Patent Publication Number: US-2007102190-A1

Title: Circuit device and method of manufacturing the same

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
      Priority is claimed to Japanese Patent Application Number JP2005-066828 filed on Mar. 10, 2005, the disclosure of which is incorporated herein by reference in its entirety.  
     BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates to a circuit device and a method of manufacturing the same, and more particularly, relates to a circuit device having both of a high heat releasing property and a high withstand voltage property, and a method of manufacturing the same.  
      2. Description of the Related Art  
      Referring to  FIG. 7 , a configuration of a conventional hybrid integrated circuit device  100  will be explained. This technology is described for instance in Japanese Patent Application No. 5-102645. Conductive patterns  103  are formed on a surface of a rectangular board  101  with an insulating layer  102  interposed therebetween. A circuit element  105  is fixed to a desired position of the conductive patterns  103  to form a predetermined electric circuit. As a circuit device, a semiconductor element and a chip element are connected with the conductive patterns  103 . Leads  104  are connected with the conductive patterns  103  formed on the peripheral portion of the board  101 , and function as an external terminal. A sealing resin  108  has a function to seal the electric circuit formed on the surface of the board  101 .  
      As for a structure of the sealing resin  108 , There are two kinds of structures of the sealing resin  108 . A first structure is a method that the sealing resin  108  is formed so as to expose a rear surface of the board  101 . This structure allows an efficient heat release through the exposed rear surface of the board  101 . A second structure is a method that the sealing resin  108  is formed so as to cover the entire board  101  inclusive of the rear surface thereof. According to this structure, a sufficient withstand voltage property and a moisture resistance of the board  101  can be ensured. In  FIG. 7 , the sealing resin covers the entire board  101  inclusive of the rear surface thereof. A thickness of the sealing resin  108  covering the rear surface of the board  101  is, for example, about 0.5 mm. Especially, in a case where the board  101  is connected with grounding potential, the above second structure is adopted, and thus the board  101  is insulated from an outside.  
      However, in a case where the sealing resin  108  covers the rear surface of the board  101 , there has been a problem that the heat releasing property of the entire device drops due to a low heat conductivity of the sealing resin  108  that covers the rear surface of the board  101 .  
      When the thickness (T 5 ) of the sealing resin  108  covering the rear surface of the board  101  is decreased, it can be expected that the heat releasing property be improved. However, if the thickness T 5  of the sealing resin  108  covering the rear surface of the board  101  is set to 0.5 mm or smaller, there arises a problem that the resin cannot completely cover the rear surface of the board  101  in a molding step where the sealing resin  108  is formed through a injection-molding.  
      Furthermore, when the rear surface of the board  101  is exposed to the outside in order to improve the heat releasing property, there arises a problem that an insulating property between the board  101  and a radiation fin, which comes into contact with the board  101 , can not be ensured. There is another problem that the bonding strength between the board  101  and the sealing resin is lowered.  
     SUMMARY OF THE INVENTION  
      The present invention has been accomplished in a view of the above problems. The present invention provides a circuit device having both of a high heat releasing property and a high withstand voltage, and a method of manufacturing the same.  
      A circuit device according to the present invention includes: a circuit board having a first insulating layer formed on a front surface and a second insulating layer formed on a rear surface; an electric circuit including conductive patterns and a circuit element which are formed on a surface of the first insulating layer; a metal board stuck to a surface of the second insulating layer; and a sealing resin for sealing the electric circuit. The sealing resin covers at least a front surface, side surfaces, and peripheral portions of a rear surface of the circuit board.  
      Furthermore, in the circuit device according to the present invention, the metal board is stuck by curing a B-stage resin.  
      Furthermore, in the circuit device according to the present invention, burrs are formed at peripheral edges of the metal board, and a surface opposite to a surface where the burrs protrude is stuck to the surface of the second insulating layer.  
      Furthermore, in the circuit device according to the present invention, a rear surface of the metal board is exposed from the sealing resin.  
      Furthermore, in the circuit device according to the present invention, the rear surface of the metal board and the sealing resin form a flat surface.  
      A manufacturing method of a circuit device according to the present invention includes: sticking a metal board to a rear surface of a circuit board with an insulating layer interposed therebetween and sticking a conductive foil to a front surface of the circuit board with an insulating layer interposed therebetween; patterning the conductive foil to form conductive patterns; configuring an electric circuit including the conductive pattern and a circuit element which are formed on the front surface of the circuit board; and forming a sealing resin using a molding die so as to cover at least the front surface of the circuit board. The metal board is stuck to the rear surface of the circuit board with a B-stage resin interposed therebetween.  
      In the manufacturing method of a circuit device according to the present invention, the B-stage resin is applied to a front surface of the metal board, and the metal board is stuck to the circuit board through a thermocompression bonding.  
      Furthermore, in the manufacturing method of a circuit device according to the present invention, the B-stage resin is applied to the front surface of the metal board, the metal board is cut into a desired shape such that burrs are formed on a rear surface of the metal board, and the front surface of the metal board is stuck to the rear surface of the circuit board.  
      According to the present invention, a metal board is bonded to a rear surface of a circuit device. Accordingly, it is possible to enhance the property of releasing heat that is generated from a circuit element incorporated in the circuit device. In addition, a sealing resin covers the front surface, the side surfaces, and peripheral portions of the rear surface of the circuit board in a manner that a metal board is exposed. Consequently, an anchor effect is generated by the sealing resin, and it is possible to improve the bonding strength between the sealing resin and the circuit board.  
      Moreover, according to the present invention, the B stage resin is used as a binder for fixing the metal board to the circuit board, whereby the binder can be applied without a leakage and an unevenness, and contributes to an improvement in quality of the circuit device.  
      Furthermore, the surface opposite to the surface having burrs is adhered to a circuit board surface. Hence, it is possible to prevent a withstand voltage from being deteriorated due to that the burrs damage an insulating layer, and an electricity flows between the metal board and the circuit board.  
      Furthermore, according to the present invention, the metal board can withstand an externally applied voltage in a state that the rear surface of the metal board is exposed to the outside from a sealing resin. Consequently, it is possible to provide a circuit device having both of a high heat releasing property and a high withstand voltage property.  
      Furthermore, according to the manufacturing method of the circuit device of the present invention, a sheet-like metal board applied with a B stage resin is stuck to a circuit board. Hence, the total thickness of the metal board and the resin can be made uniform, whereby a dimensional stability of the circuit device can be improved.  
      Furthermore, according to the manufacturing method of the circuit device of the present invention, the peripheral portions of the rear surface of a circuit board is covered with the sealing resin. Hence, the anchor effect is generated by the sealing resin covering the rear surface, and it is possible to improve the bonding strength between the sealing resin and the circuit board. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1A  is a perspective view showing a circuit device according to a preferred embodiment of the invention, and  FIG. 1B  is a cross-sectional view thereof;  
       FIGS. 2A and 2B  are cross-sectional views showing a manufacturing method of a circuit device according to the preferred embodiment of the invention;  
       FIGS. 3A and 3B  are cross-sectional views showing a manufacturing method of a circuit device according to the preferred embodiment of the invention;  
       FIG. 4  is a cross-sectional view showing a manufacturing method of a circuit device according to the preferred embodiment of the invention;  
       FIGS. 5A  to  5 C are cross-sectional views showing a manufacturing method of a circuit device according to the preferred embodiment of the invention;  
       FIGS. 6A and 6B  are cross-sectional views showing a manufacturing method of a circuit device according to the preferred embodiment of the invention; and  
       FIG. 7  is a cross-sectional view showing a conventional hybrid integrated circuit device. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
      Referring to  FIGS. 1A and 1B , a circuit device according to a preferred embodiment of the invention will be explained. Hereinafter, a hybrid integrated circuit device  10  having a plurality of semiconductor chips mounted onto the same board will be explained as an example.  
      First, a first insulating layer  12 A is formed on a front surface of a rectangular circuit board  11 . Then, conductive patterns  13  of a predetermined shape are formed on the surface of the first insulating layer  12 A. Furthermore, a semiconductor element  15 A and a chip element  15 B are electrically connected with predetermined positions of the conductive patterns  13  through a solder, a conductive paste, or a thin metal wire. The conductive patterns  13 , the semiconductor element  15 A, and the chip element  15 B which are formed on the front surface of the circuit board  11  are covered with a sealing resin  14 . In addition, the sealing resin  14  covers only peripheral portions of the rear surface of the circuit board  11 , and thus a metal board  16  stuck to the circuit board  11  is exposed to an outside. To be more specific, the metal board  16 , which is exposed from the sealing resin  14 , is stuck to a second insulating layer  12 B that covers the rear surface of the circuit board  11 , with a resin  19  interposed therebetween.  
      The circuit board  11  is made of a metal such as aluminum or copper. If an aluminum-made board is used as the circuit board  11 , for example, the surface of the circuit board  11  is subjected to an alumite treatment or a chemical oxidation. This improves an adhesion property between the first insulating layer  12 A and the circuit board  11 . Concretely, the circuit board  11  has a dimension: for example, about 61 mm (length)×42.5 mm (width)×1.5 mm (thickness). If a Cu-made circuit board is adopted, its surface may undergo surface roughening for a purpose of improving the adhesion. In particular, it is effective to roughen the rear surface in consideration of the adhesion to the metal board.  
      The first insulating layer  12 A is formed to cover the entire front surface of the circuit board  11 . The insulating layer  12  is formed of an epoxy resin highly filled with a filler excellent in a heat conductivity, such as Al 2 O 3  or SiO 2 . This promotes releasing of heat generated in the incorporated circuit element to the outside through the circuit board  11 . A specific thickness of the first insulating layer  12 A is, for example, about 50 μm. By the insulating layer  12  having this thickness, a withstand voltage of 4 KV (breakdown voltage) can be ensured.  
      The second insulating layer  12 B is formed to cover the rear surface of the circuit board  11 . The second insulating layer  12 B may have the same composition as that of the first insulating layer  12 A. The rear surface of the circuit board  11  is covered with the second insulating layer  12 B and thus a sufficient withstand voltage property of the rear surface of the circuit board  11  can be ensured. Accordingly, even if heat releasing means such as a radiation fin comes into contact with the rear surface of the circuit board  11 , the second insulating layer  12 B insulates the radiation fin from the circuit board  11 .  
      The conductive patterns are made of a metal such as copper, and formed on the surface of the first insulating layer  12 A to realize a predetermined electric circuit. Further, a pad composed of the conductive patterns  13  is formed on one side from which a lead  25  is derived.  
      Circuit elements such as the semiconductor element  15 A and the chip element  15 B are fixed to predetermined positions of the conductive patterns  13 . As the semiconductor element  15 A, a transistor, an LSI chip, or a diode is used. In this example, the semiconductor element  15 A is connected with the conductive patterns  13  through a thin metal wire  17 . As the chip element  15 B, a chip resistor or a chip capacitor is used. To give another example of the chip element  15 B, an element having electrode portions on both sides thereof such as an inductance, a thermistor, an antenna, or an oscillator is used. In addition, a resin-seal type package as a circuit element may be fixed to the conductive patterns  13 .  
      The lead  25  is fixed to the pad provided at the peripheral portion of the circuit board  11 , and has a function for performing input-output. In the illustrated example, a plurality of the leads  25  are fixed to one side of the board. Further, the leads  25  may be derived from four sides or two sides opposite to each other in the circuit board  11 .  
      Although not shown, the conductive patterns  13  may be formed in multiple layers. Needless to say, an insulating layer is interposed between a first wiring layer and a second wiring layer formed thereon, between the second wiring layer and a third wiring layer formed thereon.  
      The sealing resin  14  is formed by a transfer-molding using a thermosetting resin. In  FIG. 1B , the sealing resin  14  seals the conductive patterns  13 , the semiconductor element  15 A, the chip element  15 B, and the thin metal wire  17 . Furthermore, the sealing resin  14  covers the front and side surfaces of the circuit board  11 . In addition, on the rear surface of the circuit board  11 , the sealing resin  14  covers peripheral portions thereof and the side surfaces of the metal board. The rear surface of the metal board  16  is exposed from the sealing resin  14 . In this way, the peripheral portion of the rear surface of the circuit board  11  is covered with the sealing resin  14 , thereby an anchor effect is generated to improve a bonding strength between the circuit board  11  and the sealing resin  14 . The rear surface of the metal board  16  is exposed, and thus heat generated at the time of driving the semiconductor element  15 A can be sufficiently released to the outside through the metal board  16 . The second insulating layer  12 B and the resin  19  are inferior in a heat conductivity. However, this drawback is overcome by reducing their thicknesses and optionally filling a filler in them.  
      As a material for the metal board  16 , a metal having a sufficient heat conductivity such as copper or aluminum is used. In this embodiment, aluminum is adopted, and the aluminum board is stuck to the rear surface of the circuit board  11  with the resin  19  interposed therebetween. Furthermore, a sheet-like aluminum board (a thickness of about 0.5 mm) coated with the resin  19  of a B-stage (Partially cured) is stuck to the circuit board  11 , thereby the unevenness or leakage of the resin is suppressed. Furthermore, the sheet-like board coated with the resin  19  is stuck to the circuit board, thereby the total thickness of the metal board  16  and the resin  19  is uniform, therefore, a dimensional stability of the circuit device is excellent. The sheet-like resin-coated aluminum board is cut into a desired shape through press-cutting. At this time, burrs are formed on one side of the aluminum board; as a result of the press-cutting, the burrs protrude to a surface opposite to a surface coated with the resin  19 . This prevents such a situation that the burrs pass through a second insulating layer  18  to come into contact with the circuit board  11 , and thereby the withstand voltage is deteriorated.  
      Furthermore, the sealing resin  14  that covers the peripheral portion of the circuit board  11  and the rear surface of the metal board  16  form the flat rear surface of the circuit device. Thus, the rear surface of the hybrid integrated circuit device  10  can be easily brought into contact with the heat releasing means such as a radiation fin.  
      In this embodiment, no insulating layer is formed on the rear surface of the metal board.  
      As described above, in this embodiment, no insulating layer is formed on the rear surface of the metal board  16 , but an insulating layer such as an oxide film may be provided. For example, the oxide film is formed of an alumite film prepared through an anodic oxidation. The circuit board  11  has the thickness of about 1.5 mm, while the metal board  16  has the thickness of about 0.5 mm. The thickness of the oxide film is set to, for example, about 10 μm. The oxide film formed on the rear surface of the metal board  16  can protect the exposed rear surface of the metal board  16  from damages.  
      In this embodiment, the sealing resin  14  covers the peripheral portion of the rear surface of the circuit board  11 , therefore, a sufficient withstand voltage property of an end portion P of the circuit board  11  can be ensured. More specifically, the first insulating layer  12 A and the second insulating layer  12 B are formed on the entire front surface and the entire rear surface of the circuit board  11 , respectively. Therefore, sufficient withstand voltage properties in the front and rear surfaces of the circuit board  11  are ensured. In contrast, no resin layer covers the side surfaces of the circuit board  11 , and the metal surface is exposed. As a result, in order to securely insulate the circuit board  11  from the outside, it is necessary to prevent the side surfaces (especially, end portion P) of the circuit board  11  from being short-circuited to the outside (chassis or radiation fin fixed to the metal board) through a interface between the circuit board  11  and the sealing resin  14 . In this embodiment, the sealing resin  14  is formed on the peripheral portion of the rear surface of the circuit board  11  in a manner that the end portion P is separated from the outside. That is, the sealing resin  14  is formed so as to cover the end portion P. To be more specific, as shown in  FIG. 1B , a width of a region covered with the sealing resin  14  is denoted by L 1 . A length of the width L 1  is preferably from about 2 mm to 3 mm or longer although it varies depending on a required withstand voltage. Thus, a sufficient withstand voltage in the end portion P of the circuit board  11  can be ensured. For example, if the length of L 1  is 2 mm, the end portion P can withstand a voltage of 2 KV. If the length of L 1  is 3 mm, the end portion P can withstand a voltage of 3 KV. Incidentally, a thickness Ti of the sealing resin  14  that covers the rear surface of the circuit board  11  is equivalent to that of the metal board  16 , for example, about 0.5 mm. As mentioned above, a sufficient withstand voltage property of the entire circuit board  11  is ensured.  
      As described above, the circuit device using the circuit board  11  and the metal board excels in the heat releasing property, and is thus applied to an in-vehicle module, for example. In other words, a high-density modularization of a high-output power element, and a circuit for controlling the power element or a microcomputer requires a package having a high heat releasing property and a high-sealing property.  
      Referring to  FIGS. 2A  to  6 B, a manufacturing method of the hybrid integrated circuit device  10  having the structure described above will be explained.  
      Referring to  FIG. 2A , first, a first insulating layer  12 A is formed on the front surface of a circuit board  11 , and a second insulating layer  12 B is formed on the rear surface of the circuit board.  
      The circuit board  11  has a size enough to arrange several ten units  32  in matrix thereon. The term “unit” means a portion that constitutes one hybrid integrated circuit device. The circuit board  11  may be formed of aluminum, copper, or iron. As an example of this embodiment, an aluminum board is adopted as the circuit board  11 . Furthermore, an aluminum board having front and rear surfaces, which are treated with an alumite treatment, may be adopted. The thickness of the circuit board  11  is about 1.5 mm. In addition, the thicknesses of the first insulating layer  12 A and the second insulating layer  12 B are about 50 μm to 60 μm. Furthermore, an oxide film may cover the front and the rear surfaces of the circuit board  11 . As an example of the oxide film, an alumite film containing Al 2 O 3  is adopted and has the thickness of about 1 μm to 5 μm. By forming the thin oxide film as described, a thermal resistance can be reduced.  
      Referring  FIG. 2B , a metal board  16  is stuck to the surface of the second insulating layer  12 B with a resin  19  interposed therebetween. In this example, a sheet-like sticking board  31  prepared by applying the B-stage (partially cured) resin  19  to the surface of the metal board  16  having the thickness of about 0.5 mm is used. The resin  19  is, for example, an epoxy resin, and is cured through a heat-pressing. In this embodiment, the resin  19  undergoes the heat-pressing at 150° for about 1 hour, and thus is completely cured to stick the metal board  16  to the surface of the second insulating layer  12 B. The sheet-like sticking board  31  is cut into a desired shape and then stuck to a desired position of each unit  32 . The resin  19  serves as an adhesive and as an insulating layer. In order to further improve the insulating property, an insulating film may be attached to the resin  19 .  
      Although depending on the thickness of the metal board  16 , without being cut, the sticking board may be stuck to each unit, and then cured and etched into a small piece.  
      Referring to  FIGS. 3A  to  4 , the sticking board  31  will be explained in detail.  
      Referring now to  FIG. 3A , the sticking board  31  is prepared by applying the resin  19  onto the surface of the metal board  16 . Then, the bonding board  31  is press-cut by use of a mold  29  into a desired shape. However, by the press working, burrs are formed at the peripheral edge portions of the metal board  16 . Thus, the press working is executed in a manner that the burrs are formed on the surface opposite to the surface coated with the resin  19 . As a result, the sticking board  31  of  FIG. 3B  is formed. As shown in  FIG. 4 , this aims at preventing such a situation that burrs  30  penetrate the second insulating layer  12 B at a time of sticking the sticking board  31  to the circuit board  11 , and thus a withstand voltage property in the damaged portion is reduced to generate short-circuiting.  
      Furthermore, the resin  19  is a B-stage resin and thus excels in a processability. The resin  19  is neither damaged nor peeled off through pressing. Accordingly, reliability in adhesion between the metal board  16  and the circuit board  11  can be improved. Furthermore, even if cracks are generated at the end face of the resin  19 , the resin  19  is softened in a thermocompression bonding step, therefore, the cracks can be removed. Thus, the insulating layer made of the resin  19  can be reliably formed on the entire surface of the metal board  16 .  
      Referring to  FIG. 5A , a conductive foil  26  is stuck to the front surface of the circuit board  11 . In this example, the conductive foil  26  is stuck to the front surface of the circuit board  11  with the first insulating layer  12 A interposed therebetween. As an example, the thickness of the conductive foil  26  is about 70 μm. Furthermore, a distance between the stuck metal boards  16  is set about twice or more as large as the width L 1  of  FIG. 1B . To be more specific, the distance is about 4 mm to 6 mm.  
      Referring to  FIG. 5B , the conductive foil  26  is patterned through etching and then conductive patterns  13  are formed. The conductive patterns  13  are prepared by etching the conductive foil  26  through a resist formed thereon. In  FIG. 5B , the conductive patterns are formed in a single layer. However, the conductive patterns may be formed in two or more layers that are laminated with an insulating layer interposed between two layers.  
      Referring to  FIG. 5C , the circuit boards  11  in each unit  32  are separated. The circuit boards are separated through a press-cutting, a dicing, and a bending. If the circuit boards  11  are separated through the dicing or the bending, an isolation groove may be formed on the front and rear surfaces at the interface between the circuit boards  11  in the respective units  32 . This facilitates the separation of the circuit boards.  
      Referring to  FIG. 6A , a circuit element is electrically connected with the conductive patterns  13 . In this example, the semiconductor element  15 A and the chip element  15 B are fixed to the conductive patterns. The semiconductor element  15 A is electrically connected with the conductive patterns  13 A through the thin metal wire  17 . This step may be carried out before the separation of the units  32 .  
      Referring to  FIG. 6B , a sealing resin is formed to cover the circuit board  11 . First, the rear surface of the metal board  16  which is positioned at the undersurface of the circuit board  11  comes into contact with a lower mold  22 B. Then, an upper mold  22 A comes into contact with the lower mold  22 B to encapsulate the circuit board  11  into a cavity  23 . A size of the metal board  16  is smaller than that of the circuit board  11 , therefore, the peripheral portion of the circuit board  11  is separated from the lower mold  22 B so as to have a distance depending on the thickness of the metal board  16 . Hence, a sealing resin injected into the cavity  23  reaches a region A 1  below the circuit board.  
      Through the aforementioned steps, the hybrid integrated circuit device  10  as shown in  FIGS. 1A and 1B  is manufactured.  
      Another advantage will be explained below. As shown in  FIG. 7 , at a time of sealing a cavity with a resin  108 , the resin  108  should be filled in between a lower mold and a board  101 . However, the larger the size of circuit board  101  is, the more difficult the impregnation of the resin is. This is because a larger area leads to an increase in a heat releasing property of the board and thus causes the resin having fluidity to start solidifying. However, in the embodiment of the present invention, as shown in  FIG. 6B , the metal board  16  limits a sealing area of the resin, and the resin needs only to be filled in a portion surrounding the rear surface of the circuit board. Hence, formation of a portion not filled with the resin is suppressed.  
      Referring back to  FIGS. 1A and 1B , effects of the embodiment of the invention will be explained. In general, when the circuit board  11  is press-cut, the insulating layers  12 A and  12 B exist at a cutting portion and its surroundings, therefore cracks tend to be generated in such portions. However, the metal board  16  uses the B-stage resin  19 , and thus, even if the circuit board is press-cut, the board is hardly cracked. This is because the resin is solidified at room temperatures but is melted when heated, therefore, even if cracks are generated, the cracks are buried with the resin at a time when the resin is heated to be softened and solidified. Accordingly, one of the paths that is short-circuited can be further improved in a withstand voltage property since the generation of the cracks are suppressed. In addition, the B-stage resin  19  is applied throughout the surface with a uniform thickness, therefore, the distance between the circuit board and the metal board can be made uniform across the whole surface without voids. Accordingly, a variation of the heat conductivity can be suppressed.