation structure 1, the thickness of the coating layer in the wettable regions 4a and 5a is preferably 0.2 μm or more and 5 μm or less. The thickness of the coating layer in the non-wettable regions 4b and 5b may be the same as the film thickness of the wettable regions 4a and 5a or may be larger than the film thickness of the wettable regions 4a and 5a.

As described above, the wettable region 4a and the wettable region 5a, and the non-wettable region 4b and the non-wettable region 5b each have differences in wettability with the heat dissipation material 6. Such a difference in wettability can be expressed by using a contact angle with the heat dissipation material 6. When the wettable region 4a and the wettable region 5a are made of the same material and the non-wettable region 4b and the non-wettable region 5b are made of the same material, and a contact angle between the heat dissipation material 6 and the wettable region 4a (or wettable region 5a) is taken as θ₁, and a contact angle between the heat dissipation material 6 and the non-wettable region 4b (or non-wettable region 5b) is taken as θ₂, it is preferable to satisfy the following formula (1).

cos ⁢ θ 1 - cos ⁢ θ 2 ≥ 0.19 ( 1 )

Formula (1) means that the difference between the contact angle θ₁ and θ₂ is increased, that is, the difference in wettability between the wettable region 4a and the non-wettable region 4b is increased.

Further, in the case where the wettable region 4a and the wettable region 5a are made of different materials and the non-wettable region 4b and the non-wettable region 5b are made of the same material, and the case where the wettable region 4a and the wettable region 5a are made of the same material and the wettable region 4b and the non-wettable region 5b are made of different materials, and in the case where the wettable region 4a and the wettable region 5a are made of different materials and the non-wettable region 4b and the non-wettable region 5b are made of different materials, it is sufficient that there is a difference in wettability with the heat dissipation material 6. In the heating element 2, the contact angle of the wettable region 4a and the contact angle of the non-wettable region 4b may satisfy the formula (1). In the case where the contact angle between the heat dissipation material 6 and the wettable region 5a is θ₃ and the contact angle between the heat dissipation material 6 and the non-wettable region 5b is θ₄, the heat dissipator 3 may satisfy the following formula (2).

cos ⁢ θ 3 - cos ⁢ θ 4 ≥ 0.19 ( 2 )

The difference between the wettability of the wettable region and the wettability of the non-wettable region is relative. Therefore, in the heating surface 4, the wettability of the wettable region 4a may be improved, or the wettability of the non-wettable region 4b may be reduced, or both of them may be achieved. Similarly, in the heat absorbing surface 5, the wettability of the wettable region 5a may be improved, or the wettability of the non-wettable region 5b may be reduced, or both of them may be achieved. The improvement in the wettability of the wettable region 4a and the wettable region 5a and the reduction in the wettability of the non-wettable region 4b and the non-wettable region 5b can be controlled by, for example, surface roughness of the heating surface 4 and the heat absorbing surface 5. The surface roughness of the wettable regions 4a and 5a is 0.01 μm or more and 4 μm or less, preferably 0.01 μm or more and 0.07 μm or less in the Rq value (root mean square height, JIS B 0601: 2013), and the surface roughness of the non-wettable regions 4b and 5b is 0.10 μm or more and 40 μm or less, preferably 0.20 μm or more and 4 μm or less in the Rq value.

In the case where the contact angle of the heat dissipation material 6 on a flat surface is larger than 90°, the heat dissipation material 6 is a material that is difficult to wet, and the larger the surface roughness, the more difficult it is to wet the heat dissipation material 6. Here, the flat surface means that the surface roughness is approximately 0.1 μm in terms of Ra value (arithmetic mean roughness, JIS B 0601: 2013). Examples of the material that is difficult to wet with respect to the heat dissipation material 6 include a base material such as copper, stainless steel, glass, and silicon, an oxide material, a polymer material, and a metal material used in the coating layer described above, and a porous material described later. The surface roughness of the wettable regions 4a and 5a is preferably smaller than the surface roughness of the non-wettable regions 4b and 5b in the case where the wettable regions 4a and 5a and the non-wettable regions 4b and 5b are difficult to wet. In this case, although depending on a relationship between the material and the viscosity of the heat dissipation material 6, for example, the surface roughness of the non-wettable region 4b and the non-wettable region 5b is set to 0.20 μm or more in terms of the Rq value (root mean square height, JIS B 0601: 2013), and the surface roughness of the wettable region 4a and the wettable region 5a is preferably set to 0.07 μm or less in terms of the Rq value.

In order to obtain the relationship between the wettable region 4a and the non-wettable region 4b and the relationship between the wettable region 5a and the non-wettable region 5b, the heating surface and the heat absorbing surface may be processed so as to have the surface roughness as described above. In order to increase the surface roughness of the surface of the non-wettable region 4b or the non-wettable region 5b, for example, it is preferable that a film having a controlled surface roughness is formed on the heating surface 4 or the heat absorbing surface 5. As a film body (also referred to as a coating layer) formed on the non-wettable region 4b or the non-wettable region 5b, for example, a porous film in which through holes having an inner diameter of 10 nm or less are dispersed and arranged so that porosity is 5% or more can be applied. By adopting and arrangement in which the through holes are dispersed, a lotus effect can be obtained by a large number of openings on the surface of the film body, and the wettability can be lowered. The wettability of the heat dissipation material 6 can also be controlled by changing the porosity and the inner diameter of the through hole. Further, it is preferable that the thickness of the porous film is set to 0.2 μm to 50 μm or less so as not to increase the thermal resistance of the heat dissipation structure 1.

The porous film described above can be formed by, for example, an aerosol deposition method (Aerosol Deposition Method: AD method). In the aerosol deposition method, ceramic particles accelerated by a carrier gas strike a substrate placed in a vacuum at high speed to form a porous film on the substrate. Porosity and the like can be controlled as described above depending on the particle size of the ceramic particles of the raw material, the speed of striking, and the like. The porous film formed by the AD method is referred to as a porous AD film. As the porous film, alumina (Al₂O₃), aluminum hydroxide (Al (OH)₃, zirconia (ZrO₂), aluminum nitride (AlN), silicon carbide (SiC), and silicon nitride (Si₃N₄) can be used. Further, a particle diameter of the ceramic particles is preferably 0.2 μm or more and 1 μm or less.

The surface of the non-wettable region 4b and the non-wettable region 5b may be treated by increasing the surface roughness of the surface of the non-wettable region 4b or the non-wettable region 5b. The heating surface and the heat absorbing surface may be patterned by laser patterning, nanoimprinting patterning, plasma etching, chemical etching, or the like in regions where the non-wettable region 4b or the non-wettable region 5b is formed. For example, a linear convex shape, a cylindrical shape, or a circular hole-like pattern may be formed on the heating surface and the heat absorbing surface in the region where the non-wettable region 4b or the non-wettable region 5b is formed. In the case of forming a linear convex pattern, the pattern preferably has a width of 1 μm to 50 μm, and a distance between the patterns is preferably 1 μm to 50 μm. In the case of forming a cylindrical pattern, it is preferable to have a width of 1 μm to 50 μm, and it is preferable to provide a pattern spacing of 1 μm to 50 μm. In the case of forming a circular hole-shaped pattern, it is preferable to have an inner diameter of 1 μm to 50 μm, and it is preferable to provide a pattern interval of 1 μm to 50 μm. Further, the heating surface and the heat absorbing surface may be roughened by grinding the surface with sand blasting or sand, or sand paper may be bonded to the region where the non-wettable region 4b or the non-wettable region 5b is formed. Accordingly, the surface roughness of the non-wettable region 4b and the non-wettable region 5b can be set to 0.20 μm or more in the Rq value. This makes it possible to reduce the wettability of the non-wettable region 4b and the non-wettable region 5b with respect to the wettability of the wettable regions 4a and 5a.

In addition, instead of increasing the surface roughness of the surface of the non-wettable region 4b or the non-wettable region 5b, it is also possible to use methods of chemically reducing the wettability. For example, a fluorine-based functional group having low wettability to the liquid metal or a hydroxyl group using oxygen plasma may be added to the region in which the non-wettable region 4b or the non-wettable region 5b is to be formed. Accordingly, it is possible to increase the difference between the contact angle of the non-wettable region 4b with respect to the heat dissipation material and the contact angle of the non-wettable region 5b with respect to the heat dissipation material.

On the other hand, as methods for improving the wettability of the wettable region 4a and the wettable region 5a, the wettable region 4a and the wettable region 5a may be subjected to application of a silane-based functional group or surface smoothing by polishing. In addition, coating layers may be provided on the wettable region 4a and the wettable region 5a by the heat dissipation material 6 and a highly wettable material. In the case where a Ga alloy is used for the heat dissipation material 6, Ni capable of forming an alloy with Ga, a Ga material such as gallium, gallium oxide, or gallium nitride, diamond, DLC, graphene, gold, or the like can be used as the material of the coating layer. Further, platinum or the like which is less likely to be corroded with respect to the heat dissipation material 6 may be used. In the case where a coating layer is provided on the wettable regions 4a and 5a, the coating layer preferably has a thickness of 5 μm or less, and preferably does not increase the thermal resistance of the heat dissipation structure 1. The film thickness of the wettable regions 4a and 5a is preferably the same as the film thickness of the non-wettable regions 4b and 5b, or the film thickness of the wettable regions 4a and 5a is preferably smaller than the film thickness of the non-wettable regions 4b and 5b.

Depending on the material used as the heat dissipation material 6, the contact angle of the heat dissipation material 6 on the flat surface may be 90° or less. In this case, the larger the surface roughness is, the more easily it is to wet the heat dissipation material 6. Therefore, in the wettable regions 4a and 5a, the surface roughness of the wettable regions 4a and 5a may be larger than the surface roughness of the non-wettable regions 4b and 5b in the case where a material having a contact angle of the heat dissipation material 6 of 90° or less is used and a material having a contact angle of the heat dissipation material 6 greater than 90° is used in the non-wettable regions 4b and 5b.

For example, a semiconductor element used in a data server or the like has an increased amount of heat generation during use. In order to efficiently transfer the heat of the semiconductor element to the heat dissipator side, it is preferable that the thermal conductivity of the heat dissipation material arranged between the semiconductor element and the heat dissipator is high, and it is preferable that thermal resistance at an interface between the semiconductor element and the heat dissipation material and an interface between the heat dissipator and the heat dissipation material is low. Conventionally used greases have lower thermal conductivity compared to liquid metals. Carbon sheets have a higher thermal conductivity than grease. However, due to roughness of the surface of the carbon sheet, a region containing air is formed at the interface with the semiconductor element and the interface with the heat dissipator. As a result, there is a problem that the thermal resistance increases due to an increase in contact resistance of the interface.

In the case where a liquid metal is used as the heat dissipation material 6, a contact area between the heat dissipation material 6 and the heating surface 4 and a contact area between the heat dissipation material 6 and the heat absorbing surface 5 can be increased by increasing the wettability of the region where the heat dissipation material 6 of the heating surface 4 is arranged and the region where the heat dissipation material 6 of the heat absorbing surface 5 is arranged. This makes it possible to reduce the thermal resistance at the interface between the heat dissipation material 6 and the heating surface 4 and the interface between the heat dissipation material 6 and the heat absorbing surface 5. Therefore, the heat dissipation performance of the heat dissipation structure 1 can be improved.

In the present embodiment, although a process of increasing the wettability of the wettable regions 4a and 5a, a process of reducing the wettability of the non-wettable regions 4b and 5b, or a process of forming a film in the heating surface 4 and the heat absorbing surface 5 has been described, an embodiment of the present disclosure is not limited thereto. If the formula (1) and the formula (2) representing the difference in the contact angle with respect to the heat dissipation material 6 are satisfied, the treatment of increasing the wettability of the wettable regions 4a and 5a or the formation of a film may be performed alone, or the treatment of decreasing the wettability of the non-wettable regions 4b and 5b or the formation of a film may be performed alone.

Even in the case where the non-wettable regions 4b and 5b are formed by controlling the surface roughness, the surface roughness may change over time if the non-wettable region is corroded by the heat dissipation material 6. As a result, the wettability of the non-wettable regions 4b and 5b increases, so that the heat dissipation material 6 that wets and spreads to the region on the non-wettable regions 4b and 5b cannot be returned to the wettable regions 4a and 5a, which may cause pump-out and voids. By coating at least the non-wettable regions 4b and 5b with a material that is resistant to corrosion against the heat dissipation material 6, it is possible to suppress the surface roughness of the non-wettable regions 4b and 5b from changing over time. As a result, in the heat dissipation structure 1, the occurrence of pump-out and voids can be suppressed, and thus the reliability is improved.

Specifically, the non-wettable region 4b may be provided with a coating layer using a material selected from the group consisting of an oxide material including SiO₂, Al₂O₃, and TiO₂, a polymeric material including polytetrafluoroethylene (PTFE), polypropylene, polyethylene, natural rubber, polyurethane rubber, acrylonitrile-butadiene rubber, and silicone rubber, a metallic material including Ni, Pt, and Au, and a porous material including Al₂O₃, Al(OH)₃, ZrO₂, AlN, SiC, Si₃N₄. In addition, the wettable region 4a may be provided with the same coating layers as the non-wettable region 4b. In the case where coating layers of the same material are provided in the wettable region 4a and the non-wettable region 4b, the contact angles of the wettable region 4a and the non-wettable region 4b may be controlled by performing a process of controlling the surface roughness on at least one of the wettable region 4a and the non-wettable region so as to satisfy the formula (1) or the formula (2).

The non-wettable region 5b may also be provided with a coating layer using a material selected from the group consisting of an oxide material including SiO₂, Al₂O₃, and TiO₂, a polymeric material including polytetrafluoroethylene (PTFE), polypropylene, polyethylene, natural rubber, polyurethane rubber, acrylonitrile-butadiene rubber, and silicone rubber, a metallic material including Ni, Pt, and Au, and a porous material including Al₂O₃, Al(OH)₃, ZrO₂, AlN, SiC, Si₃N₄. In addition, the wettable region 5a may be provided with the same coating layers as the non-wettable region 5b. In the case where coating layers of the same material are provided in the wettable region 5a and the non-wettable region 5b, the contact angles of the wettable region 5a and the non-wettable region 5b may be controlled by performing a process of controlling the surface roughness on at least one of the wettable region 5a and the non-wettable region so as to satisfy the formula (1) or the formula (2).

The non-wettable region 4b and the non-wettable region 5b may be provided with a coating layer using a material selected from the group consisting of an oxide material including SiO₂, Al₂O₃, and TiO₂, a polymeric material including polytetrafluoroethylene (PTFE), polypropylene, polyethylene, natural rubber, polyurethane rubber, acrylonitrile-butadiene rubber, and silicone rubber, a metallic material including Ni, Pt, and Au, and a porous material including Al₂O₃, Al(OH)₃, ZrO₂, AlN, SiC, Si₃N₄. In addition, the wettable regions 4a and 5a may be provided with the same coating layers as the non-wettable regions 4b and 5b. In the case where coating layers of the same material are provided in the wettable regions 4a and 5a and the non-wettable regions 4b and 5b, the contact angles of the wettable regions 4a and 5a and the non-wettable regions 4b and 5b may be controlled by performing the process to control the surface roughness of at least one of the wettable regions 4a and 5a and the non-wettable regions 4b and 5b so as to satisfy the formula (1) or the formula (2).

The shape of the heating element 2 and the heat dissipator 3 may be a shape other than a plane as long as the elastic deformation behavior of the heat dissipation material 6 is not hindered. FIG. 5 is a cross-sectional side view showing another embodiment of the heat dissipation structure. FIG. 5 shows a heat dissipation structure 10 in which a recess region 14c and a recess region 15c are arranged at positions facing each other in each of a heating element 12 and a heat dissipator 13.

Specifically, as shown in FIG. 5A, the recess region 14c is formed on a heating surface 14 of the heating element 12, and the recess region 15c is formed on a heat absorbing surface 15 of the heat dissipator 13. The heat dissipation material 6 is applied to the recess region 14c of the heating element 12, and the recess region 14c and the recess region 15c face each other to sandwich the heat dissipation material 6 between the recess region 14c and the recess region 15c. A vicinity of the center of the recess region 14c is a wettable region 14a, and a non-wettable region 14b is formed so as to surround a periphery of the wettable region 14a. The non-wettable region 14b is arranged over an entire region of the heating surface 14 excluding the wettable region 14a in a region where the heat dissipation material 6 can be contacted. For example, a bottom surface of the recess region 14c, a rising portion of an outer periphery of the recess region 14c, a surface that can be contacted with the heat absorbing surface 15, and the like are also included in the non-wettable region 14b. Similarly, a vicinity of the center of the recess region 15c is a wettable region 15a, and a non-wettable region 15b is formed so as to surround a periphery of the wettable region 15a. The non-wettable region 15b is arranged over an entire region of the heat absorbing surface 15 excluding the wettable region 15a in a region where the heat dissipation material 6 can be contacted. For example, a rising part of an outer periphery of the recess region 15c, a surface that can be contacted with the heating surface 14, and the like are also included in the non-wettable region 15b. In this case, the heat dissipation material 6 is filled so as to fill a gap in which the wettable region 14a and the wettable region 15a face each other.

As shown in FIG. 5B, it is assumed that the heating element 12 and the heat dissipator 13 are close to each other due to deformation of the heat dissipation structure 10. In this case, the heat dissipation material 6 maintains its volume, and spreads so as to touch the non-wettable region 14b and the non-wettable region 15b in the recess region 14c and recess region 15c. The heat dissipation material 6 may be filled with a quantity that fills an accommodation space formed by the recess region 14c of the heating element 12 and the recess region 15c of the heat dissipator 13 by contacting the heating element 12 and the heat dissipator 13. A gap d between the heating element 12 and the dissipation element 13 may be 500 μm or less.

Also in this case, a force that tries to return from the non-wettable region 14b and the non-wettable region 15b to the wettable region 14a and the wettable region 15a acts on the heat dissipation material 6. As shown in FIG. 5A, when the deformation of the heat dissipation structure 10 returns to its original state, the heat dissipation material 6 returns from opposite regions of the non-wettable region 14b and the non-wettable region 15b to opposite regions of the wettable region 14a and the wettable region 15a due to its elastic deformation behavior. Although it is preferable that the non-wettable region 14b is formed so as to surround the wettable region 14a on a bottom surface of the recess region 14c, and the non-wettable region 15b is formed so as to surround the wettable region 15a on a bottom surface of the recess region 15c, the non-wettable region 14b and the non-wettable region 15b may not be arranged on the bottom surface of the recess regions 14c and 15c.

With respect to materials used for the heating element 12 and the heat dissipation element 13, and control of contact angles of the heating surface 14 and the heat absorbing surface 15, the description of the materials of the heating element 2 and the heat dissipator 3 shown in FIG. 1 to FIG. 4, and the control of the contact angles of the heating surface 4 and the heat absorbing surface 15 may be referred to. According to the heat dissipation structure 10, it is possible to prevent a liquid metal 16 from leaking to the outside of the heat dissipation structure 10 in the case where a strong load is applied so that a shape of the liquid metal cannot be controlled even by controlling the wettability between the wettable region and the non-wettable region in the heat dissipation structure 1.

EXAMPLES

In this embodiment, various tests performed by actually manufacturing the heat dissipation structure 1 according to an embodiment of the present invention will be described.

Verification of Surface Roughness and Contact Angle

A film body serving as a non-wettable region was produced on a substrate by the aerosol deposition method. As the substrate, a substrate made of stainless steel (SUS (registered trademark) 304 steel) was used. In the aerosol deposition method, a ceramic powder (a mixture of alumina particles and aluminum hydroxide particles) accelerated at room temperature (about 300 K) using dry air at a 6 L/min flow rate in a vacuum of 400 Pa was discharged from a nozzle having an opening with a size of 4 mm and collided with a substrate of 40 mm×40 mm. The nozzle was moved along the substrate at a speed of 10 mm/sec, and the ceramic particles were sprayed onto an entire main surface of the substrate to obtain a film. In the following explanation, the obtained film is also referred to as a porous AD film. A surface roughness of the film was measured by 1 cm lengths using a surface roughness measuring instrument (SURFTEST SV-3100, Mitutoyo Corp.), and the film was formed to have a Rq value of 0.26 μm, a thickness of 3 μm, and a large number of through holes having an inner diameter of 10 nm or less. Porosity was measured using a mercury porosimeter (BELPORE, Micro track) and the porosity of the film was 5%.

FIG. 6A and FIG. 6B show electron-micrographs of ceramics powders used as raw materials. In FIG. 6C, a stainless-steel substrate is shown on the left, and a film formed on the substrate is shown on the right. FIG. 6D is an enlarged electron micrograph of a part 302 of the film shown in FIG. 6C. As shown in FIG. 6D, the film formed on the substrate is porous.

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

FIG. 7A is a photograph showing a state in which the liquid metal is dropped on a silicone. FIG. 7B is a photograph showing a state in which the liquid metal is dropped on copper. FIG. 7C is a photograph showing a state in which the liquid metal is dropped onto the porous AD film. FIG. 7D is a photograph showing a state in which the liquid metal is dropped on a glass. FIG. 7E is a photograph showing a state in which the liquid metal is dropped on a stainless-steel plate. It was also confirmed that the liquid metal did not enter the through holes of the porous AD film.

FIG. 8 is a list of a surface roughness of each material and a contact angle θ of the liquid alloy with respect thereto. As shown in FIG. 8, in the porous AD film, a contact angle of the liquid metal is 169°, which is larger than contact angles of the other materials. It is probable that the porous AD film had a lotus effect.

For example, in the case where a glass is used as the heating element 2, the porous AD film is formed on a portion of the heating surface 4 to form the non-wettable region 4b, and a portion where the glass is exposed is defined as the wettable region 4a. Since a contact angle of the glass is 133° and the contact angle of the porous AD film is 169°, θ₁=133°, θ₂=169° in the formula (1) described above. Therefore, it can be seen that a left-hand side cos θ₁-cos θ₂ of the formula (1) is −0.682−(−0.982)=0.300, which satisfies the formula (1). In addition, copper, stainless steel, glass, and silicon each have a surface roughness of 0.07 μm or less in the Rq value, and can be used as the wettable region 4a in the same manner as the glass.

Further, as shown in FIG. 9, the contact angle of the liquid metal on the surface of the silicon on which various patterns of a linear shape, a cylindrical shape, and a circular hole shape of 10 μm were formed was measured. Ga—In eutectic was used as the liquid metal, similar to the above description. A pattern in which a distance between end portions of the convex regions or the concave regions of the linear convex shape (width: 10 μm), the cylindrical shape (diameter: 10 μm), and the circular hole shape (inner diameter: 10 μm) was changed was formed and used for measuring the contact angle. As shown in FIG. 9, in the case of a linear or circular hole-shaped pattern, the contact angle tended to increase when the distance between the patterns was increased. In addition, in the cylindrical pattern, a large contact angle of 150° or more was maintained regardless of the distance between the patterns.

Results shown in FIG. 9 show that the contact angle with the liquid metal can be controlled according to the shape of the pattern and the spacing of the patterns.

About Pump-Out

Next, results of the pump-out test on the heat dissipation structure will be described.

FIG. 10 is a side view showing a state of a pump-out test by a compression testing machine. As shown in FIG. 10, a gap between the glass plates sandwiching the liquid metal was changed by using a compression testing machine 20 (manufactured by A & Day Co., Ltd., single-column-type material tester; STB-1225L), and the state of pump-out was examined. A thickness of the glass plate is 1 mm, and 50 μL of Ga—In eutectic liquid metal was applied on a lower glass plate 24 arranged on a compression plate 22 on a lower side, and an upper glass plate 25 arranged on a lower surface of a compression plate 23 on an upper side was lowered by a crosshead 21 to compress the liquid metal. In addition, a position of the crosshead 21 when the lowered upper glass plate 25 was brought into contact with the liquid metal applied on the lower glass plate 24 was set as a reference position, and a direction (downward) for compressing the liquid metal from the reference position was set as a positive direction, and a direction (upward) for separating the upper glass plate 25 from the lower glass plate 24 was set as a negative direction.

First, a simple unidirectional compression test was carried out for both the case where no non-wettable region was formed and an entire surface of the glass plate was uniform, and the case where a non-wettable region was formed. A major surface dimension of the glass sheet is a rectangle of 3.5 cm×2.5 cm.

FIG. 11 is a perspective view of the lower glass plate 24 having a non-wettable region 24b formed thereon and the upper glass plate 25 having a non-wettable region 25b formed thereon. As shown in FIG. 11, when forming the non-wettable region, the porous AD film described above was formed on the lower glass plate 24 as the non-wettable region 24b and the upper glass plate 25 as the non-wettable region 25b. Further, the main surface dimension of the glass plate is also the rectangle of 3.5 cm×2.5 cm, and the non-wettable region 24b and the non-wettable region 25b was formed around a periphery of the lower glass plate 24 and the upper glass plate 25 so as to have a size of 2.5 cm×2.0 cm.

In the case where the liquid metal was compressed with a compressive rate of 1 mm/s in a uniform glass plate, the liquid metal spilled from the periphery of the glass plate under a load of 0.5 N. On the other hand, in the case of the glass plate having the non-wettable region formed thereon, the liquid metal spilled under a load of 6.0 N. That is, it has been found that by forming the wettable region, it is possible to withstand a load of 10 times or more without causing a pump-out.

The liquid metal was then compressed and unloaded 5 times using the uniform glass plate. In the pressurization, a load of 0.3 N was applied as a load that did not cause pump-out, and a position of the crosshead in this case was 0.2 mm to 0.25 mm. At the time of unloading, when the load became zero, movement of the crosshead was stopped, and compression was started.

FIG. 12 is a graph showing a relationship between a load and a crosshead position in a repeated compression test using a uniform glass plate without forming a non-wettable region. FIG. 13A to FIG. 13C are side-sectional views showing changes in the heat dissipation configuration of a repeated compression test performed without forming a non-wettable region. Referring to both FIG. 12 and FIG. 13, the liquid metal that had been pressurized and spread out in the first cycle (see FIG. 13B) from the initial state (see FIG. 13A) was unloaded without returning to its original thickness (crosshead position is zero) (see FIG. 13C), and compressed and unloaded repeatedly. In addition, it was found that the crosshead position only moved about 0.05 mm between compressing and unloading after the second cycle, and further, the crosshead position at the time of unloading was progressively advanced. This shows a general behavior of the liquid metal as a viscous fluid that wets and spreads under pressure.

Next, the non-wettable regions 24b and 25b were repeatedly subjected to compressing and unloading of the liquid metal with the formed glass plate. FIG. 14 is a graph showing a relationship between a load and a crosshead position in a repeated compression test performed by forming a non-wettable region. A horizontal axis represents the crosshead position (mm), and a vertical axis represents the load (N). Unlike the case where the non-wettable region shown in FIG. 12 is not arranged, the crosshead position is restored almost to the initial position (zero) at the time of unloading. In addition, in the pressurization, a load of about 4 N was applied at most, and the crosshead position in this case was 0.07 mm to 0.08 mm. That is, the displacement of the crosshead was small even if a large load was applied as compared with the case where the non-wettable region was not arranged. From this, it was confirmed that, in the elastic behavior of the liquid metal, a force for pushing back the load in the thickness direction is also generated. That is, it was shown that the wet spreading when the liquid metal is pressurized is also suppressed.

FIG. 15 is a cross-sectional side view showing a change in the heat dissipation structure of the repeated compression test performed by forming the non-wettable region. Referring to both FIG. 14 and FIG. 15, the liquid metal that has been pressurized from the initial state (see FIG. 15A) and spread to the regions of the non-wettable regions 24b and 25b (see FIG. 15B) has returned to its original thickness by unloading and has returned from the non-wettable regions 24b and 25b to regions of wettable regions 24a and 25a (see FIG. 15A). This was the same for 5 cycles.

As described above, it has been found that the non-wettable regions 24b and 25b are formed on the heat dissipator and the heating element, thereby causing an elastic deformation behavior of the liquid metal. In particular, it was confirmed that the liquid metal can follow the dimensional change of the gap between the heating surface and the heat absorbing surface while suppressing the wetting spread during pressurization by generating a force for pushing back the load in the thickness direction.

Thermal Resistance

Next, results of measuring the thermal resistance of the heat dissipation structure will be described.

FIG. 16 is a side view of a sample used to measure thermal resistance. As a sample, a heat dissipation structure in which a liquid metal is sandwiched between two Si plates 31 was prepared. A thickness of each of Si plate 31 was 525 μm, and a thickness of the liquid metal was 50 μm. The liquid metal is Ga—In eutectic as described above. As a comparative example, a heat dissipation structure consisting of only Si plates without the liquid metal, and a heat dissipation structure in which a carbon sheet (VB-200, manufactured by Nippon Zeon Co., Ltd.) was sandwiched between the Si plates 31 instead of the liquid metal, were produced.

Thermal resistance was measured for each of the heat dissipation structures described above. First, using a xenon flash analyzer (HyperFlash) manufactured by NETZSCH, thermal diffusivity in the thickness direction was measured at room temperature (25° C.) under no pressure. Thermal conductivity was calculated by multiplying the thermal diffusivity by a specific heat and density obtained from a literature value, and the thermal resistance (cm²K/W) in the thickness direction was calculated from a thickness of the sample sandwiched between the Si plates. Thermal resistance was measured at three points for each heat dissipation structure produced.

FIG. 17 is a graph showing a list of calculated thermal resistance values. The thermal resistance shown in FIG. 17 is the thickness of the entire sample including the Si plate. As shown in FIG. 17, the thermal resistance of a sample with only the Si plate without liquid metal was 1.37 cm²K/W to 1.38 cm²K/W. In addition, the thermal resistance in the heat dissipation structure in which the carbon sheet is sandwiched between the Si plates is 3.55 cm²K/W to 3.90 cm²K/W. In addition, the thermal resistance in the heat dissipation structure in which the liquid metal is sandwiched between the Si plates is 0.11 cm²K/W. The heat dissipation structure using liquid metal as a sample showed the lowest value.

For each of the heat dissipation structures described above, the thermal resistance was measured in the case where a repeated temperature change was applied from −20° C. to 120° C. The thermal resistance was obtained by measuring the thermal resistance at three points for each of the produced heat dissipation structures and averaging the three points. FIG. 18 is a graph showing temperature and thermal resistance of a sample in which nothing was sandwiched between the Si plates. FIG. 19 is a graph showing temperature and thermal resistance of a sample in which a carbon sheet is sandwiched between the Si plates. FIG. 20 is a graph showing thermal resistance of a sample with the liquid metal sandwiched between the Si plates. As shown in FIG. 18 to FIG. 20, the thermal resistance of the heat dissipation structure in which the liquid metal is sandwiched between the Si plates 31 is the lowest at all temperatures. In addition, the liquid metal used in the present example changes to a solid at 15° C. As shown in FIG. 20, even if the liquid metal is changed to a solid, a low thermal resistance is maintained and the fluctuation of the thermal resistance is suppressed as compared with FIG. 18 and FIG. 19.

Next, in the heat dissipation structure, the thermal resistance of the sample in which the thickness of the liquid metal was changed to 10 μm, 40 μm, and 100 μm was measured. The thermal resistance was measured at 25° C. FIG. 21 is a graph showing a relationship between a thickness of a liquid metal and thermal resistance of a sample. The thermal resistance shown in FIG. 21 is the combined thermal resistance between the liquid metal and the interface. As shown in FIG. 21, the lower the thickness of the liquid metal, the lower the thermal resistance. Also, in the case where the thickness of the liquid metal was 10 μm, the thermal resistance was 0.001 cm²K/W. In the case where the thickness of the liquid metal was 40 μm, the thermal resistance was 0.048 cm²K/W. In the case where the liquid metal depth was 100 μm, the thermal resistance was 0.078 cm²K/W. Further, it was confirmed that voids did not occur even if the thickness of the liquid metal was 10 μm.

Stability of Heat Dissipation Performance

Next, results of the heat dissipation test in which stability of the heat dissipation performance in a situation simulating an actual use environment of the heat dissipation structure is investigated will be described.

FIG. 22 is a perspective view showing positions of non-wettable regions of an Si plate and a Cu plate in the heat dissipation structure. The Si plate was used as a heating element, and the Cu plate was used as a heat dissipator. Each of the Si plate and the Cu plate is a square having a size of 3 cm square. For both the Si plate and the Cu plate, a width of 2.5 mm from an outer periphery was a non-wettable region so that a wettable region was 2.5 cm square. As the non-wettable region, a porous film (porous AD film) having a thickness of 3 μm was formed. After applying 0.5 g of the liquid metal (Ga—In eutectic) as the heat dissipation material 6 to a wettable region 35a, a Si plate 32 was stacked on an upper side of a Cu plate 33 to prepare a heat dissipation structure.

Next, a heat dissipation test was performed on the heat dissipation structure. FIG. 23A is a perspective view of a heat dissipation test apparatus, and FIG. 23B is a side view showing a stacking order from the Si plate to the Cu plate. As shown in 23A, in a heat dissipation test apparatus 40, a heating plate 42 made of Cu 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 a lower surface. The heating plate 42 is heated at one end by a heater 43 with an output of 20 W, and the temperature of the end at an opposite position can be measured by a thermometer 44 using a thermocouple. On the other hand, the Cu plate 33 is fixed to a cool plate 45 maintained at 15° C. below the Si plate 32. The heating plate 42 is lowered together with the load cell 41, a point in time at which the Si plate 32 comes into contact with the heat dissipation material 6 is set as a zero point, and the Si plate 32 is repeatedly subjected to a displacement of ±200 μm (width 400 μm) up and down from the zero point using the automatic stage, with one cycle occurring every 2 seconds. This simulates the repeated thermal stress and thermal strain of a power semiconductor.

First, as a comparative test, a similar configuration to that shown in FIG. 23B was used, but no non-wetted portions were formed on the Si plate 32 and the Cu plate 33 and 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 a heat dissipation test of the thermal grease. As shown in FIG. 24, the temperature continued to rise immediately after the start of the test, resulting in a rapid temperature rise in several hundred cycles, reaching 100° C. in approximately 1200 seconds (approximately 600 cycles), and the test was terminated by the operation of a safety device. The rapid temperature rise was probably due to poor thermal contact.

Next, as a comparative test, a similar configuration to that shown in FIG. 23B was used, but no non-wetted portions were formed on the Si plate 32 and the Cu plate 33 and a liquid metal was used as a heat dissipation material, and the temperature was measured. The load change of the load cell was also measured. FIG. 25 is a graph showing time, temperature, and load of a heat dissipation test of a liquid metal performed without forming a non-wettable region. A temperature rise of about 2° C. occurred in about 100 seconds after the start of the test, and a temperature rise of about 4° C. occurred in about 7000 seconds. This temperature rise is considered to be caused by the deterioration of a thermal contact state due to the generation of voids. In addition, the reason why the temperature rise is lower than the thermal grease is considered to be that the thermal conductivity of the liquid metal is 10 times or more higher and that the fluidity is higher. In addition, the load is slightly lower at the end of the test than immediately after the start of the test. This also indicates the generation of voids.

FIG. 26 is an external photograph of a Cu plate after the heat dissipation test. As shown in FIG. 26, a Cu plate 33 after the test showed that the liquid metal-free parts were spotted. That is, the generation of voids was confirmed.

Finally, a non-wettable region was formed on the Si plate 32 and the Cu plate 33, and a heat dissipation structure using the liquid metal as a heat dissipation material was measured. A load change of the load cell was also measured. FIG. 27 is a graph showing time, temperature, and load of a heat dissipation test of the liquid metal performed by forming the non-wettable region. As shown in FIG. 27, the temperature rise of about 2° C. after the start of the test occurred in the same manner as in the case where the wettable region was not formed, but thereafter maintained at a substantially constant temperature up to about 7000 seconds. A temperature change during this period was within ±1° C. That is, it has been found that there is almost no change in the thermal contact state, and the thermal contact state can be stably maintained over a long period of time. In addition, it is probable that the amount of the liquid metal was kept constant because there was almost no change in the load.

FIG. 28 is an external photograph of the Cu plate after the heat dissipation test. As shown in FIG. 28, on a surface of the Cu plate 33 after the test, it was observed that the liquid metal uniformly spread on the wetted surface and hardly leaked out on the non-wetted surface. That is, it can be seen that generation of voids and pump-out was sufficiently suppressed.

As described above, according to the heat dissipation structure of the present embodiment, it was confirmed that the high heat dissipation performance by the liquid metal can be favorably maintained.

Thermal Resistance to Heat Dissipation Material

Next, results of measuring the thermal resistance of the heat dissipation material will be described.

FIG. 29 is a side view of a steady-state thermal resistance measurement apparatus. As shown in FIG. 29, a heat dissipation material 60 is sandwiched between measurement rods 53 and 54. The measurement rod 53 includes a thermocouple 56 and the measurement rod 54 includes a thermocouple 52. FIG. 30 is a perspective view of the measurement rod 53. On a surface of the measurement rods 53, gold was coated as a wettable region 53a and sandpapers (TRUSCO Sheet Paper Particle Size #400, manufactured by Trusco Nakayama Co.) were adhered as a non-wettable region 53b, thereby controlling wettability. Although not shown, the measurement rod 54 also formed a wettable region and a non-wettable region in the same manner as the measurement rod 53. As the heat dissipation material 60, a thermal grease (heat dissipation oil compound G746, manufactured by Shin-Etsu Chemical Co., Ltd.) and a Ga-In eutectic liquid metal were used.

In the steady-state thermal resistance measurement apparatus, an upper measurement rod 57 was heated to 100° C. by a heating plate, and the lower measurement rod 53 was cooled to 20° C. by a cooling plate 58. The thermal resistance was measured in accordance with ASTM D5470 at a thickness of 200 μm using the liquid metal as the heat dissipation material 60. Thereafter, the upper measurement rod 54 was displaced by a mechanical testing machine 59 up and down by ±30 μm and 100 times to impart deformation to the heat dissipation material 60 repeatedly. Thereafter, by measuring the thermal resistance of the heat dissipation material 60 in accordance with the ASTM D5470 at a thickness of 200 μm, the thermal resistance change of the heat dissipation material 60 in the case where the repeated deformation under actual use was given was evaluated. In addition, a thermal grease was used as the heat dissipation material 60, and thermal resistance was measured as in the case of using the liquid metal with a thickness of 200 μm. Further, the heat dissipation material 60 is sandwiched between the measurement rods 53 and 54 without controlling the wettability of the wettable region and the non-wettable region on the surfaces thereof. 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 using liquid metal as the heat dissipation material 60, and setting the thickness to 200 μm.

FIG. 31 shows the measurement results of the thermal resistance of the liquid metal sandwiched between thermal greases, the measurement rods 53 and 54 in which the wettability differences are not controlled, and the measurement rods 53 and 54 in which the wettability differences are controlled. It was found that the liquid metal sandwiched by both of the thermal greases and the measurement rods 53 and 54 which had no controlled wettability difference had increased thermal resistance after 100 displacements. On the other hand, it was found that in the liquid metal sandwiched by the measurement rods 53 and 54 whose wettability difference was controlled, there was no change in the thermal resistance even after 100 displacements, and the liquid metal exhibited the same thermal resistance as the initial state.

According to the present example, it is shown that the heat dissipation structure according to the embodiment of the present invention has low thermal resistance and can suppress fluctuation in thermal resistance. Thus, even if a heating element such as a semiconductor element generates heat, heat can be efficiently released. Therefore, reliability of a device on which the semiconductor element is mounted can be improved.

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