Abstract:
A laser welding method using a laser welding jig having a plurality of pressing parts, includes a step of placing a second member on a first member; a step of pressing the second member with the plurality of pressing parts in a direction toward the first member to thereby form a gap between the first member and the second member at most 300 μm; and a first welding step of laser-welding the first member and the second member by irradiating on a surface of the second member at a location between the pressing parts with laser light while conducting the step of pressing.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    The present application is a divisional application of U.S. Ser. No. 14/528,537, filed on Oct. 30, 2014, which claims priority to, Japanese Patent Application No. 2013-262226, filed on Dec. 19, 2013, contents of which are incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to a laser welding method for achieving high joining strength between members of the semiconductor device. 
         [0004]    2. Description of the Related Art 
         [0005]      FIG. 15  shows a construction of an ordinary power semiconductor module  500 . This power semiconductor module  500  comprises: heat radiating base  51 , a direct copper bonding (DCB) substrate  54  having a rear surface conductor pattern  53  fixed to the heat radiating base  51  through a joining material  52  such as solder, a semiconductor chip  57  fixed onto a front surface conductor pattern  55  of the DCB substrate  54  through a joining material  56  such as solder, a terminal-inserted type resin casing  58  fixed to the heat radiating base  51 , and terminals  59 ,  60 , and  61  fixed to the terminal-inserted type casing  58 . 
         [0006]    The power semiconductor module  500  further comprises a surface electrode and a gate pad (which are not depicted) of the semiconductor chip  57 , and bonding wires  62  for connection among the surface electrodes, the gate pad, the front surface conductor pattern  55 , and terminals  59 ,  60 , and  61 . The power semiconductor module  500  contains sealing gel material  63  filling the terminal-inserted type resin casing  58 . 
         [0007]    The DCB substrate  54  is composed of a ceramic insulated substrate  54   a , a rear surface conductor pattern  53 , and a front surface conductor pattern  55 . 
         [0008]    In a power semiconductor module  500  today, high current carrying capacity and small size are required. Thus, the semiconductor chip  57  of the module is used at a high current density for carrying heavy current and for down-sizing. Consequently, one of the most important issues with the recent power semiconductor module  500  is effective radiation of heat generated in the semiconductor chip  57  in order to secure reliability in high power operation. 
         [0009]    To solve this issue, a lead-frame, which has a larger cross-sectional area of the current path and greater heat capacity than those of conventional bonding wires  62 , is used in a known power semiconductor nodule  57  in place of the bonding wires  62 . The lead-frame is partly utilized for wiring terminals and externally leading-out terminals. The wiring terminal is joined to the surface of the upper main electrode of the semiconductor chip  57  and utilized for a heat removing path. Thus, the heat generated in the semiconductor chip  57  is removed also from the upper surface thereof, as well as from the lower surface. 
         [0010]    Patent Document 1 discloses a means for improving heat removal and averaging the temperature distribution on the chip. The means is composed of a heat spreader with high thermal conductivity joined on the upper surface of the semiconductor chip to spread the heat in the central region of the semiconductor chip toward surroundings. 
         [0011]    In the wiring construction as described above, the joining process between the externally leading out terminal and the wiring terminal using a lead frame is carried out by means of laser welding as disclosed in Patent Document 2 as well as resistance welding and ultrasonic bonding. The laser welding joins two materials to be welded by first melting the materials with the energy of laser light and then cooling down them to a solid state. The laser welding has a characteristic feature, unlike the resistance welding and ultrasonic bonding, that the welding process can be carried out without contacting the joining apparatus the materials to be welded. Known laser welding techniques include seam welding and spot welding. In the seam welding, the material to be welded melts by continuously irradiating laser light, and in the spot welding, the material to be welded melts by irradiating high power pulse laser light on a spot of the material. Because the spot welding uses laser light with a high energy density, deep weld penetration is achieved as compared with the seam welding. Thus, two sheets of relatively thick metallic plates can be welded putting one on the other. 
         [0012]    Patent Document 3 discloses laser welding at a plurality of places on lead-frame terminal and positioning of the material to be welded by preliminarily forming protruding parts and recessed parts at the position of laser welding of two sheets of materials. 
         [0013]    Patent Document 4 discloses laser welding in which the laser welding place of the lead frame terminal is exposed out of the sealing resin. 
         [0014]    Patent Document 5 discloses laser welding in which the lead frame to be welded is pressed and the vicinity of laser welding spot is pressed by a heat removing probe with a configuration of a circular or rectangular column. 
         [0015]    Patent Document 6 discloses plating on laser welding places and covering the semiconductor chip with sealing resin to prevent sputtered particles from scattering onto the semiconductor chip. 
         [0016]    Patent Document 7 discloses use of plating material of nickel, palladium, and gold. 
         [0017]    Patent Document 8 discloses locally reducing the thickness of a lead frame to decrease rigidity of the lead frame. 
         [0018]    Patent Document 9 discloses laser welding conducted through a light transmitting window of a slit. 
         [0019]    [Patent Document 1] Japanese Unexamined Patent Application Publication No. 2000-307058 
         [0020]    [Patent Document 2] Japanese Unexamined Patent Application Publication No. 2004-096135 
         [0021]    [Patent Document 3] Japanese Unexamined Patent Application Publication No. 2001-045634 (FIGS. 1 through 7, in particular) 
         [0022]    [Patent Document 4] Japanese Unexamined Patent Application Publication No. 2007-265962 (FIG. 2 and FIG. 8, in particular) 
         [0023]    [Patent Document 5] Japanese Unexamined Patent Application Publication No. 2009-190067 (Paragraph 0007, paragraph 0008, and paragraph 0017, in particular) 
         [0024]    [Patent Document 6] International Patent Application Publication WO 2009-081723 (FIG. 1, FIG. 8, and Paragraph 0024, in particular) 
         [0025]    [Patent Document 7] Japanese Unexamined Patent Application Publication No. 2011-077278 (Paragraph 0026, in particular) 
         [0026]    [Patent Document 8] Japanese Unexamined Patent Application Publication No. 2008-098585 (Paragraph 0043, in particular) 
         [0027]    [Patent Document 9] Japanese Unexamined Patent Application Publication No. H07-014969 
         [0028]    Power semiconductor modules  500 , which can control a heavy current, are used in a wide range of industrial fields including motor control of electric vehicles and controllers for motor driving. In application in those fields, further down-sizing and enhancement of reliability of the power semiconductor module  500  are required. Down-sizing of the semiconductor chip  57  is being promoted in order to cope with down-sizing of the semiconductor module. This needs high current density in the semiconductor chip  57 . Thus, effective removal of the heat generated in the semiconductor chip  57  is a key issue for ensuring high reliability in high power operation. 
         [0029]    When a wiring terminal is joined with the main electrode surface of a semiconductor chip  57  and the wiring terminal is laser welded with the externally leading out terminal for the purpose of improving heat dissipation, the joined condition of the laser welding part affects the long term reliability of heat dissipation property and current carrying characteristic at the joint part. 
         [0030]    If the joining area of the laser welded part is small, not only the heat removal path is narrow, but the welded part would become a spot of abnormal heating on running a heavy electric current. 
         [0031]    A semiconductor device like the power semiconductor module  500  may suffer from thermal deformation in the whole module or a part thereof due to operation in an environment of violent temperature change and caused by Joule heating due to heavy current through the device. This thermal deformation can cause stress concentration at the laser welded part. 
         [0032]    The main factors that impair stability of the joint condition of the laser welded part are the positional shift between the externally leading out terminal and the wiring terminal, and the gap between these terminals. The positional shift and the gap occur depending on a variety of factors including dimensional tolerance of these members (the externally leading out terminal and the wiring terminal), tolerance in assembling process, ease of assembling work, and thermal deformation of the module. The positional shift between the overlapped terminals may cause penetration of laser light through the terminal or damage of the module. If the gap between the terminals is too large, the thermal energy by the laser light is not transferred to the lower terminal, causing failure of joining. 
         [0033]    Therefore, it is important for obtaining a stable laser welding part to minimize the positional shift and a gap between the terminals in the construction for laser welding the externally leading out terminal and the wiring terminal. 
         [0034]    The Patent Documents 1 through 9 all fail to mention about using a laser welding jig having a plurality of pressing parts and laser welding while pressing one of the overlapped members to be welded. 
       SUMMARY OF THE INVENTION 
       [0035]    The present invention has been made to solve the problems described above and an object of the present invention is to provide a laser welding method in which joining strength at a welding part is enhanced. 
         [0036]    To attain the above object, a semiconductor device of the invention comprises: a ceramic insulated substrate; a front surface conductor pattern fixed on a front surface of the ceramic insulated substrate; a semiconductor chip electrically connected to the front surface conductor pattern; a first member electrically connected to the semiconductor chip; and a second member laser-welded to electrically connect to the first member at one or more places or portions; wherein a gap between the first member and the second member at each of the places of laser welding is at most 300 μm. 
         [0037]    This structure of the invented semiconductor device enhances joining strength between the first member and the second member. 
         [0038]    In the invented semiconductor device, preferably, a distance between an outer periphery of one of the places of laser welding and an outer periphery of another of the places of laser welding is at most 2 mm. 
         [0039]    This structure has a high joining strength between the first member and the second member because the distance between the places of laser welding is small. 
         [0040]    In the invented semiconductor device, preferably, a thickness of the first member is at least 0.5 mm and a thickness of the second member is at most 1 mm. 
         [0041]    This structure allows the energy of laser light to be sufficiently transferred to the first member, achieving a high joining strength between the first member and the second member. 
         [0042]    In the invented semiconductor device, preferably, the first member and the second member are planar at least at a location around the piece of laser welding. 
         [0043]    This structure easily makes the gap between the first member and the second member be not larger than 300 μm. 
         [0044]    In the invented semiconductor device, preferably, each of the first and second member has an engaging part for positioning the first member and the second member, and the place of laser welding is located at a different position from the engaging part. 
         [0045]    This structure allows the first member and the second member to be positioned at the engaging part. Thus, positional shift scarcely occurs in the welding process. 
         [0046]    The invented semiconductor device, preferably, further comprises a sealing material for sealing the ceramic insulated substrate, the front surface conductor pattern, the semiconductor chip, and a part of the first member, wherein the places of laser welding are not sealed with the sealing material. 
         [0047]    This structure seals the ceramic insulated substrate, the front surface conductor pattern, and the semiconductor chip with the resin. Thus, the sputtered particles in the laser welding process do not generate a harmful effect such as short-circuiting. 
         [0048]    A laser welding method of the present invention uses a laser welding jig having a plurality of pressing parts and comprises: a step of placing a second member on a first member; a step of pressing the second member with the plurality of pressing parts toward the first member to make a gap between the first member and the second member be at most 300 μm; and a first welding step of laser welding the first member and the second member by irradiating a surface of the second member at a location between the pressing parts with laser light while conducting the step of pressing. 
         [0049]    This structure of the laser welding method achieves high joining strength between the first member and the second member. 
         [0050]    The invented laser welding method, preferably, further comprises a second welding step of laser welding the first member and the second member in a region with a gap between the first member and the second member of at most 300 μm after the first welding step. 
         [0051]    This structure of the laser welding method achieves high joining strength in the second welding step. The second welding step can be performed eliminating a pressing step. 
         [0052]    In the invented laser welding method, preferably, a distance between centers of the places of laser welding is at most 4 mm. 
         [0053]    The laser welding method achieves high joining strength because a distance between the laser welding places is short. 
         [0054]    In the invented laser welding method, preferably, the method includes laser welding the first member and the second member having engaging parts for positioning the first member and the second member, and the place of laser welding is different from the engaging parts. 
         [0055]    The laser welding method avoids direct deformation of the engaging part by laser welding, thus, positional shift between the first member and the second member scarcely occurs. 
         [0056]    In the invented laser welding method, preferably, the laser welding method conducts laser welding for such a semiconductor device that comprises a ceramic insulated substrate, a front surface conductor pattern fixed on a front surface of the ceramic insulated substrate, a semiconductor chip electrically connected to the front surface conductor pattern, a first member electrically connected to the semiconductor chip, and a second member electrically connected to the first member that is laser welded to the second member at one or more places, the method comprising a step of sealing the ceramic insulated substrate, the front surface conductor pattern, the semiconductor chip, and a part of the first member with a sealing material before the step of putting the second member on the first member. 
         [0057]    The laser welding method seals the ceramic insulated substrate, the front surface conductor pattern, the semiconductor chip with the resin. Thus, the sputtered particle in the laser welding process does not generate a harmful effect such as short-circuiting. 
         [0058]    A laser welding jig of the present invention comprises a plurality of pressing parts, a distance between the pressing parts being a length of a width of laser welded region plus distance in two sides of the welded region, each distance being in the range of 0.5 mm to 3 mm. 
         [0059]    This construction of the laser welding jig allows pressing a vicinity of the welding place preventing the pressing parts from contacting the region melted by the laser light in the laser welding process. Thus, positional shift between the first member and the second member scarcely occurs. 
         [0060]    The present invention provides a laser welding method, a laser welding jig, and a semiconductor device manufactured using the laser welding method, in which the joining strength of the welded parts is enhanced. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0061]      FIGS. 1A and 1B  show a step in a laser welding method of the first embodiment of the present invention. 
           [0062]      FIGS. 2A and 2B  show a step in a laser welding method following the steps of  FIGS. 1A and 1B . 
           [0063]      FIGS. 3A and 3B  show a step in a laser welding method following the steps of  FIGS. 2A and 2B . 
           [0064]      FIGS. 4A and 4B  show a step in a laser welding method following the steps of  FIGS. 3A and 3B . 
           [0065]      FIGS. 5A, 5B, and 5C  show a construction of an essential part of a laser welding jig of the second embodiment of the present invention. 
           [0066]      FIG. 6  is a sectional view showing an assembling step of a semiconductor device that is the third embodiment of the present invention. 
           [0067]      FIG. 7  is a sectional view showing an assembling step following the step of  FIG. 6  of the semiconductor device. 
           [0068]      FIG. 8  is a sectional view showing an assembling step following the step of  FIG. 7  of the semiconductor device. 
           [0069]      FIG. 9  is a sectional view showing an assembling step following the step of  FIG. 8  of the semiconductor device. 
           [0070]      FIGS. 10A, 10B, and 10C  are enlarged views of the part where the upper emitter terminal is put overlapped on the lower emitter terminal, each terminal having an engaging part. 
           [0071]      FIG. 11  shows a relationship between a gap between terminals and shear strength of the joined part. 
           [0072]      FIGS. 12A, 12B, and 12C  show a relationship between the gap between the terminals and the configuration of the welded part. 
           [0073]      FIG. 13  shows a sputtered particle generated in the case without resin sealing. 
           [0074]      FIG. 14  is a sectional view of an essential part of a semiconductor device of the fourth embodiment of the present invention. 
           [0075]      FIG. 15  shows a construction of a power semiconductor module of a general type. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0076]    Some preferred embodiments of the present invention will be described in detail in the following with reference to the accompanying drawings. 
       First Embodiment 
       [0077]      FIGS. 1A and 1B  through  FIGS. 4A and 4B  show a laser welding method according to the first embodiment of the present invention in the sequence of the laser welding method.  FIGS. 1A, 2A, 3A, and 4A  are plan views and  FIGS. 18, 2B, 38, and 4B  are sectional views cut along the line Y-Y in  FIGS. 1A, 2A, 3A , and  4 A, respectively. 
         [0078]    Referring to  FIGS. 1A and 1B , a second member at the upper position, which is a second member  2  to be welded, is placed on a first member at the lower position, which is a first member  1  to be welded, with the first and second members overlapping each other. A gap  10  between the two members  1  and  2  to be welded can be larger than 300 μm in some cases. The first and second members  1  and  2  to be welded are composed of a high thermal conductivity material exhibiting a thermal conductivity of at least 100 W/(m·K). The material can be selected from copper (Cu), a copper-molybdenum (Cu—Mo) alloy, a copper-tungsten (Cu—W) alloy, molybdenum (Mo), tungsten (W), an aluminum-silicon carbide (Al—SiC) alloy, and a silicon-silicon carbide (Si—SiC) alloy, for example. The surface of the second member  2  to be welded on which laser light is irradiated is preferably plated with electroless nickel-phosphorus (Ni—P) plating, electroless nickel-boron (Ni—B) plating, or electrolytic nickel (Ni) plating, for example. The plating is favorable because it exhibits improved absorption coefficient of laser light. 
         [0079]    Then as shown in  FIGS. 2A and 2B , a support column  40  of a laser welding jig  100  is pressed by a pressing device such as a pneumatically driven cylinder (not shown in the figure), for example. The laser welding jig  100  is moved to such a position that a welding part  5  (indicated in  FIGS. 3A and 3B ) falls between two claws  39 , which are pressing parts, formed at the tip of the support column  40 . The laser welding jig presses the second member  2  to be welded with a force  4  of about 3×9.8 N to make the gap  10  between the two members  1  and  2  to be welded be at most 300 μm. The pair of claws  39  is provided for making the gap  10  between the first and second members to be welded be sufficiently small in the overlapped state. The places of the claws that contact the second member  2  to be welded is a straight line with a length L of about 1 mm. In  FIGS. 2A and 2B , the two contact places are aligned on a common straight line, but they can be arranged in parallel facing each other. 
         [0080]    Then as shown in  FIGS. 3A and 3B , an irradiation surface  6  is irradiated with laser light  35  to execute laser welding. The laser light  35  has a pulse waveform. The pair of claws  39  keeps its configuration until the welded place solidifies. The welded place cools down and solidifies rapidly through heat diffusion toward the welded members  1  and  2  in the surroundings. 
         [0081]    If there are two or more welding places, as shown in  FIGS. 4A and 4B , after a first laser welding has finished, the pair of claws  39  is moved to a different place from the welded part  5  of first laser welding, and laser welding is conducted at a place next to the first welded part  5 . Laser welding of second and thereafter are carried out without pressing the second member  2  to be welded with the pair of claws  39 . Because the gap between the two members  1  and  2  has fixed to be not larger than 300 μm during the first laser welding, the laser welding of second and thereafter does not need to press the second member  2  to be welded with the pair of claws  39 . Places of laser welding of second and thereafter can be any place as long as the gap between the two members  1  and  2  to be welded is ensured at a distance not larger than 300 μm. 
         [0082]    According to the welding procedure, the step of pressing by the pair of claws  39  can be eliminated in laser welding of second and thereafter. Thus, a dead time can be shortened. When the places of laser welding of second and thereafter are set at a vicinity of within 2 mm from the first laser welding place, the distance between welded places  5  is sufficiently small, enhancing joint strength. 
         [0083]    Laser welding can of course be conducted by pressing with the pair of claws  39  after moving the pair of claws  39  and the laser light  35  together. 
         [0084]    Laser welding can be favorably carried out when the thickness of the first member  1  to be welded, which corresponds to the lower emitter terminal  30  in  FIG. 7 , is at least 0.5 m, and the thickness of the second member  2  to be welded, which corresponds to the upper emitter terminal  33  in  FIG. 8 , is at most 1 mm. If the thickness of the first member  1  to be welded is less than 0.5 mm, the welded place of the first welded member may be punched through to the back surface. If the thickness of the second member  2  to be welded is larger than 1 mm, the welded part does not sufficiently penetrate into the first welded member, lowering the welding strength. If the output power of the laser light is enhanced too high in the intention of increasing of the welding strength, larger heat is conducted to the claws  39 , resulting in deformation of the claws. 
         [0085]    The distance between centers of laser welded parts is desirably not larger than 4 mm. If the distance is larger than 4 mm, the joining strength between the first member and the second member tends to decrease. 
       Second Embodiment 
       [0086]      FIGS. 5A, 5B, and 5C  show a construction of an essential part of a laser welding jig  100  according to the second embodiment of the present invention, in which  FIG. 5A  is a perspective view of the whole structure,  FIG. 5B  is a side view of a pair of claws  39 , and  FIG. 5C  is a front view of the pair of claws  39 . 
         [0087]    The laser welding jig comprises a support column  40  and two claws  39  facing each other and connected to the support column  40 . The pair of claws  39  pushes the both ends of an area of a welding part. The edges of the pair of claws are linear like a knife edge, but blunt. The pair of claws  39  depicted in  FIGS. 5A, 5B, and 5C  has linear edges in the direction 90 degrees-revolved in the horizontal direction with respect to the direction of extension of the claws  39 . But the claws can be arranged in the direction rotated by 90 degrees from the direction depicted in  FIGS. 5A, 5B , and SC. 
         [0088]    As shown in  FIG. 3A , the distance T between the two claws  39  is wide enough not to interfere with laser light  35  and wider than the size of the welded part  5  that is a mark of fusion solidified after melted by the laser light  35 . More specifically, when the irradiation area  6  by the laser light  35  is 0.4 mm in diameter D 1 , the diameter D 2  of the welded part  5  is about 1.5 mm. Separating each of the claws  39  from the welded part  5  by a distance in the range from 0.5 mm to 3 mm, the interval T between the two claws  39  becomes 2.5 mm to 7.5 mm. The dimensions in these ranges are favorable because the pair of claws  39  does not interfere with the laser light  35 , which means the laser light  35  does not irradiate the claws, and the welded part  5  does not contact the claws  39 . The laser light  35  irradiates the place between the two claws  39 . The locations pressed by the claws  39  are in the vicinity of the irradiation area  6  of the laser light  35 , and the two claws  39  press the vicinity of the region to become the welded part  5  of the second member  2  to be welded to make the gap between the member  1  to be welded and the member  2  to be welded be at most 300 μm. In this arrangement, the laser light  35  irradiates the area between the two claws  39  to laser-weld the welding members  1  and  2 . 
         [0089]    If the interval T between the two claws  39  is too wide, the first member  1  to be welded and the second member  2  to be welded are insufficiently close contacting each other and the gap  10  between the two members is wider than 300 μm, lowering the joining strength of the welded part  5 . Use of the laser welding jig  100  allows to press the vicinity of the welding part  5  and to narrow the gap  10  between the members  1  and  2  to be welded within 300 μm achieving a stable joint condition. 
       Third Embodiment 
       [0090]      FIGS. 6 through 9  show a procedure of assembling a semiconductor device  200  that is the third embodiment of the present invention, and are sectional views showing assembling steps given in the sequence of the steps. These figures show assembling steps of a semiconductor device in which a wiring terminal on a semiconductor chip is joined with an externally leading out terminal by means of laser welding. In the assembling steps, the laser welding method described previously is applied. 
         [0091]    Referring to  FIG. 6 , a semiconductor device  200  comprises a DCB substrate  24  having a rear surface conductor pattern  23  disposed on a heat radiating base  21  made of a high thermal conductivity material, for example copper or aluminum, intercalating a joining material  22  such as solder between them. In the procedure of assembling the semiconductor device  200 , a semiconductor chip  27  and a collector terminal  31  are arranged on a front surface conductor pattern  25  formed on the surface of the DCB substrate  24  through joining materials  26  and  29 , respectively, such as solder. Then, a lower emitter terminal  30  is arranged on a main electrode surface (not depicted) of the semiconductor chip  27  through a joining material  28 . This lower emitter terminal  30  is a wiring terminal having a thickness of about 1.5 mm, for example. After heated to melt, the joining materials  22 ,  26 ,  28 , and  29  are cooled dad solidified to provide a monolithic body including the above mentioned components. The semiconductor chips  27  can be an insulated gate bipolar transistor (IGBT) and a free-wheeling diode (FWD) chip. The semiconductor chips  27  are electrically connected to the lower emitter terminal  30  through the joining material  28 . The lower emitter terminal  30  is a wiring terminal, which is a first member or a first member to be welded, and made of a material, for example copper, exhibiting high electric conductivity and high thermal conductivity. The DCB substrate  24  is composed of a ceramic insulated substrate  24   a , which is an insulated substrate made of ceramic, and a rear surface conductor pattern  23  and a front surface conductor pattern  25 . 
         [0092]    Then, as shown in  FIG. 7 , the components assembled to a state of  FIG. 6  are sealed with a resin sealing material  32 . The resin sealing material  32  can be an epoxy resin. The components are surrounded by a wall arranged at the periphery of the heat radiating base  21  and sealed with liquid state epoxy resin. The resin is cured to seal the components. Here, the upper surface of the lower emitter terminal  30  and the upper part of the collector terminal  31  are not sealed and are remaining exposed to the air. The semiconductor chips  27  and the joining material  28  are sealed with the resin. 
         [0093]    The resin sealing material  32  seals the upper surface of the heat radiating base  21 , the DCB substrate, the semiconductor chip  27 , and the front surface conductor pattern  25 . The surface  32   a  of the resin sealing material  32  is disposed not to contact the rear surface  30   a  of the lower emitter terminal  30 . If the surface  32   a  of the resin sealing material  32  is contacting the rear surface  30   a  of the lower emitter terminal  30 , the heat dissipation from the rear surface  30   a  of the lower emitter terminal  30  in the laser welding process is restricted, which expands the melted region and the melted region may unfavorably penetrate through the lower emitter terminal  30 . 
         [0094]    The step of resin sealing can be omitted in the case of rare sputtering particles in the laser welding process. 
         [0095]    Then as shown in  FIG. 3 , an upper emitter terminal  33  is put overlapping on the lower emitter terminal  30  indicated in  FIG. 7 . The upper emitter terminal  33  is an externally leading out terminal, which is a second member or a second member to be welded. The thickness of the upper emitter terminal  33  is 1.0 mm, for example. At this stage, the gap  10  between the upper and lower emitter terminals  30  and  33  may be larger than 300 μm. The upper emitter terminal  33  is made of a material, for example copper, exhibiting high electrical conductivity and high thermal conductivity. The surface of the upper emitter terminal  33  is plated with electrolytic nickel, for example. 
         [0096]    Then as shown in  FIG. 9 , the surface of the upper emitter terminal  33  is pressed with a pair of claws  39  of a laser welding jig  100  to make the gap  10  between the upper and lower emitter terminals  30  and  33  be at most 300 μm. Subsequently, laser light irradiates the surface of the upper emitter terminal  33  between the two claws  39  to conduct laser welding of the upper emitter terminal  33  and the lower emitter terminal  30 . When second laser welding is conducted at a nearby place, the gap  10  between the upper emitter terminal  33  and the lower emitter terminal  30  has already been made at a dimension not larger than 300 μm. Thus, the surface of the upper emitter terminal  33  is not necessarily pressed with the pair of claws  39 , although the laser welding can be conducted while pressing the upper emitter terminal  33  with the pair of claws  39 . 
         [0097]    The distance between the outer peripheries of laser welded places is preferably at most 2 mm. If the distance is larger than 2 mm, the gap between the first member and the second member is liable to increase, which decreases the joining strength of the first and second members. 
         [0098]      FIGS. 10A, 10B, and 10C  are enlarged views of the place where the upper emitter terminal  33 , a second member, and the lower emitter terminal  30 , a first member, are put overlapping with each other, in which  FIG. 10A  is a sectional view of the upper emitter terminal  33  having a protruding part  36  and the lower emitter terminal  30  having a recessed part  37 ,  FIG. 10B  is a sectional view showing the protruding part  36  of the upper emitter terminal  33  is engaged with the recessed part  37  of the lower emitter terminal  30 , and  FIG. 10C  is a sectional view showing the laser welding process while the upper emitter terminal  33  is pressed by the pair of claws  39 . The protruding part  36  formed on the upper emitter terminal  33  and the recessed part  37  formed on the lower emitter terminal  30  allows positioning of the upper emitter terminal  33 , avoiding positional shift of the emitter terminals in the assembling process. 
         [0099]    In order to reduce the gap  10  between the emitter terminals  30  and  33 , the surface of the upper emitter terminal  33  is pressed with the pair of claws  39  of the laser welding jig  100 , and laser light irradiates the upper emitter terminal  33  in the configuration with the gap  10  between the upper emitter terminal  33  and the lower emitter terminal  30  disposed under the upper emitter terminal  33  being at most 300 μm. Thus, a good welded part  34  is obtained. 
         [0100]      FIG. 11  shows a relationship between the gap  10  between the emitter terminals  30  and  33  and shear strength of the welded part  34 . An upper emitter terminal  33  and a lower emitter terminal  30  are disposed overlapping with each other intercalating a spacer between the terminals. Laser welding is conducted in this configuration in which the gap  10  between the emitter terminals  30  and  33  is controlled by the spacer.  FIG. 11  shows shear strength in terms of normalized value with reference to the value in the condition of direct close contact, which means without a gap. The shear strength here is a tensile strength when the terminals  30  and  33  are pulled in the longitudinal direction, and represents a joint strength of the welded part  34 . 
         [0101]      FIG. 11  shows that the shear strength is larger than the one in the case of no gap when the gap  10  between the terminals  30  and  33  is in the range of up to 300 μm. At the gap of 400 μm, the shear strength decreased by about 30% as compared with the case of no gap, and at the gap of 500 μm, laser welding becomes impossible. In order to obtain a shear strength larger than the value in the case of direct contact, which means no gap, the gap between the terminals  30  and  33  needs to be at most 300 μm. It has been demonstrated that a shear strength larger than the one in the case of direct contact can be readily obtained by laser welding using the laser welding jig  100  of the invention as shown in  FIGS. 5A, 5B , and  5 C. 
         [0102]    A gap larger than 300 μm causes adverse effect of increased heat resistance at the welded part  34  as well as decrease in the shear strength. Therefore, the gap between the terminals  30  and  33  is necessarily at most 300 μm. 
         [0103]      FIGS. 12A, 12B, and 12C  show relationship between the gap  10  between the terminals  30  and  33  and the configuration of the welded part  34 , in which  FIG. 12A  is a sectional view without gap  10  between the terminals  30  and  33 ,  FIG. 12B  is a sectional view with the gap  10  between the terminals  30  and  33  not larger than 300 μm, and  FIG. 12C  is a sectional view with the gap  10  between the terminals  30  and  33  larger than 300 μm. 
         [0104]    Referring to  FIG. 12A , in the case of no gap, which is a direct contact case, the welded part  34   a  cannot expand at the boundary  49  between the terminals  30  and  33 . 
         [0105]    Referring to  FIG. 12B , in the case the gap between the terminals  30  and  33  is not larger than 300 μm, the welded part expands at the gap, increasing the area of the welded part  34   b . As a consequence, the shear strength becomes larger than the case of no gap. 
         [0106]    Referring to  FIG. 12C , when the gap  10  between the terminals  30  and  33  exceeds 300 μm, the welded part  34   c  at the gap  10  shrinks in the center portion of the welded part, and thus, the shear strength is lowered. 
         [0107]      FIG. 13  shows scattering of sputtered metallic particles  42  in the laser welding process without resin sealing. The sputtered particles  42  attach onto the semiconductor chip  27  and the front surface conductor pattern  25 , which may cause failure of electric insulation. This failure can be avoided by sealing the semiconductor chip  27  with a resin up to the level of joining material  28 . 
         [0108]    Stable joining strength is ensured by pressing the upper emitter terminal  33  using the laser welding jig  100  of the invention to make the gap between the upper emitter terminal  33  and the lower emitter terminal  30  be at most 300 μm, and joining the lower emitter terminal  30  and the upper emitter terminal  33  according to the laser welding method of the invention. 
         [0109]    The lower emitter terminal  30  and the upper emitter terminal  33  are made of a high thermal conductivity material exhibiting a thermal conductivity of at least 100 W/(m·K). Preferable materials include: copper (Cu), a copper-molybdenum (Cu—Mo) alloy, a copper-tungsten (Cu—W) alloy, molybdenum (Mo), tungsten (W), an aluminum-silicon carbide (Al—SiC) alloy, and a silicon-silicon carbide (Si—SIC) alloy, for example. 
         [0110]    The thickness of the lower emitter terminal  30  is preferably at least 0.5 mm, and the thickness of the upper emitter terminal  33  is at most 1 mm. If the thickness of the lower emitter terminal  30  less than 0.5 mm causes the laser welded part penetrates through the lower emitter terminal  30 . The thickness of the lower emitter terminal  30  is more preferably, not smaller than 0.8 mm. 
         [0111]    The thickness of the upper emitter terminal  33  thicker than 1 mm increases the proportion of the energy of laser welding consumed in the upper emitter terminal  33  and decreases the proportion of the energy consumed in the lower emitter terminal  30 . As a consequence, the area of melted part of the lower emitter terminal decreases resulting in lowered shear strength. 
         [0112]    Both of the lower emitter terminal  30  and the upper emitter terminal  33 , or only the upper emitter terminal  33 , which is irradiated by laser light is preferably plated with a material that absorbs much energy of laser light. Preferable plating includes electroless nickel-phosphorus (Ni—P) plating, electroless nickel-boron (Ni—B) plating, and electrolytic nickel (Ni) plating, for example. The plating is favorable because it improves absorption coefficient of laser light. 
       Fourth Embodiment 
       [0113]      FIG. 14  is a sectional view of an essential part of a semiconductor device  200  according to the fourth embodiment of the present invention. The semiconductor device  200  is manufactured according to the laser welding method of the first embodiment and using the laser welding jig  100  of the second embodiment. 
         [0114]    The semiconductor device  200  comprises a heat radiating base  21 , a DCB substrate  24  having a rear surface conductor pattern  23  that is fixed through a joining material  22  such as solder to the heat radiating base  21 , and a semiconductor chip  27  fixed to a front surface conductor pattern  25  through a joining material  26  such as solder. The DCB substrate  24  is composed of a ceramic insulated substrate  24   a , a rear surface conductor pattern  23  fixed on the rear surface of the ceramic insulated substrate  24   a , and a front surface conductor pattern  25  fixed on the front surface of the ceramic insulated substrate  24   a . The semiconductor device  200  further comprises a lower emitter terminal  30  fixed to the surface electrode of the semiconductor chip  27  through a joining material  28  such as solder, an upper emitter terminal  33  fixed to the lower emitter terminal by means of laser welding, and a collector terminal  31  fixed to the front surface conductor pattern  25  through a joining material  29  such as solder. The semiconductor device  200  also comprises a resin sealing material  32  that seals the whole device excepting the side face and rear face of the heat radiating base  21 , the front surface of the lower emitter terminal  30 , a tip portion of the collector terminal  31 , and the upper emitter terminal  33 , which are exposed to the air. 
         [0115]    A high shear strength is obtained by ensuring the gap  10  between lower emitter terminal  30  and the upper emitter terminal  33  to be within 300 μm. Positional shift of the upper emitter terminal  33  in assembling process of the semiconductor device  200  is avoided by forming a recessed part  37  on the lower emitter terminal  30  and a protruding part  36  on the upper emitter terminal  33  and positioning a welding place of the terminals  30  and  33  with the aid of the protruding part and the recessed part. Alternatively, positional shift of the upper emitter terminal  33  in assembling process of the semiconductor device  200  is likewise avoided by forming a protruding part on the lower emitter terminal  30  and a recessed part on the upper emitter terminal  33  and positioning a welding place of the terminals  30  and  33  with the aid of the protruding part and the recessed part. The recessed part can be a through hole. 
         [0116]    In some cases in the semiconductor device  200 , two linear compression marks with a length of about 1 mm may be formed on the upper emitter terminal  33  at two places on either side of the welded part  34 . 
       DESCRIPTION OF SYMBOLS 
       [0000]    
       
           1 : first welded member, first member 
           2 : second welded member, second member 
           4 : force 
           5 ,  34 : welded part 
           6 : irradiated surface 
           10 : gap 
           21 ,  51 : heat radiating base 
           22 ,  26 ,  28 ,  29 ,  52 ,  56 : joining material 
           23 ,  53 : rear surface conductor pattern 
           24 ,  54 : DCB substrate 
           24   a ,  54   a : ceramic insulated substrate 
           25 ,  55 : front surface conductor pattern 
           27 ,  57 : semiconductor chip 
           30 : lower emitter terminal, first member 
           30   a : rear surface 
           31 : collector terminal 
           32 : resin sealing material 
           32   a : front surface 
           33 : upper emitter terminal, second member 
           35 : laser light 
           36  protruding part 
           37 : recessed part 
           39 : claw, pressing part 
           40 : support column 
           42 : sputtered particle 
           49 : boundary region 
           58 : terminal-inserted type resin casing 
           59 ,  60 ,  61 : terminal 
           62 : bonding wire 
           63 : gel sealing material 
           100 : laser welding jig 
           200 : semiconductor device 
           500 : power semiconductor module 
         D 1 , D 2 : diameter 
         L: length 
         T: interval