JP6603098B2 - Circuit board and electronic device - Google Patents

Description

  The present invention relates to a circuit board used for an electronic device such as a power module, and an electronic device using the circuit board.

2. Description of the Related Art In recent years, a circuit in which a metal plate such as copper or aluminum is bonded to both upper and lower surfaces of a ceramic substrate in an electronic device such as a power module or switching module on which a semiconductor element such as an IGBT (Insulated Gate Bipolar Transistor) is mounted and a large current flows A substrate is used (for example, see Patent Document 1 below). The demand for such circuit boards is increasing in applications such as control devices for electric vehicles or power generation devices using thermoelectric conversion.

  Such a circuit board generally has a ceramic substrate and a metal plate provided on the main surface of the ceramic substrate. The metal plate includes, for example, an upper surface metal plate provided on the upper surface of the ceramic substrate and a lower surface metal plate provided on the lower surface of the ceramic substrate. The upper surface metal plate and the lower surface metal plate are bonded to the ceramic substrate on the surface facing the ceramic substrate. The upper metal plate functions as a conductive path for connecting a base metal layer for mounting electronic components such as semiconductor elements or electronic components to an external electric circuit, for example. The lower metal plate functions as, for example, a heat radiating plate for radiating heat to the outside.

JP-A-2-183557 JP 2003-86747 A

  In the conventional circuit board, the entire surface of each of the upper metal plate and the lower metal plate facing each other is bonded to the ceramic substrate, so that thermal stress is generated due to the difference in linear expansion coefficient between the ceramic substrate and the metal plate. Cheap. Therefore, when the circuit board is cooled, there is a possibility that tensile stress is generated in the ceramic substrate near the end of the portion where the metal plate is joined. This tensile stress may cause mechanical destruction such as cracks in the ceramic substrate. In addition, the upper surface metal plate and the lower surface metal plate may be separated from the insulating substrate.

  As means for reducing the possibility of such mechanical destruction of the ceramic substrate, for example, means for dividing the upper surface metal plate into a plurality of parts is conceivable (see, for example, Patent Document 1). However, in this case, since the bonding area with respect to the ceramic substrate is different between the upper surface side and the lower surface side of the ceramic substrate, the thermal stresses generated on the lower surface side and the upper surface side of the ceramic substrate are different from each other. Therefore, there is a possibility that the ceramic substrate is likely to warp due to this thermal stress difference. Due to the occurrence of such warpage, tensile stress is applied to the joint portion between the upper metal plate and the ceramic substrate, and cracks may occur in the ceramic substrate at the outer periphery of the joint portion. Further, there is a possibility that the metal plate is peeled off from the ceramic substrate.

  The present invention has been completed in view of the above problems in the prior art, and an object of the present invention is to provide a machine for a ceramic substrate in a circuit board having a ceramic substrate, and an upper metal plate and a lower metal plate bonded to the ceramic substrate. It is an object of the present invention to provide a circuit board in which an effective destruction and peeling of an upper metal plate and a lower metal plate from a ceramic substrate are effectively suppressed. Moreover, it is providing the electronic device which has such an effect.

A circuit board according to an aspect of the present invention has an upper surface, a lower surface, and side surfaces, and a plurality of ceramic substrates arranged such that the side surfaces are separated from each other and face each other, and the plurality of ceramic substrates Each of the upper surfaces of the ceramic substrate and a lower surface metal plate having an upper surface to which the lower surfaces of the plurality of ceramic substrates are joined. Of these, the exposed portion between the opposing side surfaces of the ceramic substrate has a groove provided along the outer periphery of the ceramic substrate in plan view, and the upper surface metal plates are adjacent to each other. A connecting portion connected to each other between the ceramic substrates, and the connecting portion is curved upward or downward in a portion facing the connecting portion; The upper surface of the lower surface metal plate has a recess, or the lower surface metal plate is characterized by having a penetration portion penetrating in the thickness direction lower surface metal plate.

  An electronic device according to an aspect of the present invention includes the circuit board configured as described above, and an electronic component mounted on the circuit board and electrically connected to the upper metal plate. .

The circuit board according to one aspect of the present invention has the above-described configuration, and the lower surface metal plate is disposed on the outer periphery of the ceramic substrate in a plan view in a portion exposed between the mutually facing side surfaces of the ceramic substrate. Since it has the groove | channel provided along, it is easy to deform | transform in the direction which a groove | channel closes or opens on both sides which pinched | interposed this groove | channel. This direction in which deformation is likely to occur is the same as the direction of action of thermal stress caused by the difference in thermal expansion coefficient between the ceramic substrate and the lower metal plate. Therefore, the thermal stress can be effectively relieved by deformation mainly of the groove portion of the lower metal plate. Moreover, since the connection part is included, the freedom degree of the wiring in a circuit board and an electronic device is raised, and practicality further improves. And when a connection part is curving, it is easy to change to the length direction, ie, a horizontal direction. Therefore, it is possible to provide a circuit board in which mechanical destruction of the ceramic substrate and separation of the upper surface metal plate and the lower surface metal plate from the ceramic substrate are effectively suppressed. When the lower surface metal plate has a recess or a through portion, the electrical insulation between the lower surface metal plate and the connecting portion becomes higher, and the breakdown voltage as the circuit board and the electronic device is improved. be able to.

  Further, in the electronic device according to one aspect of the present invention, since the electronic component is mounted on the circuit board having the above configuration, the ceramic substrate is mechanically broken, and the upper surface metal plate and the lower surface metal plate are separated from the ceramic substrate. An electronic device in which peeling is effectively suppressed can be provided.

(A) is a top view which shows the circuit board and electronic device in the 1st Embodiment of this invention, (b) is sectional drawing in the AA of (a). (A) A mantle (b) is an enlarged sectional view showing an example of a portion B in FIG. 1 (b). (A) A mantle (b) is an enlarged sectional view showing an example of a portion B in FIG. 1 (b). (A) And (b) is sectional drawing which shows the modification of FIG.1 (b), respectively. (A) is a top view which shows the circuit board and electronic device in the 2nd Embodiment of this invention, (b) is a top view which shows the modification of (a). (A) is a top view which shows the circuit board and electronic device in the 3rd Embodiment of this invention, (b) is sectional drawing in the AA of (a). (A) is sectional drawing which expands and shows an example of B part of FIG.6 (b), (b) is sectional drawing which shows the modification of (a). It is sectional drawing which expands and shows the principal part of the circuit board and electronic device in the 4th Embodiment of this invention.

  A circuit board and an electronic device according to an embodiment of the present invention will be described with reference to the drawings.

(First embodiment)
FIG. 1A is a plan view showing a circuit board and an electronic device according to a first embodiment of the present invention.
FIG.1 (b) is sectional drawing in the AA of Fig.1 (a).

  The circuit board 10 is gathered together on a plurality of ceramic substrates 1 arranged side by side in a horizontal direction, an upper surface metal plate 2 provided on each upper surface of the plurality of ceramic substrates 1, and a lower surface of the plurality of ceramic substrates 1. And a single lower metal plate 3 joined by a brazing material 4 or the like. The electronic component 5 such as a semiconductor element is mounted on the circuit board 10 to form an electronic device 20 such as a semiconductor device. The electronic device 20 is, for example, a power module.

  Each of the plurality of ceramic substrates 1 has a function as, for example, a mounting substrate for mounting the electronic component 5 and a connection substrate for bonding and holding the upper surface metal plate 2 and the lower surface metal plate 3. is doing. The upper surface metal plate 2 and the lower surface metal plate 3 are, for example, a base metal material for connection of the electronic component 5, a heat dissipation material for radiating heat generated by the operation of the electronic component 5, and the electronic component 5 to the outside. It functions as a conductive path or the like for electrical connection with an electric circuit (not shown). For example, the electronic component 5 mounted on the ceramic substrate 1 via the upper surface metal plate 2 is electrically connected to an external electric circuit by the upper surface metal plate 2. Heat generated by operations including transmission / reception of signals to / from an external electric circuit is dissipated to the outside from the upper surface metal plate 2, the lower surface metal plate 3, the exposed surface of the ceramic substrate 1, and the like.

  A plurality of ceramic substrates 1 are arranged in accordance with the functions of the mounting substrate and the connection substrate, for example. Each ceramic substrate 1 has an upper surface, a lower surface, and side surfaces, and has, for example, a flat plate shape such as a square shape in plan view.

  The plurality of ceramic substrates 1 are arranged such that the respective side surfaces are opposed to each other. In other words, the plurality of ceramic substrates 1 are arranged side by side in the lateral direction. In the circuit board 10 of the first embodiment, the upper surface and the lower surface of each of the plurality of ceramic substrates 1 can be regarded as being arranged on a virtual plane (not shown).

  The ceramic substrate 1 is formed of a ceramic material such as an aluminum oxide sintered body, a mullite sintered body, a silicon carbide sintered body, an aluminum nitride sintered body, or a silicon nitride sintered body. . Of these ceramic materials, a silicon nitride sintered body, an aluminum nitride sintered body, and a silicon carbide sintered body are preferable in terms of thermal conductivity (heat dissipation). In terms of mechanical strength, a silicon nitride sintered body and a silicon carbide sintered body are preferable.

When the ceramic substrate 1 is made of a ceramic material having a relatively high mechanical strength such as a silicon nitride sintered body, the ceramic substrate 1 is used when the upper metal plate 2 and the lower metal plate 3 having a larger thickness are used. 1 is less likely to crack. Therefore, it is more advantageous in realizing the circuit board 10 that can flow a larger current even if it is small. The thickness of the ceramic substrate 1 may be smaller in terms of thermal conductivity, but may be appropriately set according to a predetermined external dimension (size) of the ceramic substrate 1 and is, for example, about 0.1 to 1 mm. Is set.

Each of the plurality of ceramic substrates 1 is manufactured as follows if it is made of, for example, silicon nitride ceramics. That is, first, an appropriate organic binder, plasticizer, solvent, and the like are added to and mixed with raw material powders such as silicon nitride, aluminum oxide, magnesium oxide, and yttrium oxide to prepare a ceramic slurry. Next, the ceramic slurry is formed into a sheet by a forming method such as a doctor blade method or a calender roll method to produce a band-shaped ceramic green sheet (ceramic green sheet). Next, the band-shaped ceramic green sheet is appropriately punched to have a predetermined shape, and a plurality of sheets are laminated as necessary to form a formed body. Then, the ceramic substrate 1 can be manufactured by firing this molded body at a temperature of 1600 to 2000 ° C. in a non-oxidizing atmosphere such as a nitriding atmosphere.

  The plurality of upper surface metal plates 2 are joined to the upper surface of the ceramic substrate 1 by a brazing material 4. Some of the plurality of upper surface metal plates 2 bonded to the plurality of ceramic substrates 1 function as a base metal layer for mounting and bonding the electronic component 5 as described above, for example. The upper surface metal plate 2 also includes terminals that are electrically connected to the electronic component 5 via bonding wires 6, for example. In the example of FIG. 1, an electronic component 5 is mounted on and bonded to an upper surface metal plate 2 on one ceramic substrate 1 arranged at the center. Bonding wires 6 are connected to the upper metal plate 2 on the other ceramic substrates 1.

  The lower metal plate 3 has an upper surface bonded to the lower surfaces of the plurality of ceramic substrates 1 by a brazing material 4. That is, the circuit board 10 of the first embodiment has one lower surface metal plate 3 having an upper surface to which the lower surfaces of the plurality of ceramic substrates 1 are bonded. A plurality of ceramic substrates 1 are collectively joined to the lower metal plate 3 to form an independent circuit board 10 as one structure.

  For example, as described above, the lower surface metal plate 3 is a portion that becomes a part of the heat radiation path from the electronic product to the outside (heat radiation material). The lower surface metal plate 3 can also function as a mounting pad when the circuit board 10 and the electronic device 20 are mounted by being bonded to an external electric circuit or the like. Since the plurality of ceramic substrates 1 are joined to one lower surface metal plate 3, heat dissipation as the circuit substrate 10 is enhanced. In addition, the electronic device 20 including the circuit board 10 can be easily connected to and mounted on an external electric circuit.

  The upper metal plate 2 and the lower metal plate 3 are made of metal such as copper or aluminum, for example. The upper surface metal plate 2 and the lower surface metal plate 3 are subjected to a conventionally known metal processing method such as mechanical processing such as rolling or punching or chemical processing such as etching, for example, on a copper ingot. By, for example, it can be manufactured by forming it into a predetermined pattern in a flat plate shape having a thickness of 0.05 to 5 mm. At this time, the upper surface metal plate 2 and the lower surface metal plate 3 may be formed in a predetermined pattern shape in advance, or a metal plate having the same size and shape as the ceramic substrate 1 is joined to the ceramic substrate 1. You may process into a predetermined pattern shape by an etching later.

  When the upper surface metal plate 2 and the lower surface metal plate 3 are made of copper, it is preferably formed of oxygen-free copper. When formed of oxygen-free copper, when the ceramic substrate 1 and the upper surface metal plate 2 or the lower surface metal plate 3 are joined, the surface of the copper is not oxidized by oxygen present in the copper, and the brazing material 4 Since the wettability becomes good, the bonding becomes stronger.

  The ceramic substrate 1 and the upper surface metal plate 2 or the lower surface metal plate 3 are joined by, for example, a brazing material 4. The brazing filler metal 4 may contain an active metal or may be a normal one containing no active metal. As the brazing material 4, the brazing material 4 containing active metal may be used for all the above-mentioned joining, but considering the productivity and economic efficiency as the circuit board 10, a normal brazing material containing no active metal is acceptable. It is preferable to use more material 4.

In order to join each member with the brazing material 4, first, a brazing material paste is printed and applied in a predetermined pattern with a thickness of, for example, 30 to 50 μm to at least one of the joining surfaces of the respective members by screen printing or the like. Next, these members are aligned with a brazing filler paste so as to have a predetermined structure. Then, while applying a load of 5 to 10 kPa on the metal plate, heat in a non-oxidizing atmosphere such as a hydrogen gas atmosphere or a hydrogen-nitrogen gas atmosphere at 780 ° C. to 900 ° C. for 10 to 120 minutes. The above-mentioned members can be joined with the brazing material 4 by changing the organic solvent, the solvent and the dispersing agent into a gas to emit the gas and melting the brazing material 4.

  When the upper surface metal plate 2 and the lower surface metal plate 3 are made of copper, an ordinary brazing material paste containing no active metal is, for example, silver made of silver and copper powder, silver-copper alloy powder, or a mixed powder thereof. A brazing material (for example, silver: 72% by mass—copper: 28% by mass) is prepared by adding an appropriate binder, an organic solvent, and a solvent to a powder and kneading. The brazing material paste containing the active metal is prepared by adding 2 to 5% by mass of an active metal such as titanium, hafnium or zirconium or a hydride thereof to the normal brazing material paste, and mixing and kneading. Make it.

  When the upper surface metal plate 2 and the lower surface metal plate 3 are made of aluminum, an aluminum brazing material (for example, aluminum: 88 mass%-silicon: 12 mass%) may be used instead of the silver brazing material. In this case as well, a brazing material paste and a brazing material paste containing an active metal may be produced in the same manner and joined in the same manner. When an aluminum brazing material is used, bonding can be performed at about 600 ° C., which is lower than copper.

  In order to join the upper surface metal plate 2 or the lower surface metal plate 3 to the ceramic substrate 1 with a normal brazing material paste containing no active metal, a metallized layer (not shown) is formed on the ceramic substrate 1, and the metallized layer is formed. A brazing paste may be disposed between the upper metal plate 2 and the lower metal plate 3. The metallized layer on the ceramic substrate 1 can be formed by forming the ceramic substrate 1 by printing and applying a metallized paste in a predetermined pattern shape on the ceramic green sheet and firing it. it can. Alternatively, after the ceramic substrate 1 is manufactured, the metallized paste may be printed on the ceramic substrate 1 in a predetermined pattern shape and baked. The metallized paste is prepared, for example, by adding and mixing a metal powder composed of tungsten (W), molybdenum (Mo), manganese (Mn) or a mixed powder thereof, an appropriate binder, an organic solvent / solvent, and kneading. To do.

  In addition, after the upper surface metal plate 2 is bonded to the ceramic substrate 1, a metal (not shown) having high conductivity such as a nickel plating layer on the surface and having good corrosion resistance and good wettability with the brazing material 4 is plated. You may make it adhere by. In this case, the electronic component 5 such as a semiconductor element can be more firmly bonded to the upper metal plate 2 via solder. Further, the electrical connection between the top metal plate 2 and the external electric circuit can be made better.

In this case, when the thickness of the nickel plating layer is 1.5 μm or more, the exposed surfaces of the upper surface metal plate 2 and the lower surface metal plate 3 (parts of the surface that are not joined to the ceramic substrate 1) are completely formed. It becomes easy to coat and oxidative corrosion of the upper metal plate 2 can be reduced more effectively. In addition, if the thickness is 10 μm or less, the residual stress inside thereof is relatively small. For example, even if the thickness of the ceramic substrate 1 is less than 300 μm, the ceramic substrate 1 warps due to the stress inside the plating layer. Or possibility that mechanical destruction, such as a crack, will occur can be suppressed lower.

When the upper surface metal plate 2 and the lower surface metal plate 3 are formed in a predetermined pattern in advance and then bonded to the ceramic substrate 1, the following may be performed. First, a plurality of ceramic substrates 1 are prepared. Further, the metal plate is processed into a predetermined pattern shape of the upper surface metal plate 2 and the lower surface metal plate 3 using press processing or etching processing. Next, a brazing material paste containing an active metal, which becomes the brazing material 4 after joining, is screen-printed in a predetermined shape on at least one of the joint portions where the ceramic substrate 1, the upper metal plate 2 and the lower metal plate 3 face each other. Apply with etc. A brazing material 4 is formed in advance on the lower surface of the upper metal plate 2 and the upper surface of the lower metal plate 3. The brazing material 4 may be provided at a predetermined position by screen printing, or by punching a metal plate having a predetermined thickness clad with the brazing material 4 into the shapes and dimensions of the upper surface metal plate 2 and the lower surface metal plate 3. It may be formed. Each member is placed at a predetermined position, and a temperature is raised to a temperature at which the brazing filler metal 4 melts in a vacuum while applying a load using a jig or the like so that the position does not shift, and the members are joined. As described above, the upper surface metal plate 2 and the lower surface metal plate 3 are bonded to the ceramic substrate 1 to manufacture the circuit board 10.

  The electronic component 5 is mounted on the upper surface metal plate 2 and is bonded to one upper surface metal plate 2 by a bonding material such as solder or Au—Si alloy. The electronic component 5 is electrically connected to the plurality of upper surface metal plates 2 by bonding wires 6. Examples of the electronic component 5 include semiconductor elements such as transistors, LSIs (Large Scale Integrated circuits) for CPUs (Central Processing Units), IGBTs (Insulated Gate Bipolar Transistors), or MOS-FETs (Metal Oxide Semiconductors-Field Effect Transistors). It is done.

  In the circuit board 10 of the first embodiment, the lower surface metal plate 3 is along the outer periphery of the ceramic substrate 1 in a plan view in a portion of the upper surface exposed between the mutually facing side surfaces of the ceramic substrate 1. A groove 3a is provided. Therefore, the lower surface metal plate 3 is easily deformed in a direction in which the groove 3a is closed or opened on both sides of the groove 3a. This direction in which deformation is likely to occur is the same direction as the direction of action of thermal stress caused by the difference in thermal expansion coefficient between the ceramic substrate 1 and the lower metal plate 3. The acting direction of the thermal stress is a direction (lateral direction) along the upper and lower surfaces of the surface of the lower surface metal plate 3 having a large area and a large absolute amount of thermal expansion and contraction. Therefore, the thermal stress can be effectively relieved by deformation mainly of the groove 3a portion of the lower surface metal plate 3. That is, the thermal stress generated between the ceramic substrate 1 and the lower metal plate 3 is effectively reduced. Therefore, mechanical destruction of the ceramic substrate 1 due to the thermal stress is effectively suppressed. Moreover, about each of the upper surface metal plate 2 and the lower surface metal plate 3, peeling from the ceramic substrate 1 is suppressed effectively.

  The groove 3a can be provided in the lower surface metal plate 3 by mechanical processing such as grinding or metal processing such as etching.

  The width of the groove 3a (the dimension of the groove 3a in the direction orthogonal to the direction in which the groove 3a extends in plan view) is, for example, the dimension in plan view of the ceramic substrate 1, the thickness of the lower metal plate 3, and the electronic component 5 What is necessary is just to set suitably according to conditions, such as the emitted-heat amount. For example, the electronic component 5 is an IGBT semiconductor element, the upper metal plate 2 and the lower metal plate 3 are made of oxygen-free copper, the outer dimensions of the ceramic substrate 1 are 20 × 20 mm, and the lower surface When the outer dimension of the metal plate 3 is a square plate shape of 50 × 100 × 0.5 mm, the width of the groove 3a is about 0.1 to 0.4 mm at room temperature (for example, 20 to 25 ° C.). You only have to set it.

  Further, the distance between the ceramic substrate 1 and the groove 3a in plan view is preferably as small as possible in terms of relaxation of thermal stress, but may be appropriately set in consideration of workability and the like when forming the groove 3a. it can. For example, in the condition of the above example, it may be set to about 0.1 to 0.5 mm.

  Further, the depth of the groove 3a is preferably as large as possible in terms of relaxation of thermal stress, but can be appropriately set in consideration of workability and the like when forming the groove 3a. For example, in the condition of the above example, it may be set to about 0.1 to 0.4 mm.

  The grooves 3a are provided along the entire outer periphery of the plurality of ceramic substrates 1 in a plan view (when the circuit board 10 is viewed from above), for example. Thus, if the groove 3a is provided along the entire outer periphery of the ceramic substrate 1, the lower surface metal plate 3 is more easily deformed, which is particularly effective for the relaxation of thermal stress.

  However, with regard to the effect of relaxing the thermal stress as described above, if the groove 3a is provided at least between the ceramic substrates 1 adjacent to each other, a certain effect can be obtained. In addition, for example, when the interval between adjacent ceramic substrates 1 is narrow, only one groove 3a may be provided between adjacent ceramic substrates 1. Further, the groove 3a is not limited to one (two in total) along each ceramic substrate 1 in plan view between adjacent ceramic substrates 1, but for example, an allowable range of productivity and economy. Etc., three or more may be provided.

  Moreover, the longitudinal cross-section (cross section in the direction orthogonal to the length direction of the groove | channel 3a) shape of the groove | channel 3a may be suitably set according to the formation method etc. of the groove | channel 3a. For example, the bottom surface and the inner surface of the groove 3a may be in contact with each other at a right angle (for example, a rectangular shape in the longitudinal section). Moreover, the groove | channel 3a may be smoothly connected between the bottom face and the inner side surface in the shape of a curved surface, so-called R surface shape. This is preferable from the viewpoint of suppressing the fracture because the fatigue fracture hardly occurs when the thermal stress is repeatedly generated.

  Further, as described above, the electronic device 5 of the first embodiment is manufactured by mounting the electronic component 5 on the circuit board 10 configured as described above. Since the electronic device 20 includes the circuit board 10 having the above-described configuration, it is possible to effectively suppress mechanical destruction of the ceramic substrate 1 and peeling of the upper surface metal plate 2 and the lower surface metal plate 3 from the ceramic substrate 1. Is possible.

  2 (a) and 2 (b), and FIGS. 3 (a) and 3 (b) are cross-sectional views showing an example of the B portion of FIG. 1 (b) in an enlarged manner. 2 and 3, the same parts as those in FIG. 1 are denoted by the same reference numerals. In each example of FIGS. 2 and 3, for example, a brazing material 4 containing an active metal is used.

  In the example of FIG. 2A, the outer circumferences of the upper and lower brazing materials 4 of the ceramic substrate 1 are located on the inner side (center side of the ceramic substrate 1) with substantially the same dimensions as the outer circumference of the ceramic substrate 1. In the circuit board 10 and the electronic device 20 of this example, the brazing material 4 cannot be seen when viewed from above. In this case, mechanical destruction of the ceramic substrate 1 is more effectively suppressed. This is due to the following reason.

  In general, in a structure (not shown) in which a flat metal plate is joined to the main surface (upper and lower surfaces) of a flat ceramic substrate, the phenomenon that the ceramic substrate breaks due to tensile stress is This is not only due to the fact that the tensile thermal stress due to the difference in thermal expansion corresponding to the difference in linear expansion coefficient from the substrate acts parallel to the main surface of the ceramic substrate. That is, when a difference occurs in the tensile stress applied to the upper and lower surfaces of the ceramic substrate due to the difference in the area and thickness of the metal plates joined to the upper and lower sides of the ceramic substrate, the ceramic substrate warps. As a result, the ceramic substrate is placed in the ceramic substrate at a portion outside the bonding portion between the ceramic substrate and the metal plate (and the brazing material bonding the metal plate and the ceramic substrate) on the side where the ceramic substrate warps in a concave shape. As a result, a stress is generated in which the ceramic substrate returns to a flat surface. This stress includes a component in a direction perpendicular to the main surface of the ceramic substrate and opposite to the metal plate. Since such a stress is also applied together with the tensile thermal stress, the metal plate is peeled off. In addition, the ceramic substrate is destroyed.

  On the other hand, in the example of FIG. 2A, substantially the same magnitude of stress is applied to the same upper and lower portions of the ceramic substrate 1 on the low temperature side. Thus, even if thermal stress is generated, warping of the ceramic substrate 1 is suppressed. Therefore, the stress at which the warped ceramic substrate 1 becomes flat is further reduced. Therefore, the upper surface metal plate 2 and the lower surface metal plate 3 are difficult to peel from the ceramic substrate 1. Moreover, the possibility that the ceramic substrate 1 is destroyed is more effectively reduced.

  In this case, since the dimensions of the brazing material 4 are smaller than those of the ceramic substrate 1, the upper surface metal plate 2 and the groove 3a in plan view, the ceramic substrate 1 and the upper surface metal plate 2 through the brazing material 4 are manufactured. ) Is easy to align. Therefore, it is more advantageous in terms of productivity as the circuit board 10 and the electronic device 20.

  In the example of FIG. 2B, the position of the outer periphery of the ceramic substrate 1 and the position of the outer periphery of the brazing material 4 are substantially the same position in plan view (when viewed from above). In such a case, the stress difference generated due to the difference in thermal expansion coefficient between the ceramic substrate 1 and the upper surface metal plate 2 and the lower surface metal plate 3 above and below the ceramic substrate 1 is more effective up to the outer peripheral portion of the ceramic substrate 1. Has been reduced. Further, there is no portion outside the brazing material 4 of the ceramic substrate 1 that receives tensile stress due to a temperature change to the low temperature side where the ceramic substrate 1 is easily broken. Therefore, peeling of the upper surface metal plate 2 and the like from the ceramic substrate 1 and mechanical destruction of the ceramic substrate 1 at a joint portion with the upper surface metal plate 2 and the like are more effectively suppressed.

  In the example of FIG. 3A, the outer periphery of the brazing material 4 is located outside the outer periphery of the ceramic substrate 1 (outside the ceramic substrate 1). In such a case, since the area of the portion where the stress is applied to the ceramic substrate 1 of the upper surface metal plate 2 and the area of the inner portion of the groove 3a of the lower surface metal plate 3 are closer, the stress difference between the upper and lower sides of the ceramic substrate 1 is increased. Furthermore, it is effectively reduced. Therefore, warpage of the ceramic substrate 1 caused by this stress difference can be further effectively suppressed. Therefore, in this case, mechanical destruction of the ceramic substrate 1 is more effectively suppressed.

  In the example of FIG. 3B, the outer periphery of the brazing material 4 is located outside the outer periphery of the ceramic substrate 1 (outside the ceramic substrate 1). Further, a part of the brazing material 4 extends to the side surface of the ceramic substrate 1 and is joined. In other words, at least a part of the side surface of the ceramic substrate 1 is covered with the brazing material 4. In such a case, the following effect can be obtained in addition to the effect in the example of FIG. That is, even if a fine crack is generated in the outer peripheral portion of the ceramic substrate 1, the progress of the fine crack can be prevented by the brazing material 4. Therefore, the possibility that the fine cracks generated on the outer periphery (a part of the side surface, etc.) of the ceramic substrate 1 progress due to the progress of the fine cracks, and the mechanical breakage of the ceramic substrate 1 occurs more effectively. ing.

  4 (a) and 4 (b) are cross-sectional views showing modifications of FIG. 1 (b). 4, parts similar to those in FIG. 1 are denoted by the same reference numerals. In the example of FIG. 4, the upper surface metal plate 2 has a connecting portion 2 a that is connected to each other between adjacent ceramic substrates 1. Moreover, this connection part is curving upward.

  The upper surface metal plates 2 of the adjacent ceramic substrates 1 are directly connected to each other by the connecting portion 2a and are electrically connected to each other. When such a connecting portion 2a is included, the degree of freedom of wiring in the circuit board 10 and the electronic device 20 is increased, and the practicality is further improved.

  Further, since the connecting portion 2a is curved upward, the connecting portion 2a is easily deformed in the length direction, that is, in the lateral direction. In other words, the connecting portion 2a can be easily deformed in the length direction. Therefore, when a stress such as a thermal stress is applied to the connecting portion 2a in the length direction, the stress can be effectively absorbed and relaxed by deformation in a curved portion or the like of the connecting portion 2a. Further, the effect of thermal stress on the ceramic substrate 1 is further reduced.

Therefore, in this case, peeling of the upper surface metal plate 2 due to thermal stress, mechanical destruction of the ceramic substrate 1 and the like are more effectively suppressed. In addition, the downward direction (not shown) may be sufficient as the bending direction of this connection part 2a. In this case, the same effect as described above can be obtained. However, in order to reduce the possibility that the connecting portion 2a erroneously contacts the ceramic substrate 1 or the like, the connecting portion 2a is preferably curved upward. In addition, in order to reduce the height of the circuit board 10 and the electronic device 20, the connecting portion 2a is preferably curved downward.

  In the example of FIG. 4B, the groove 3 a is also provided on the lower surface of the lower metal plate 3. In this case, the lower surface metal plate 3 can be more easily deformed between the adjacent ceramic substrates 1. Therefore, the thermal stress is more effectively relaxed and the mechanical destruction of the ceramic substrate 1 is more effectively suppressed. Moreover, peeling from the ceramic substrate 1 of the upper surface metal plate 2 and the lower surface metal plate 3 is suppressed more effectively.

(Second Embodiment)
FIG. 5A is a plan view showing a circuit board 10 and an electronic device 20 in the second embodiment of the present invention, and FIG. 5B is a plan view showing a modification of FIG. 5A. The second embodiment is different from the first embodiment in that the upper metal plate 2 has a connecting portion 2a described later, and the other points are the same. Explanation of these similar points is omitted.

  In the example of FIG. 5A, a plurality of ceramic substrates 1 each provided with an upper surface metal plate 2 includes one mounting substrate 1 (1A) on which an electronic component 5 is mounted on the upper surface metal plate 2, and The upper surface metal plate 2 includes a plurality of terminal substrates 1 (1B) to which one electrode (not shown) of the electronic component 5 is electrically connected. The mounting substrate 1 (1A) is the ceramic substrate 1 arranged at the center of the arrangement of the ceramic substrates 1 in FIG. 5A, and the other ceramic substrate 1 is the terminal substrate 1 (1B).

  That is, this example can also be regarded as an example in which the upper metal plate 2 connected to the plurality of electrodes of the electronic component 5 is divided so as to correspond to the individual electrodes. Further, it can be regarded as an example in which the area in plan view of the upper surface metal plate 2 on which the electronic component 5 is mounted is suppressed as much as possible. In other words, in this example, the bonding area of the plurality of upper surface metal plates 2 and the plurality of ceramic substrates 1 to which these are bonded to the respective lower surface metal plates 3 is minimized.

  Therefore, the thermal stress generated between the individual ceramic substrates 1 (the mounting substrate 1A and the terminal substrate 1B) and the lower metal plate 3 can be suppressed to be smaller. Accordingly, in this case, the ceramic substrate 1 (the mounting substrate 1A and the terminal substrate 1B) caused by the thermal stress generated between the ceramic substrate 1 (the mounting substrate 1A and the terminal substrate 1B) and the lower surface metal plate 3 is used. Mechanical destruction and peeling of the lower metal plate 3 are more effectively suppressed. Further, since the thermal stress generated between the upper surface metal plate 2 and the ceramic substrate 1 (the mounting substrate 1A and the terminal substrate 1B) is further reduced, the ceramic substrate 1 (the mounting substrate 1A and the mounting substrate 1A and It is also more advantageous in terms of suppressing mechanical destruction of the terminal board 1B).

  In the example of FIG. 5B, the plurality of terminal substrates 1B (ceramic substrates 1) have the same shape and dimensions in plan view. In this case, the thermal stress generated between the plurality of terminal substrates 1B (ceramic substrate 1) and the lower surface metal plate 3 is suppressed from being partially biased. Therefore, mechanical destruction of the terminal substrate 1B (ceramic substrate 1) due to the concentration of thermal stress on a part of the terminal substrate 1B (ceramic substrate 1) is more effectively suppressed.

  In addition, since a plurality of terminal substrates 1B (ceramic substrate 1) to which the upper surface metal plate 2 is bonded can be manufactured together with the same shape and dimensions, the productivity as the circuit board 10 and the electronic device 20 can be improved. It is also advantageous in terms of economy.

(Third embodiment)
6A is a plan view showing a circuit board 10 and an electronic device 20 according to a third embodiment of the present invention, and FIG. 6B is a cross-sectional view taken along the line AA of FIG. 6A. . In FIG. 6, the same parts as those in FIG.

  In the circuit board 10 and the electronic device 20 shown in FIGS. 6A and 6B, the upper surface metal plate 2 has a connecting portion 2a connected to each other between the ceramic substrates 1 adjacent to each other. The upper surface of the lower metal plate 3 has a recess 3b at a portion facing the connecting portion 2a. The recess 3b in the example shown in FIG. 6 has a shape and a dimension that protrude slightly outward from the connecting portion 2a in plan view. The concave portion 3b may have a shape and a dimension so as to overlap the connecting portion 2a in plan view. In other words, the recessed part 3b is formed so that it may cover the whole area of the upper surface of the lower surface metal plate 3, for example, at least facing the lower surface of the connecting part 2a (the connecting part 2a).

  The circuit board 10 and the electronic device 20 of the third embodiment are different from the above-described embodiments only in having the concave portion 3b, and the other points are the same. Explanation of these similar points is omitted.

  FIG. 7A is an enlarged cross-sectional view showing a portion B in FIG. In FIG. 7A, the same reference numerals are given to the same parts as in FIG. In the circuit board 10 and the electronic device 20 of the third embodiment, as shown in FIGS. 6 and 7A, the lower surface and the lower surface metal plate of the connecting portion 2a of the upper surface metal plate 2 by the depth d of the recess 3b. The distance between the upper surface 3 and the bottom surface of the recess 3b is large. As the distance increases, the electrical insulation between the connecting portion 2a and the lower surface metal plate 3 is improved.

  That is, the recess 3 b is provided to increase the dielectric breakdown voltage between the connecting portion 2 a of the upper surface metal plate 2 and the lower surface metal plate 3. The depth d of the recess 3b is, for example, the distance between the position of the upper surface of the lower metal plate 3 and the lower surface of the connecting portion 2a (assuming that the height of the connecting portion 2a is assumed). ), The magnitude of the current flowing through the connecting portion 2a, and the thickness (mechanical strength) of the lower surface metal plate 3 may be set as appropriate.

For example, when the electronic component 5 is an IGBT, it is desirable that the dielectric breakdown voltage between the connecting portion 2a and the lower surface metal plate 3 is about 10 KV or more. On the other hand, since the dielectric breakdown voltage of air is approximately 3 KV / mm, a part of the lower surface metal plate 3 facing the bottom surface of the connecting portion 2a and the concave portion 3b of the upper surface metal plate 2 (that is, the lower surface of the connecting portion 2a). ) Is preferably set to about 3.4 to 3.8 mm. Therefore, for example, if the thickness of the ceramic substrate 1 is about 0.3 mm, the depth d of the recess 3b may be about 3.1 to 3.5 mm. Moreover, the thickness of the lower surface metal plate 3 having the recess 3b having such a depth d may be about 4 to 5 mm.

  In the example shown in FIGS. 6 and 7A, the end portion on the ceramic substrate 1 side of the outer periphery of the recess 3 b is located below the ceramic substrate 1. That is, the outer periphery of the recess 3 b includes a portion located on the lower side of the ceramic substrate 1. In such a configuration, the electrical insulation between the lower metal plate 3 and the connecting portion 2a becomes higher. Thereby, the dielectric breakdown voltage as the circuit board 10 and the electronic device 20 can be improved.

This is due to the following reason. That is, the distance between the connecting portion 2a of the upper surface metal plate 2 and the lower surface metal plate 3 is the closest to the upper end of the side surface of the recess 3b. In particular, it is an end portion on the ceramic substrate 1 side of the outer periphery of the recess 3b, which is a portion where the connecting portion 2a and the upper end of the side surface of the recess 3b intersect when viewed from above (in plan perspective). On the other hand, if it is the said structure, the side surface upper end of the recessed part 3b will be located in the lower side of the ceramic substrate 1 (that is, hidden), and will not be visible from the upper side. In other words, the upper end of the side surface of the lower metal plate 3 and the connecting portion 2a do not face each other. Therefore, setting the upper end of the side surface of the recess 3b to the inside of the side surface of the ceramic substrate 1 is effective for improving the dielectric breakdown voltage.

  The distance between the side surface of the ceramic substrate 1 and the side surface of the recess 3b when viewed from above with respect to the depth d of the recess 3b may be about the same (d).

  Further, when such a recess 3b is provided, if the brazing material 4 protrudes from the upper end of the side surface of the recess 3b to the side surface of the ceramic substrate 1, a dielectric breakdown occurs between the brazing material 4 and the connecting portion 2a. It is likely to occur. In order to reduce this possibility, therefore, the brazing material 4 should not protrude from the side surface of the ceramic substrate 1.

  Note that the portion of the upper end of the side surface of the recess 3b that cannot be covered with the ceramic substrate 1 in plan view, that is, the portion of the recess 3b that is away from the ceramic substrate 1 is connected as shown in the example of FIG. It forms in the position away from the side surface of the part 2a.

  When the thickness of the ceramic substrate 1 is t, it is preferable that the distance between the outer peripheral position of the connecting portion 2a and the outer peripheral position of the concave portion 3b in a plan view is t + d or more in all portions. .

  In addition, the portion of the upper end of the side surface of the recess 3b that is not covered with the ceramic substrate 1 in plan view may have a shape that is gently connected from the upper end of the side surface of the recess 3b to the upper surface. Moreover, the corner | angular part between the side surface and lower surface may connect gently also about the connection part 2a. That is, the corner between the side surface of the recess 3b and the upper surface of the lower metal plate 3 may be rounded, and the corner between the connecting portion 2a and the side surface and the lower surface may be rounded. With such a shape, there is no corner portion where charges are likely to concentrate between the upper end of the side surface of the recess 3b and the connecting portion 2a, so that it is difficult for discharge to occur. Therefore, the circuit board 10 and the electronic device 20 are advantageous in improving the dielectric breakdown voltage. In order to form the upper end portion of the side surface of the concave portion 3b and the corner portion of the connecting portion 2a in this way, for example, before the plating layer such as the nickel plating layer is applied to the circuit substrate 10, the circuit substrate 10 is mounted. What is necessary is just to immerse in etching liquid suitably. The corners of the metal plate are preferentially etched in the etchant and formed into a round shape.

  In the example shown in FIG. 6, the circuit board 10 is provided with holes 3 c at four corners of the square plate-like bottom metal plate 3. The hole 3c is provided for fixing the circuit board 10 to a heat dissipation member (not shown). Further, a hole 3d is also provided at a position closer to the electronic component 5 than the holes 3c for fixing these four corners. These holes 3d are provided, for example, for improving thermal conductivity. The hole 3d for improving thermal conductivity may be used for fixing the circuit board 10 to the heat radiating member with a screw or the like (not shown). As a result, heat generated in the electronic component 5 is efficiently conducted from the circuit board 10 and the electronic device 20 to the heat radiating member not only through the hole 3c but also through the hole 3d (and screws, etc.) for improving heat conduction. .

The circuit board 10 and the electronic device 20 having such a plurality of holes 3c and 3d are advantageous in improving heat dissipation even when the ceramic substrate 1 is not on the entire upper surface of the lower metal plate 3. This is due to the following reason. When the ceramic substrate 1 having a relatively large elastic modulus is not on the entire upper surface of the lower metal plate 3, the elastic modulus is relatively small in the circuit board 10 and the electronic device 20. That is, the adhesion between the circuit board 10 and the heat radiating member tends to decrease due to the elastic deformation of the circuit board 10. On the other hand, if the circuit board 10 is fixed to the heat radiating member by the plurality of holes 3c, 3d and screws as described above, the adhesion between the circuit board 10 and the heat radiating member is improved. Therefore, in the circuit board 10 and the electronic device 20, the heat generated in the electronic component 5 can be efficiently transmitted to the heat radiating member.

FIG. 7B shows an example in which the recess 3 b is filled with the filler 7. As the filler 7, epoxy resin (dielectric breakdown voltage: about 20 kV / mm), polyethylene resin (dielectric breakdown voltage: 30 kV / mm)
Etc.), polyvinyl chloride resin (dielectric breakdown voltage: about 30 kV / mm), polyimide resin (dielectric breakdown voltage: about 20 kV / mm), silicone resin (dielectric breakdown voltage: about 25 kV / mm), etc. can be used. That is, the resin material (filler 7) is disposed between the recess 3b and the connecting portion 2a of the upper surface metal plate 2. By using such a filler 7 (resin material), air (
With respect to the dielectric breakdown voltage: about 3 kV / mm), the dielectric breakdown voltage can be increased several times to about 10 times.

  In the example shown in FIG. 6B, the filling material 7 is filled up to the middle of the side surface of the ceramic substrate 1, but the filling range, in other words, the height of the filling material 7 is set to a desired dielectric breakdown voltage or What is necessary is just to determine suitably according to conditions, such as workability | operativity of the filling operation | work of the resin material. The filler 7 may be filled in the entire area from the concave portion 3b to the connecting portion 2a, or may cover the upper surface of the connecting portion 2a.

  When silicone resin is used as the filler 7, since the heat resistance of the silicone resin is relatively high and the Young's modulus is small, it is difficult for large stress to be applied to the lower surface metal plate 3 even when the silicone resin is filled. Good sex. The electronic device 20 may be fixed to the heat radiating plate with screws or the like using the holes 3c and 3d and then filled with a filler 7 such as silicone resin so as to cover the entire electronic device 20. In this way, not only the dielectric breakdown voltage between the connecting portion 2a and the lower metal plate 3 is increased, but the electronic device 20 can be more effectively protected from the external environment.

  The filler 7 may be one in which fillers such as inorganic particles and additives such as a plasticizer are appropriately selected and added to the resin material. With these additives, for example, the thermal expansion coefficient and Young's modulus of the filler 7 may be adjusted.

(Fourth embodiment)
FIG. 8 is an enlarged cross-sectional view showing the main parts of the circuit board 10 and the electronic device 20 in the fourth embodiment of the present invention. This main part is a part corresponding to part B in FIG. In the example shown in FIG. 8, the upper surface metal plate 2 has a connecting portion 2 a, and the lower surface metal plate 3 penetrates the lower surface metal plate 3 in the thickness direction at a portion facing the connecting portion 2 a. This is an example having the part 3e. That is, in the lower side of the connecting portion 2a, the lower metal plate 3 is provided with a through portion 3e instead of the concave portion 3b.

  The circuit board 10 and the electronic device 20 according to the fourth embodiment are different from the circuit board 10 and the electronic device 20 according to the third embodiment only in having a through-hole 3e. Is the same. Explanation of these similar points is omitted.

  In the case of the example shown in FIG. 8 as well, the distance between the connecting portion 2a and the lower surface metal plate 3 is increased by the through portion 3e, as in the example shown in FIGS. 6 and 7 (third embodiment). . Therefore, the dielectric breakdown voltage between the connection part 2a and the lower surface metal plate 3 is effectively increased.

  That is, the example shown in FIG. 8 can be regarded as a form in which the recess 3b in the example shown in FIGS. Therefore, as described above, the same effect as the example shown in FIGS. 6 and 7 can be obtained, and the effect can be further enhanced.

  Similarly to the recess 3b, the through-hole 3e may have a shape and a size that protrudes slightly outside the connecting portion 2a in a plan view, for example.

  As for the circuit board 10 and the electronic device 20 of the fourth embodiment, the end portion on the ceramic substrate 1 side of the outer periphery may be located below the ceramic substrate 1. That is, the outer periphery of the penetrating part 3 e may include a part located on the lower side of the ceramic substrate 1. Thereby, the dielectric breakdown voltage as the circuit board 10 and the electronic device 20 can be improved.

  That is, dielectric breakdown is likely to occur at the upper end of the side surface of the through-hole 3e. On the other hand, if the through-hole 3e and the ceramic substrate 1 have the same positional relationship as the positional relationship between the upper end of the side surface of the recess 3b and the ceramic substrate 1 shown in FIG. The effect of improving the breakdown voltage is enhanced.

  Moreover, you may arrange | position a filler (not shown) in the penetration part 3e similarly to the example shown in FIG.7 (b). The dielectric breakdown voltage can be increased by filling the filler up to a place covering at least a part of the side surface of the ceramic substrate 1.

  In addition, this invention is not limited to the example of said embodiment, A various change is possible if it is in the range of the summary of this invention. For example, the electronic component 5 may be bonded not to the upper surface of the ceramic substrate 1 but to the upper surface of the lower metal plate 3. In this case, it is preferable in terms of heat dissipation.

  Further, for example, the form in which the connecting portion 2a is curved upward (the example shown in FIG. 4) and the form having the recess 3b or the through portion 3e (the examples shown in FIGS. 6 to 8) are combined. It may be a circuit board 10 or the like. Thereby, the improvement of the dielectric breakdown voltage between the connecting portion 2a and the lower surface metal plate 3 is further improved.

DESCRIPTION OF SYMBOLS 1 ... Ceramic substrate 1A ... Mounting board 1B ... Terminal substrate 2 ... Upper surface metal plate 2a ... Connection part 3 ... Lower surface metal plate 3a ... · Groove 3b ··· recess 3c, 3d ··· hole 3e ··· through portion 4 ··· brazing material 5 ··· electronic component 6 ··· bonding wire 7 ··· filling Material
10 ... Circuit board
20 ... Electronic device d ... Depth (concave) depth t ... Ceramic substrate thickness

Claims (9)

A plurality of ceramic substrates each having an upper surface, a lower surface, and side surfaces, each of the side surfaces being arranged to face each other apart from each other;
A top metal plate provided on the top surface of each of the plurality of ceramic substrates;
A lower metal plate having an upper surface to which the lower surfaces of the plurality of ceramic substrates are joined, and
The lower surface metal plate has a groove provided along an outer periphery of the ceramic substrate in a plan view in a portion exposed between the side surfaces of the ceramic substrate facing each other on the upper surface .
The circuit board according to claim 1, wherein the upper metal plate has a connecting portion connected to each other between the ceramic substrates adjacent to each other, and the connecting portion is curved upward or downward . The plurality of ceramic substrates each provided with the upper metal plate include at least one mounting substrate on which an electronic component is mounted on the upper metal plate, and one electrode of the electronic component on the upper metal plate. The circuit board according to claim 1, comprising a plurality of terminal boards electrically connected. The circuit board according to claim 2 , wherein the plurality of terminal boards have the same shape and dimensions in plan view. A plurality of ceramic substrates each having an upper surface, a lower surface, and side surfaces, each of the side surfaces being arranged to face each other apart from each other;
A top metal plate provided on the top surface of each of the plurality of ceramic substrates;
A lower metal plate having an upper surface to which the lower surfaces of the plurality of ceramic substrates are joined, and
The lower surface metal plate has a groove provided along an outer periphery of the ceramic substrate in a plan view in a portion exposed between the side surfaces of the ceramic substrate facing each other on the upper surface.
The upper surface metal plate has a connecting portion connected to each other between the ceramic substrates adjacent to each other, and the upper surface of the lower surface metal plate has a recess in a portion facing the connecting portion. it characterized circuitry substrate that you are. The circuit board according to claim 4 , wherein a resin material is disposed between the concave portion and the connecting portion of the upper surface metal plate. A plurality of ceramic substrates each having an upper surface, a lower surface, and side surfaces, each of the side surfaces being arranged to face each other apart from each other;
A top metal plate provided on the top surface of each of the plurality of ceramic substrates;
A lower metal plate having an upper surface to which the lower surfaces of the plurality of ceramic substrates are joined, and
The lower surface metal plate has a groove provided along an outer periphery of the ceramic substrate in a plan view in a portion exposed between the side surfaces of the ceramic substrate facing each other on the upper surface.
The upper surface metal plate has a connecting portion connected to each other between the ceramic substrates adjacent to each other, and the lower surface metal plate has a thickness direction of the lower surface metal plate in a portion facing the connecting portion. it characterized circuitry board that has a through portion that penetrates the. The outer periphery of the concave portion, the circuit board according to claim 4 or claim 5, characterized in that it includes a portion located on the lower side of the ceramic substrate. The circuit board according to claim 6 before the outer periphery of Kinuki communicating portion, characterized in that it includes a portion located on the lower side of the ceramic substrate. A circuit board according to any one of claims 1 to 8,
An electronic device comprising: an electronic component mounted on the circuit board and electrically connected to the upper metal plate.

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