Patent Publication Number: US-2009218386-A1

Title: Soldering Method, Soldering Apparatus and Method for Manufacturing Semiconductor Device

Description:
TECHNICAL FIELD 
     The present invention relates to a soldering method of soldering a semiconductor element to a circuit board, a soldering apparatus, and a method for manufacturing semiconductor devices. 
     BACKGROUND ART 
     A conventional semiconductor module includes a ceramic substrate, a wiring layer, which is a metal plate joined to a surface of the ceramic substrate, and a joining layer, which is a metal plate joined to a back surface of the ceramic substrate. A semiconductor element is soldered (joined) to the wiring layer. A heat radiating device, that is, a heat sink for radiating heat generated by the semiconductor element is joined to the joining layer. 
     At a time of soldering, it is often the case that a void is generated in the solder in the process of solidification of the solder after the solder is molten. In the case that a lot of voids are generated in the solder, resistances of electricity and heat passing through the solder become higher. Further, if the size of one void becomes equal to or more than a certain degree, the electricity and the heat flow through the wiring layer and the circuit board while bypassing the void from the semiconductor element. Accordingly, a hot spot, which is a locally high temperature region, is generated about a void of the semiconductor element. As a result, the semiconductor element can be destroyed. 
     Accordingly, Patent Document 1 and Patent Document 2 propose a technique of suppressing the void generation. These publications propose a technique of soldering by evacuating a chamber at a time of heating solder so as to depressurize, and melting the solder in a state in which the degree of vacuum is high. 
     However, as shown in  FIGS. 7A and 7B , the inventor conducted an experiment and verified the fact that the void is generated even in the case where soldering is carried out by melting solder in a high degree of vacuum. Accordingly, it is hard to say that the soldering method in the publications mentioned above can suppress the generation of voids. 
     Patent Document 1: Japanese Laid-Open Patent Publication No. 2005-230830 
     Patent Document 2: Japanese Laid-Open Patent Publication No. 2005-271059 
     DISCLOSURE OF THE INVENTION 
     An objective of the present invention is to provide a soldering method and a soldering apparatus that suppress the generation of voids. 
     In accordance with one aspect of the present invention, there is provided a soldering method of soldering a semiconductor element to a circuit board. The soldering method includes a step of accommodating an object-to-be-soldered in a chamber. The object-to-be-soldered includes the circuit board, the semiconductor element, and solder arranged between the circuit board and the semiconductor element. The solder has a melting temperature. The soldering method includes a step of achieving a reduction state in which the chamber is filled with an atmospheric gas at least including a reducing gas. The soldering method includes a step of melting the solder by heating the solder in such a manner as to raise a temperature of the solder to the melting temperature or higher, in the chamber in the reduction state. The soldering method includes a step of soldering the semiconductor element to the circuit board by solidifying the solder by lowering the temperature of the molten solder to lower than the melting temperature. The soldering method includes a step of structuring the chamber to be sealable, and a step of raising an internal pressure of the chamber to a melting start pressure equal to or higher than a normal pressure by the atmospheric gas until the rising temperature of the solder reaches the melting temperature. The melting start pressure is the internal pressure of the chamber at the time when the solder starts melting. The soldering method includes a step of achieving a pressurization state in which the internal pressure of the chamber is set to be equal to or higher than the melting start pressure, in a solder melting period. The solder melting period corresponds to a period until the molten solder is solidified after the solder starts melting. The soldering method includes a step of soldering the semiconductor element to the circuit board in the pressurization state. 
     Further, in accordance with another aspect of the present invention, there is provided a method for manufacturing semiconductor devices including a circuit board and a semiconductor element soldered to the circuit board. 
     Further, in accordance with another aspect of the present invention, there is provided a soldering apparatus for soldering a semiconductor element to a circuit board. The soldering apparatus includes a sealable chamber. A heating apparatus heats a solder arranged between the circuit board and the semiconductor element so as to melt the solder. The circuit board, the semiconductor element and the solder construct an object-to-be-soldered. The solder has a melting temperature. A gas introduction portion introduces an atmospheric gas at least including a reducing gas to the chamber. The gas introduction portion introduces the atmospheric gas to the chamber in a state in which the object-to-be-soldered is accommodated. The heating apparatus raises a temperature of the solder, to which the atmospheric gas has been introduced, to the melting temperature or higher so as to melt the solder. The gas introduction portion raises an internal pressure of the chamber to a melting start pressure equal to or higher than the normal pressure by the atmospheric gas, until the rising temperature of the solder reaches the melting temperature. The soldering apparatus is structured such as to achieve a pressurization state in which the internal pressure of the chamber is set to a pressure equal to or higher than the melting start pressure, in a solder melting period until the molten solder is solidified after the solder starts melting. The soldering apparatus is structured such as to solder the semiconductor element to the circuit board in the pressurization state. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a plan view of a semiconductor module manufactured by a manufacturing method in accordance with the present invention; 
         FIG. 2  is a cross-sectional view taken along line  2 - 2  in  FIG. 1 ; 
         FIG. 3  is a vertical cross-sectional view of a soldering apparatus according to the present invention; 
         FIG. 4(   a ) is a plan view of the jig shown in  FIG. 3 ; 
         FIG. 4(   b ) is a perspective view of the weight shown in  FIG. 3 ; 
         FIG. 5  is a plan view showing a layout of a high-frequency heating coil with respect to the semiconductor module shown in  FIG. 3 ; 
         FIG. 6A  is a graph showing a transition of a pressure and an X-ray photograph of a manufactured semiconductor module in a first experimental example of the soldering apparatus in  FIG. 3 ; 
         FIG. 6B  is a graph showing a transition of a pressure and an X-ray photograph of a manufactured semiconductor module in a second experimental example of the soldering apparatus in  FIG. 3 ; 
         FIG. 7A  is a graph showing a transition of a pressure and an X-ray photograph of a manufactured semiconductor module in a first comparative example; 
         FIG. 7B  is a graph showing a transition of a pressure and an X-ray photograph of a manufactured semiconductor module in a second comparative example; and 
         FIG. 8  is a graph showing a transition of a pressure in a modified embodiment in accordance with the present invention. 
     
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     A description will be given below of one embodiment according to the present invention with reference to  FIGS. 1 to 7B . 
     As shown in  FIGS. 1 and 2 , a semiconductor module  10 , which is a semiconductor device, includes a circuit board  11 , semiconductor elements  12  joined to the circuit board  11 , and a heat sink  13 , which is a heat radiating device. The circuit board  11  includes a ceramic substrate  14 , wiring layers  15  joined to a surface of the ceramic substrate  14 , that is, a top surface in  FIG. 2 , and a bonding layer  16  joined to a back surface of the ceramic substrate  14 , that is, a lower surface in  FIG. 2 . The ceramic substrate  14  is formed, for example, by aluminum nitride, alumina, or silicon nitride. The wiring layers  15  are formed, for example, by aluminum (pure aluminum and an aluminum alloy) or copper. The semiconductor elements  12  are soldered to the wiring layers  15 . Solder layers H are positioned between the semiconductor elements  12  and the wiring layer  15 . The semiconductor elements  12  and the wiring layers  15  correspond to a joining member to which the solder is joined. 
     The semiconductor elements  12  include an insulated gate bipolar transistor (IGBT) or a diode. A plurality of, four in the present embodiment, semiconductor elements  12  are joined to the circuit board  11 . The bonding layer  16  joins the heat sink  13  to the ceramic substrate  14 . The bonding layer  16  is formed, for example, by aluminum or a copper. The heat sink  13  is joined to the bonding layer  16 . 
     As shown in  FIG. 3 , a soldering apparatus HK solders the semiconductor elements  12  to the circuit board  11 .  FIG. 5  shows a large-scaled semiconductor module  100  corresponding to six semiconductor modules  10  in  FIG. 1 . In other words, the semiconductor module  100 , which is the semiconductor device, includes six circuit boards  11 , and twenty four semiconductor elements  12 . The soldering apparatus HK manufactures the semiconductor module  100 . 
     As shown in  FIG. 3 , the soldering apparatus HK is provided with a sealable chamber  17 . The chamber  17  includes a box main body  18  having an opening  18   a , and a lid  19  which can switch an open state and a closed state of the opening  18   a . A support table  20  for positioning and supporting the semiconductor module  100  is accommodated in the box main body  18 . A gasket  21  is arranged at an attaching position of the lid  19  in the box main body  18 . 
     The lid  19  has a size which can close the opening  18   a  of the box main body  18 . The box main body  18  and the lid  19  define a sealed space S within the chamber  17 . The lid  19  includes a glass plate  22  opposing to the sealed space S. The glass plate  22  serves as a nonmagnetic and electrically insulating material. 
     As shown in  FIG. 3 , a reducing gas feeding portion  23  serving as a gas introduction portion feeding a reducing gas to the chamber  17  is connected to the box main body  18 . In the present embodiment, the reducing gas is hydrogen gas (H 2 ). The reducing gas feeding portion  23  is provided with a piping  23   a , an on-off valve  23   b  of the piping  23   a , a pressure reducing valve  23   c , which is a first pressure regulating portion, and a hydrogen tank  23   d . The pressure reducing valve  23   c  feeds hydrogen gas introduced from the hydrogen tank  23   d  while passing through the on-off valve  23   b  to the chamber  17  while setting the pressure to a fixed value. 
     Inert gas feeding portion  24  for feeding an inert gas to the chamber  17  is connected to the box main-body  18 . In the present embodiment, the inert gas is nitrogen gas (N 2 ). The inert gas feeding portion  24  is provided with a piping  24   a , an on-off valve  24   b  of the piping  24   a , and a nitrogen tank  24   c . Further, a vacuum portion  25  for evacuating the inside of the chamber  17  is connected to the box main body  18 . The vacuum portion  25  is provided with a piping  25   a , an on-off valve  25   b  of the piping  25   a , and a vacuum pump  25   c.    
     Further, a gas discharge portion  26  for discharging gas filling the chamber  17  to an outside is connected to the box main body  18 . The gas discharge portion  26  is provided with a piping  26   a , an on-off valve  26   b  of the piping  26   a , and a throttle valve  26   c , which is a second pressure regulating portion. The gas within the chamber  17  is discharged to the outside while being regulated in the discharging amount by the throttle valve  26   c . The soldering apparatus HK is structured such as to be capable of regulating the pressure in the sealed space S by being provided with the reducing gas feeding portion  23 , the inert gas feeding portion  24 , the vacuum portion  25  and the gas discharge portion  26 . In other words, the soldering apparatus HK pressurizes or depressurizes the sealed space S on the basis of pressure regulation. 
     A temperature sensor  27  measuring the temperature T within the chamber  17  is arranged in the box main body  18 . The temperature sensor  27  is, for example, a thermo couple. In the present embodiment, the temperature sensor  27  is arranged in such a manner as to be capable of measuring the temperature T at a joining section of a semiconductor element  12  to a wiring layer  15 , that is, a section which is soldered. 
     A plurality of high-frequency heating coils  28  serving as a heating device are located in an upper portion of the soldering apparatus HK, that is, above the lid  19 . Six high-frequency heating coils  28  are arranged over the circuit boards  11  in such a manner as to individually correspond to six circuit boards  11  shown in  FIG. 5 . Each of the high-frequency heating coils  28  has such a size as to expand over one circuit board  11 . Further, each of the high-frequency heating coils  28  is formed larger than the outline of the upper surface of the weight  35 . 
     As shown in  FIG. 5 , each of the high-frequency heating coils  28  is formed as a spiral shape, particularly a rectangular spiral shape. Each of the high-frequency heating coils  28  is expanded two-dimensionally. Each of the high-frequency heating coils  28  is arranged in such a manner as to oppose to the glass plate  22  of the lid  19 . The soldering apparatus HK has a high-frequency generating device  29  to which each of the high-frequency heating coils  28  is electrically connected. Each of the high-frequency heating coils  28  is controlled to a predetermined temperature on the basis of a result of measurement of the temperature sensor  27 . A cooling path  30  for putting cooling water through the inside of the coil  28  is formed in each of the high-frequency heating coils  28 . The soldering apparatus HK has a cooling water tank  31  to which the cooling path  30  is connected. 
       FIG. 4(   a ) shows a jig  32  used for soldering.  FIG. 4(   b ) shows a weight  35  serving as a pressing body. The jig  32  is formed as a flat plate having the same size as the ceramic substrate  14  of the circuit board  11 . The jig  32  is made, for example, of graphite or ceramics. As shown in  FIG. 3 , each jig  32  positions a solder sheet  33 , a semiconductor element  12  and a weight  35  on the circuit board  11 , at the time of soldering. Each jig  32  has a plurality of positioning through holes  34 . Since four semiconductor elements  12  are joined onto the circuit board  11 , each jig  32  has four through holes  34 . Each of the through holes  34  corresponds to a joined section of the semiconductor elements  12  on the circuit board  11 . Each of the through holes  34  has a size corresponding to the semiconductor elements  12 . The temperature T measured by the temperature sensor  27  indicates the temperature within the chamber  17  and the temperature of the solder sheet  33 . 
     If a magnetic flux passing through each weight  35  is changed, an electric current is generated in the weight  35 . The material of the weight  35  is selected in such a manner that the weight  35  generates heat on the basis of an electric resistance of the weight  35  itself. In the present embodiment, the weight  35  is made of stainless steel. As shown in  FIG. 3 , the weights  35  are arranged immediately above the semiconductor elements  12  at the time of soldering. In other words, the weights  35  come into contact with upper surfaces, that is, non-joining surfaces  12   a  of the semiconductor elements  12 . As a result, the weights  35  press the semiconductor elements  12  toward the circuit board  11 . Each of the weights  35  is an integrated part produced by machining a material. A pressing surface  35   a  of the weight  35  can be fitted and inserted to a through hole  34  of the jig  32 . The pressing surface  35   a  of one weight  35  can come into contact with and press a non-joining surface  12   a  of four semiconductor elements  12 . The jig  32  has a partition  32   a  defining the adjacent through holes  34 . The pressing surface  35   a  has a groove  35   b  extending over the partition  32   a . The pressing surface  35   a  of the weight  35  serves as a surface which comes into contact with the non-joining surface  12   a  of each semiconductor element  12 .  FIG. 4(   a ) shows a state in which one weight  35  shown by a two-dot chain line enters four through holes  34 . 
     Next, a description will be given of a method by which the soldering apparatus HK solders the semiconductor elements  12  to the circuit board  11 . As shown in  FIG. 3 , there is previously prepared a joined object  93  including the circuit board  11 , and the heat sink  13  joined to the circuit board  11 . 
     At a time of carrying out the soldering, first, the lid  19  is detached from the box main body  18 , and the opening  18   a  is opened. As shown in  FIG. 3 , the joined object  93  is arranged on the support table  20  of the box main body  18  while being positioned. Next, the jig  32  is set on the circuit boards  11 . The solder sheets  33  and the semiconductor elements  12  are arranged within each of the through holes  34  of the jig  32  in this order. The weights  35  are set onto the semiconductor elements  12 . In other words, the solder sheets  33 , the semiconductor elements  12 , and the weights  35  are laminated on the wiring layers  15  in this order. The solder sheets  33 , the semiconductor elements  12 , and the weights  35  are laminated in a vertical direction of the soldering apparatus HK. In other words, the solder sheets  33 , the semiconductor elements  12 , and the weights  35  are laminated toward the lid  19 . The joined object  93  is arranged horizontally with respect to a ground surface. The pressing surface  35   a  of each weight  35  comes into contact with the non-joining surface  12   a  of the corresponding semiconductor element  12 , and presses the non-joining surface  12   a.    
     As mentioned above, an object-to-be-soldered  92  is arranged within the chamber  17 . The object-to-be-soldered  92  includes the joined object  93 , the solder sheets  33 , and the semiconductor elements  12 . 
     Next, the opening  18   a  is closed by attaching the lid  19  to the box main body  18 , and a sealed space S is defined within the chamber  17 . As shown in  FIG. 3 , in a state in which the object-to-be-soldered  92  is accommodated in the sealed space S, each of the high-frequency heating coils  28  is arranged above each of the weights  35 . The glass plate  22  is arranged between the high-frequency heating coils  28  and the weights  35 . In a state in which the high-frequency heating coils  28  are arranged above the weights  35 , each high-frequency heating coil  28  lies off the outline of the upper surfaces of the corresponding weights  35 . Since each high-frequency heating coil  28  is formed as the spiral shape, a lot of magnetic fluxes are generated in an area close to the center. Accordingly, it is preferable to arrange each weight  35  and the joined section to the circuit board  11  in such a manner as to correspond to the center of the high-frequency heating coil  28 . The joined section of the circuit board  11  refers to a section at which the semiconductor elements  12  are joined. 
     Next, the gas within the chamber  17  is replaced. First, the inside of the chamber  17  is evacuated by operating the vacuum portion  25 . Nitrogen gas is fed into the chamber  17  by operating the inert gas feeding portion  24 . In other words, the sealed space S is filled with inert gas. Hydrogen gas is fed into the chamber  17  by repeating the evacuation and the feed of the nitrogen gas several times, and thereafter operating the reducing gas feeding portion  23 . In other words, the inside of the chamber  17  is set to a reducing gas atmosphere. 
     Next, a high-frequency current is circulated to each of the high-frequency heating coils  28  by actuating the high-frequency generating device  29 . Then, there is generated a high-frequency magnetic flux passing through each weight  35 . An eddy current is generated in the weight  35 . In other words, the weight  35  exposed to the magnetic flux generated by the high-frequency heating coil  28  generates heat on the basis of an electromagnetic induction effect. The heat of the weight  35  is transmitted to the corresponding semiconductor element  12  from the pressing surface  35   a . As a result, the heat of the weight  35  is intensively, that is, locally transmitted to the joined section with the solder sheet  33  in the circuit board  11 . As a result, the temperature T of the solder sheet  33  becomes equal to or higher than a melting temperature Tm, and the solder sheet  33  melts. The semiconductor elements  12  are pressed toward the circuit board  11  by the weight  35 . Accordingly, the semiconductor elements  12  are prevented from being lifted up or being moved by a surface tension of the melting solder. 
     If the solder sheets  33  completely melt, the high-frequency generating device  29  is stopped. The solder is cooled until the melting solder is solidified. The melting solder is solidified by being cooled to the temperature lower than the melting temperature Tm, and joins the semiconductor elements  12  to the wiring layers  15 . If the semiconductor elements  12  are joined to the wiring layers  15 , the semiconductor module  100  is finished. Thereafter, the lid  19  is taken out from the box main body  18 , the jigs  32  and the weights  35  are detached, and the semiconductor module  100  is taken out from the inside of the chamber  17 . At a time of taking out the semiconductor module  100  from the chamber  17 , the gas discharge portion  26  releases the gas within the chamber  17  to the atmospheric air. 
     The internal pressure P of the chamber  17  is raised and lowered on the basis of the measured temperature of the temperature sensor  27  and an elapse of a time. Since the reducing gas feeding portion  23 , the inert gas feeding portion  24 , the vacuum portion  25 , and the gas discharge portion  26  feed gas into the chamber  17  or discharge gas from the inside of the chamber  17 , the internal pressure P of the chamber  17  is raised or lowered. The pressure reducing valve  23   c  of the reducing gas feeding portion  23 , and the throttle valve  26   c  of the gas discharge portion  26  circulate the reducing gas to the inside of the chamber  17  at a time of heating and cooling the solder. 
     A description will be given of a manner of regulating the atmospheric air within the chamber  17  at a time of heating and cooling the solder, in a first experimental example shown in  FIG. 6A , and a second experimental example shown in  FIG. 6B . 
     Dimensions of each of the semiconductor modules  10  used in the first experimental example and the second experimental example were as follows. 
     The ceramic substrate  14  was made of aluminum nitride. The ceramic substrate  14  was a quadrangular plate of 30 mm×30 mm having a thickness 0.635 mm. Each of the wiring layer  15  and the bonding layer  16  was made of pure aluminum, for example, 1000 series aluminum, which is an industrial pure aluminum. Each of the wiring layer  15  and the bonding layer  16  was a rectangular plate of 27 mm×27 mm having a thickness 0.4 mm. The thickness of the semiconductor elements  12  was 0.35 mm. The solder sheet  33  was made of a Sn—Cu—Ni—P based lead-free solder. The thickness of the solder sheet  33  was between 0.1 mm and 0.2 mm. 
     First, as shown by a graph in  FIG. 6A , a description will be given of a transition, that is, a regulation of the internal pressure P of the chamber  17  in the first experimental example. 
     Since the chamber  17  was evacuated, the internal pressure P of the chamber  17  at an initial time “to” indicated a state having a high degree of vacuum. The ambient atmosphere in the chamber  17  was replaced by a reducing gas atmosphere having a set pressure P 1  which was higher than a normal pressure Po, at a first point in time t 1 . In the present specification, the normal pressure Po, that is, the atmospheric pressure was about 0.1023 MPa. The set pressure P 1  was 0.13 MPa. The set pressure P 1  was the internal pressure of the chamber  17  at a time when the solder sheet  33  starts melting, that is, a melting start pressure. 
     Heating of the solder sheet  33  was started at a second point in time t 2  after the first point in time t 1 . In other words, the ambient atmosphere in the chamber  17  was replaced by reducing gas atmosphere having the set pressure P 1  before the second point in time t 2  at which the heating of the solder sheet  33  was started. 
     The temperature T of the solder sheet  33  reached a melting temperature Tm at a third point in time t 3 . In other words, the internal pressure P of the chamber  17  was raised to the normal pressure Po or higher at the first point in time t 1 , before the third point in time t 3  at which the temperature T of the solder sheet  33  reached the melting temperature Tm. In the present specification, the melting temperature Tm of the solder sheet  33  is 217° C. 
     The solder  33  was heated until the temperature T of the solder sheet  33  reaches a set temperature T 1  at a fourth point in time t 4 . The set temperature T 1  was higher than the melting temperature Tm. The set temperature T 1  was 250° C. In other words, the solder sheet  33  was heated between the second point in time t 2  and the fourth point in time t 4 , after the gas replacement of the inside of the chamber  17  at the first point in time t 1 . The temperature T of the solder sheet  33  was maintained to the set temperature T 1  between the fourth point in time t 4  and a fifth point in time t 5 . 
     The internal pressure P of the chamber  17  was regulated between the first point in time t 1  and a seventh point in time t 7  in such a manner as to maintain the set pressure P 1 . In other words, the internal pressure P of the chamber  17  in the first experimental example was maintained at the set pressure P 1  without being lowered to the normal pressure Po or lower (vacuum) between the second point in time t 2  and the fifth point in time t 5  at which the solder sheet  33  was heated. The heating of the solder sheet  33  was finished at the fifth point in time t 5 . In the first experimental example, the internal pressure P of the chamber  17  was maintained at the set pressure P 1  even at a time of cooling the solder between the fifth point in time t 5  and the seventh point in time t 7 . At a sixth point in time t 6 , the temperature T of the solder was lowered than the melting temperature Tm. When the solder was solidified, the internal pressure P of the chamber  17  was temporarily lowered to the normal pressure Po or less at the point in time t 7 , whereby the reducing gas was discharged. Thereafter, at an eighth seven point in time t 8 , the internal pressure P of the chamber  17  was recovered to the normal pressure Po by introducing the atmospheric air to the chamber  17 . 
     An X-ray photograph on the right side of the graph in  FIG. 6A  shows a back surface, that is, a joining surface of a semiconductor element  12  soldered in the first experimental example. A portion which is deepest in color in the X-ray photograph indicates the solder layer H. In accordance with the X-ray photograph in  FIG. 6A , an unwet spot was observed in a part of the solder layer H, however, no voids were observed. 
     Next, a description will be given of a transition, that is, a regulation of the internal pressure P of the chamber  17  in the second experimental example, as shown in a graph in  FIG. 6B . 
     As shown in  FIG. 6B , the internal pressure P of the chamber  17  was regulated in the same manner as the first experimental example in  FIG. 6A , between the initial point in time t 0  and the fifth point in time t 5  of the second experimental example. In other words, in the second experimental example, the internal pressure P of the chamber  17  at a time of heating the solder was maintained at the set pressure P 1  (0.13 MPa) which was higher than the normal pressure Po. As shown in  FIG. 6B , when the heating of the solder was finished at the fifth point in time t 5 , the internal pressure P of the chamber  17  was raised to a second set pressure P 2  from the set pressure P 1 . The second set pressure P 2  is 0.2 MPa. The second set pressure P 2  was maintained between the fifth point in time t 5  and the seventh point in time t 7  during the solder cooling. In other words, at a sixth point in time t 6  at which the temperature T of the solder was lowered than the melting temperature Tm, the internal pressure P of the chamber  17  was the second set pressure P 2 . When the solidification of the solder was completed at the seventh point in time t 7 , the internal pressure P of the chamber  17  was temporarily lowered to the normal pressure Po or less (vacuum). In other words, the reducing gas of the chamber  17  was discharged to the outside. The internal pressure P after the seventh point in time t 7  in the second experimental example in which the internal pressure P of the chamber  17  was recovered to the normal pressure Po was the same as the first experimental example, by introducing the atmospheric air to the chamber  17  at an eighth point in time t 8 . 
     An X-ray photograph on the right side of the graph in  FIG. 6B  shows a back surface, that is, a joining surface of a soldered semiconductor element  12  in the second experimental example. In accordance with the X-ray photograph in  FIG. 6B , neither unwet spots nor voids were observed in the entire solder layer H. 
       FIG. 7A  shows a first comparative example in which the internal pressure P of the chamber  17  was set to the normal pressure Po or less at a time of heating and cooling the solder, just for reference.  FIG. 7B  shows a second comparative example in which the internal pressure P of the chamber  17  was set to the normal pressure Po or less at a time of cooling the solder. 
     As shown in  FIG. 7A , the ambient atmosphere of the chamber  17  in the first comparative example was replaced by the reducing gas atmosphere before heating the solder from the second point in time t 2 . In other words, the internal pressure P of the chamber  17  was set to the set pressure P 1  (0.13 MPa) at the first point in time t 1 . 
     However, as shown in  FIG. 7A , the internal pressure P of the chamber  17  was lowered to the normal pressure Po or less at a point in time t 23  existing between the second point in point in time t 2  and the third point in time t 3 . In other words, the internal pressure P of the chamber  17  was maintained in a vacuum state between the point in time t 23  and the eighth point in time t 8 . 
     In other words, the internal pressure P in the first comparative example was lowered to the normal pressure Po or less before the third point in time t 3  at which the temperature T of the solder reached the melting temperature Tm at a time of heating the solder. The internal pressure P of the chamber  17  in accordance with the first comparative example was equal to or less than the normal pressure Po in both of a period between the second point in time t 2  and the fourth point in time t 4 , which is the solder heating period, and a period between the fifth point in time t 5  and the eighth point in time t 8 , which is the solder cooling period. The seventh point in time t 7  does not exist in the graph in  FIG. 7A . In other words, the seventh point in time t 7  for maintaining the internal pressure P of the chamber  17  at the set pressure P 1  after the sixth point in time t 6  at which the solidification of the solder was completed does not exist in the graph in  FIG. 7A . 
     An X-ray photograph on the right side of the graph in  FIG. 7A  shows the back surface, that is, the joining surface of a semiconductor element  12  soldered in the first comparative example. In accordance with the X-ray photograph, it was found out that voids were generated in all the solder layer H. Further, the voids were generated in a wide region. In other words, it was observed that voids were generated even in a state having a high degree of vacuum. The first comparative example strongly suggests that the gas hardly exists in voids. 
     In the second comparative example shown in  FIG. 7B , the internal pressure P of the chamber  17  was the same as the first embodiment in  FIG. 6A  between the initial point in time t 0  and the fifth point in time t 5 . In other words, the ambient atmosphere of the chamber  17  was replaced by the reducing gas atmosphere at the first point in time t 1  before heating the solder. The internal pressure P of the chamber  17  was maintained at the set pressure P 1  (0.13 MPa) between the second point in time t 2  and the fifth point in time t 5  at a time of heating the solder. 
     However, as shown in  FIG. 7B , the internal pressure P of the chamber  17  was lowered to the normal pressure Po or lower at the fifth point in time t 5 . In other words, the internal pressure P of the chamber  17  was maintained at the normal pressure Po or lower (vacuum) between the fifth point in time t 5  and the eighth point in time t 8  corresponding to the solder cooling period. The seventh point in time t 7  does not exist in the graph in  FIG. 7B . 
     An X-ray photograph on the right side of the graph in  FIG. 7B  shows the back surface, that is, the joining surface of ah semiconductor element  12  soldered in the second comparative example. In accordance with the X-ray photograph, the generating amount of the void was reduced in comparison with the first comparative example in  FIG. 7A , and was improved. However, the void remains to be generated in all the solder layers H. The unwet spots were observed in a part of the solder layer H. 
     It is quite obvious that the first experimental example in  FIG. 6A  and the second experimental example in  FIG. 6B  inhibited the generation of voids in comparison with the first comparative example in  FIG. 7A  and the second comparative example in  FIG. 7B . In the first experimental example and the second experimental example, the solder melting period t 3  to t 7  corresponds to a period until the solder was solidified at the seventh point in time t 7  after the solder started melting at the third point in time t 3 . The molten solder was kept being pressurized in such a manner that the internal pressure P of the chamber  17  was maintained at the set pressure P 1 , which was higher than the normal pressure Po, in the solder melting period t 3  to t 7 . The generation of voids was suppressed by pressurizing the molten solder as mentioned above. 
     The reducing gas having the higher pressure than the set pressure P 1  (0.13 MPa) was fed from the hydrogen tank  23   d  in the solder melting period t 3  to t 7  of the first experimental example and the second experimental example. The pressure reducing valve  23   c  kept the internal pressure P of the chamber  17  at a fixed value, that is, the set pressure P 1 . The throttle valve  26   c  of the gas discharge portion  26  discharged a fixed amount of gas to the outside of the chamber  17 . The reducing gas feeding portion  23  fed the reducing gas to the chamber  17  in such a manner as to compensate for a pressure reduction component of the internal pressure P of the chamber  17  caused by the gas discharge. As a result, the internal pressure P of the chamber  17  was kept at the fixed value. Further, the gas was circulated within the chamber  17 . The internal pressure P of the chamber  17  in the solder melting period t 3  to t 7  was kept at a fixed value while taking into consideration a rising component of the internal pressure P of the chamber  17  caused by a temperature rising in the chamber  17  by heating the solder. 
     A consideration will be given to causes of the generation of voids on the basis of the results of experiments. 
     A surface tension of the molten solder is lowered as the temperature T of the solder rises. Since oxide exists on the surface of the solder, and the surface of the joined member (the semiconductor elements  12  and the wiring layers  15 ), a wettability of these surfaces is not good. Three kinds of materials including the solder, the joined member, and the atmospheric gas (the reducing gas in the present embodiment) cross in an interface in which the solder gets wet. On a cross line, or the line on which three kinds of materials cross, there exist a first surface tension applied between the joined member (solid body) and the atmospheric gas (gaseous body), a second surface tension applied between the molten solder (liquid body) and the atmospheric gas (gaseous body), and a boundary tension applied between the joined member (solid body) and the molten solder (liquid body). Each of the first surface tension, the second surface tension and the boundary tension is applied toward the corresponding interface direction from the cross line. 
     In many cases, the second surface tension between the molten solder and the atmospheric gas is great, and the boundary tension between the molten solder and the joined member has a negative value, immediately after the solder is molten. In this case, the solder is hard to be spread. The solder rather has a tendency to conglobate so that joined areas between the solder and the joined member are reduced. In order to suppress this tendency, it is effective to solder in a state in which the solder is pressurized by the weight  35  as in the present embodiment. For example, in a state in which a soft ball is sandwiched between a pair of upper and lower plates, the ball is collapsed by mounting a weight on the upper plate. Accordingly, the theory mentioned above is easy to understand. However, it is hard to prevent the conglobation tendency molten solder, only by the pressure of the ambient atmosphere. For example, even if the pressure of the atmospheric gas of a ball filled with water is raised, the ball is hard to be deformed from the spherical shape, however, the ball is easily collapsed by mounting a weight on the ball. Accordingly, the theory mentioned above is easy to understand. 
     As described in the background art of the present specification, the conventional void generation countermeasure heats the solder in the state in which the internal pressure P of the chamber  17  is set to the normal pressure Po or less (vacuum). This is based on the thinking that voids are generated by the atmospheric gas, or the gas generated from the residual gas, the solder or the like. In other words, it is thought that the generation of the void can be suppressed in the vented vacuum state. 
     However, as shown in  FIG. 7A , the inventor of the present invention found out that voids are generated in solder even if soldering is carried out in the normal pressure Po or less (vacuum), on the basis of the experiments. In the case of joining the semiconductor element such as a power transistor of which one side is about 10 mm to the circuit board by the solder sheet, solder is dotted with voids. Many of the voids are of a cylindrical type passing through the solder having a thickness between 100 and 200 μm. In other words, the inventor of the present invention found out that voids were connected to both the surfaces of the joined portion. The fact that the solder existing between the semiconductor element and the circuit board before being heated disappears in the void portions through the heating means that the solder existing in the void portions was pushed away to the void peripheral portions by some force. 
     On the basis of these results, the inventor of the present invention considered that the inside of a void is in a low pressure state (a state having a high degree of vacuum), and the force generating voids is surface tension. The surface tension refers to a force minimizing the surface area of the liquid. The inventor of the present invention found out that a cylinder having a diameter of 1 mm, a height of 100 μm and a surface area 1×n×0.1 mm 2  is more stable than the case where a unwet portion having a diameter of 1 mm exists in a spherical state being close to a close contact without being joined, that is, the case where the spherical surface area is 0.025×n mm 2 . Accordingly, if the inside of the void is in a state having a high degree of vacuum, it is considered that the void will disappears by applying a pressure overcoming the surface tension of the molten solder to the molten solder. In the case where soldering is carried out in the pressurized state as shown in  FIGS. 6A and 6B  on the basis of the theory, it was possible to achieve a state in which no voids were observed, that is, a zero void state. 
     If the state of the void does not depend on presence or absence of gas, but depends on surface tension, the factors such as the material of the solder, the surface state of the semiconductor elements  12  and the wiring layers  15 , the temperature T, the thickness of the solder and the like are expected to dominate the state of the void. In the case where the soldering was experimented under the same conditions by using the solder sheet  33  having a thickness of 100 μm and the solder sheet  33  having a thickness of 150 μm, a better result was obtained in the solder sheet  33  having a thickness of 150 μm. 
     For example, if two plates are dipped into a liquid body in a state in which a gap is formed between the plates, a liquid surface rises along the gap in the case that two plates are well wetted. The smaller the gap is, the higher the liquid surface rises. If the plate repels the liquid body due to poor wetting, the liquid surface is depressed. If the gap is small, the liquid surface is depressed. However, if the gap is large, the liquid surface is not significantly depressed. 
     The solder sheet  33  having a thickness of 100 μm corresponds to a case where the gap between a semiconductor element  12  and a wiring layer  15 , which are two plates, is small. The solder sheet  33  having a thickness of 150 μm corresponds to a case where the gap between two plates is large. Accordingly, it is clear that the solder sheet  33  having a thickness of 150 μm suppresses voids on the basis of pressure more effectively than the solder sheet  33  having a thickness of 100 Mm. The inventor of the present invention believes that the generation of voids is suppressed by increasing the internal pressure P of the chamber  17 . 
     The present embodiment has the following advantages. 
     (1) In the solder melting period t 3  to t 7  until the solder is solidified after starting melting, the soldering is carried out in the ambient atmosphere of the set pressure P 1  equal to or higher than the normal pressure Po. Accordingly, the force overcoming the surface tension of the solder is applied to the molten solder. Therefore, it is possible to suppress the influence of the surface tension which is considered as the factor of the void generation, and it is possible to inhibit voids from being generated. 
     (2) In order to keep the internal pressure P of the chamber  17  at the fixed value, the reducing gas feeding portion  23  has the pressure reducing valve  23   c . Accordingly, in the solder melting period t 3  to t 7 , the stable pressurized state is achieved within the chamber  17 , and it is possible to reliably inhibit voids from being generated. Particularly, in the case of solidifying the molten solder, the internal pressure P of the chamber  17  is lowered in accordance with the reduction of the temperature T within the chamber  17 . It is possible to keep the internal pressure P of the chamber  17  at the normal pressure Po or higher by feeding the reducing gas to the chamber  17  from the pressure reducing valve  23   c.    
     (3) The throttle valve  26   c  of the gas discharge portion  26  discharges the gas within the chamber  17  to the outside. Accordingly, the reducing gas is circulated between the inside and outside of the chamber  17 . As a result, the water content within the chamber  17  generated by the reducing effect is removed by discharging the gas. 
     (4) As shown in  FIG. 6B , in the second experimental example, the internal pressure P of the chamber  17  is raised further from the set pressure P 1  at a time of finishing the solder heating. Accordingly, even if the void is generated in the molten solder, it is possible to eliminate voids until the molten solder is solidified. Accordingly, the generation of voids is easily suppressed. 
     (5) The weights  35  are heated by the high-frequency heating coils  28  which are away from the weights  35 . Accordingly, in the case where a plurality of semiconductor elements  12  are soldered to the circuit board  11  all at once, it is not necessary to provide the high-frequency heating coil  28  per weight  35 . In other words, the high-frequency heating coils  28 , the number of which is less than the number of the weights  35 , can heat a greater number of joined sections on the circuit board  11  all at once. 
     Further, since the high-frequency heating coils  28  are away from the weights  35 , it is possible to handle the high-frequency heating coils  28  independently from the weights  35  and the circuit board  11 , at the time of cooling the molten solder. Accordingly, for example, in the case where a plurality of semiconductor modules  100  are arranged within the chamber  17 , it is possible to improve the operating efficiency of the high-frequency heating coils  28  by moving the high-frequency heating coils  28  from a certain semiconductor module  100  to another semiconductor module  100 . 
     Further, the present embodiment heats the joined section of the circuit board  11  by heating the weights  35  pressing the semiconductor elements  12 . Accordingly, it is possible to concentrically transmit the heat to the joined section. Therefore, it is possible to improve the heating efficiency, for example, in comparison with the case of heating the entire circuit board  11  or the entire chamber  17 . 
     (6) One high-frequency heating coil  28  is arranged above a plurality of weights  35  on one circuit board  11 . Accordingly, it is possible to two-dimensionally transmit heat to a plurality of joined sections in one circuit board  11 . Therefore, it is possible to uniformly heat a plurality of joined sections. As a result, it is possible to approximate the melting start timings of the solder sheets  33  arranged at the respective joined sections in such a manner as to be substantially simultaneous. Further, it is possible to approximate the timings at which all the solder sheets  33  finish melting in such a manner as to be substantially simultaneous. Therefore, it is possible to make the soldering work efficient. 
     (7) The high-frequency heating coils  28  are arranged outside of the chamber  17 . Accordingly, the high-frequency heating coils  28  do not need to be held in positions in the soldering work except for the heating period. In other words, the high-frequency heating coils  28  can be detached from the chamber  17  except for the heating period. Therefore, it is possible to improve the production efficiency of the semiconductor modules  100  by moving one high-frequency heating coil  28  to other chambers  17  one after the other. 
     Further, a volumetric capacity of the chamber  17  in accordance with the present embodiment, in which the high-frequency heating coils  28  are arranged outside of the chamber  17 , is smaller than, for example, the case where a heating members, that is, the high-frequency heating coils  28  are arranged within the chamber  17 . Accordingly, it is possible to achieve a downsizing of the chamber  17 . 
     The regulation of the ambient atmosphere mainly includes discharging the air from the chamber  17 , that is, vacuuming, and feeding and discharging the inert gas such as nitrogen gas or the like and reducing gas such as hydrogen gas or the like. Accordingly, it is possible to reduce the time and the energy consumption necessary for discharging the air by reducing the volumetric capacity of the chamber  17 . For example, it is possible to reduce the operating energy of the vacuum pump  25   c . Further, it is possible to reduce the time and the energy consumption necessary for feeding or discharging inert gas or reducing gas to and from the chamber  17 . It is also possible to reduce the consumption of inert gas and reducing gas. 
     (8) The lid  19 , which is a portion of the chamber  17  that faces the high-frequency heating coils  28 , is formed by the glass plate  22 , which is an electrically insulating material. Accordingly, it is possible to prevent the chamber  17  from generating heat. Further, since the magnetic flux passes through the chamber  17 , it is possible to heat the weights  35 . 
     (9) One pressing surface  35   a  of the weight  35  can come into contact with the non-joining surfaces  12   a  of a plurality of semiconductor elements  12 . In other words, one weight  35  corresponds to an assembly obtained by collecting a plurality of sub weights each of which is provided for pressing one semiconductor element  12 . Accordingly, it is possible to enlarge the pressing surface  35   a  of one weight  35 . Therefore, in comparison with the case where the pressing surface  35   a  is small, the weights  35  stably press each of the semiconductor elements  12 . Accordingly, each of the semiconductor elements  12  is hardly affected by the surface tension of the molten solder, and the soldering work is stably carried out. 
     (10) One high-frequency heating coil  28  is allocated to one circuit board  11 . Accordingly, the heat generating efficiency of the weights  35  is higher than the case where one high-frequency heating coil  28  is allocated to a plurality of circuit boards  11 . 
     The embodiment mentioned above may be modified as follows. 
     As shown in  FIG. 8 , the internal pressure P of the chamber  17  may be gradually increased in the solder melting period t 3  to t 7 . In other words, the internal pressure P of the chamber  17  is gradually increased from the set pressure P 1  at the second point in time t 2  toward the second set pressure P 2  at the seventh point in time t 7 . In other words, the internal pressure P of the chamber  17  in the solder melting period t 3  to t 7  is not limited to be kept at the set pressure P 1  or the second set pressure P 2 , which is a fixed value. 
     The set pressure P 1  may be set higher than 0.13 MPa. The second set pressure P 2  may be changed from 0.2 MPa. The set pressure P 1  and the second set pressure P 2  are set taking the durability of the chamber  17  into consideration. 
     The set pressure P 1  is not limited to 0.13 MPa. In correspondence to the materials of the wiring layer  15  and the semiconductor elements  12  and the condition of the surface treatment, the set pressure P 1  may be set to a range between 0.11 MPa and 0.13 MPa, inclusive. The closer to the normal pressure Po outside of the chamber  17 , the more advantageous the internal pressure P of the chamber  17  in terms of the durability of the chamber  17 . Further, the set pressure P 1  can be changed in correspondence to the wettability and the surface tension of the molten solder. The inert gas has been conventionally fed to a reflow furnace used for soldering, for preventing the atmospheric air from making an intrusion into the reflow furnace. However, the feed of the inert gas only sets the internal pressure of the reflow furnace to about the normal pressure. 
     The internal pressure P of the chamber  17  in the solder melting period t 3  to t 7  may be kept at the set pressure P 1 , which is a fixed value, by introducing the reducing gas having a higher pressure than the set pressure P 1  to the chamber  17  on the basis of the pressure value obtained by monitoring the internal pressure P of the chamber  17 . Alternatively, the internal pressure P of the chamber  17  may be gradually increased by feeding the reducing gas. 
     In the embodiment mentioned above, a throttle valve may be connected to the reducing gas feeding portion  23 . The internal pressure P of the chamber  17  may be gradually increased by feeding the reducing gas to the chamber  17  by the throttle valve. 
     In the embodiment mentioned above, the gas atmosphere of the chamber  17  at the time of heating or cooling the solder is the reducing atmosphere having 100% hydrogen. This may be changed, for example, to a mixed gas atmosphere in which hydrogen gas, that is, reducing gas is set to 3%, and the remainder is nitrogen gas, that is, inert gas. 
     The reducing gas is not limited to hydrogen gas, but may be, for example, gas of which the composition includes formaldehyde. 
     The heating system of the solder is not limited to the high-frequency induction heating by the high-frequency heating coils  28 , but may be structured, for example, such that a heating device is provided within the chamber  17 . Further, a heat transfer medium may be circulated in the heat sink  13 . The heat sink  13  may heat the solder by transferring the heat to the solder sheet  33 . 
     The soldering apparatus HK may be provided with an ambient atmosphere regulating device regulating an internal atmosphere of the chamber  17 . The ambient atmosphere regulating device is connected to each of the on-off valves  23   b ,  24   b ,  25   b  and  26   b  and the vacuum pump  25   c . The ambient atmosphere regulating device controls the reducing gas feeding portion  23 , the inert gas feeding portion  24 , the vacuum portion  25  and the gas discharge portion  26 . As a result, it is possible to feed reducing gas and inert gas to the chamber  17 , and it is possible to discharge gas and air from the chamber  17 . 
     The joined object  93 , to which the semiconductor elements  12  are soldered, may be a circuit board  11  to which no heat sink  13  is joined. In this case, the semiconductor device including the circuit board  11  and the semiconductor elements  12  is manufactured in the chamber  17 . The number of the circuit boards  11  included in the semiconductor module  100  is not limited to six, but may be changed. 
     The lid  19  may be detachably mounted to the box main body  18 , or may be structured as an opening and closing type. 
     The portion facing the high-frequency heating coil  28  in the lid  19  may be formed by an electrically insulating material other than glass, for example, ceramics or resin. In order to ensure that the lid  19  has a strength that stands against the atmospheric pressure difference between the inside and the outside of the chamber  17 , it is preferable that the lid  19  be structured by a compound material, for example, of glass fiber and resin, that is, a glass fiber reinforced plastic (GFRP). Further, the lid  19  may be formed by a nonmagnetic metal. In the case where the lid  19  is formed by magnetic metal, it is preferable to employ a material having a higher electric resistivity than the weights  35 . The lid  19  may be structured by a compound material of metal and insulating material. It is preferable to arrange an electromagnetic steel plate of a ferromagnetic material in a portion immediately above the weight in order to effectively introduce magnetic flux to the weights  35 . 
     The weights  35  are not limited to integrated parts each formed by machining a material. Each weight  35  may be one assembly formed by joining a plurality of divided bodies. 
     In place of the weights  35 , a plurality of sub weights each corresponding to one semiconductor element  12  may be employed. Specifically, four sub weights are prepared in correspondence to four semiconductor elements  12  joined to one circuit board  11 . Each of the sub weights is arranged immediately above the corresponding semiconductor element  12 . 
     The component of the solder sheet  33  is not limited to the embodiment mentioned above. In order to inhibit voids from being generated, it is preferable to pressurize the molten solder, that is, the solder in the molten state, and the component of the solder sheet  33  is not limited. 
     In the embodiment mentioned above, the reducing gas feeding portion  23  connected to the gas inlet of the chamber  17  has the pressure reducing valve  23   c . The gas discharge portion  26  connected to the outlet of the chamber  17  has the throttle valve  26   c . However, the layout of the pressure reducing valve and the throttle valve may be changed. For example, the reducing gas feeding portion  23  may have the pressure reducing valve  23   c  and a throttle valve, and the gas discharge portion  26  may also have a pressure reducing valve and the throttle valve  26   c . Inversely to the embodiment mentioned above, the reducing gas feeding portion  23  may have only a throttle valve, and the gas discharge portion  26  may have only a pressure reducing valve. Only the reducing gas feeding portion  23  may have one of the pressure reducing valve and the throttle valve. 
     In this case, the pressure reducing valve can keep the internal pressure P of the chamber  17  constant. In the case where the throttle valve  26   c  is provided without the provision of the pressure reducing valve  23   c , it is possible to gradually increase the internal pressure P of the chamber  17  by setting the flow rate of the gas fed to the chamber  17  higher than the flow rate of the gas discharged from the chamber  17 . The pressure regulating portion connected to the inlet of the chamber  17  serves as the first pressure regulating portion. The pressure regulating portion connected to the outlet of the chamber  17  serves as the second pressure regulating portion.