JP2012244006A - Circuit board, and electronic device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a circuit board that can improve durability against temperature change while improving thermal conductivity, and an electronic device including the circuit board.SOLUTION: A crystal 23 of hexagonal boron nitride is oriented so that its a-axis and b-axis directions are along a surface direction of an insulation layer 13, in a circuit board 12 including the insulation layer 13 made of a thermosetting epoxy resin 24 containing the crystal 23 of hexagonal boron nitride.

Description

  The present invention relates to a circuit board and an electronic apparatus including the circuit board.

  2. Description of the Related Art Conventionally, in an electronic device in which various electronic components are mounted on a circuit board, the operation of each electronic component is stabilized by radiating heat generated when the electronic component is driven. As a circuit board used in such an electronic device, for example, a board in which a hexagonal boron nitride thin film (hBN layer) is formed on an insulating substrate has been proposed (Patent Document 1). In the circuit board of Patent Document 1, heat generated by driving the electronic component is released in the surface direction of the substrate by the hBN layer, so that the heat dissipation of the electronic component is improved.

JP 2010-161168 A

  In the circuit board of Patent Document 1, since the thermal expansion coefficient is different between the insulating board and the hBN layer, the hBN layer is formed on the board by repeating the temperature rise and temperature drop accompanying driving and stopping of the electronic component. There is a possibility that it will be easily peeled off. For this reason, it was expected to improve durability against temperature change while improving thermal conductivity.

  The present invention has been made paying attention to such problems existing in the prior art, and the purpose thereof is a circuit board capable of improving durability against temperature change while improving thermal conductivity, And providing an electronic apparatus including the circuit board.

  In order to solve the above problems, the invention of claim 1 is directed to a circuit board having an insulating layer made of an insulating resin containing hexagonal boron nitride fine particles, wherein the hexagonal boron nitride fine particles are the hexagonal boron nitride fine particles. The gist is that the a-axis and b-axis directions corresponding to the surface direction of the six-membered ring of boron nitride are aligned along the surface direction of the insulating layer.

  According to this, since the insulating layer contains hexagonal boron nitride, the hexagonal boron nitride has a temperature higher than that of the conventional configuration in which a thin film of hexagonal boron nitride is formed on the surface of the insulating layer. The separation from the insulating layer due to the change is suppressed. Further, since the hexagonal boron nitride contained in the insulating layer is oriented so that the a-axis and b-axis directions corresponding to the plane direction of the six-membered ring are along the plane direction of the insulating layer, the plane direction of the insulating layer The thermal conductivity with respect to can be improved. Therefore, according to invention of Claim 1, durability with respect to a temperature change can be improved, improving thermal conductivity.

  The invention according to claim 2 is characterized in that, in the circuit board according to claim 1, the content of the hexagonal boron nitride in the insulating layer is 50 volume% or more and 80 volume% or less.

  According to this, the thermal conductivity can be suitably improved as compared with the case where the content of hexagonal boron nitride is less than 50% by volume, while the strength of the insulating layer is compared with the case where the content exceeds 80% by volume. The shortage can be suppressed.

  According to a third aspect of the present invention, in the circuit board according to the first or second aspect, the lengths of the fine particles of the hexagonal boron nitride in the a-axis and b-axis directions correspond to the stacking direction of the six-membered ring. The gist is that the aspect ratio, which is a value divided by the length in the c-axis direction, is 2 or more and 10 or less.

  According to this, compared with the case where the aspect ratio of hexagonal boron nitride is less than 2, the thermal conductivity in the plane direction of the insulating layer can be improved, while the strength of the insulating layer is higher than when the aspect ratio exceeds 10. It can suppress that it falls.

  According to a fourth aspect of the present invention, heat exchange is performed between the circuit board according to any one of the first to third aspects, a heat generating component mounted on the circuit board and generating heat by driving, and a heat exchange medium. In the electronic device provided with the heat exchange member for carrying out, it makes a summary that the said heat-emitting component and the said heat radiating member are thermally connected through the said circuit board.

  According to this, heat exchange can be performed between the heat generating component and the heat exchange medium via the circuit board according to any one of claims 1 to 3. For this reason, it can suppress that a number of parts increases compared with the structure which provides the exclusive part separately for transferring the heat which generate | occur | produced in the heat-emitting part to a heat exchange medium.

  According to the present invention, it is possible to improve durability against temperature changes while improving thermal conductivity.

The model perspective view of a DC / DC converter. The ABCD schematic cross section shown in FIG. The schematic cross section of an insulating layer. FIG. 2 is a schematic perspective view of a hexagonal boron nitride crystal. The graph which shows the simulation result of the heat conductivity in case the content of hexagonal boron nitride and alumina is changed.

Hereinafter, an embodiment embodying the present invention will be described with reference to FIGS.
As shown in FIG. 1, a DC / DC converter 10 as an electronic apparatus according to the present embodiment includes a housing 11 having a bottomed rectangular box shape, and various electronic components are disposed inside the housing 11. The circuit board 12 is accommodated. The housing 11 is made of a material having high thermal conductivity such as aluminum.

  As shown in FIG. 2, the circuit board 12 is a double-sided board in which a thick copper foil pattern 14 having a thickness of 0.1 to 0.5 mm is bonded to both sides of an insulating layer 13 having insulating properties. In the following description, among the surface directions of the insulating layer 13 shown in FIG. 1, the direction indicated by the arrow Ya is the front-rear direction, the direction indicated by the arrow Yb is the left-right direction, and the direction indicated by the arrow Yc is the vertical direction. .

  As shown in FIG. 2, the insulating layer 13 is formed in a flat plate shape (or substantially flat plate shape) as a whole while the upper surface and the lower surface are parallel to each other. The insulating layer 13 is formed with a plurality of insertion holes 13a through which the screws Sc are inserted. A plurality of fixing portions 11a are formed on the bottom surface of the housing 11 so as to protrude toward the opening of the housing 11, and screw holes 11b are provided in the fixing portion 11a. And the circuit board 12 is being fixed to the housing 11 in the state closely_contact | adhered to the fixing | fixed part 11a by screwing each screw Sc inserted in the penetration hole 13a in the corresponding screw hole 11b, respectively. A plurality of power semiconductor elements Sd such as IGBTs and MOSFETs are soldered to the thick copper foil pattern 14 and mounted on the circuit board 12.

  The circuit board 12 is provided with a transformer 15. The transformer 15 includes a core 16 made of a soft magnetic material such as ferrite. The core 16 lies sideways (cross-sectional view), has an approximately E shape, and is disposed on the upper surface side of the circuit board 12. The core 16 has a generally bowl shape and is disposed on the lower surface side of the circuit board 12. The lower core member 18 is provided. Both core members 17 and 18 are disposed so as to sandwich the circuit board 12 from above and below the circuit board 12.

  The upper core member 17 includes a base portion 17a having a substantially rectangular flat plate shape, a substantially columnar first leg portion 17b protruding from the center portion of the base portion 17a toward the insulating layer 13 side, and both ends of the base portion 17a. And a pair of second leg portions 17c projecting toward the insulating layer 13 side. The lower core member 18 includes a base portion 18a having a substantially rectangular flat plate shape, and a pair of wall portions 18b protruding from both ends of the base portion 18a toward the insulating layer 13 side.

  The upper core member 17 is assembled to the upper surface side of the circuit board 12 in a state where the first leg portion 17b is inserted into the circular through hole 13b formed in the insulating layer 13 in a plan view. In this state, the flat surface formed at the distal end portion of the first leg portion 17b is disposed at a position slightly spaced from the base portion 18a of the lower core member 18, and at the distal end portion of the second leg portion 17c. The formed plane is in close contact with the upper surface of the insulating layer 13.

  On the other hand, the lower core member 18 is assembled to the lower surface side of the insulating layer 13 so that the flat surface formed at the tip of the wall portion 18 b is in close contact with the lower surface of the insulating layer 13. The second leg portion 17c of the upper core member 17 and the wall portion 18b of the lower core member 18 are arranged so that the planes formed at the tip portions thereof face each other with the insulating layer 13 therebetween.

  A heat conductive sheet 20 is disposed on the lower surface side of the lower core member 18 so as to thermally connect the lower core member 18 and the housing 11. Each core member 17, 18 is disposed away from the thick copper foil pattern 14 and is not in contact with the thick copper foil pattern 14.

  A coil 21 is formed (patterned) on the upper and lower surfaces of the insulating layer 13 so as to surround the periphery of the through hole 13b of the insulating layer 13. The coil 21 is formed by punching a copper foil (copper plate). Further, the number of turns (turns) of the coil 21 formed on the upper surface of the insulating layer 13 is five, while the number of turns (turns) of the coil 21 formed on the lower surface of the insulating layer 13 is one. The number of turns is different. Each coil 21 is disposed inside the core members 17 and 18 so as to be separated from the core members 17 and 18, and is not in contact with the core members 17 and 18.

Next, the insulating layer 13 will be described in detail.
As shown in FIG. 3, the insulating layer 13 of this embodiment includes a thermosetting epoxy resin 24 as an insulating resin containing crystals (hereinafter simply referred to as “boron nitride”) 23 as hexagonal boron nitride fine particles. Formed from. That is, the boron nitride 23 is blended (added) to the thermosetting epoxy resin 24 as an inorganic filler (filler).

  Further, as shown in FIG. 4, the boron nitride 23 has a length in the a-axis direction indicated by the arrow Ca and the b-axis direction indicated by the arrow Cb, preferably 1 to 300 μm, whereas the length in the c-axis direction indicated by the arrow Cc. The length is preferably 0.5 to 150 μm, and the aspect ratio is preferably 2 to 10. Here, the aspect ratio is a value obtained by dividing the length in the a-axis and / or b-axis, or any of the directions by the length in the c-axis direction. When the aspect ratio of the boron nitride 23 is less than 2, it is difficult to improve the thermal conductivity with respect to the surface direction of the insulating layer 13, while when it exceeds 10, the strength of the insulating layer 13 tends to decrease. The length of the boron nitride 23 in the a-axis direction or the length in the b-axis direction is preferably 10% or more and 40% or less of the length (thickness) of the insulating layer 13 in the vertical direction. . As described above, each boron nitride 23 has a flat plate shape (a scale shape) extending in the a-axis and b-axis directions.

Here, the boron nitride 23 is a compound composed of nitrogen and boron, and has a network structure in which six-membered rings in which nitrogen atoms and boron atoms are alternately arranged are continuous in the a-axis and b-axis directions. Are stacked in the c-axis direction. That is, the plane direction of the crystal structure six-membered ring of boron nitride 23 corresponds to the a-axis and b-axis directions, and the stacking direction of the six-membered ring corresponds to the c-axis direction. For this reason, in boron nitride 23, the thermal conductivity in the a-axis and b-axis directions is 20 to 30 W / mK, which is compared with the thermal conductivity of other inorganic fillers such as alumina (Al ₂ O ₃ ) and glass fiber. High value. In boron nitride 23, the thermal conductivity in the c-axis direction is lower than the thermal conductivity in the a-axis and b-axis directions.

Further, the thermal expansion coefficient of the boron nitride 23 is 0.5 to 0.7 × 10 ⁻⁶ / ° C., which is smaller than, for example, alumina (thermal expansion coefficient = 5 to 7 × 10 ⁻⁶ / ° C.). Dimensional change due to temperature change hardly occurs. Further, the thermal expansion coefficient of the boron nitride 23 is substantially the same as the thermal expansion coefficient of the thermosetting epoxy resin 24 after thermosetting. Further, the hardness of the boron nitride 23 is lower than that of alumina.

  As shown in FIG. 3, in the insulating layer 13, the boron nitride 23 is oriented so that the a-axis and the b-axis are along the plane direction (front-rear direction and left-right direction) of the insulating layer 13. In other words, “along the plane direction of the insulating layer 13” means that the ab-axis plane defined from the a-axis and the b-axis of the boron nitride 23 and the surface (upper surface and lower surface) of the insulating layer 13 are completely parallel. In addition to the above state, the ab axis plane also includes a state where the ab axis plane intersects the surface (upper surface and lower surface) of the insulating layer 13 with a slight angle.

  For example, the boron nitride 23 a is arranged in the insulating layer 13 so that the a axis and the b axis are parallel to the surface direction of the insulating layer 13. On the other hand, the boron nitride 23 b is arranged in the insulating layer 13 so that the a-axis or the b-axis intersects the surface direction of the insulating layer 13, but its inclination is slight, and the boron nitride 23 b extends along the surface direction of the insulating layer 13. It can be said that.

  Thus, in this embodiment, the boron nitride 23 parallel to the surface direction of the insulating layer 13 and the intersecting boron nitride 23 are mixed. For this reason, in the insulating layer 13, the boron nitrides 23 may be in contact with each other in addition to the case where the boron nitrides 23 are completely separated by the thermosetting epoxy resin 24.

  The content of boron nitride 23 in the insulating layer 13 (thermosetting epoxy resin 24) is preferably 50% by volume to 80% by volume, more preferably 60% by volume to 70% by volume. .

  When the insulating layer 13 contains alumina, as indicated by the alternate long and short dash line in the simulation results of FIG. 5, the thermal conductivity increases only slowly even when the content is increased. On the other hand, when the insulating layer 13 contains boron nitride 23, as shown by a solid line in FIG. 5, when the content is 50% by volume or more, compared with the case where the same amount of alumina is blended. The thermal conductivity is significantly improved. The difference becomes more prominent when the content of boron nitride 23 in the insulating layer 13 is 60% by volume or more.

  Moreover, when the content of boron nitride 23 in the insulating layer 13 exceeds 80% by volume, the strength of the insulating layer 13 is reduced due to the shortage of the thermosetting epoxy resin 24, while the content thereof is When the content is preferably 80% by volume or less, more preferably 70% by volume or less, the strength of the insulating layer 13 can be sufficiently maintained.

Next, a method for manufacturing the circuit board 12 and the DC / DC converter 10 will be described.
First, 50% by volume or more and 80% by volume or less of boron nitride 23 is added to the epoxy resin before thermosetting, and the boron nitride 23 is kneaded so that it is uniformly dispersed. A mixture of boron nitride 23 is obtained. In this state, the boron nitride 23 is randomly dispersed in the thermosetting epoxy resin 24 and is not oriented in a specific direction.

  Next, a mask screen is placed on a metal tray (printing stand), and the above-described mixture is stretched and applied uniformly over the mask screen using a scraper (squeegee) (application step). Thus, by extending the mixture using a scraper in the coating process, boron nitride 23 has an a-axis and a b-axis in the insulating layer 13 formed on the metal tray, as shown in FIG. It is oriented along the surface direction of the insulating layer 13.

  The screen mask has a configuration in which a transmission portion that transmits the mixture is provided so as to have the same shape as the insulating layer 13 with respect to the screen that does not transmit the mixture. Further, the thickness of the screen mask matches the thickness of the insulating layer 13, and the mixture can be applied at a thickness necessary for forming the insulating layer 13.

  Next, the metal tray on which the insulating layer 13 is formed is heated at 80 ° C. for 10 minutes in an air atmosphere and temporarily fired (first firing step). Next, after sticking the thick copper foil pattern 14 to both surfaces of the pre-fired insulating layer 13 via an adhesive sheet made of a thermosetting epoxy resin, the substrate is heated at 200 ° C. for 10 minutes in an air atmosphere. The thick copper foil pattern 14 is completely thermoset while adhering to the insulating layer 13 to complete the circuit board 12 (second firing step).

  Subsequently, the insertion hole 13a is formed in the insulating layer 13 (circuit board 12) using a lathe or the like. Further, by assembling the core 16 and the coil 21, the transformer 15 is formed on the circuit board 12, and the power semiconductor element Sd is surface-mounted on the circuit board 12 using solder or the like. Then, the heat conductive sheet 20 is disposed at a portion corresponding to the lower core member 18 on the bottom surface of the housing 11, and the circuit board 12 is fixed to the housing 11 with screws Sc, whereby the DC / DC converter 10 is completed.

Next, the operation of the circuit board 12 and the DC / DC converter 10 will be described.
As shown in FIG. 2, the heat generated by driving the power semiconductor element Sd is mainly in the plane direction of the circuit board 12 through the thick copper foil pattern 14 having a high thermal conductivity, as indicated by an arrow Yx. At the same time, it is transmitted to the housing 11 through the insulating layer 13 from the vicinity of the fixed portion by the screw Sc.

  In addition, heat is generated in each of the core members 17 and 18 as the coil 21 is energized. However, as described above, each of the core members 17 and 18 is disposed so as to be separated from the thick copper foil pattern 14 and the coil 21, so that the generated heat is hardly transmitted to the thick copper foil pattern 14. . For this reason, the heat generated in the lower core member 18 is transmitted to the housing 11 mainly via the heat conductive sheet 20 as indicated by the arrow Yy.

  On the other hand, in the upper core member 17, the tip of the first leg portion 17 b is separated from the lower core member 18, so that the generated heat is difficult to be transmitted to the lower core member 18. On the other hand, in the insulating layer 13 of this embodiment, the thermal conductivity in the surface direction of the insulating layer 13 is increased. For this reason, the heat generated in the upper core member 17 is transmitted from the second leg portion 17c to the insulating layer 13 as well as in the surface direction of the insulating layer 13 as indicated by an arrow Yz, and finally by the screw Sc. It is transmitted to the housing 11 via the fixed part. Similarly, part of the heat generated in the lower core member 18 is transmitted to the housing 11 via the insulating layer 13.

  The heat transferred to the housing 11 is dissipated by heat exchange with a heat exchange medium such as air or water that flows outside the housing 11. As described above, in this embodiment, the transformer 15 (core 16 and coil 21) and the power semiconductor element Sd are heat-generating components that generate heat when driven, and the housing 11 performs heat exchange for heat exchange with the heat exchange medium. It becomes a member. The power semiconductor element Sd and the transformer 15 (core 16 and coil 21) are thermally connected to the housing 11 via the circuit board 12 (insulating layer 13).

  Further, the insulating layer 13 of the circuit board 12 is configured such that a boron nitride 23 is contained in a thermosetting epoxy resin 24. For this reason, in the circuit board 12 (insulating layer 13) of this embodiment, for example, even if the temperature rise due to the driving of the power semiconductor element Sd and the temperature fall due to the stop are repeated, it is caused by such a temperature change. Thus, the boron nitride 23 does not peel from the thermosetting epoxy resin 24, and durability against temperature changes is improved.

Therefore, according to the above embodiment, the following effects can be obtained.
(1) Since the insulating layer 13 contains boron nitride 23, the boron nitride 23 is insulated due to a temperature change as compared with the conventional configuration in which a boron nitride layer is formed on the surface of the insulating layer 13. Peeling from the layer 13 is suppressed. Furthermore, the boron nitride 23 contained in the insulating layer 13 is oriented so that the a-axis and b-axis directions corresponding to the plane direction of the six-membered ring are along the plane direction of the insulating layer 13. The thermal conductivity with respect to the surface direction can be improved. Therefore, durability against temperature change can be improved while improving thermal conductivity.

  (2) The content of boron nitride 23 in the insulating layer 13 (thermosetting epoxy resin 24) was set to 50% by volume or more and 80% by volume or less. Therefore, the thermal conductivity can be preferably improved as compared with the case where the content of boron nitride 23 is less than 50% by volume, while the insulating layer 13 (circuit board 12) is compared with the case where the content exceeds 80% by volume. Insufficient strength can be suppressed.

  (3) The aspect ratio of boron nitride 23 contained in the insulating layer 13 is 2 or more and 10 or less. For this reason, the thermal conductivity with respect to the surface direction of the insulating layer 13 can be improved as compared with the case where the aspect ratio of the boron nitride 23 is less than 2, while the strength of the insulating layer 13 is decreased as compared with the case where the aspect ratio exceeds 10. Can be suppressed.

  (4) The transformer 15 (core 16 and coil 21) or the power semiconductor element Sd and the housing 11 are thermally connected by the circuit board 12 (insulating layer 13), and the transformer 15 and the power semiconductor element are connected via the circuit board 12. Heat exchange (radiation) is performed between Sd and the heat exchange medium outside the housing 11. For this reason, it can suppress that the number of parts which comprise the DC / DC converter 10 increases compared with the structure which provides the exclusive component for transmitting the heat which generate | occur | produced with the transformer 15 and the power semiconductor element Sd to the housing 11 separately. .

  (5) In particular, the heat generated in the thick copper foil pattern 14, the lower core member 18, and the upper core member 17 in a non-contact state with the coil 21 is transferred to the housing 11 via the circuit board 12 (insulating layer 13). Can be transmitted to and dissipated.

  (6) In the coating step, the mixture is stretched with a scraper to orient the boron nitride 23 in the insulating layer 13 so that the a-axis and the b-axis are along the plane direction. Therefore, the insulating layer 13 (circuit board 12) can be easily manufactured.

(7) The hardness of the boron nitride 23 is lower than that of alumina, and the insertion hole 13a can be easily formed in the insulating layer 13.
In addition, you may change this embodiment as follows.

  The boron nitride 23 may be a material in which the length in the a-axis direction, the b-axis direction, and the c-axis direction and the aspect ratio thereof are appropriately changed. The boron nitride 23 may be appropriately changed in content in the insulating layer 13, but is preferably configured as in the embodiment from the viewpoint of ensuring thermal conductivity and strength.

  The upper surface and the lower surface of the insulating layer 13 may not be parallel. In this case, the boron nitride 23 has only to be oriented in the insulating layer 13 so that its a-axis and b-axis extend along either the upper surface or the lower surface of the insulating layer 13.

O A phenol resin or a fluorine resin may be used as the insulating resin for forming the insulating layer 13.
The housing 11 may be provided with a flow path for flowing the heat exchange medium, and may be provided with cooling fins in addition to or instead of the housing 11. In this case, the insulating layer 13 may be fixed and thermally connected to the cooling fin.

The heating temperature, heating time, and heating atmosphere in the first and second baking steps may be changed as appropriate as long as the thermosetting epoxy resin 24 is cured.
○ The heat conductive sheet 20 may be omitted. Even with this configuration, the heat generated in the lower core member 18 can be transmitted to the housing 11 by the insulating layer 13.

The circuit board 12 may be fixed and thermally connected to the housing 11 using a joining method that does not use the screws Sc. For example, it may be fixed by rivets or fittings.
The insulating layer 13 (thermosetting epoxy resin 24) may further contain another inorganic filler such as alumina or glass fiber.

  In the circuit board 12, electronic components different from these may be disposed (mounted) instead of or in addition to the transformer 15 and the power semiconductor element Sd. That is, the circuit board 12 (insulating layer 13) may be used for other electronic devices such as an inverter.

Next, the technical idea that can be grasped from the above embodiment and other examples will be described below.
(A) The hexagonal boron nitride fine particles have a length in the a-axis direction or a length in the b-axis direction that is not less than 10% and not more than 40% of the thickness of the insulating layer. Item 4. The circuit board according to any one of Items 1 to 3.

  DESCRIPTION OF SYMBOLS 10 ... DC / DC converter (electronic device), 11 ... Housing (heat exchange member), 12 ... Circuit board, 13 ... Insulating layer, 15 ... Transformer (heat-generating component), 16 ... Core, 17 ... Upper core member, 18 ... Lower core member, 21 ... coil, 23 ... hexagonal boron nitride crystal (hexagonal boron nitride fine particles), 24 ... thermosetting epoxy resin (insulating resin), Sd ... power semiconductor element (heat generating component).

Claims (4)

In a circuit board provided with an insulating layer made of an insulating resin containing fine particles of hexagonal boron nitride,
The fine particles of the hexagonal boron nitride are oriented so that the a-axis and b-axis directions corresponding to the plane direction of the six-membered ring of the hexagonal boron nitride are along the plane direction of the insulating layer. Circuit board.   2. The circuit board according to claim 1, wherein a content of the hexagonal boron nitride in the insulating layer is 50 volume% or more and 80 volume% or less.   An aspect ratio, which is a value obtained by dividing the lengths of the hexagonal boron nitride fine particles in the a-axis and b-axis directions by the length in the c-axis direction corresponding to the stacking direction of the six-membered ring, is 2 or more and 10 or less. The circuit board according to claim 1, wherein the circuit board is provided. The circuit board according to any one of claims 1 to 3,
A heat-generating component mounted on the circuit board and generating heat by driving;
In an electronic device comprising a heat exchange member for exchanging heat with a heat exchange medium,
The electronic device, wherein the heat generating component and the heat exchange member are thermally connected through the circuit board.

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