Abstract:
A semiconductor device is provided which includes a plurality of semiconductor units each including a semiconductor chip, a support plate, a contact terminal block and a positioning guide, a flat package including a pair of common electrode plates and an insulating outer sleeve, and positioning and thermal-stress reducing means for positioning the support plates of the semiconductor elements in a horizontal direction, without interfering with the positioning guides, and reducing a thermal stress applied to a peripheral portion of a contact interface between the contact terminal block and the semiconductor chip due to heat generated during intermittent flow of current through a load. The contact terminal block is disposed on a first main electrode of the semiconductor chip, while the support plate is secured to a second main electrode of the chip, such that the semiconductor chip is placed under pressure between the contact terminal block and support plate that also serve as conductors and heat radiators. The semiconductor units are incorporated in the flat package, such that the pair of common electrode plates are in pressure contact with the contact terminal blocks and the support plates, respectively, of all of the semiconductor units.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a semiconductor device, and in particular to a pressure contact type semiconductor device in which a plurality of insulated gate bipolar transistors (IGBT) are incorporated in the same flat package. 
     BACKGROUND OF THE INVENTION 
     Insulated gate bipolar transistors (IGBT) have been widely used as power switching devices in such applications, for example, as motor PWM (pulse width modulation) control inverters. The IGBT is a voltage driven type device, which is easy to handle as compared with a current driven type device, and therefore there has been a strong demand in the market for an increase in the current capacity of the IGBT. To meet with the demand of the market, a modular structure has been employed in which a plurality of IGBT chips are integrated together within the same package. In a MOS control type device, such as an IGBT, an emitter electrode and a gate electrode are located side by side on the main surface of a semiconductor chip. In general, when such IGBT chips are incorporated or mounted in a package or case, a collector electrode formed on the lower surface of each chip is soldered to a support plate as a metallic base that also serves as a heat radiator, and connected to an external conductor, while the emitter electrode and gate electrode are separately connected to external lead terminals through bonding wires, or the like. The bonding wires consist of aluminum lead wires having a diameter of about 300 μm, and therefore heat dissipation mainly takes place at one surface on which the collector electrode is formed, thus resulting in reduced efficiency and reduced operating reliability. 
     In view of the above situation, there has been proposed a pressure contact type IGBT in which a plurality of IGBT chips for achieving a larger current capacity are mounted in a flat package having an insulating outer sleeve made of a ceramic material, in a similar manner to conventional thyristors or GTO (gate turn-off) thyristors. 
     Here, a known IGBT having a flat structure as proposed in Japanese Patent Application No. 8-79743 will be described. 
     FIG. 4 shows a cross section of a principal part of the known IGBT having a flat structure. In FIG. 4, a plurality of IGBT chips  102  are mounted in a single package  101 . Each IGBT chip  102  has an emitter contact terminal block  104  made of molybdenum, which block is accurately positioned by a positioning guide  103  having an insulating property. The emitter contact terminal blocks  104  of the respective IGBT chips  102  are in contact with a single thermal buffer plate  105  also made of molybdenum, and the buffer plate  105  is in contact with an upper common electrode plate  106  made of copper. The collector sides of the IGBT chips  102  are soldered to a single collector substrate  107  made of molybdenum, and the collector substrate  107  is in contact with a lower common electrode plate  108  made of copper. The upper common electrode plate  106  and the lower common electrode plate  108  are secured to an insulating sleeve  109  made of a ceramic material, to thus provide a package  101 . A pressure contact type semiconductor device having desired current-carrying characteristics is fabricated by applying a pressure from the upper and lower sides of the upper common electrode plate  106  and lower common electrode plate  108 , respectively, to the whole assembly of the thermal buffer plate  105 , emitter contact terminal blocks  104 , IGBT chips  102 , and the collector substrate  107 . 
     The pressure contact type semiconductor device having the flat structure as described above exhibits excellent electric conductivity and thermal conductivity due to the use of copper for the upper common electrode plate  106  and lower common electrode plate  108 . Also, the emitter contact terminal blocks  104  and collector substrate  107  are made of molybdenum whose coefficient of thermal expansion is close to that of silicon, and therefore thermal stresses applied to the IGBT chips  102  can be desirably alleviated or reduced. Furthermore, the thermal buffer plate  105  that is made of hard molybdenum and inserted between the emitter contact terminal blocks  104  and the upper common electrode plate  106  can suppress or reduce extraordinary strains, which would otherwise arise in the IGBT chips  102  when the emitter contact terminal blocks  104  bite into and are pulled by the upper common electrode plate  106  having a large coefficient of thermal expansion during heat cycles. Thus, the pressure contact type IGBT has a high resistance to power cycles (in which current intermittently flows through a load), which is five times or more as high as that of the bonding-type modular structure. 
     The above type of IGBT is constructed such that a plurality of IGBT chips are secured to the single collector substrate  107 . Where at least one defective IGBT chip is present among the IGBT chips mounted in the package, the whole IGBT becomes a defective, resulting in a reduced yield in the manufacture of IGBT. In order to improve the manufacturing yield, another type of IGBT having a flat structure as disclosed in Japanese Patent Application No. 7-328462 has been proposed. 
     FIG. 5 shows a cross section of a principal part of the IGBT as disclosed in the above-identified application. As shown in FIG. 5, each IGBT chip  111  is soldered to a collector substrate  112  to provide a semiconductor element. The semiconductor element and an emitter contact terminal block  114  are fitted in a positioning guide  113 , so as to provide a single semiconductor unit. A plurality of such semiconductor units are arranged in contact with each other within a frame  115 , and an upper common electrode plate  116  and a lower common electrode plate  117  are disposed on the upper and lower surfaces of the semiconductor units. Also, outer peripheral portions of the upper and lower common electrode plates  116 ,  117  are closed by an insulating outer sleeve  118 , to thus provide a single IGBT. 
     Where at least one defective is present among the plural semiconductor units arranged in the above manner, the defective semiconductor unit may be easily replaced by a non-defective one, so as to provide a non-defective IGBT. This leads to an improved yield in the manufacture of IGBT, and reduced manufacturing cost. 
     The flat type IGBT having the known structure as shown in FIG. 4, however, suffers from a problem that the IGBT chips are broken in a power cycle test. More specifically, the coefficient of thermal expansion of the upper and lower common electrode plates having large areas is different from the coefficient of thermal expansion of the collector substrate similarly having a large area, and the emitter contact terminal block makes a seesaw motion on the surface of the corresponding IGBT chip, due to the difference in the coefficients of thermal expansion, thus causing a thermal stress at the interface between the emitter contact terminal block and the IGBT chip. In particular, the thermal stress is concentrated at the surface of the IGBT chip in the vicinity of the outer periphery or edge of the emitter contact terminal block, with the result of breakage of the IGBT chip. While the thermal stress is more or less suppressed by the thermal buffer plate between the emitter contact terminal blocks and the upper common electrode plate, the IGBT chips located in the peripheral portion of the device are particularly subjected to large thermal stresses, which cause a reduction in the service life of the IGBT. 
     In the IGBT as shown in FIG. 5, on the other hand, the collector substrate that contacts with the lower common electrode plate is provided for each semiconductor unit, and this structure, which is advantageous in terms of an improved yield in the manufacture of IGBT, provides a secondary effect of reducing thermal stresses and improving the resistance to power cycles. Where the semiconductor units are assembled together at room temperature, the positioning guides  113  of the respective semiconductor units are fitted in the frame  115  such that the guides  113  are in contact with each other, and thus positioned by themselves in horizontal directions. If the coefficient of thermal expansion of the positioning guides  113  is different from that of the upper and lower common electrode plates, the positioning guide of a certain semiconductor unit pushes the positioning guides of its adjacent semiconductor units in horizontal directions, whereby stresses are applied to the corresponding IGBT chips through the emitter contact terminal blocks, which may result in breakage of the IGBT chips. 
     SUMMARY OF THE INVENTION 
     The present invention was developed in the light of the above-described situations. It is therefore an object of the present invention to provide a semiconductor device including a plurality of semiconductor elements that are arranged side by side, wherein thermal stresses applied from common electrode plates are advantageously reduced, and the semiconductor elements can be positioned in horizontal directions without contacting with each other. 
     To accomplish the above object, the present invention provides a semiconductor device comprising a plurality of semiconductor units, a flat package, and positioning and thermal-stress reducing means. Each of the semiconductor units includes a semiconductor element, a contact terminal block, and a positioning guide, and the semiconductor element includes a semiconductor chip and a support plate. The semiconductor chip includes a first main electrode and a control electrode disposed on a first main surface thereof, and a second main electrode disposed on a second main surface thereof. The support plate is secured to a surface of the second main electrode for applying a pressure to the semiconductor chip and serving as a conductor and a heat radiator, and the contact terminal block is disposed on a surface of the first main electrode of the semiconductor chip, for applying a pressure to the semiconductor chip and serving as a conductor and a heat radiator. The positioning guide serves to position the support plate and the contact terminal block on the surface of the first main electrode. The flat package comprises first and second common electrode plates and an insulating outer sleeve that surrounds the first and second common electrode plates, and incorporates therein the semiconductor units that are arranged side by side. The first and second common electrode plates are in pressure contact with the contact terminal blocks and the support plates, respectively, of all of the semiconductor units. The positioning and thermal-stress reducing means is provided for positioning the support plates of the semiconductor elements in a horizontal direction, without interfering with the positioning guides, and reducing a thermal stress applied to a peripheral portion of a contact interface between the contact terminal block and the semiconductor chip due to heat generated during intermittent flow of current through a load. 
     In the semiconductor device constructed as described above, the semiconductor elements are positioned by the positioning and thermal-stress reducing means such that these elements are spaced from each other, and therefore the semiconductor elements do not interfere with each other even when the temperature changes cyclically in a power cycle test, for example. With this arrangement, the semiconductor elements are prevented from being pressed by adjacent semiconductor elements, and thus thermal stresses applied from the common electrode plates can be suppressed or reduced. Since the individual chips are allowed to move in accordance with thermal expansion of common electrodes even upon cyclic changes in the temperature in a power cycle test, it is possible to prevent large thermal stresses from being applied from the common electrode plates to the semiconductor chips, particularly those located in the peripheral part of the device. Thus, the present invention provide a pressure contact type semiconductor device having a high resistance to power cycles, and a high operating reliability. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will be described in greater detail with reference to preferred embodiments thereof and the accompanying drawings, wherein: 
     FIG.  1 (A) is a cross-sectional view of a principal part of an IGBT according to the first embodiment of the present invention, and 
     FIG. 1 (B) is a plan view showing a positioning frame used in the IGBT of the first embodiment; 
     FIG.  2 (A) is a cross-sectional view of a principal part of an IGBT according to the second embodiment of the present invention, and 
     FIG.  2 (B) is a plan view showing a positioning frame used in the IGBT of the second embodiment; 
     FIG.  3 (A) is a cross sectional view of a principal part of an IGBT according to the third embodiment of the present invention, and 
     FIG.  3 (B) is a plan view showing positioning springs mounted in the IGBT of the third embodiment; 
     FIG. 4 is a view showing a cross section of a principal part of a known IGBT having a flat structure; and 
     FIG. 5 is a view showing a cross section of a principal part of another known IGBT having a flat structure. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Some preferred embodiments of the present invention, when applied to pressure contact type IGBT having a flat structure, will be described in detail. 
     FIG.  1 (A) is a cross-sectional view of a principal part of an IGBT according to the first embodiment of the present invention, and FIG.  1 (B) is a plan view of a positioning frame used in the IGBT of FIG.  1 (A). As shown in FIG.  1 (A), a plurality of IGBT chips  1  are respectively soldered to individual collector substrates  2  each serving as a support plate and made of molybdenum having a coefficient of thermal expansion close to that of silicon, such that each collector substrate  2  is in contact with a surface of a collector electrode as a second main electrode provided on a second main surface of the corresponding IGBT chip  1 . Thus, the IGBT chip  1  and the collector substrate  2  constitute a semiconductor element. An emitter contact terminal block  4  also made of molybdenum is accurately positioned by an insulating positioning guide  3 , on a surface of an emitter electrode as a first main electrode provided on a first main surface of the corresponding IGBT chip  1 . Thus, the IGBT chip  1 , collector substrate  2 , and the emitter contact terminal block  4  constitute an individual semiconductor unit. A single positioning frame  5  as shown in FIG.  1 (B) has a plurality of openings  6  formed at the locations of the individual semiconductor units. The semiconductor units are respectively fitted in the openings  6  of the positioning frame  5 , so that the units are positioned in horizontal directions while being spaced from each other at predetermined intervals, as shown in FIG.  1 (A). The plural semiconductor units thus positioned in the horizontal directions by the positioning frame  5  are sandwiched between upper thin plate  7  and lower thin plate  8  that serve as lids, and the outer peripheral portions of the upper and lower thin plates  7 ,  8  are supported by an insulating outer frame  9  so that the assembly of the semiconductor units is appropriately positioned in the vertical and horizontal directions. A gate liner  10  is provided in the outer peripheral portion of the insulating outer frame  9 , such that the gate liner  10  is connected to a gate electrode as a control electrode located beside the first main electrode on the first main surface of each IGBT chip  1 . The assembly of the semiconductor units is accurately positioned by the insulating outer frame  9 , and placed within a flat package  14  that consists of upper common electrode plate  11  and lower common electrode plate  12  that are made of copper, and an insulating sleeve  13  made of a ceramic material and disposed between the upper and lower electrode plates  11 ,  12 . In this manner, an IGBT having a flat structure is provided. Here, the collector substrate  2  and emitter contact terminal block  4  apply pressures to the corresponding IGBT chip  1 , and function to electrically connect the main electrodes of the IGBT chip  1  with the upper and lower thin plates  7 ,  8  while permitting heat dissipation. 
     In FIG. 1, the positioning guides  3  and the insulating outer frame  9  are illustrated as if each of these members were formed as an integral body, for the sake of simplification. The positioning guide  3 , however, is actually divided into a lower portion that houses the IGBT chip  1  and the collector substrate  2 , and an upper portion that positions the emitter contact terminal block  4 . The insulating outer frame  9  is also divided into a lower portion that houses the lower thin plate  8 , positioning frame  5 , semiconductor units, and the upper thin plate  7 , and an upper portion that presses the outer peripheral portion of the upper thin plate  7  from the above. 
     The positioning frame  5  is formed of a material (copper in this embodiment) whose coefficient of thermal expansion is equivalent to that of the upper and lower common electrode plates  11 ,  12 , and the upper thin plate  7  and the lower thing plate  8  are formed of a material (copper in this embodiment) whose coefficient of thermal expansion is also equivalent to that of the upper and lower electrode plates  11 ,  12 . The insulating outer frame  9  is made of a plastic having a high heat resistance. 
     In the present embodiment, the positioning frame  5  has sixteen openings  6  that receive sixteen semiconductor elements, as shown in FIG.  1 (B). The semiconductor elements include four flywheel diodes (that do not have control terminals) which are received in four openings  6  located in the inner part of the frame  5 , and twelve IGBT and flywheel diodes as needed, which are received in twelve openings  6  located in the peripheral part of the frame  5  and surrounding the above four openings  6 . In this case, the flywheel diodes are oriented such that the anode of each diode contacts with the corresponding emitter contact terminal block  4 . The gate electrodes of the IGBT chips are connected to the gate liner  10  provided on the insulating outer frame  9 . 
     In fabrication of the IGBT as described above, a solder sheet is sandwiched between the IGBT chip  1  and collector substrate  2  of each semiconductor element, and the corresponding emitter contact terminal block  4  is guided and positioned by the positioning guide  3 . In this state, a pressure soldering apparatus applies a pressure to the semiconductor units each consisting of the IGBT chip  1 , collector substrate  2 , and the emitter contact terminal block  4 , and heat them so that the IGBT chips  1  are secured to the corresponding collector substrates  2 . The pressure soldering apparatus is provided with a stopper that serves to limit the stroke or moving distance of a pressurizing member to a predetermined value, thereby to adjust the height of the semiconductor units at the same time. 
     In a power cycle test, current is repeatedly caused to flow through a load at certain time intervals while a pressure is applied to the upper common electrode plate  11  and the lower common electrode plate  12 , and the resistance to power cycles is evaluated based on the number of cycles counted until the service life of the IGBT expires due to temperature cycles caused by the intermittent current flow through the load. 
     As described above, each set of the IGBT chip  1 , collector substrate  2 , positioning guide  3  and the emitter contact terminal block  4  are assembled into a semiconductor unit, and the positioning of the individual units in horizontal directions is accomplished by the positioning frame  5  that does not cause thermal stresses. Further, the positioning of the semiconductor units in the vertical direction is accomplished by the upper thin plate  7  and lower thin plate  8  that do not cause thermal stresses, like the positioning frame  5 . Thus, the plural IGBT chips are arranged in rows and columns between the opposite common electrode plates having large areas. With this arrangement, even where the temperature changes cyclically in a power cycle test, the individual chips are allowed to move in accordance with thermal expansion of the common electrodes, and therefore thermal stresses from the common electrode plates can be prevented, particularly for the chips located in the peripheral portion of the device. The resulting pressure contact type semiconductor device exhibits a high operating reliability. 
     While the same material, i.e., copper, is used for forming the positioning frame  5  and the upper and lower common electrode plates  11 ,  12  in the present embodiment, other materials may be employed provided that these frame and electrode plates  5 ,  11 ,  12  have substantially the same coefficient of thermal expansion. TABLE 1 below shows some materials that can be used for the positioning frame  5 . 
     
       
         
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 Material 
                 Coefficient of Linear Expansion (10 −6  K −1 ) 
               
               
                   
               
             
             
               
                 Copper 
                 17.1 
               
               
                 Silver 
                 19.6 
               
               
                 SUS 304 
                 17.3 
               
               
                 SUS 316 
                 16.0 
               
               
                 Glass-filled polyimide resin 
                 20.0 
               
               
                 (filling rate 30%) 
               
               
                 Glass-filled polyimide resin 
                 14.0 
               
               
                 (filling rate 40%) 
               
               
                   
               
             
          
         
       
     
     As shown in TABLE 1 above, copper has a coefficient of linear expansion of 17.1×10 −6  K −1  and materials having equivalent coefficients of linear expansion include silver, stainless steel (SUS 304, SUS 316), and heat resistant plastics. In addition, copper alloys and silver alloys whose coefficients of linear expansion are equivalent to that of copper may also be used. Examples of heat resistance plastics include glass-filled polyimide resins (filling rate: 30%, 40%) whose coefficients of linear expansion are equivalent to that of copper. 
     While copper is used for the upper thin plate  7  and lower thin plate  8  in the present embodiment, the materials as indicated in TABLE 1, except the heat resistant plastics, may be used since the thin plates  7 ,  8  are required to show electric conductivity, as well as having a coefficient of linear expansion that is equivalent to that of the upper and lower common electrode plates  11 ,  12 . 
     FIG.  2 (A) is a cross-sectional view showing a principal part of IGBT according to the second embodiment of the present invention, and FIG.  2 (B) is a plan view of a positioning frame used in the IGBT. In FIG.  2 (A) and  2 (B), the same reference numerals as used in FIG. 1 are used for identifying corresponding elements, of which no detailed description will be provided herein. In the second embodiment, an individual positioning frame  15  made of copper as used for the upper common electrode plate  11  and lower common electrode plate  12  is disposed around each collector substrate  2 , and adjacent faces of the individual positioning frames  15  are brought into contact with each other, so that the individual semiconductor elements are accurately positioned in horizontal directions. Each of the positioning frames  15  is formed at its middle portion with an opening  16  in which the corresponding collector substrate  2  is inserted, as shown in FIG.  2 (B). 
     The positioning frames  15  provided for the respective semiconductor elements are disposed on the lower thin plate  8  within the insulating outer frame  9  such that the outer faces of the adjacent positioning frames  15  are held in contact with each other, so as to yield the same effect as provided in the first embodiment in which a single positioning frame  5  is used. The positioning frames  15  of the present embodiment may be used in common for different types of semiconductor devices. Where IGBTs having different numbers of chips are fabricated, for example, there is no need to prepare a plurality of types of positioning frames having different numbers of openings depending upon the number of chips mounted in the device. 
     FIG.  3 (A) is a cross-sectional view showing a principal part of an IGBT according to the third embodiment of the present invention, and FIG.  3 (B) is a plan view showing a part of the IGBT in which positioning springs are mounted. In FIGS.  3 (A) and  3 (B), the same reference numerals as used in FIG. 1 are used for identifying corresponding elements, of which no detailed description will be provided. In the third embodiment, positioning springs  20  are inserted between adjacent collector substrates  2 , and between the outermost collector substrates  2  and the inner wall of the insulating outer frame  9 , so as to position the individual semiconductor elements in horizontal directions. Each of the positioning springs  20  consists of two arc-shaped sheet springs that are bonded to each other at their opposite ends. The positioning springs  20  are inserted between the adjacent collector substrates  2 , and between the collector substrates and the inner wall of the insulating outer frame  9 , while a pressure is being applied to each positioning spring  20  in the direction of its thickness to thus deform the spring  20 . As a result, the positioning of the individual semiconductor elements in the horizontal directions is accomplished when the spring forces of the positioning springs  20  are balanced. 
     While some preferred embodiments of the present invention have been described above, the present invention is not limited to these particular embodiments. Although copper is used for forming the positioning frames  5 ,  15  so as to provide substantially the same coefficient of thermal expansion as that of the upper common electrode plate  11  and lower common electrode plate  12 , the positioning frames  5 ,  15  may also be made of a material, such as silicone rubber, that has a relatively small modulus of elasticity. With a decrease in the modulus of elasticity, the force due to a difference in the coefficients of thermal expansion is less likely to be produced even if the material has a relatively large coefficient of thermal expansion, and therefore thermal stresses can be alleviated or reduced. While sixteen semiconductor chips are mounted in the same flat package in the illustrated embodiments, the number of semiconductor chips may be changed as desired depending upon the required current capacity. Further, the semiconductor chips used in the present invention are not limited to IGBT chips as illustrated above, but may be other types of semiconductor chips, such as MOS control thyristors, and MOS transistors. 
     According to the present invention as described above, a plurality of semiconductor chips, such as IGBT and flywheel diodes, are mounted in a flat package, such that each of the semiconductor chips is secured to a collector substrate provided for each chip, and an emitter contact terminal block is positioned relative to the corresponding semiconductor chip and collector substrate by means of a positioning guide, to thus provide an individual semiconductor unit. A plurality of semiconductor units each consisting of the semiconductor chip, collector substrate, and the emitter contact terminal block are positioned in horizontal directions, using a positioning frame(s) that does not produce thermal stresses, and interposed between thin plates that do not produce thermal stresses like the positioning frame(s), so as to be positioned in the vertical direction. With this arrangement, the individual chips are allowed to move in accordance with thermal expansion of common electrode plates when the temperature changes cyclically in a power cycle test, for example, thereby to avoid large thermal stresses applied from the common electrode plates to the chips, particularly those in the peripheral portion of the semiconductor device. Thus, the pressure contact type semiconductor device of the present invention exhibits high resistance to power cycles, and a high operating reliability. Also, the above arrangement permits the use of common electrodes having larger areas than conventional ones, and the number of semiconductor chips mounted in the same package can be accordingly increased, so as to provide an IGBT of a flat structure having a significantly increased current capacity.