Patent Publication Number: US-2021180876-A1

Title: Vapor chamber

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application is a continuation of International application No. PCT/JP2020/012857, filed Mar. 24, 2020, which claims priority to Japanese Patent Application No. 2019-115877, filed Jun. 21, 2019, the entire contents of each of which are incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to a vapor chamber. 
     BACKGROUND OF THE INVENTION 
     In recent times, mobile terminals such as smartphones or tablet PCs have been increasing heat generation due to high integration and high performance of devices. As size reduction of products proceeds, heat-generation density increases further. Thus, heat dissipation performs an important role for such products. 
     Examples of a heat dissipater with high heat dissipation capability include a vapor chamber, which is a planar heat pipe. A vapor chamber has an apparent coefficient of entire thermal conductivity of several to several tens of times higher than that of metals such as copper or aluminum. 
     As an example of a heat dissipater including a vapor chamber, Patent Document 1 describes a planar heat pipe including a container with a protrusion having a cavity portion defined by two opposing plates at a center portion, and an operation fluid enclosed in the cavity portion. The cavity portion has a wick structure. The protrusion is sealed by laser welding at the outer periphery. 
     Patent Document 1: Japanese Unexamined Patent Application Publication No. 2016-35348 
     SUMMARY OF THE INVENTION 
     In the planar heat pipe described in Patent Document 1, the protrusion is sealed with laser welding at the periphery. Here, the plates are heated while undergoing laser welding which results in the entirety of the planar heat pipe being undesirably warped. 
     The present invention has been made in view of these circumstances, and aims to provide a vapor chamber having smaller warpage caused by joining involving heating. 
     A vapor chamber according to the present invention includes a housing including a first sheet and a second sheet opposing each other and having respective outer edges of the first sheet and the second sheet joined together; an operation fluid enclosed in the housing; a wick on an inner surface of the first sheet and/or an inner surface of the second sheet; a first pillar on the inner surface of the first sheet and/or the inner surface of the second sheet and that defines a cavity in the housing; and a second pillar on the inner surface of the first sheet and/or the inner surface of the second sheet and configured to prevent the housing from being warped when heated to join the first sheet and the second sheet together. The first pillar preferably has an area that is equal to or smaller than 0.05% of an area of the housing in a plan view. The second pillar preferably has an area that is 0.5% to 7.0% of the area of the housing in the plan view. A profile of the vapor chamber in the plan view is in the form of a rectangle or a combination of a plurality of rectangles. The second pillar is located on an inner side of one of the rectangles longest in a longitudinal direction so as to extend in the longitudinal direction of the longest rectangle at a position passing a center point of the longest rectangle. The second pillar has a length in the longitudinal direction of the longest rectangle that is 30% to 70% of the length of the longest rectangle. The second pillar has a width in a width direction of the longest rectangle that is 5% to 10% of the width of the longest rectangle. 
     The present invention can provide a vapor chamber having smaller warpage caused by joining involving heating. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic cross-sectional view of an example of a structure of a vapor chamber. 
         FIG. 2  is a schematic top view of an example of a vapor chamber. 
         FIGS. 3A, 3B, and 3C  are schematic cross-sectional views of an example of a second pillar constituting a vapor chamber. 
         FIG. 4  is a top view illustrating the positional relationship between the plan-view profile of the vapor chamber and the second pillar. 
         FIGS. 5A, 5B, 5C, 5D, and 5E  are schematic top views of other examples of a vapor chamber. 
         FIG. 6  is a schematic top view of another example of a vapor chamber. 
         FIG. 7  is a schematic top view of another example of a vapor chamber. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A vapor chamber of the present invention will be described, below. 
     The present invention, however, is not limited to the following structure, and may be changed as appropriate within the scope not departing from the gist of the present invention. A combination of two or more of individual desirable components of the present invention described below is also included in the present invention. 
     The following embodiments are naturally mere examples, and components of different embodiments may be partially replaced with each other or combined together. 
     A vapor chamber according to the present invention includes a housing, including first and second sheets opposing each other with their outer edges joined together with a joining operation that includes heating, an operation fluid enclosed in the housing, a wick on an inner surface of the first sheet and/or an inner surface of the second sheet, a first pillar on the inner surface of the first sheet and/or the inner surface of the second sheet and that defines a cavity in the housing, and a second pillar on the inner surface of the first sheet and/or the inner surface of the second sheet and configured to prevent the housing from being warped when the first sheet and the second sheet are joined together by heating. The first pillar has an area that is equal to or smaller than 0.05% of the area of the housing in a plan view of the housing. The second pillar has an area that is 0.5% to 7.0% of the area of the housing in the plan view. A profile of the vapor chamber in the plan view is a substantial rectangle or a combination of substantial rectangles. The second pillar is located on an inner side of one of the substantial rectangles that is longest in a longitudinal direction and aligned so as to extend in the longitudinal direction of the longest substantial rectangle at a position passing a center point of the longest substantial rectangle. The second pillar has a length in the longitudinal direction of the longest substantial rectangle that is 30% to 70% of the length of the longest substantial rectangle. The second pillar has a width in a width direction of the longest substantial rectangle that is 5% to 10% of the width of the longest substantial rectangle. 
       FIG. 1  is a schematic cross-sectional view of an example of a structure of a vapor chamber. 
     A vapor chamber  1  illustrated in  FIG. 1  includes a housing  10 , including a first sheet  11  and a second sheet  12  opposing each other, an operation fluid  20  enclosed in the housing  10 , a wick  30  disposed on a main surface  11   a  of the first sheet  11  opposing the second sheet  12  (inner surface  11   a  of the first sheet  11 ), and multiple pillars  40  (first pillars  41  and a second pillar  42 ) disposed on a main surface  12   a  of the second sheet  12  opposing the first sheet  11  (inner surface  12   a  of the second sheet  12 ). 
     The housing  10  has a cavity  13  inside. The first sheet  11  and the second sheet  12  are supported by the first pillars  41  to secure the cavity  13 . 
     The first sheet  11  and the second sheet  12  are joined with joining operations including heating at their outer edges that are sealed together. 
     In the vapor chamber  1  illustrated in  FIG. 1 , the wick  30  includes a mesh  32  disposed on the inner surface  11   a  of the first sheet  11 . 
     The pillars  40  may be integrated with the second sheet  12 . For example, the pillars  40  may be formed by etching the inner surface  12   a  of the second sheet  12 . 
     Alternatively, the pillars  40  may be formed by subjecting the second sheet  12  to a process of forming protrusions and depressions. 
     The portion that joins the outer edges of the first sheet  11  and the second sheet  12  is a sealing portion  50 . 
     The operation fluid  20  resides in the wick  30  in a liquid phase. The operation fluid  20  resides in the cavity  13  mainly in a gaseous phase (e.g., water vapor when the operation fluid is water). 
       FIG. 1  illustrates a heat source  120  installed on the main surface (outer surface) of the first sheet  11  not opposing the second sheet  12 . Alternatively, the surface on which the heat source  120  is installed may be a main surface of the second sheet  12  not opposing the first sheet  11 . 
     Heat generated by the heat source  120  vaporizes the operation fluid  20  residing in the wick  30  immediately above the heat source  120 , the vaporization takes away heat from the heat source  120 , and the vaporized operation fluid moves from the mesh  32  to the cavity  13 . 
     The vaporized operation fluid  20  moves into the housing  10 , and condenses near the outer edge of the housing  10  into the liquid phase. 
     The operation fluid  20  in the liquid phase is absorbed by the wick  30  with the capillary force of the wick  30 , and moves through the wick  30  toward the heat source  120  again to take away heat from the heat source  120 . 
     This circulating movement of the operation fluid in the housing cools the heat source with the vapor chamber. 
       FIG. 2  is a schematic top view of an example of a vapor chamber. 
       FIG. 2  is a top view of the vapor chamber  1  viewed from above the second sheet  12 , while illustrating the second sheet  12  in a see-through form, to indicate the positions of the first pillars  41 , the second pillar  42 , and the wick  30 . 
       FIG. 1  is a cross-sectional view of the vapor chamber taken along line A-A in  FIG. 2 . 
     In the vapor chamber  1  illustrated in  FIG. 2 , the housing is a substantial rectangle in a top plan view, and the sealing portion  50  has a shape extending along the outer peripheral sides of the substantial rectangle. 
     The outer edges of the first sheet  11  and the second sheet  12  are joined with joining operations including heating to form the sealing portion  50 . 
     A second pillar  42  is disposed at substantially the center of the vapor chamber  1  illustrated in  FIG. 2 . 
     The position of the second pillar  42  in the vapor chamber  1  will be described in detail, later. 
     The inside of the second pillar may be porous or hollow. 
       FIGS. 3A, 3B, and 3C  are schematic cross-sectional views of examples of a second pillar constituting the vapor chamber. 
     A second pillar  42   a  illustrated in  FIG. 3A  is integrated with the second sheet  12 . 
     A second pillar  42   b  illustrated in  FIG. 3B  is porous inside. 
     A second pillar  42   c  illustrated in  FIG. 3C  is hollow inside. 
     The second pillar integrated with the second sheet can be obtained by, for example, a method of leaving the portion forming the second pillar while other part of the second sheet is removed by cutting or etching. 
     Examples of a method of obtaining a second pillar that is porous inside include a method of sintering metal particles or metal fiber on the surface of the second sheet and a method of welding metal particles or a porous sintered body of metal particles onto the surface of the second sheet. 
     Examples of a method of obtaining a second pillar that is hollow inside include a method of forming ribs on the surface of a flat second sheet to form protrusions on the surface of the second sheet, and a method of forming a non-through hole by laser processing in the second pillar  42   a  illustrated in  FIG. 3A . 
     The pillars other than the second pillar may also be porous or hollow inside, as in the case of the second pillar. 
     In the vapor chamber, the first sheet and the second sheet may be formed from any material having characteristics appropriate for forming the vapor chamber, including heat conductivity, strength, and flexibility. Examples of the preferable material for the first sheet and the second sheet include metals, such as copper, nickel, aluminum, magnesium, titanium, iron, and alloys mainly containing any of these. A particularly preferable material for the first sheet and the second sheet is copper. 
     In the vapor chamber, the operation fluid may be any fluid that can cause gas-liquid phase changes under the environments in the housing. Examples usable as the operation fluid include water, alcohols, and CFC substitutes. The operation fluid is preferably an aqueous compound, and more preferably, water. 
     In the vapor chamber, the wick may be in any form that has a capillary structure that allows the operation fluid to move with the capillary force. The capillary structure of the wick may be any known structure used in an existing vapor chamber. Examples of the capillary structure include a microstructure having unevenness including pores, grooves, and protrusions. Examples of the microstructure include a porous structure, a fiber structure, a groove structure, and a network structure. 
     In the vapor chamber, the wick is preferably continuously formed inside the housing from an evaporator to a condenser. At least part of the wick may be integrated with the housing. 
     In the vapor chamber, the wick may include a mesh, non-woven fabric, or a porous body on the surface of the first sheet opposite to the inner surface. For example, the wick may include multiple protrusions arranged at predetermined intervals on the inner surface of the first sheet, and a mesh, non-woven fabric, or a porous body disposed over the protrusions. Alternatively, the wick may include a mesh, non-woven fabric, or a porous body directly disposed on the inner surface of the first sheet. 
     The vapor chamber is not limited to the above embodiment, and may be modified or applied to various other forms within the scope of the present invention in relation to, for example, the structure or manufacturing conditions of the vapor chamber. 
     For example, the vapor chamber may include a wick on the inner surface of the second sheet. Here, the pillars may support the second sheet with the wick interposed therebetween without directly coming into contact with the second sheet. 
     When viewed in a plan view, the vapor chamber according to the present invention has a profile of a substantial rectangle or a combination of multiple substantial rectangles. 
     When the profile is formed from a combination of multiple substantial rectangles, the number of substantial rectangles forming the profile is preferably equal to or smaller than five, and more preferably equal to or smaller than three. 
     A method for identifying the number of substantial rectangles forming the plan-view profile of the vapor chamber will be described, below. 
     In the vapor chamber according to the present invention, the pillars disposed on the inner surface of the first sheet and/or the inner surface of the second sheet include the first pillars defining the cavity in the housing, and the second pillar for preventing the housing from being warped due to heating performed when joining the first sheet and the second sheet together. 
     The first pillars are pillars for defining a cavity in the housing, and the area of each of the first pillars is equal to or smaller than 0.05% of the area of the housing in a plan view. 
     The entire area of all of the first pillars is preferably 1% to 20% of the area of the housing in the plan view. 
     The second pillar is located on the inner side of the longest substantial rectangle forming the profile of the vapor chamber in the plan view (hereinafter also referred to as a plan-view profile) and extends in the longitudinal direction of the longest substantial rectangle at a position passing the center point of the longest substantial rectangle. The center point of the substantial rectangle is a point where the diagonals of the substantial rectangle cross. 
     The longest substantial rectangle refers to one of the rectangles forming the plan-view profile of the vapor chamber longest in the longitudinal direction. 
     The length of the second pillar in the longitudinal direction of the longest substantial rectangle is 30% to 70% of the length of the longest substantial rectangle. 
     The width of the second pillar in the width direction of the longest substantial rectangle is 5% to 10% of the width of the longest substantial rectangle. 
     The area of the second pillar is 0.5% to 7.0% of the area of the housing in the plan view. 
     Installing the second pillar of the above dimensions at the above position prevents warpage caused by heating performed to join the first sheet and the second sheet together. 
     The longest substantial rectangle will now be described with reference to  FIG. 4 . 
       FIG. 4  is a top view illustrating the positional relationship between the plan-view profile of the vapor chamber and the second pillar. 
     As illustrated in  FIG. 4 , the plan-view profile of the vapor chamber  1  is one substantial rectangle T 1 . Thus, the substantial rectangle T 1  serves as the longest substantial rectangle. 
     Here, the plan-view profile of the vapor chamber  1  indicates the profile of the entirety of the vapor chamber  1 , instead of indicating only the area surrounded by the sealing portion  50 . 
     The second pillar  42   a  is disposed on the inner side of the substantial rectangle T 1  to extend in the longitudinal direction of the substantial rectangle T 1  and to pass a center point C T1  of the substantial rectangle T 1 . The second pillar  42   a  is disposed to extend in the longitudinal direction of the substantial rectangle T 1 , and at substantially the center of the substantial rectangle T 1  in the width direction. 
     The plan-view profile of the vapor chamber  1  corresponds to the plan-view profile of the housing  10  constituting the vapor chamber  1 . When the plan-view profile of the vapor chamber  1  is formed from a single substantial rectangle (longest substantial rectangle), the area of the substantial rectangle corresponds to the area of the housing  10  in a plan view. 
     A length L 2a  of the second pillar  42   a  in the longitudinal direction is 30% to 70% of a length L T1  of the substantial rectangle T 1  in the longitudinal direction. 
     A width W 2a  of the second pillar  42   a  in the width direction is 5% to 10% of a width W T1  of the substantial rectangle T 1  in the width direction. 
     The second pillar  42   a  having predetermined dimensions and disposed to extend in the longitudinal direction of the substantial rectangle can prevent warpage caused by heating performed to join the first sheet and the second sheet together. 
     The plan-view profile of the vapor chamber according to the present invention is not limited to a substantial rectangle, and may be a combination of multiple substantial rectangles. 
     For the case where the plan-view profile of the vapor chamber is formed from a combination of multiple substantial rectangles, a method for selecting the longest substantial rectangle will be described with reference to  FIGS. 5A, 5B, 5C, 5D, and 5E . 
       FIGS. 5A, 5B, 5C, 5D, and 5E  are schematic top views of different examples of a vapor chamber. 
     As illustrated in  FIGS. 5A, 5B, 5C, and 5D , the plan-view profile of a vapor chamber  2  is formed from a combination of a substantial rectangle T 2 , a substantial rectangle T 3 , and a substantial rectangle T 4 . The reason why the entirety of the plan-view profile of the vapor chamber  2  is not formed from a single substantial rectangle will be described, later. 
     Here, the shape and the number of substantial rectangles forming a combination to constitute the plan-view profile of the vapor chamber are determined such that the number of substantial rectangles is minimum, and the sum of the lengths of all the substantial rectangles in the longitudinal direction is largest. 
     The substantial rectangles may have different longitudinal directions with reference to which the sum of lengths of the substantial rectangles in the longitudinal direction is calculated. The multiple substantial rectangles forming the plan-view profile of the vapor chamber do not have to be arranged without leaving a gap between each other, and may overlap each other. 
     In the vapor chamber  2  illustrated in  FIG. 5A , the substantial rectangle T 2  and the substantial rectangle T 3  overlap each other in an area X 1 , and the substantial rectangle T 2  and the substantial rectangle T 4  overlap each other in an area X 2 . 
     As illustrated in  FIGS. 5B, 5C, and 5D , the lengths of the substantial rectangle T 2 , the substantial rectangle T 3 , and the substantial rectangle T 4  in the longitudinal direction are denoted with L T2 , L T3 , and L T4 , among which L T2  is the longest. Thus, the substantial rectangle T 2  serves as the longest substantial rectangle. In the vapor chamber  2 , the second pillar  42   b  is disposed to extend in the longitudinal direction of the substantial rectangle T 2  at a position passing a center point C T2  of the substantial rectangle T 2 . 
     A dimension L 2b  of the second pillar  42   b  in the longitudinal direction is 30% to 70% of the dimension L T2  of the substantial rectangle T 2  in the longitudinal direction, and a dimension W 2b  of the second pillar  42   b  in the width direction is 5% to 10% of a dimension W T2  of the substantial rectangle T 2  in the width direction. 
     In the present description, the longitudinal direction of the substantial rectangle refers to the direction in which the distance between opposing two sides is longer. For a regular square, the distance between opposing two sides is equal to the distance of the other pair, and both directions serve as the longitudinal direction. 
     Thus, the substantial rectangles include a regular square. 
     In the present description, the plan-view profile of the vapor chamber is formed from a substantial rectangle or a combination of multiple substantial rectangles. 
     Here, the number of substantial rectangles forming the plan-view profile of the vapor chamber is preferably equal to or smaller than three, and more preferably, equal to or smaller than two. 
     When the number of substantial rectangles forming the plan-view profile of the vapor chamber is equal to or greater than four, the vapor chamber has a complex shape, and involves an increase in manufacturing costs. 
     In the present description, when the plan-view profile of the vapor chamber has a cutout (missing portion), whether the cutout is to be taken into consideration is determined depending on the area of the cutout and the area of the substantial rectangle assumed to have no cutout. 
     Specifically, when the rate of the area of the cutout to the area of the substantial rectangle assumed to have no cutout is equal to or smaller than 10%, the substantial rectangle is regarded as having no cutout. Thus, the entirety of the plan-view profile of the vapor chamber is regarded a single longest substantial rectangle. 
     On the other hand, when the rate of the area of the cutout to the area of the substantial rectangle assumed to have no cutout exceeds 10%, the substantial rectangle is divided into multiple substantial rectangles. 
     When the area of any of the divided substantial rectangles is equal to or smaller than 10% of the area of the plan-view profile of the vapor chamber, the substantial rectangle is excluded from the rectangles constituting the plan-view profile of the vapor chamber. 
     As illustrated in  FIG. 5E , in the vapor chamber  2 , the rate of the area of a cutout T 6  to the area of a substantial rectangle T 5  assumed to have no cutout exceeds 10% (approximately 11% in  FIG. 5E ). Thus, the plan-view profile of the vapor chamber  2  is regarded as being constituted of a combination of the three substantial rectangles T 2 , T 3 , and T 4 , instead of the single substantial rectangle T 5 . 
     If, in the vapor chamber  2  illustrated in  FIG. 5E , the rate of the area of the cutout T 6  to the area of the substantial rectangle T 5  assumed to have no cutout is equal to or smaller than 10%, the substantial rectangle T 5  serves as the longest substantial rectangle. 
     In the vapor chamber according to the present invention, the pillars may include an even number of third pillars arranged to be substantially line symmetric with the second pillar and without in contact with the second pillar when the vapor chamber is viewed in a plan. 
     The area of each of the third pillars is 0.5% to 2.0% of the area of the housing in the plan view. 
     Installation of the third pillars prevents warpage in the width direction. 
     If the third pillars are in contact with the second pillar, flow of the operation fluid in the housing may be blocked, and may degrade the cooling efficiency. 
     The length of each of the third pillars in the width direction of the longest substantial rectangle is preferably 10% to 20% of the width of the longest substantial rectangle. 
     The width of each of the third pillars in the longitudinal direction of the longest substantial rectangle is preferably 2.5% to 10% of the length of the longest substantial rectangle. 
     Preferably, the third pillars are not in contact with the outer edge of the housing. 
     If the third pillars are in contact with the outer edge of the housing, flow of the operation fluid in the housing may be blocked and the cooling efficiency may be degraded. 
     The third pillars may be disposed to extend in the width direction or in the longitudinal direction of the longest substantial rectangle. Preferably, the third pillars extend in the width direction of the longest substantial rectangle. 
     As to the direction in which the third pillars extend, the dimensions of the third pillars in the longitudinal direction and the width direction are compared, and the third pillars are disposed to extend in the direction of the longer one of the dimensions. 
     An example of a vapor chamber including the third pillars will be described with reference to  FIG. 6 . 
       FIG. 6  is a schematic top view of another example of a vapor chamber. 
     A vapor chamber  3  illustrated in  FIG. 6  is a substantial rectangle when viewed in the plan view. 
     The longest substantial rectangle constituting the vapor chamber  3  is a substantial rectangle T 7 . 
     The second pillar  42  is disposed to extend in the longitudinal direction of the substantial rectangle T 7  to pass a center point of the substantial rectangle T 7 . Thus, the second pillar  42  is disposed at substantially the center of the substantial rectangle T 7  in the width direction. 
     An even number (two in  FIG. 6 ) of third pillars  43  are arranged to be substantially line symmetric with respect to the second pillar  42  and without in contact with the second pillar  42 . 
     The area of each of the third pillars  43  is preferably 0.5% to 2.0% of the area of the housing in the plan view. 
     The third pillars are preferably arranged to extend in the width direction of the substantial rectangle. 
     A length L 3a  of each of the third pillars  43  in the width direction of the substantial rectangle T 7  is preferably 10% to 20% of a width W T7  of the substantial rectangle T 7  (approximately 10.7% in  FIG. 6 ). A width W 3a  of each of the third pillars in the longitudinal direction of the substantial rectangle T 7  is preferably 2.5% to 10% of a length L T7  of the substantial rectangle T 7  (approximately 3.0% in  FIG. 6 ). 
     When the width W 3a  and the length L 3a  of each third pillar  43  are compared, the length L 3a  is longer than the width W 3a . Thus, the third pillars  43  are disposed to extend in the width direction of the substantial rectangle T 7 , serving as the longest substantial rectangle. 
     When the plan-view profile of the vapor chamber is constituted of a combination of multiple substantial rectangles, a fourth pillar may also be disposed on the inner side of a substantial rectangle other than the longest substantial rectangle to extend in the longitudinal direction of the substantial rectangle at a position to pass the center point of the substantial rectangle. 
     The length of the fourth pillar in the longitudinal direction of the substantial rectangle is preferably 30% to 70% of the length of the substantial rectangle. Preferably, the width of the fourth pillar in the width direction of the substantial rectangle is 5% to 10% of the width of the substantial rectangle. 
     An example of the vapor chamber including the fourth pillar will be described with reference to  FIG. 7 . 
       FIG. 7  is a schematic top view of another example of a vapor chamber. 
     A vapor chamber  4  illustrated in  FIG. 7  corresponds to the vapor chamber  2  illustrated in  FIG. 5A  to which the fourth pillar is added. 
     As illustrated in  FIG. 7 , the vapor chamber  4  includes the second pillar  42   b  and a fourth pillar  44 . 
     As in the case of the vapor chamber  2  illustrated in  FIGS. 5A, 5B, 5C, 5D, and 5E , the vapor chamber  4  has a plan-view profile formed from a combination of the substantial rectangle T 2 , the substantial rectangle T 3 , and the substantial rectangle T 4 . Similarly to the vapor chamber  2 , the substantial rectangle T 2  serves as the longest substantial rectangle. In addition, the vapor chamber  4  includes the fourth pillar  44  disposed to pass the center point of the substantial rectangle T 3 , other than the longest substantial rectangle, to extend in the longitudinal direction of the substantial rectangle T 3 , to pass a center point C T3  of the substantial rectangle T 3 . 
     The fourth pillar may be in contact with the second pillar, but preferably not in contact with the second pillar. 
     [Method for Manufacturing Vapor Chamber] 
     A method for manufacturing a vapor chamber may be any method with which the above structure is obtained. For example, a first sheet on which a wick is disposed and a second sheet on which the pillars including the first pillars and the second pillar are disposed are stacked one on the other, the operation fluid is poured between the sheets, and then the first sheet and the second sheet are joined together to form a vapor chamber. 
     A method of joining the first sheet and the second sheet together may be any method involving heating. Examples of the method include laser welding, resistance welding, diffusion bonding, solder joining, brazing, tungsten inert gas (TIG) welding, and ultrasonic joining. Among these, laser welding, brazing, or diffusion bonding is preferable. 
     EXAMPLES 
     Embodiments more specifically describing a vapor chamber according to the present invention will be described below. The present invention is not limited to these embodiments. 
     Comparative Example 1 
     (Manufacture of Vapor Chamber) 
     A copper foil sheet with plan-view dimensions of a width of 60 mm, a length of 100 mm, and a thickness of 0.2 mm was prepared to serve as a first sheet. 
     A copper foil sheet with plan-view dimensions of a width of 60 mm, a length of 100 mm, and a thickness of 0.08 mm was prepared to serve as a second sheet. 
     The first sheet was etched with persulfuric acid soda to form protrusions serving as first pillars, the first sheet and the second sheet were then bonded together while holding a mesh in between, and the outer edges of the sheets were laser-welded to obtain a housing including the first sheet and the second sheet bonded together. After welding, the operation fluid was poured through a pipe. 
     (Warpage Check) 
     The surface roughness was checked with a laser range finder and expressed in numeric form, and the difference between the highest protrusion and the lowest depression was calculated to obtain warpage of the vapor chamber. 
     (Heat Characteristics Check) 
     The vapor chamber according to Comparative Example 1 was brought into contact with a ceramic heater in the conditions of an outside temperature of 25° C., and a difference ΔT between a temperature at a position of the vapor chamber immediately above the heat source and a temperature at a position of the vapor chamber farthest from the heat source was obtained. 
     Comparative Examples 2 to 3 and Examples 1 to 2 
     Vapor chambers according to Comparative Examples 2 to 3 and Examples 1 to 2 were formed by changing the etching pattern of the second sheet, and forming the second pillar in addition to the first pillars. Except above, the vapor chambers according to Comparative Examples 2 to 3 and Examples 1 to 2 were formed in the same procedure as in Comparative Example 1 by welding the first sheet and the second sheet together to have the second pillar at the same position as in the vapor chamber  1  illustrated in  FIG. 4 . 
     Table 1 shows the rate of the length of the second pillar in the longitudinal direction of the substantial rectangle forming the plan-view profile of the vapor chamber, the rate of the width of the second pillar in the width direction of the substantial rectangle, and the rate of the area of the second pillar with respect to the area of the housing in a plan view. 
     The height of the second pillar was 150 μm, as in the first pillars. 
     The rates [%] of the length and the width of the second pillar were calculated assuming that the plan-view profile of the vapor chamber is a rectangle with a width of 60 mm and a length of 100 mm. 
     Example 3 
     A vapor chamber according to Example 3 was formed by changing the etching pattern of the second sheet, and forming the second pillar and the third pillars in addition to the first pillars. Except above, the vapor chamber according to Example 3 was formed in the same procedure as in Comparative Example 1 by welding the first sheet and the second sheet together to have the second pillar and the third pillar at the same positions as in the vapor chamber  3  illustrated in  FIG. 6 . 
     Table 1 shows the rates of the length of the second pillar and the width of the third pillars in the longitudinal direction of the substantial rectangle forming the plan-view profile of the vapor chamber, the rates of the width of the second pillar and the length of the third pillars in the width direction of the substantial rectangle, and the rates of the area of each of the second pillar and the third pillar with respect to the area of the housing in a plan view. 
     The height of each of the second pillar and the third pillars was 150 μm, as in the first pillars. 
     The rates [%] of the length and the width of each of the second pillar and the third pillar were calculated assuming that the plan-view profile of the vapor chamber is a rectangle with a width of 60 mm and a length of 100 mm. 
     (Comparison in Warpage and Heat Characteristics) 
     Warpage of the vapor chamber indicates the rate obtained with reference to warpage of Comparative Example 1 defined as 1.00, and warpage with the value closer to zero expresses less warpage. Thus, the rate with a value smaller than 0.9 is evaluated as “acceptable”, and the rate with a value greater than 0.9 is evaluated as “unacceptable”. 
     Heat characteristics indicate the rate obtained with reference to ΔT in Comparative Example 1 defined as 1.00, and heat characteristics with the value closer to 1.00 express a lower temperature at the position farthest from the heat source, that is, having preferable heat characteristics. Thus, the rate with a value equal to or greater than 0.9 is evaluated as “acceptable”, and the rate with a value smaller than 0.9 is evaluated as “unacceptable”. 
     Table 1 shows the comparison between warpage and heat characteristics. 
     
       
         
           
               
               
               
               
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Comparative 
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                 Example 1 
                 Example 2 
                 Example 1 
                 Example 2 
                 Example 3 
                 Example 3 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                 First Pillar 
                 Area Rate/ 
                 0.01 
                 0.01 
                 0.01 
                 0.01 
                 0.01 
                 0.01 
               
               
                   
                 Column (%) 
               
               
                 Second Pillar 
                 Length Rate 
                 — 
                 20 
                 50 
                 50 
                 80 
                 50 
               
               
                   
                 (%) 
               
               
                   
                 Width Rate 
                 — 
                 5 
                 5 
                 10 
                 5 
                 5 
               
               
                   
                 (%) 
               
               
                   
                 Area Rate/ 
                 — 
                 1 
                 2.5 
                 5 
                 4 
                 2.5 
               
               
                   
                 Column (%) 
               
               
                 Third Pillar 
                 Length Rate 
                 — 
                 — 
                 — 
                 — 
                 — 
                 10 
               
               
                   
                 (%) 
               
               
                   
                 Width Rate 
                 — 
                 — 
                 — 
                 — 
                 — 
                 5 
               
               
                   
                 (%) 
               
               
                   
                 Area Rate/ 
                 — 
                 — 
                 — 
                 — 
                 — 
                 0.5 
               
               
                   
                 Column (%) 
               
               
                 Warpage 
                 Rate 
                 1.00 
                 0.95 
                 0.60 
                 0.55 
                 0.42 
                 0.55 
               
               
                   
                 Evaluation 
                 Unacceptable 
                 Unacceptable 
                 Acceptable 
                 Acceptable 
                 Acceptable 
                 Acceptable 
               
               
                 Heat 
                 Rate 
                 1.00 
                 0.99 
                 0.95 
                 0.91 
                 0.79 
                 0.92 
               
               
                 Characteristics 
                 Evaluation 
                 Acceptable 
                 Acceptable 
                 Acceptable 
                 Acceptable 
                 Unacceptable 
                 Acceptable 
               
               
                   
               
            
           
         
       
     
     The results in Table 1 show that the vapor chambers according to the present invention can reduce warpage caused by joining involving heating. The results have also revealed that, the second pillar having a length in the longitudinal direction of the substantial rectangle that is smaller than 30% of the length of the substantial rectangle has scarcely any effect on warpage prevention, and the second pillar having a length in the longitudinal direction of the substantial rectangle that exceeds 70% of the length of the substantial rectangle degrades heat characteristics. 
     REFERENCE SIGNS LIST 
     
         
         
           
               1 ,  2 ,  3 ,  4  vapor chamber 
               10  housing 
               11  first sheet 
               11   a  inner surface of first sheet 
               12  second sheet 
               12   a  inner surface of second sheet 
               13  cavity 
               20  operation fluid 
               30  wick 
               32  mesh 
               40  pillar 
               41  first pillar 
               42 ,  42   a ,  42   b ,  42   c  second pillar 
               43  third pillar 
               44  fourth pillar 
               50  sealing portion 
               120  heat source 
             C T1 , C T2 , C T3 , C T7  center point of substantial rectangle 
             T 1 , T 2 , T 3 , T 4 , T 5 , T 7  substantial rectangle 
             T 6  cutout 
             L T1 , L T2 , L T3 , L T4 , L T7  length in longitudinal direction of substantial rectangle 
             L 2a , L 2b  length in longitudinal direction of second pillar 
             L 3a  length in longitudinal direction of third pillar 
             W T1 , W T2 , W T7  width in width direction of substantial rectangle 
             W 2a , W 2b  width in width direction of second pillar 
             W 3a  width in width direction of third pillar 
             X 1  area where substantial rectangle T 2  and substantial rectangle T 3  overlap 
             X 2  area where substantial rectangle T 2  and substantial rectangle T 4  overlap