Manufacturing method of the bonded body

Abstract

To provide a method for manufacturing a joined body by joining a copper plate to an aluminum plate by solid phase diffusion bonding.SOLUTION: A method for manufacturing a joined body made by joining a copper plate 42A made of copper or copper alloy and an aluminum plate made of aluminum or aluminum alloy includes a copper plate forming step of punching and forming a copper plate using press, and a joining step of joining the copper plate and the aluminum plate by solid phase diffusion bonding. The copper plate forming step forms a copper plate having a tapered surface where the peripheral area of the surface to be joined to the aluminum plate is thicker on the inner side and thinner on the edge side. The tapered surface has a width dimension of 1.80 mm or less in the in-plane direction of the copper plate, and the depth dimension at the periphery in the surface direction of the copper plate is 20.0 μm or less, and in the joining step, the tapered surface is joined so as to face the aluminum plate.SELECTED DRAWING: Figure 4

Description

[Technical Field]

[0001] The present invention relates to a method for manufacturing a bonded body made of a plurality of different plate members, and particularly to a manufacturing method suitable for bonding a copper plate and an aluminum plate by solid-state diffusion bonding. [Background technology]

[0002] Insulated substrates mounted with various semiconductor elements are used in modern automobiles, railway vehicles, elevators, industrial equipment, and more. Power semiconductor elements used to control high-power electric vehicles and hybrid vehicles generate a lot of heat, so the insulating substrate on which these elements are mounted must have high heat dissipation properties. These insulating substrates include a ceramic substrate made of, for example, AlN (aluminum nitride) or Si3N4 (silicon nitride), a circuit layer formed by bonding a metal plate with excellent conductivity and heat dissipation properties to one side of the ceramic substrate, and a heat dissipation layer with excellent heat dissipation properties on the other side.

[0003] For example, Patent Document 1 proposes an insulating substrate in which copper plates constituting circuit layers and heat dissipation layers are directly bonded to a ceramic substrate using the DBC (Direct Bonded Copper) method. This DBC method utilizes a eutectic reaction between copper and copper oxide to generate a liquid phase at the interface between the copper and the ceramic substrate, thereby bonding the copper plate and the ceramic substrate. However, in recent years, as power modules have become smaller and thinner, the amount of heat generated by the elements themselves has also increased. Therefore, insulating substrates are required to have higher heat dissipation properties than before.

[0004] Although high heat dissipation can be achieved by thickening the circuit layer to improve heat dissipation, the difference in thermal expansion coefficient between the ceramic substrate and the copper plate that makes up the circuit layer increases the stress on the copper plate and the solder joints that mount the ceramic substrate and semiconductor elements. This generates tensile stress that exceeds the flexural strength of the ceramic substrate, causing cracks in the ceramic substrate and increasing thermal resistance due to cracks in the solder joints. This can lead to chip failure.

[0005] To prevent the above-mentioned problems, Patent Document 2 discloses a structure in which aluminum plates (with a purity of 99.99% by mass or higher), which have relatively low deformation resistance, are bonded to the top and bottom surfaces of a ceramic substrate, allowing the aluminum plates to absorb the thermal stress that occurs in the ceramic substrate during thermal cycles. This prevents cracks from occurring in the ceramic substrate. Furthermore, the aluminum plates also absorb the thermal stress applied to the soldered parts, thereby preventing cracks in the soldered parts.

[0006] In the power module substrate disclosed in Patent Document 3, the circuit layer is made of a bonded body of an aluminum plate and a copper plate. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Application Publication No. 01-251781 [Patent Document 2] Patent No. 3171234 [Patent Document 3] JP 2017-228693 A Summary of the Invention [Problem to be solved by the invention]

[0008] When forming a bonded body of an aluminum plate and a copper plate as in Patent Document 3, the copper plate is formed by press working, and it is generally known that the peripheral regions of the punch-side and die-side surfaces develop characteristic shapes. On the punch-side surface, the peripheral region becomes a droopy surface, and on the die-side surface, the peripheral region becomes a tapered surface and burrs are formed.

[0009] To bond a copper plate and an aluminum plate by solid-state diffusion bonding, the bonding surfaces must be in contact with each other. Therefore, a load is applied during bonding to reduce non-contact areas. However, as described above, a copper plate formed by press working has a tapered surface. Even if a load is applied with the tapered surface aligned with the surface of the aluminum plate, the tapered surface does not contact the surface of the aluminum plate, resulting in insufficient solid-state diffusion bonding and resulting in non-bonded areas around the periphery of the copper plate. In particular, when the thickness of the copper plate is increased to allow for high current flow or to improve heat dissipation, the problem of an increase in such non-bonded areas is likely to occur.

[0010] If such unbonded regions remain, not only will heat dissipation performance decrease, but cracks may occur between the copper plate and the aluminum plate, or the copper plate and the aluminum plate may peel off when subjected to a thermal cycle.

[0011] Therefore, an object of the present invention is to provide a method for producing a bonded body in which unbonded regions are reduced by solid-state diffusion bonding of a copper plate and an aluminum plate. [Means for solving the problem]

[0012] The method for manufacturing a bonded body of the present invention is a method for manufacturing a bonded body formed by bonding a copper plate made of copper or a copper alloy and an aluminum plate made of aluminum or an aluminum alloy, and includes a copper plate forming step of punching and shaping the copper plate with a press, and a bonding step of bonding the copper plate and the aluminum plate by solid-state diffusion bonding, wherein the copper plate forming step forms a copper plate having a tapered surface in the peripheral region of the surface to be bonded to the aluminum plate, the tapered surface being thicker on the inner region side and thinner on the edge side, and the tapered surface has a width dimension in the surface direction of the copper plate of 1.80 mm or less and a depth dimension at the peripheral edge in the surface direction of the copper plate of 20.0 μm or less, and the bonding step bonds the tapered surface toward the aluminum plate.

[0013] The copper plate forming process forms the tapered surface of the copper plate with the width and depth dimensions. This allows the copper plate to fit the entire surface to the aluminum plate or further reduces unbonded areas. If the width is greater than 1.8 mm, unbonded areas will frequently occur at the periphery during manufacturing. If the depth exceeds 20.0 μm, unbonded areas will also occur.

[0014] In the present invention, preferably, the copper plate forming step may include forming a punched plate from a raw plate made of copper or a copper alloy by pressing, and then shaving the peripheral edge of the punched plate to form the tapered surface having the width and depth dimensions. [Effects of the Invention]

[0015] According to the present invention, it is possible to manufacture a bonded body in which the unbonded area at the periphery of the copper plate is reduced. [Brief explanation of the drawings]

[0016] [Figure 1] 1A and 1B are diagrams illustrating a power module substrate according to an embodiment of the present invention. [Figure 2] 2(a) and 2(b) are diagrams for explaining a method for manufacturing the power module substrate of FIG. [Figure 3] 2(a) is a plan view showing a copper plate used in manufacturing the power module substrate of FIG. 1, and FIG. 2(b) is an enlarged schematic cross-sectional view of the copper plate taken along line S1-S1 of FIG. [Figure 4] 4(a) to 4(c) are diagrams illustrating the steps of shaping the copper plate of FIG. [Figure 5] FIG. 10 is a graph showing the measured contour lines of the peripheral area of ​​a copper plate. [Figure 6] 1(a) is a scanning electron microscope (SEM) image showing the bonded body of Sample 1, and FIG. 1(b) is a SEM image showing the bonded body of Sample 7. FIG. DETAILED DESCRIPTION OF THE INVENTION

[0017] Hereinafter, an embodiment of the present invention will be described with reference to the drawings. This embodiment is an example in which a power module substrate is used as a bonded body. As shown in Fig. 1, this power module substrate 1 includes a ceramic substrate 10 which is an insulating layer, a circuit layer 20 bonded to one surface of the ceramic substrate 10, and a heat dissipation layer 30 bonded to the other surface of the ceramic substrate 10. The ceramic substrate 10 may be made of nitride ceramics such as AlN (aluminum nitride) or Si3N4 (silicon nitride), or oxide ceramics such as Al2O3 (alumina). The thickness of the ceramic substrate 10 is set to 0.2 mm or more and 1.5 mm or less.

[0018] The circuit layer 20 and the heat dissipation layer 30 each have a two-layer structure consisting of a first metal layer 41 made of aluminum or an aluminum alloy and a second metal layer 42 made of copper or a copper alloy, with the first metal layer 41 formed on both sides of the ceramic substrate 10 and the second metal layer 42 formed on the side of the first metal layer 41 opposite the ceramic substrate 10.

[0019] The first metal layer 41 can be made of pure aluminum having a purity of 99% by mass or more (for example, pure aluminum in the 1000 series according to the JIS standard, particularly 1N90 (purity of 99.9% by mass or more: so-called 3N aluminum) or 1N99 (purity of 99.99% by mass or more: so-called 4N aluminum), or an aluminum alloy such as A6063). In order to buffer the difference in thermal expansion and contraction between the second metal layer 42 and the ceramic substrate 10, it is preferable to use pure aluminum, particularly 4N aluminum, for the first metal layer 41. The second metal layer 42 is preferably made of, for example, copper with a purity of 99.96% by mass or more (oxygen-free copper) or copper with a purity of 99.90% by mass or more (tough pitch copper).

[0020] The thickness of the first metal layer 41 and the second metal layer 42 is, for example, 0.1 mm to 3.0 mm for the first metal layer 41, and 2.0 mm to 5.0 mm for the second metal layer 42. The first metal layer 41 and the second metal layer 42 may have the same thickness for the circuit layer 20 and the heat dissipation layer 30, or may have different thicknesses. In the illustrated example, the first metal layer 41 and the second metal layer 42 are denoted by the same reference numerals without distinguishing between the circuit layer 20 and the heat dissipation layer 30.

[0021] A method for manufacturing the power module substrate 1 configured as above will be described. First, as shown in FIG. 2(a), aluminum plates 41A made of aluminum or an aluminum alloy are laminated on both sides of a ceramic substrate 10 via a brazing filler metal 43, and the laminate is pressurized and heated to bond the ceramic substrate 10 and the aluminum plates 41A together, forming first metal layers 41 on both sides of the ceramic substrate 10 (first bonding step). The aluminum plates 41A are formed to correspond to the pattern shape of the circuit, and in this embodiment, an aluminum plate 41A having a rectangular outline in a plan view will be described as an example. The bonding load is 0.2 MPa to 0.9 MPa (2 kgf / cm 2 ~10kgf / cm 2 ) and the bonding temperature is preferably in the range of 640° C. to 650° C. In the first bonding step, a first bonded body 11 is produced in which the first metal layers 41 are provided on both sides of the ceramic substrate 10.

[0022] Next, as shown in Figure 2(b), a copper plate 42A made of copper or a copper alloy is laminated on the first metal layer 41, and the laminate is pressurized and heated to solid-state diffusion bond the aluminum and copper, thereby forming a second metal layer 42 on the surface of the first metal layer 41 opposite the ceramic substrate 10 (second bonding process).

[0023] In this second joining step, the following copper plate 42A is used. The copper plate 42A is formed to have the same planar shape as the aluminum plate 41 A. In this embodiment, the copper plate 42A having a rectangular outline in plan view will be described as an example.

[0024] As shown in the plan view of Figure 3(a) and the enlarged cross-sectional view of Figure 3(b), the peripheral portion 420 of this copper plate 42A forms a surface that is inclined toward the opposite lower surface 422 so that the thickness gradually decreases from an inner region 421B, where the peripheral region 421A is formed approximately flat, to an outer side.

[0025] Similarly, lower surface 422 of peripheral portion 420 also forms a surface that is inclined toward upper surface 421 so that the thickness gradually decreases outward from inner region 422B, where inner region 421B of upper surface 421 is formed substantially flat, in an area along the edge of the rectangle (peripheral region 422A). Note that inner region 421B of upper surface 421 and inner region 422B of lower surface 422 are formed on parallel surfaces.

[0026] 3(b), the peripheral edge 420 of the copper plate 42A has a wall surface 423 adjacent to the inner regions 421B and 422B, and the surface direction of the wall surface 423 is perpendicular to the surface direction of the inner region 421B and also intersects with the surface direction of the inner region 422B. The wall surface 423 forms the edge of the copper plate 42A and defines the outline in a plan view.

[0027] In the following description, peripheral region 421A of upper surface 421 will be referred to as the "sagging surface," and peripheral region 422A of lower surface 422 will be referred to as the "tapered surface." Tapered surface 422A has a wider width along the surface direction than sagging surface 421A.

[0028] As shown in FIG. 3(b), the sagging surface 421A has a width dimension w1 in the surface direction of 0.80 mm or less and a depth dimension h1 of 200.0 μm or less.

[0029] As shown in FIG. 3(b), the tapered surface 422A has a width dimension w2 in the surface direction of 1.80 mm or less and a depth dimension h2 of 20.0 μm or less.

[0030] The dimensions (w1, w2, h1, h2) of sagging surface 421A and tapered surface 422A of peripheral edge portion 420 can be measured using a surface roughness meter, a laser microscope, or the like.

[0031] (Forming of copper plate 42A) The copper plate 42A used in manufacturing the power module substrate 1 of this embodiment is formed by punching using a press (copper plate forming step). The copper plate forming step will be described below. (First molding method) First, as shown in FIG. 4(a), a blank made of copper or a copper alloy is formed into a punched plate 61 of a predetermined shape using a die 51 and a punch 53. Reference numeral 52 in the figure denotes a plate holder that holds down the blank. The blank 61 formed by punching with the die 51 and punch 53 has a sag surface 61A on one side and a burr 61B and a tapered surface 61C on the opposite side at its peripheral edge, as shown in FIG. 4(b). The tapered surface 61C is formed when the punch 53 is lowered to punch out the blank and then moved back up to its original position before lowering. For blanks with a thickness of 2.0 mm to 5.0 mm, the clearance d1 between the die 51 and punch 53 is 0.07 mm to 1.5 mm.

[0032] Next, as shown in FIG. 4(c), the peripheral edge of the punching plate 61 is shaved using a die 71 and a punch 73 to produce a copper plate 42A. Reference numeral 72 in the figure denotes a plate holder. This shaving process separates an outer portion 63 from the dashed line L in FIG. 4(b). The machining allowance w3 of the outer portion 63 is set so that burrs 61B are removed and the tapered surface 61C becomes the tapered surface 422A with the width w2 and depth h2 described above. The machining allowance w3 is, for example, 0.4 mm in the width direction of the punching plate 61. The clearance d2 between the punch 73 and the die 71 is 0.4 mm to 2.5 mm. The clearance d2 during shaving is set to be smaller than the clearance d1 (d1 > d2).

[0033] (Second molding method) In the first forming method, after forming the punched plate 61, further processing of the peripheral edge is performed, but in the second forming method, a copper plate 42A is produced by punching a raw plate made of copper or a copper alloy in a single operation, the copper plate 42A having the tapered surface 422A with the width dimension w2 and depth dimension h2 and the sagging surface 421A with the width dimension w1 and depth dimension h1 on the peripheral edge 420. Although not shown in the drawings, in the second forming method, the clearance between the punch 73 and the die 71 is preferably 0.01 mm.

[0034] In this way, the copper plate 42A having the sagging surface 421A and the tapered surface 422A is used as a member to be joined to the first metal layer 41 in manufacturing the power module substrate 1 (second bonded body: corresponding to the bonded body of the present invention).

[0035] In the second joining step, as shown in Fig. 2(b), two copper plates 42A formed as described above are stacked in the thickness direction, one on each of the first metal layers 41 of the first joined body 11. Each copper plate 42A is stacked so that its tapered surface faces the first metal layer 41. Then, this stack is heat-treated in a vacuum heating furnace while being pressurized.

[0036] The load applied at this time is preferably 2.4 MPa or less. A load greater than 2.4 MPa is not preferred because it will cause significant deformation of first metal layer 41. The temperature for solid-phase diffusion bonding of copper plate 42A and first metal layer 41 is within the temperature range of 528°C to 543°C. This is because the eutectic temperature of aluminum constituting first metal layer 41 and copper constituting copper plate 42A is 548°C, and therefore solid-phase diffusion bonding is preferably performed within a temperature range that does not exceed the eutectic temperature of aluminum and copper.

[0037] In this way, by forming the second metal layer 42 by solid-state diffusion bonding the copper plate 42A to the first metal layer 41, the power module substrate 1 can be configured as a second bonded body.

[0038] In the manufacturing method of this embodiment, the tapered surface 422A of the copper plate 42A before bonding has a width dimension w2 and a depth dimension h2. This allows the copper plate 42A to be bonded to the first metal layer 41 without leaving any unbonded areas on the lower surface 422 or with the unbonded areas further reduced. Furthermore, by widening the inner region 421B (reducing the sagging surface 421A) on the upper surface 421, which is not in contact with the first metal layer 41, a highly flat surface suitable for mounting elements, etc. can be formed. Note that if the width dimension w2 is greater than 1.80 mm, unbonded areas frequently occur around the periphery during manufacturing. If the depth dimension h2 exceeds 20.0 μm, unbonded areas also occur.

[0039] The second bonded body (bonded body of the present invention) is not limited to the power module substrate 1 of the above-described embodiment, but may be configured as an insulated circuit board formed by sequentially bonding an aluminum plate and a copper plate to an insulating plate other than a ceramic substrate, and the present invention is also applicable to solid-state diffusion bonding of a copper plate and an aluminum plate in the manufacture of such an insulated circuit board. Furthermore, in the above-described embodiment, the first metal layer 41 and the second metal layer 42 bonded thereto are provided on both sides of the ceramic substrate, but the first metal layer 41 and the second metal layer 42 may be provided on only one side of the ceramic substrate. The dimensions and ratios are not limited to those shown in the drawings, and the second metal layer 42 may be formed smaller than the first metal layer 41. [Example]

[0040] A confirmatory experiment was carried out to confirm the effect of the present invention. (1) First, aluminum plates were laminated on both sides of a ceramic substrate via a brazing material, and then heated while being pressed in the lamination direction to form a first metal layer on each of the two sides of the ceramic substrate, thereby producing a first bonded body. A mask was printed on one of the first metal layers and etched with iron chloride, and then the mask was removed to form circuit patterns with a distance between the circuit patterns of 1.00 mm. (1-1) Material of the first bonding body Ceramic substrate: Si3N4 (thickness 0.32 mm) Aluminum plate: 4N aluminum (thickness 0.4 mm) Brazing filler metal: Brazing filler metal foil (thickness 0.017 mm) made of Al-7.5 mass% Si alloy (1-2) Manufacturing conditions of the first bonded body Bonding load: 0.2MPa Heating temperature: 650℃ Holding time: 30 minutes Bonding atmosphere: vacuum

[0041] (2) Next, a copper plate having a tapered surface as shown in Table 1 was bonded to the circuit patterned first metal layer on one side of the first bonded body and the first metal layer on the other side by the solid-state diffusion bonding described above to form a second metal layer, thereby producing a second bonded body. (2-1) Copper plate used for joining The copper plates were each made of oxygen-free copper and 2.0 mm thick. Samples with varying peripheral dimensions were selected and used as samples. Samples 1 and 2 were fabricated using the first fabrication method described in the embodiment. Unlike Samples 1 and 2, Samples 3 to 8 were each fabricated by punching the raw plate in a single operation without shaving. Samples 3 and 4 were fabricated using the second fabrication method described in the embodiment. The clearance d3 for Samples 3 and 4 was 0.01 mm, and the clearance d3 for Samples 5 to 8 was 0.07 mm. The copper plates used had tapered surface widths and depths determined by the measurement methods described below.

[0042] (2-2) Measuring tapered surfaces The dimensions of the tapered surface on each copper plate were measured using a surface roughness meter (Mitutoyo Corporation, SURFTEST SJ-410). The scanning direction of the contact needle was from the inner region of the copper plate to the edge (wall surface) of the copper plate, and measurements were taken from the inner region to the periphery. Specifically, the measurement started 4 mm from the edge of the copper plate and ended when the measured value reached the over-range. In other words, measurements were taken until the contact needle of the surface roughness meter left the edge of the copper plate and reached the lowest limit of the needle height. Figure 5 is a graph showing the contour line drawn by the contact needle's trajectory in the peripheral region of the copper plate. The width and depth dimensions of the tapered surface were calculated from the contour line.

[0043] The tapered surface was defined as the point on the contour line below the surface reference line that was the start point of the tapered surface, the corner where the edge meets the wall surface as the end point, and the start point of the burr as the end point. For samples 3 to 8, the corner where the edge meets the wall surface (not shown) was used as the end point instead of the start point of the burr. The average value obtained from multiple measurements was used as the width dimension, and the maximum value from the surface reference line to the contour line was used as the depth dimension of the tapered surface.

[0044] (2-3) Manufacturing conditions of the second bonded body Bonding load: 2.4MPa Heating temperature: 533℃ Holding time: 120 minutes Bonding atmosphere: vacuum

[0045] (3) Evaluation of tapered surface bonding rate (3-1) Evaluation method The interface between the first metal layer and the second metal layer after bonding was evaluated using an ultrasonic flaw detector. As an evaluation method, the bonding rate was calculated from the following formula (1) (initial bonding). Bonding rate (%) = [{(bonding area) - (peeling area)} / (bonding area)] × 100 (1) Here, the bonding area was evaluated as the region at the interface between the first metal layer and the second metal layer, extending 1 mm inward from the edge (wall surface) of the copper plate. Since the peeled areas are shown as white in the ultrasonic flaw detection image (binarized image), the total area of ​​these white areas was taken as the peeled area. For the bonded bodies of Samples 1 to 6, the bonding ratio after baking at 300°C was also calculated. These results are shown in Table 1.

[0046] [Table 1]

[0047] (3-2) Evaluation As shown in Table 1, in the bonded bodies of Samples 1 to 6, the bonding rate of the second metal layer was 100% after the initial bonding and baking, which was good, and it was confirmed that there were no unbonded areas on the tapered surface and that delamination was unlikely to progress. 6(a) is an SEM image of Sample 1, in which the first metal layer is labeled Al and the second metal layer is labeled Cu. It can be seen that the second metal layer 42 is bonded to the first metal layer 41 without any gaps or cracks. In contrast, in the bonded bodies of Samples 7 and 8, the bonding rate of the second metal layer was 91% to 98%, and non-bonding and peeling of the tapered surface were confirmed. FIG. 6(b) is an SEM image of sample 7, which shows that cracks have occurred on the tapered surface between second metal layer 42 and first metal layer 41, and that unbonded regions remain. [Explanation of symbols]

[0048] 1 Power module substrate (joint) 10 Ceramic substrate 20 circuit layers 30 Heat dissipation layer 41 First metal layer 41A Aluminum Plate 42 Second metal layer 42A copper plate 421 Top surface 421A Peripheral area (sagging surface) 421B,422B Inner area 422 Bottom surface 422A Peripheral area (tapered surface) 423 Wall 51,71 Die 53,73 punch 61 punching board

Claims

1. A method for manufacturing a joined body obtained by joining a copper plate made of copper or a copper alloy and an aluminum plate made of aluminum or an aluminum alloy, comprising: The method comprises a copper plate forming step of punching and forming the copper plate having a thickness of 2.0 mm or more and 5.0 mm or less using a press having a die and a punch, and a joining step of joining the copper plate and the aluminum plate by solid-state diffusion bonding, In the copper plate forming step, punching is performed with a clearance between the die and the punch set to 0.01 mm or more and 0.07 mm or less, and a copper plate having a tapered surface in which the inner region is thicker and the edge is thinner in the peripheral region of the surface to be joined to the aluminum plate is formed, the tapered surface has a width dimension of 1.80 mm or less in the surface direction of the copper plate and a depth dimension of 20.0 μm or less at the peripheral edge in the surface direction of the copper plate; The method for manufacturing a joined body, wherein the joining step joins the tapered surface facing the aluminum plate.

2. A method for manufacturing a joined body formed by joining a copper plate made of copper or a copper alloy and an aluminum plate made of aluminum or an aluminum alloy, comprising: The method comprises a copper plate forming step of punching and forming the copper plate having a thickness of 2.0 mm or more and 5.0 mm or less using a press having a press die and a press punch, and a joining step of joining the copper plate and the aluminum plate by solid-state diffusion bonding, The copper plate forming process includes forming a punched plate from a raw plate made of copper or a copper alloy using a press having a press die and a press punch, and then shaving the peripheral portion of the punched plate using a shaving die and a shaving punch, and setting a clearance between the shaving die and the shaving punch smaller than the clearance between the press die and the press punch to form a copper plate having a tapered surface in which the inner region is thicker and the edge is thinner in the peripheral region of the surface to be joined to the aluminum plate, the tapered surface has a width dimension of 1.80 mm or less in the surface direction of the copper plate and a depth dimension of 20.0 μm or less at the peripheral edge in the surface direction of the copper plate; The method for manufacturing a joined body, wherein the joining step joins the tapered surface facing the aluminum plate.

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