Bipolar battery structure

Abstract

This invention provides a bipolar battery structure that can suppress the leakage of flux into the conductive adhesive during brazing. [Solution] The bipolar battery structure consists of battery modules and coolers stacked alternately. The cooler comprises a first metal plate having a convex portion bonded to the bottom surface of the upper battery module via a conductive adhesive and a pair of first flange portions arranged on both sides of the convex portion, a second metal plate having a concave portion bonded to the top surface of the lower battery module via a conductive adhesive and a pair of second flange portions arranged on both sides of the concave portion, and a metal corrugated sheet arranged between the first and second metal plates to form a coolant flow path. The first metal plate, the second metal plate, and the metal corrugated sheet are joined by brazing. A first engaging projection that engages with the bottom recess of the upper battery module is arranged on the outside of the first flange portion, and a second engaging projection that engages with the top recess of the lower battery module is arranged on the outside of the second flange portion.

Description

Technical Field

[0001] The present invention relates to a bipolar battery structure.

Background Art

[0002] Conventionally, as such a technical field, for example, there is one described in Patent Document 1. The bipolar battery structure described in Patent Document 1 is formed by laminating a battery module and a cooler, and attempts to cool the battery module using a refrigerant flowing through the cooler.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] When manufacturing the above-described bipolar battery structure, from the viewpoint of ensuring sealing performance, it is desirable to join a plurality of metal plates by brazing to form a cooler. Further, in order to fix the cooler and the battery module, a method of applying a conductive adhesive between the cooler and the battery module and bonding them is used. However, when brazing the metal plates, it is necessary to apply flux to the metal plates for the purpose of removing the natural oxide film. Therefore, during brazing, the flux may bleed out onto the surface where the conductive adhesive is applied, remain on the application surface as a residue, and cause a problem of increasing the electrical resistance.

[0005] The present invention has been made to solve such technical problems, and an object thereof is to provide a bipolar battery structure capable of suppressing the bleeding of flux to the conductive adhesive side during brazing.

Means for Solving the Problems

[0006] The bipolar battery structure according to the present invention is a bipolar battery structure in which a plurality of battery modules and a plurality of coolers are alternately stacked, wherein the cooler comprises a first metal plate having a convex portion bonded to the bottom surface of one of a pair of battery modules adjacent in the stacking direction via a conductive adhesive and a pair of first flange portions arranged on both sides of the convex portion, a second metal plate having a concave portion bonded to the top surface of the other of the pair of adjacent battery modules via a conductive adhesive and a pair of second flange portions arranged on both sides of the concave portion, and disposed between the first metal plate and the second metal plate, and The invention comprises a metal corrugated sheet that forms a refrigerant flow path with a convex portion and a concave portion, wherein the first metal sheet, the second metal sheet, and the metal corrugated sheet are joined by brazing with the end of the metal corrugated sheet sandwiched between the first flange portion and the second flange portion, and a first engaging projection that engages with a bottom recess provided on the bottom surface of the battery module is disposed on the outside of the first flange portion of the first metal sheet, and / or a second engaging projection that engages with a top recess provided on the top surface of the battery module is disposed on the outside of the second flange portion of the second metal sheet.

[0007] In the bipolar battery structure according to the present invention, a first engaging projection is provided on the outside of the first flange portion of the first metal plate, which engages with a recess on the bottom surface of the battery module bonded to the first metal plate, and / or a second engaging projection is provided on the outside of the second flange portion of the second metal plate, which engages with a recess on the top surface of the battery module bonded to the second metal plate. By utilizing the first engaging projection and / or the second engaging projection, the leakage of flux into the conductive adhesive side during brazing can be suppressed. As a result, an increase in electrical resistance caused by flux leakage can be prevented, and conductivity can be ensured.

[0008] In the bipolar battery structure according to the present invention, it is preferable that the first engaging projection is arranged on the outside of the first flange portion of the first metal plate, and the second engaging projection is arranged on the outside of the second flange portion of the second metal plate. In this case, the first engaging projection is used to prevent flux from seeping into the conductive adhesive between the first metal plate and the bottom surface of the battery module, and the second engaging projection is used to prevent flux from seeping into the conductive adhesive between the second metal plate and the top surface of the battery module, thereby enhancing the effect of suppressing flux seepage into the conductive adhesive during brazing. Furthermore, the engagement of the first engaging projection with the recess on the bottom surface of the battery module, and the engagement of the second engaging projection with the recess on the top surface of the battery module, makes it easy to position the battery module and the cooler in the stacking direction, and also prevents misalignment of the battery module and the cooler in a direction perpendicular to the stacking direction.

[0009] In the bipolar battery structure according to the present invention, the first engaging projection has a first bent portion that bends from the first flange portion toward the bottom surface side of the battery module, and the second engaging projection has a second bent portion that bends from the second flange portion toward the top surface side of the battery module, and when the first metal plate, the second metal plate and the metal corrugated plate are each formed of aluminum, the metal corrugated plate is arranged inside the first bent portion and the second bent portion in a direction perpendicular to the stacking direction such that the distance from its end face to the first bent portion and the distance from the end face to the second bent portion are both L or more, and it is preferable that L = (σcosθ) / (4.9ρt), where σ is the surface tension coefficient of the flux liquid and aluminum, θ is the contact angle between the flux liquid and aluminum, ρ is the density of the flux liquid, and t is the thickness of the metal corrugated plate in the stacking direction. In this way, the first engaging projection can be used to reliably prevent the flux from seeping into the conductive adhesive between the first metal plate and the bottom surface of the battery module, and the second engaging projection can be used to reliably prevent the flux from seeping into the conductive adhesive between the second metal plate and the top surface of the battery module. [Effects of the Invention]

[0010] According to the present invention, it is possible to suppress the leakage of flux into the conductive adhesive side during brazing. [Brief explanation of the drawing]

[0011] [Figure 1] This is a schematic cross-sectional view showing a bipolar battery structure according to an embodiment. [Figure 2] This is an enlarged view of section A in Figure 1. [Figure 3] This is a schematic cross-sectional view illustrating the length L at which flux leakage does not occur. [Figure 4] This is a schematic diagram illustrating the amount of liquid entering due to capillary action. [Figure 5] This is a schematic cross-sectional view illustrating the problems (flux leakage) in conventional bipolar battery structures. [Figure 6] This is a schematic cross-sectional view illustrating the problems (positional misalignment) of conventional bipolar battery structures. [Modes for carrying out the invention]

[0012] Hereinafter, embodiments of the bipolar battery structure according to the present invention will be described with reference to the drawings. In the description of the drawings, the same elements are denoted by the same reference numerals, and redundant explanations are omitted.

[0013] Figure 1 is a schematic cross-sectional view showing a bipolar battery structure according to an embodiment, and Figure 2 is an enlarged view of part A in Figure 1. The bipolar battery structure 1 of this embodiment is formed by alternately stacking a plurality of battery modules 2 and a plurality of coolers 3 along one direction (stacking direction). In the following description, the bipolar battery structure 1 will be explained assuming that the stacking direction of the battery modules 2 and coolers 3 is along the vertical direction in each drawing, and the longitudinal direction of the battery modules 2 or coolers 3 is along the left-right direction in each drawing. However, these directions are merely convenient directions corresponding to the state shown in the drawings and do not limit the orientation or arrangement of the bipolar battery structure 1.

[0014] The battery module 2 is a module in which a plurality of battery cells are stacked. Although not shown, each battery cell has a bipolar electrode, and a current collector coated with a positive electrode and a negative electrode active material on both sides and a separator are alternately stacked. Further, each battery cell is formed in a structure that directly conducts an electric current in the stacking direction of the bipolar electrodes. In the battery module 2, the stacking direction of the battery cells and the direction in which the energizing current flows are the same direction. Also, in the present embodiment, the stacking direction of the battery cells and the stacking direction of the battery module 2 and the cooler 3 are the same.

[0015] On the top surface (i.e., the upper surface) 2a of the battery module 2, a top surface recess 21 that can engage with a second engaging protrusion 323 of the cooler 3 described later is provided. The top surface recesses 21 are respectively arranged at both edge portions in the longitudinal direction of the battery module 2 (see FIG. 1). The top surface recess 21 extends along the width direction of the battery module 2 and is formed over the entire width of the battery module 2. That is, the top surface recess 21 is a concave groove formed over the entire width of the battery module 2 on the top surface 2a of the battery module 2. Here, the width direction refers to a direction orthogonal to the stacking direction and the longitudinal direction.

[0016] Corresponding to the top surface recess 21, on the bottom surface (i.e., the lower surface) 2b of the battery module 2, a bottom surface recess 22 that can engage with a first engaging protrusion 313 of the cooler 3 described later is provided. The bottom surface recesses 22 are respectively arranged at both edge portions in the longitudinal direction of the battery module 2 so as to be in the same position as the top surface recess 21 in the stacking direction (see FIG. 1). This bottom surface recess 22 is also a concave groove formed over the entire width of the battery module 2, similar to the top surface recess 21.

[0017] The cooler 3 has a flat box shape so as to have a space for circulating a refrigerant inside, and absorbs heat from the battery module 2 with the refrigerant flowing inside, thereby cooling the battery module 2. As shown in FIG. 1, the cooler 3 is disposed between a pair of upper and lower battery modules 2 adjacent in the stacking direction and is configured to absorb heat from the pair of upper and lower battery modules 2.

[0018] This cooler 3 includes a first metal plate 31 adhered to the bottom surface 2b of the upper battery module 2 among the pair of upper and lower battery modules 2, a second metal plate 32 adhered to the top surface 2a of the lower battery module 2, and a metal corrugated plate 33 disposed between the first metal plate 31 and the second metal plate 32.

[0019] The first metal plate 31 has a convex cross-sectional shape, and has a convex portion 311 disposed at the central position of the first metal plate 31 and overlapping with the bottom surface 2b of the upper battery module 2, and a pair of first flange portions 312 disposed on both the left and right sides of the convex portion 311. The convex portion 311 is a flat portion that is in surface contact with the bottom surface 2b of the upper battery module 2 among the first metal plate 31, and is formed larger than other portions such as the first flange portion 312 so as to improve the heat absorption rate. On the other hand, the first flange portion 312 is parallel to the convex portion 311.

[0020] Further, outside the first flange portion 312 of the first metal plate 31, a first engaging protrusion 313 that engages with the bottom recess 22 of the upper battery module 2 is disposed. The first engaging protrusion 313 is integrally formed with the first flange portion 312 and is located on the side away from the convex portion 311. This first engaging protrusion 313 protrudes from the first flange portion 312 to the bottom surface 2b of the upper battery module 2 and has a U-shaped cross-section. And in the state where the first engaging protrusion 313 engages with the bottom recess 22, the tip of its U-shape is inserted into the inside of the bottom recess 22. Further, the first engaging protrusion 313 has a first bent portion 313a that bends from the first flange portion 312 toward the bottom surface 2b side of the upper battery module 2 (see FIG. 2).

[0021] The first metal plate 31 having such a structure is formed by pressing a metal flat plate such as aluminum, an aluminum alloy, or stainless steel.

[0022] The second metal plate 32 has a concave cross-section and includes a concave portion 321 positioned in the center of the second metal plate 32 that overlaps with the top surface 2a of the lower battery module 2, and a pair of second flange portions 322 positioned on both the left and right sides of the concave portion 321. The concave portion 321 is the planar portion of the second metal plate 32 that is in surface contact with the top surface 2a of the lower battery module 2, and is formed to be larger than other parts such as the second flange portions 322 in order to improve the lower heat absorption rate. On the other hand, the second flange portions 322 are parallel to the concave portion 321.

[0023] Furthermore, a second engaging projection 323 is positioned on the outside of the second flange portion 322 of the second metal plate 32, which engages with the top surface recess 21 of the lower battery module 2. The second engaging projection 323 is integrally formed with the second flange portion 322 and is located on the side away from the concave portion 321. This second engaging projection 323 protrudes from the second flange portion 322 to the top surface 2a of the lower battery module 2 and has a V-shaped cross-section. When the second engaging projection 323 is engaged with the top surface recess 21, its V-shaped tip is inserted into the interior of the top surface recess 21. The second engaging projection 323 also has a second bent portion 323a that bends from the second flange portion 322 toward the top surface 2a of the lower battery module 2 (see Figure 2).

[0024] The second metal plate 32 having such a structure is formed by press-forming a metal plate such as aluminum, an aluminum alloy, or stainless steel.

[0025] The metal corrugated sheet 33 is formed in a corrugated cross-section by alternating multiple recesses 331 and protrusions 332. The bottom of each recess 331 is flat and in surface contact with the concave portion 321 of the second metal sheet 32. On the other hand, the top of each protrusion 332 is also flat and in surface contact with the convex portion 311 of the first metal sheet 31. These recesses 331 and protrusions 332 are arranged in the space surrounded by the convex portion 311 of the first metal sheet 31 and the concave portion 321 of the second metal sheet 32, and the convex portion 311 and the concave portion 321 form a refrigerant flow path.

[0026] Furthermore, the corrugated metal sheet 33 has a pair of third flange portions 333 arranged on both the left and right sides of the region where multiple recesses 331 and protrusions 332 are formed. The corrugated metal sheet 33 having such a structure is formed, for example, by press-forming a metal sheet such as aluminum, an aluminum alloy, or stainless steel, similar to the first metal sheet 31 and the second metal sheet 32.

[0027] As shown in Figure 1, the first metal plate 31, the second metal plate 32, and the metal corrugated plate 33 are joined by brazing with the end of the third flange portion 333 sandwiched between the first flange portion 312 and the second flange portion 322. Specifically, the first flange portion 312 and the second flange portion 322 are joined to the first engaging projection 313, respectively, via a brazing material containing flux (in other words, brazing material 34 and flux 35).

[0028] When brazing, as shown in Figure 3(a), first, brazing material 34 is placed on both the upper and lower surfaces of the third flange portion 333 of the corrugated metal sheet 33, and flux 35 is applied to the surface of each brazing material 34 facing the first flange portion 312 or the second flange portion 322. Next, with the brazing material 34 and flux 35 interposed between the third flange portion 333 and the first flange portion 312, and between the third flange portion 333 and the second flange portion 322, the corrugated metal sheet 33 is sandwiched between the first metal plate 31 and the second metal plate 32, and the first flange portion 312, the second flange portion 322, and the third flange portion 333 are joined by brazing.

[0029] Furthermore, the brazing material 34 is made of a metal material (for example, an aluminum alloy) with a lower melting point than the first metal plate 31, the second metal plate 32, and the corrugated metal plate 33. For the flux 35, for example, a non-corrosive flux is used.

[0030] The cooler 3, configured as described above, is sandwiched between the upper and lower battery modules 2 and fixed to them with conductive adhesive 36. Specifically, the cooler 3 is fixed to the upper and lower battery modules 2 by bonding the convex portion 311 of the first metal plate 31 to the bottom surface 2b of the upper battery module 2 with conductive adhesive 36, and the concave portion 321 of the second metal plate 32 to the top surface 2a of the lower battery module 2 with conductive adhesive 36.

[0031] As shown in Figures 1 and 2, the conductive adhesive 36 is applied intermittently to the convex portion 311 or the concave portion 321 at regular intervals. The conductive adhesive 36 may also be applied to the entire surface of the convex portion 311 or the entire surface of the concave portion 321. Furthermore, the conductive adhesive 36 may also be applied to the first engaging projection 313 and the second engaging projection 323, although this application may be omitted. For example, silver paste can be used as the conductive adhesive 36.

[0032] In the bipolar battery structure 1 according to this embodiment, a first engaging projection 313 is provided on the outside of the first flange portion 312 of the first metal plate 31, which engages with the bottom recess 22 of the upper battery module 2 bonded to the first metal plate 31. A second engaging projection 323 is provided on the outside of the second flange portion 322 of the second metal plate 32, which engages with the top recess 21 of the lower battery module 2 bonded to the second metal plate 32. By utilizing the first engaging projection 313 and the second engaging projection 323, it is possible to suppress the leakage of flux 35 into the conductive adhesive 36 during brazing. As a result, it is possible to prevent an increase in electrical resistance caused by the leakage of flux 35 and ensure the conductivity of the bipolar battery structure 1.

[0033] For example, in the conventional cooler 3A shown in Figure 5(a), neither the first metal plate 31A nor the second metal plate 32A has the aforementioned engaging protrusions (first engaging protrusion 313 and second engaging protrusion 323). Also, in the longitudinal direction, the end faces of the first metal plate 31A, the second metal plate 32A, and the metal corrugated sheet 33 are flush. When the third flange portion 333 is sandwiched between the first flange portion 312A and the second flange portion 322A and brazed together, the flux 35 seeps out through capillary action onto the upper surface of the first flange portion 312A and the lower surface of the second flange portion 322A, respectively (see Figure 5(b)), and further seeps out onto the conductive adhesive side, becoming a residue and causing a problem of increased electrical resistance.

[0034] To solve these problems, in this embodiment, the first metal plate 31 is provided with a first engaging projection 313 that engages with the bottom recess 22 of the upper battery module 2, and the second metal plate 32 is provided with a second engaging projection 323 that engages with the top recess 21 of the lower battery module 2. The first engaging projection 313 prevents the flux 35 from seeping into the conductive adhesive 36 between the convex portion 311 and the bottom surface 2b of the upper battery module 2, and the second engaging projection 323 prevents the flux 35 from seeping into the conductive adhesive 36 between the concave portion 321 and the top surface 2a of the lower battery module 2.

[0035] Furthermore, the engagement of the first engaging projection 313 with the bottom recess 22 of the upper battery module 2, and the engagement of the second engaging projection 323 with the top recess 21 of the lower battery module 2, facilitates the positioning of the battery module 2 and the cooler 3 in the stacking direction, and prevents misalignment of the battery module 2 and the cooler 3 in the longitudinal direction.

[0036] For example, in the conventional bipolar battery structure 1A shown in Figure 6, when stacking multiple battery modules 2 and coolers 3A, there is no structure for positioning the battery modules 2 and coolers 3A in the stacking direction, nor is there a structure for restricting the movement of the battery modules 2 and coolers 3A in the longitudinal direction. As a result, misalignment between the battery modules 2 and coolers 3A is likely to occur. If misalignment occurs between the battery modules 2 and coolers 3A, the current-carrying area decreases, leading to a decrease in the capacity of the battery pack.

[0037] To solve these problems, in this embodiment, the top surface 2a of the battery module 2 is provided with a top surface recess 21, the bottom surface 2b is provided with a bottom surface recess 22, the first metal plate 31 is provided with a first engaging projection 313 that engages with the bottom surface recess 22, and the second metal plate 32 is provided with a second engaging projection 323 that engages with the top surface recess 21. When stacking the battery module 2 and the cooler 3, the first engaging projection 313 is engaged with the bottom surface recess 22 of the upper battery module 2, and the second engaging projection 323 is engaged with the top surface recess 21 of the lower battery module 2, thereby making it easy to position the battery module 2 and the cooler 3 in the stacking direction and preventing misalignment of the battery module 2 and the cooler 3 in the longitudinal direction.

[0038] Furthermore, in the bipolar battery structure 1 of this embodiment, when the first metal plate 31, the second metal plate 32, and the metal corrugated plate 33 are each formed of aluminum, as shown in Figure 3, the distance from the end face 33a of the metal corrugated plate 33 to the first bent portion 313a and the distance from the end face 33a of the metal corrugated plate 33 to the second bent portion 323a are both L or greater. In this embodiment, L is the length at which flux seepage does not occur, and is calculated as L = (σcosθ) / (4.9ρt). σ is the surface tension coefficient between the flux liquid and aluminum, θ is the contact angle between the flux liquid and aluminum, ρ is the density of the flux liquid, and t is the thickness of the metal corrugated plate 33 in the lamination direction. In Figure 3, (a) shows the state with flux applied (i.e., before brazing), and (b) shows the state with diluted flux (i.e., after brazing).

[0039] Here, we will explain in detail the length L at which flux leakage does not occur, based on Figure 4.

[0040] As shown in Figure 4, generally, the amount h that penetrates the gap between two flat plates 10 by capillary action can be calculated as h = (2σcosθ) / (gρt) = (σcosθ) / (4.9ρt). Here, σ is the surface tension coefficient (unit: N / m), θ is the contact angle (unit: deg), and ρ is the density of the liquid (unit: g / m³). 3 ), t is the gap amount (unit: m), g is the acceleration due to gravity (9.8 kg / m²). 2 )

[0041] Referring to the penetration amount h described above, in this embodiment, the distance from the end face 33a of the metal corrugated sheet 33 to the first bent portion 313a, and the distance from the end face 33a of the metal corrugated sheet 33 to the second bent portion 323a are both set to be L or greater. Then, L = (σcosθ) / (4.9ρt), where σ is the surface tension coefficient of the flux solution and aluminum (unit: N / m), θ is the contact angle between the flux solution and aluminum (unit: deg), and ρ is the density of the flux solution (unit: g / m³). 3 ), where t is the thickness of the metal corrugated sheet in the lamination direction (unit: m).

[0042] Furthermore, the surface tension coefficient between the flux solution and aluminum is σ = 23 mN / m, the contact angle between the flux solution and aluminum is θ = 10°, and the density of the flux solution is ρ = 0.79 g / cm³. 3 When the thickness of the metal corrugated sheet in the lamination direction is t=0.8mm, L=7mm.

[0043] Thus, by setting the distance from the end face 33a of the metal corrugated sheet 33 to the first bent portion 313a, and the distance from the end face 33a of the metal corrugated sheet 33 to the second bent portion 323a to both be greater than or equal to L, the intrusion of flux 35 due to capillary action is limited to the above-mentioned range L. In other words, even if flux 35 penetrates beyond the end face 33a due to capillary action, the intrusion will not exceed the above-mentioned L. Therefore, it is possible to reliably suppress the flux 35 from seeping out to the conductive adhesive 36 side between the convex portion 311 and the bottom surface 2b of the upper battery module 2, and from seeping out to the conductive adhesive 36 side between the concave portion 321 and the top surface 2a of the lower battery module 2.

[0044] Furthermore, even if the flux 35 seeps out beyond L, gravity will cause it to accumulate in the recessed portion of the V-shaped second engaging projection 323, thus preventing further seepage toward the conductive adhesive 36.

[0045] Although embodiments of the present invention have been described in detail above, the present invention is not limited to the embodiments described above, and various design modifications can be made without departing from the spirit of the invention as described in the claims. [Explanation of Symbols]

[0046] 1: Bipolar battery structure, 2: Battery module, 2a: Top surface, 2b: Bottom surface, 3: Cooler, 21: Top surface recess, 22: Bottom surface recess, 31: First metal plate, 32: Second metal plate, 33: Metal corrugated plate, 33a: End surface, 34: Brazing material, 35: Flux, 311: Convex portion, 312: First flange portion, 313: First engaging projection, 313a: First bent portion, 321: Concave portion, 322: Second flange portion, 323: Second engaging projection, 323a: Second bent portion, 331: Recess, 332: Convex portion, 333: Third flange portion

Claims

1. A bipolar battery structure in which multiple battery modules and multiple coolers are stacked alternately, The aforementioned cooler is, A first metal plate having a convex portion bonded to the bottom surface of one of a pair of battery modules adjacent in the stacking direction via a conductive adhesive, and a pair of first flange portions arranged on both sides of the convex portion, A second metal plate having a concave portion bonded to the top surface of the other of the pair of adjacent battery modules via a conductive adhesive, and a pair of second flange portions arranged on both sides of the concave portion, A metal corrugated sheet is placed between the first metal sheet and the second metal sheet, and the convex portion and the concave portion form a refrigerant flow path. Equipped with, The first metal plate, the second metal plate, and the corrugated metal sheet are joined by brazing with the end of the corrugated metal sheet sandwiched between the first flange portion and the second flange portion. A first engaging projection is provided on the outer side of the first flange portion of the first metal plate, which engages with a bottom recess provided on the bottom surface of the battery module. or / and, A bipolar battery structure characterized in that a second engaging projection is provided on the outside of the second flange portion of the second metal plate, which engages with a top surface recess provided on the top surface of the battery module.

2. The bipolar battery structure according to claim 1, wherein a first engaging projection is arranged on the outside of the first flange portion of the first metal plate, and a second engaging projection is arranged on the outside of the second flange portion of the second metal plate.

3. The first engaging projection has a first bent portion that bends from the first flange portion toward the bottom surface side of the battery module, and the second engaging projection has a second bent portion that bends from the second flange portion toward the top surface side of the battery module. When the first metal plate, the second metal plate, and the corrugated metal plate are each made of aluminum, In a direction perpendicular to the stacking direction, the metal corrugated sheet is arranged inside the first and second bent portions such that the distance from its end face to the first bent portion and the distance from the end face to the second bent portion are both L or greater, wherein L = (σcosθ) / (4.9ρt), where σ is the surface tension coefficient of the flux liquid and aluminum, θ is the contact angle between the flux liquid and aluminum, ρ is the density of the flux liquid, and t is the thickness of the metal corrugated sheet in the stacking direction, as described in claim 2.

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