Patent Publication Number: US-2006007661-A1

Title: Circuit board

Description:
BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates to a circuit board having a substrate and a circuit element mounted on this substrate.  
      2. Description of the Related Art  
      Along the increased sophistication of a circuit element, a heat dissipation value of the circuit element also increases. As a technique of cooling the circuit element, a metal heat dissipation pad (i.e., a slug) is provided on a portion (i.e., a rear surface) of the circuit element facing the substrate, and this heat dissipation slug is soldered to the metal pad disposed on the substrate, thereby releasing the heat from the circuit element.  
       FIG. 1  is a cross-sectional diagram of a circuit board to which a heat dissipation slug of a semiconductor element and a pad of a substrate are soldered.  
      A circuit board  1  shown in  FIG. 1  has a semiconductor element  10  and a substrate  20 . The semiconductor element  10  is covered with a package  11  which has leads  12  connected to the package  11  by wire bonding. A metal heat dissipation slug  13  is provided on the semiconductor element  10  at a portion facing the substrate. The substrate  20  shown in  FIG. 1  has metal pads disposed on a mounting surface  20   a  on which the semiconductor element  10  is mounted. A conductive layer  201  is provided between the mounting surface  20   a  and a rear surface  20   b  opposite to the mounting surface  20   a . The heat dissipation slug  13  of the semiconductor element  10  shown in  FIG. 1  is connected to a pad (hereinafter referred to as a heat dissipation pad  21 ) of the substrate  20  with solder  30  (refer to Japanese Patent Application Laid-open (JP-A) No. 10-79562, for example). The lead  12  of the semiconductor element  10  is also connected to a pad (hereinafter referred to as an electric connection pad  22 ) of the substrate  20  with solder. Through-holes  202  that pierce through the substrate  20  are connected to the heat dissipation pad  21 . The inner surface of each through-hole  202  is coated with a conductive material, and the through-hole  202  is brought into contact with the conductive layer  201 . Therefore, heat of the semiconductor element  10  is dissipated from the heat dissipation pad  21  and the through-holes  202  to the mounting surface  20   a  and the inside of the substrate  20 . At the same time, the heat is dissipated to the conductive layer  201  via the through-holes  202 .  
      However, as shown in  FIG. 1 , at the time of connecting the heat dissipation slug  13  to the heat dissipation pad  21  with solder, the molten solder  30  flows into the through-holes  202  shown at the left of the solder  30 . The entered solder  30  flows out and swells on the rear surface  20   b . When the solder  30  that flows and swells on the rear surface  20   b  is solidified, this constrains the mounting of elements onto the rear surface, causing problems. When the circuit board  1  is disposed on a limited space, the solder reaching the rear surface  20   b  interferes with other parts, causing problems as well.  
      When a circuit board is mounted with a CPU (Central Processing Unit) having an extremely high heat dissipation value, or when a circuit board is mounted with many circuit elements requiring heat dissipation, the conventional technique shown in  FIG. 1  cannot manage a rise (or a saturation) in a temperature of the substrate  20 , which results in reduced performance of cooling the circuit elements.  
      To overcome these problems, a technique of obtaining high cooling performance based on a provision of a heat sink on the rear surface  20   b  of the substrate  20  is proposed (refer to JP-A 11-33074, for example). According to the technique described in JP-A 11-33074, pins that pierce through the substrate are connected to the CPU. The pins transmit the heat of the CPU to the heat sink disposed on the surface opposite to the surface on the CPU is mounted.  
      According to the technique described in JP-A 11-33074, however, the pins that pierce through the substrate are connected to the CPU with an adhesive. Therefore, a connection portion between the CPU and the pins, that is, a portion of the adhesive, has poor heat conductivity and poor heat dissipation. Consequently, despite the provision of the heat sink, the cooling effect is little improved. The coating of an adhesive also becomes a trouble in the manufacturing process.  
     SUMMARY OF THE INVENTION  
      The present invention has been made in view of the above circumstances, and provides a circuit board having a satisfactory characteristic of cooling a circuit element, without a constraint of mounting the circuit element on the rear surface of the substrate, without an interference with other parts, and without a trouble in the manufacturing process.  
      According to the present invention, a circuit board includes: 
          a substrate having a pair of pads at mutually opposite positions of a front surface and a rear surface of the substrate;     a circuit element having a heat dissipation part which is soldered to one of the pair of pads; and     a heat transfer section which pierces through the substrate in a thickness direction, and both ends of which are soldered to the pair of pads respectively, wherein     at least a part of the heat transfer section has a solid structure which prevents air from passing through between the front surface and the rear surface.        

      According to the circuit board of the present invention, general pads are disposed on the substrate to dissipate heat from the circuit element. Therefore, this has no trouble in the manufacturing process. Because the connection part which transfers heat is soldered, this part has excellent heat conductivity and excellent heat dissipation, thereby satisfactorily cooling the circuit element. Because at least a part of the heat transfer section has the solid structure, the solid structure stops molten solder, and prevents the solder from flowing out and swelling on the surface opposite to the surface on which the circuit element is mounted. As a result, there are no such problems as a constraint of a mounting on the substrate rear surface or an interference with other parts.  
      The whole heat transfer section can have the solid structure.  
      According to the circuit board of the present invention, the substrate can have a conductive layer that extends in a direction orthogonal with a thickness direction, inside the substrate.  
      With the conductive layer kept in contact with the heat transfer section, heat can be released from the circuit element to the conductive layer, thereby increasing heat dissipation effect. When the temperature of the substrate is too high, the conductive layer can have an extended structure by keeping away from the heat transfer section. With this arrangement, a rise in the temperature of the substrate can be suppressed.  
      According to the circuit board of the present invention, preferably plural heat transfer sections are disposed on the pair of pads.  
      The plural heat transfer sections can uniformly transfer heat from the circuit element.  
      According to the circuit board of the present invention, preferably the heat transfer section has a head embedded in one of the pair of pads, at one end in the thickness direction, and has an end part embedded in the other pad at the other end, the head being larger than the other end.  
      With the above arrangement, the area of the heat transfer section connected to the pads increases, and heat transfer of the pads and the heat transfer section becomes satisfactory.  
      According to the circuit board of the present invention, preferably a soldered part includes one simple substance selected from a Bismuth simple substance, an Indium simple substance, and a Zinc simple substance.  
      According to the circuit board of the present invention, the pair of pads are connected to transfer heat, and heat added to one pad is dissipated by the other pad. Because of the necessity of soldering the pads at a high temperature, there is a risk of adding temperature to the circuit element in excess of a heat-resistant temperature of the circuit element. A portion of a soldered member (i.e., the pads, the heat transfer section, and the heat dissipation part) that is brought into contact with the solder is covered with one simple substance selected from the Bismuth simple substance, the Indium simple substance, and the Zinc simple substance. With this arrangement, a melting temperature of the solder can be lowered, and the adding of a temperature to the circuit element in excess of a heat-resistant temperature of the circuit element can be prevented. Therefore, the soldered part includes one simple substance selected from the Bismuth simple substance, the Indium simple substance, and the Zinc simple substance.  
      According to the circuit board of the present invention, the heat transfer section has a cylinder having an opening at a protrusion end that protrudes in the thickness direction from a first pad out of the pair of pads, the first pad being different from a second pad of which heat dissipation part is soldered.  
      The heat transfer section can have a heat dissipation member that is fixed to the internal peripheral surface of the cylinder, and has a larger capacity than that of the first pad to dissipate heat transferred from the heat transfer section, at the side where the first pad is provided.  
      According to the above structure, the use of the cylinder makes it possible to easily dispose the heat dissipation member on the substrate, thereby increasing heat dissipation using the heat dissipation member.  
      According to the present invention, a circuit substrate with a satisfactory characteristic of cooling a circuit element can be obtained, without troubles of a constraint of mounting on the rear side of the substrate, an interference with other parts, and troublesome work in the manufacturing process.  
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a cross-sectional diagram of a circuit board to which a heat dissipation slug of a semiconductor element and a pad of a substrate are soldered;  
       FIG. 2  is a cross-sectional diagram of a circuit board according to a first embodiment of the present invention;  
       FIG. 3  is a cross-sectional diagram of a circuit board according to a second embodiment of the present invention;  
       FIG. 4  is a cross-sectional diagram of a circuit board according to a third embodiment of the present invention; and  
       FIG. 5  is a perspective diagram of a heat transfer section that is provided on the circuit board shown in  FIG. 4 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      Embodiments of the present invention are explained below with reference to the drawings.  
       FIG. 2  is a cross-sectional diagram of a circuit board according to a first embodiment of the present invention.  
      In the following explanation, constituent elements having the same functions as those of the constituent elements shown in  FIG. 1  are attached with like reference numerals. (the same applies hereinafter)  
      The circuit board  1  shown in  FIG. 2  also has the semiconductor element  10  covered with the package  11 , and the substrate, like the circuit board shown in  FIG. 1 . The semiconductor element  10  has plural leads  12 , and the metal heat dissipation slug  13 . The heat dissipation slug  13  shown in  FIG. 2  is coated with a low melting-point material containing Bi (bismuth).  
      On the other hand, a pair of metal pads are provided at mutually opposite positions of a front surface and a rear surface of the substrate  20  shown in  FIG. 2 . These pads are also coated with a low melting-point material containing Bi of the front and rear surfaces, a surface on which the semiconductor element  10  is mounted is called the mounting surface  20   a , and the opposite surface is called the rear surface  20   b . Out of the pair of pads, the pad (i.e., the heat dissipation pad  21 ) disposed on the mounting surface  20   a  is connected to the heat dissipation slug  13  of the semiconductor element  10  with the solder  30 . Out of the pair of pads, the pad disposed on the rear surface  20   b  is called a heat dissipation rear-surface pad  23 . Plural metal pads are provided on the mounting surface  20   a , regardless of the opposed relationship with the rear surface  20   b . The plural leads  12  of the semiconductor element  10  are connected to these pads (i.e., metal connection pads  22 ) with the solder  30 . The substrate  20  shown in  FIG. 2  has the conductive layer  201  that extends in a direction orthogonal with a thickness direction, inside the substrate  20 .  
      The circuit board  1  shown in  FIG. 2  has plural heat transfer sections  40 . Plural through-holes, of which inner surface is coated with a conductive material, are provided on the substrate  20  shown in  FIG. 2  to pierce through the substrate  20  in a thickness direction and connect between the heat dissipation pad  21  and the heat dissipation rear-surface pad  23 . The heat transfer sections  40  are accommodated in the through-holes, and both ends of the heat transfer sections  40  are soldered to the heat dissipation pad  21  and the heat dissipation rear-surface pad  23  respectively. In other words, the heat transfer sections  40  pierce through the substrate  20  in the thickness direction, and are soldered to the heat dissipation pad  21  and the heat dissipation rear-surface pad  23  respectively. The heat transfer sections  40  are brought into contact with the conductive layer  201  inside the substrate. Therefore, heat of the semiconductor element  10  is transferred from the heat dissipation slug  13  to the heat dissipation pad  21 . The heat is further transferred to the conductive layer  201  and to the heat dissipation rear-surface pad  23 , via the heat transfer sections  40 . As a result, the heat of the semiconductor element  10  is dissipated by the heat dissipation pad  21  and the heat dissipation rear-surface pad  23 , and is also released to the conductive layer  201 . According to the circuit board  1  shown in  FIG. 2 , all the connection parts that transfer heat are soldered. Therefore, these connection parts achieve excellent heat conduction and heat dissipation, thereby cooling the semiconductor element  10  satisfactorily. Because general pads are disposed on the substrate  20  to dissipate heat from the semiconductor element  10 , no troublesome work is involved in the manufacturing process. Because plural heat transfer sections  40  are disposed on the pair of pads  21  and  23 , heat is transferred uniformly from the semiconductor element  10 .  
      Each heat transfer section  40  has a head  41  and an end part  42 , and seals air between the mounting surface  20   a  and the rear surface  20   b . In other words, each heat transfer section  40  shown in  FIG. 2  has a solid structure in total. A portion excluding the head  41  is a cylinder, and one end of the cylinder forms the end part  42 . The head  41  has a larger diameter than that of the cylinder, and is positioned opposite to the end. The head  41  of each heat transfer section  40  shown in  FIG. 2  is soldered to the heat dissipation rear-surface pad  23  in a state of being embedded in the heat dissipation rear-surface pad  23 . The end part  42  is soldered to the heat dissipation pad  21  in a state of being embedded in the heat dissipation pad  21 . As shown in  FIG. 2 , the solder  30  is solidified to cover the total surface of the heat dissipation rear-surface pad  23  in which the head  41  is embedded. The solder  30  is also solidified to cover the total surface of the heat dissipation pad  21  between the heat dissipation pad  21  in which the end part  42  is embedded and the heat transfer slug  13  of the semiconductor element  10 . Therefore, both the head  41  and the end part  42  of each heat transfer section  40  secure a sufficient area to have contact with the pads. Consequently, heat transfer between the pads and each heat transfer sections  40  is very satisfactory. Because each heat transfer section  40  has a solid structure in total, this structure prevents the molten solder, before solidification between the heat dissipation pad  21  and the heat dissipation slug  13 , from flowing out to the rear surface  20   b . Each heat transfer section  40  can omit the head  41 .  
      Because the heat dissipation slag  13 , the heat dissipation pad  21 , and the heat dissipation rear-surface pad  23  are all coated with a low melting-point material containing Bi, the soldered portions also contain the Bismuth simple substance. According to the circuit board  1  shown in  FIG. 2 , the pair of pads including the heat dissipation pad  21  and the heat dissipation rear-surface pad  23  are connected together to transfer heat. Heat added to one pad is dissipated by the other pad. While these pads require a soldering at a high temperature, the low melting-point material coated on these pads allows the solder to be melted at a temperature about the same as that of soldering the electric connection pad  22 . The low melting-point material can be covered according to a method different from the coating (such as a dropping with a dispenser or a partial printing, for example).  
      A circuit board according to a second embodiment of the present invention is explained below. A duplicate explanation of the first embodiment is omitted, and characteristic parts of the second embodiment are mainly explained.  
       FIG. 3  is a cross-sectional diagram of a circuit board according to the second embodiment of the present invention.  
      The circuit board  1  shown in  FIG. 3  also has plural heat transfer sections  40 . However, the head  41  of each heat transfer section  40  shown in  FIG. 3  is soldered to the heat dissipation pad  21  in a state of being embedded in the heat dissipation pad  21 , and the end part  42  is soldered to the heat dissipation rear-surface pad  23  in a state of being embedded in the heat dissipation rear-surface pad  23 . Each heat transfer section  40  is accommodated in a through-hole that is mechanically formed on the substrate  20 . A low melting-point material is not coated on any one of the heat dissipation slag  13 , the heat dissipation pad  21 , and the heat dissipation rear-surface pad  23  shown in  FIG. 3 . All the heads  41  and the ends  42  of the heat transfer sections  40  are coated with a low melting-point material containing Bi respectively. Therefore, the solder is melted at a temperature about the same as that of soldering the electric connection pad  22 . The heat transfer sections  40  shown in  FIG. 3  are not brought into contact with the conductive layer  201  of the substrate  20 . The semiconductor element  10  shown in  FIG. 3  is a CPU (Central Processing Unit) having a large heat dissipation value. When the heat transfer sections  40  are brought into contact with the conductive layer  201 , the temperature of the substrate  20  becomes too high. Therefore, the heat transfer sections  40  are intentionally separated from the conductive layer  201  in this example.  
      A circuit board according to a third embodiment of the present invention is explained below. A duplicate explanation of the first and the second embodiments is omitted, and characteristic parts of the third embodiment are mainly explained.  
       FIG. 4  is a cross-sectional diagram of a circuit board according to the third embodiment of the present invention.  
      The semiconductor element  10  shown in  FIG. 4  is a CPU having an extremely high heat dissipation value. According to the circuit board  1  shown in  FIG. 4 , the heat transfer sections  40  are also separated from the conductive layer  201  to avoid a rise in the temperature of the substrate, like the circuit board shown in  FIG. 3 . The circuit board  1  shown in  FIG. 4  has a heat sink  50  disposed on the rear surface  20   b . The heat sink  50  has plural fins  51  having a larger capacity than that of the heat dissipation rear-surface pad  23 . Each heat transfer section  40  shown in  FIG. 4  has a tube  430  that stretches from the heat dissipation rear-surface pad  23  in a thickness direction of the substrate  20 . The peripheral surface of the tube  430  is soldered to the heat dissipation rear-surface pad  23 .  
       FIG. 5  is a perspective diagram of a heat transfer section that is provided on the circuit board shown in  FIG. 4 .  
      The heat transfer section  40  shown in  FIG. 4  has a disk-shaped head  41  and a cylinder  43 . The head  41  is provided to seal an opening at one end of the cylinder  43 . An opening  431  at the other end of the cylinder  43  is kept open. Therefore, the head  41  of the heat transfer section  40  shown in  FIG. 5  has a solid structure, and accordingly, a part of the heat transfer section  40  has a solid structure. A thread groove  432  is provided on the internal peripheral surface of the cylinder  43 , from the opening  431  toward the head  41 . The end part of the opening  431  of the cylinder  43  is equivalent to the tube  430  shown in  FIG. 4 .  
      The heat sink  50  shown in  FIG. 5  has an insertion section that is inserted into the opening of the heat transfer section  40  shown in  FIG. 5 . The insertion section has a thread groove that meshes with the thread groove  432  provided on the internal peripheral surface of the cylinder  43 . The heat sink  50  shown in  FIG. 4  is meshed with the internal peripheral surface of the cylinder of each heat transfer section  40 . The heat sink can be easily mounted. Heat is transferred from the heat transfer sections  40  to the mounted heat sink  50 . According to the circuit board  1  shown in  FIG. 4 , heat of the semiconductor element  10  is transferred from the heat dissipation slug  13  to the heat dissipation pad  21 . Further, the heat is transferred to the heat dissipation rear-surface pad  23  via the heat transfer sections  40 , and is efficiently dissipated from the heat sink  50 . In fixing the heat sink  50  to the internal peripheral surface of the cylinder of each heat transfer section  40 , meshing is not the only method. Other method such as pushing can be used according to various mechanical fastening methods.