Abstract:
A mounting structure for a semiconductor device of the present invention includes a substrate, a semiconductor device, a plurality of connecting members and a member. The substrate has a first surface and a plurality of pads on the first surface. The semiconductor device has first and second main surfaces and a plurality of terminals provided, on the second main surface, at locations corresponding to the pads. The connecting members connect the pads to the terminals, respectively. The member has at least one surface which is coupled to the first main surface. The thermal expansion coefficient of the member is equal to, or substantially equal to, that of the substrate. A method for mounting a member and a semiconductor device on a substrate, wherein the member having at least one surface and the thermal expansion coefficient which is equal to or substantially equal to that of the substrate, includes coupling a surface of the member to the upper surface of the semiconductor device; positioning the semiconductor device so that terminals on the lower surface of the semiconductor device face pads on an upper surface of the substrate through connecting members, respectively; and heating the semiconductor device, the member, and the substrate to melt the connecting members, and thereafter, putting them back to ordinary temperature.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a mounting structure and a method of mounting a semiconductor device, and more particularly, to a mounting structure and a method of mounting a semiconductor device in which a semiconductor device and a mounting substrate having different thermal expansion coefficients are connected to each other by connecting members. 
     A conventional mounting structure for a semiconductor device of this kind is, for example, a flip chip connecting structure. In flip chip connecting structure, a semiconductor device is connected to a mounting substrate with solder or conductive adhesive. More specifically, in the structure, a plurality of input/output terminals provided on the lower surface of a semiconductor device are connected with a plurality of pads provided on the upper surface of a mounting substrate, respectively, with solder or conductive adhesive. 
     During manufacturing, in the conventional structure such as the above-mentioned flip chip connecting structure, the solder or conductive adhesive is heated to about 200 degrees centigrade to melt the solder or to cure the conductive adhesive. 
     For example, when eutectic solder is used, the eutectic solder is heated to 183 degrees centigrade or higher. Heat is also applied to the semiconductor device and the mounting substrate. Because the thermal expansion coefficients of the semiconductor device and the mounting substrate are different from each other, the input/output terminals of the semiconductor device do not align with the pads of the mounting substrate after thermal expansion. When heating stops and/or cooling starts, the connecting members begin solidifying and making the input/output terminals of the semiconductor device connect to the pads of the mounting substrate, although there is a gap between positions of the input/output terminals of the semiconductor device and the pads of the mounting substrate. However, the semiconductor device and the mounting substrate contract, causing the input/output terminals and the pads to attempt to return to their original positions before the heating. This creates a problem because the connecting members are stressed when the semiconductor device, the solder, and the mounting substrate return to their ordinary temperature. When the stress exceeds the breaking stress of the connecting members, the connection between the input/output terminals and the pads breaks. 
     Stress produces another problem because it makes the semiconductor device and/or the mounting substrate warp or distort. In particular, when a printed substrate is used as the mounting substrate, the thermal expansion coefficient of the printed substrate is about 15×10 −6 /° C. to 20×10 −6  /° C. while that of a semiconductor device made of silicon is about 2.5 to 3.5×10 −6 /° C. Therefore, the difference between the thermal expansion coefficients of the printed substrate and of the semiconductor device is about 12 to 17×10 −6 /°C., and the stress lowers the reliability of the connection of the mounted parts. Therefore, because the difference between the thermal expansion coefficients of the printed substrate and the semiconductor device is large, stress produced to the connecting members becomes large. 
     In relatively small semiconductor devices the thermal expansion problem is not significant, however, as the size of the semiconductor device increases, differences between thermal expansion coefficients become a serious problem and the reliability of the devices is reduced. 
     Japanese Patent Application Laid-open No. Hei 8-148592 discloses a semiconductor integrated circuit device which has a semiconductor device mounted on a mounting substrate with solder bumps. The semiconductor integrated circuit device has a cap formed of a material having a thermal expansion coefficient being substantially equal to that of a semiconductor device and is secured on the upper surface of the semiconductor device. When the temperature changes due to heat generated from the semiconductor device, the semiconductor device and the cap expand and contract according to the thermal expansion coefficient of the semiconductor device. In this device, however, because the semiconductor device contracts according to the thermal expansion coefficient of the semiconductor device, the above problem of stress associated with the connecting members is not solved. 
     SUMMARY OF THE INVENTION 
     An object of the invention is to provide a mounting structure and a method of mounting a semiconductor device wherein a semiconductor device and a mounting substrate having different thermal expansion coefficients are connected to each other by connecting members, without applying stress to the connecting members. 
     Another object of the invention is to provide a mounting structure and a method of mounting a semiconductor device wherein reliability is improved when a semiconductor device having a large outer shape is mounted on a mounting substrate. 
     Another object of the invention is to provide a mounting structure and a method of mounting a semiconductor device wherein a printed substrate is used as the mounting substrate, the reliability of the connection between the printed substrate and the semiconductor device is improved. 
     According to one aspect of the present invention, a mounting structure for a semiconductor device is provided which includes: a substrate which has a first surface and a plurality of pads provided on the first surface; a semiconductor device which has first and second main surfaces and a plurality of terminals provided on the second main surface at locations corresponding to the pads; a plurality of connecting members which connect the pads to the terminals, respectively; and a member which has at least one surface which is coupled with the first main surface, wherein the thermal expansion coefficient of the member is equal to, or substantially equal to, that of the substrate. 
     According to another aspect of the present invention, a mounting structure for a semiconductor device is provided which includes: a substrate which has a first surface and a plurality of pads provided on the first surface; a semiconductor device which has first and second main surfaces and a plurality of terminals provided on the second main surface at locations corresponding to the pads; a plurality of connecting members which connect the pads with the terminals, respectively; a plate which is combined with the first main surface and has a thermal expansion coefficient being equal to, or substantially equal to, that of the substrate; and a cooling member which is thermally coupled with the plate. 
     According to another aspect of the present invention, a method for mounting a member and a semiconductor device on a substrate, wherein the member having at least one surface and the thermal expansion coefficient which is equal to, or substantially equal to, that of the substrate is provided which includes: coupling a surface of the member with the upper surface of the semiconductor device; positioning the semiconductor device so that terminals on a lower surface of the semiconductor device face pads on an upper surface of the substrate through connecting members, respectively; and heating the semiconductor device, the member, and the substrate to melt the connecting members, and thereafter, returning them to their ordinary temperature. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other features and advantages of the invention will be made more apparent by the following detailed description and the accompanying drawings, wherein: 
     FIG. 1 is a sectional view of a first embodiment of the present invention; 
     FIGS. 2A to  2 D are illustrations showing a method of mounting a semiconductor device of the first embodiment of the present invention; 
     FIG. 3 is a sectional view of a second embodiment of the present invention; 
     FIG. 4 is a sectional view of a third embodiment of the present invention; 
     FIG. 5 is a sectional view of a fourth embodiment of the present invention; and 
     FIG. 6 is a sectional view of a fifth embodiment of the present invention; 
    
    
     In the drawings, the same reference numerals represent the same structural elements. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A first embodiment of the present invention will be described in detail below. 
     Referring to FIG. 1, a mounting structure for a semiconductor includes a printed substrate  10 , a semiconductor device  30 , and a metal plate  50 . 
     Printed substrate  10  has a plurality of pads  11 . Pads  11  are provided on the upper surface of printed substrate  10  and are connected with wiring disposed on the surface or in an internal layer of printed substrate  10 . 
     Semiconductor device  30  is mounted on the upper surface of printed substrate  10 . Semiconductor device  30  is, for example, an integrated circuit (IC) or a large scale integration (LSI). Semiconductor device  30  has a plurality of input/output terminals  31 . Input/output terminals  31  are provided on the lower surface of semiconductor device  30 . Input/output terminals  31  are arranged in a lattice formation or the like. Input/output terminals  31  are each provided at locations corresponding to each of pads  11  on printed substrate  10 , respectively. Input/output terminals  31  and the corresponding pads  11  are connected to each other by solder  20 . Solder  20  is formed as solder balls. 
     Semiconductor device  30  is formed of silicon, and its thermal expansion coefficient is about 3×10 −6 /° C. The thickness of semiconductor device  30  is about 0.5 millimeters. The outer shape of semiconductor device  30  is a square with each side being about 12 millimeters long. The number of input/output terminals  31  is, for example, 800. The pitch of input/output terminals  31  is about 0.25 millimeters. The diameter of the terminal  31  is about 0.15 millimeters. 
     Metal plate  50  is coupled with the upper surface of semiconductor device  30 . The thermal expansion coefficient of metal plate  50  is equal to, or substantially equal to, that of printed substrate  10 . In this embodiment, metal plate  50  is made of copper or brass because the thermal expansion coefficients of copper and brass are 16.5×10 −6 /° C. and 17.3×10 −6 /° C., respectively, which are substantially equal to the thermal expansion coefficient of printed substrate  10  (15 to 20×10 −6 /° C.). As semiconductor device  30  becomes larger, it is preferable to use a material for metal plate  50  which has a thermal expansion coefficient close to the thermal expansion coefficient of semiconductor device  30 . For example, when each side of semiconductor device  30  is about 10 millimeters long, there is no stress problem even if the difference of the thermal expansion coefficient between semiconductor device  30  and metal plate  50  is large. However, if each side is equal to or greater than about 20 millimeters long, it is preferable that the thermal expansion coefficient of metal plate  50  be substantially equal to that of semiconductor device  30  in order to avoid stress. 
     Metal plate  50  has an outer shape which is identical to the outer shape of semiconductor device  30  or has an outer shape which is larger than the outer shape of semiconductor  30 . Metal plate  50  is strong enough to cause semiconductor device  30  to expand according to the thermal expansion of metal plate  50 . However, if the strength of metal plate  50  is insufficient, metal plate  50  cannot cause semiconductor device  30  to expand according to the thermal expansion coefficient of metal plate  50 , therefore, semiconductor device  30  expands according to the thermal expansion of the semiconductor device  30  itself not according to the thermal expansion of metal plate  50 . Therefore, though it is preferable to make metal plate  50  as thin as possible, from the viewpoint of heat radiation, it is necessary that metal plate  50  be thick enough to cause semiconductor device  30  to expand according to the thermal expansion coefficient of metal plate  50 . Specifically, the thickness of metal plate  50  is about 2 millimeters to 5 millimeters, and in this embodiment, about 3 millimeters. 
     Metal plate  50  and semiconductor device  30  are coupled to each other with a bonding strength sufficient to make semiconductor device  30  expand and contract according to the thermal expansion of metal plate  50 , even after being heated and cooled. More specifically, metal plate  50  and semiconductor device  30  are coupled to each other by an adhesive  40 . It is preferable to select adhesive  40  such that it provides the necessary adhesive strength to make semiconductor device  30  expand according to the thermal expansion of metal plate  50  when heated. In this embodiment, adhesive  40  is an epoxy adhesive. Sufficient adhesive strength can be obtained with any kind of epoxy adhesive. The thickness of an adhesive layer formed between metal plate  50  and semiconductor device  30  by adhesive  40  is about 10 to 20 microns. 
     Next, a method of mounting a semiconductor device will be described in detail. 
     First, semiconductor device  30  is prepared. Solder balls  20  are provided on input/output terminals  31  on the lower surface of semiconductor device  30 , respectively. Depending on the kind of the mounting substrate on which semiconductor device  30  is mounted, a metal having a thermal expansion coefficient which is equal to, or substantially equal to, the thermal expansion coefficient of the mounting substrate is selected. Metal plate  50  is formed by forming the selected metal in a shape having a surface which is larger than, or equal in size to, the upper surface of semiconductor device  30 . 
     Referring to FIG. 2A, in a first step, epoxy adhesive  40  is applied on the upper surface of semiconductor device  30  to a thickness of about 10 to 20 microns. The upper surface of semiconductor device  30 , with adhesive  40  applied thereon, and the lower surface of metal plate  50  are positioned facing each other. Note that adhesive  40  may be applied on the lower surface of metal plate  50 . 
     In FIG. 2B, in a second step, metal plate  50  is attached to semiconductor device  30  by adhesive  40  to form a coupled body  100 . More specifically, the surface of  20  semiconductor device  30 , with adhesive  40  applied thereon, is put in contact with the lower surface of metal plate  50 . Heat is applied to semiconductor device  30 , adhesive  40 , and metal plate  50  to cure adhesive  40 , and thus, semiconductor device  30  is adhered to metal plate  50 . 
     Referring to FIG. 2C, in a third step, coupled body  100  formed in the second step  25  is positioned above the upper surface of printed substrate  10 , and is placed on printed substrate  10 . Input/output terminals  31  on the lower surface of semiconductor device  30  are positioned on pads  11  on the upper surface of printed substrate  10 , respectively. 
     In FIG. 2D, in a fourth step, printed substrate  10 , solder  20 , semiconductor device  30 , and metal plate  50  are heated. The heat causes thermal expansion of printed substrate  10 , semiconductor device  30 , and metal plate  50 . Because semiconductor device  30  is coupled with metal plate  50 , it expands according to the thermal expansion of metal plate  50  not according to the thermal expansion of semiconductor device  30  itself. The thermal expansion of semiconductor device  30  is also equal to, or substantially equal to, that of printed substrate  10  because the thermal expansion coefficient of printed substrate  10  is equal to, or substantially equal to, that of metal plate  50 . Accordingly, even after the thermal expansion of printed substrate  10  and semiconductor device  30  due to the heating, the respective pads  11  and the corresponding input/output terminals  31  are not misaligned but are aligned. Or, the misalignment is so small that input/output terminals  31  and pads  11  may be considered to be substantially properly aligned. 
     When heating is stopped and/or cooling is started, melted solder  20  begins to solidify. Solder  20  connects input/output terminals  11  to pads  31  which are aligned or are considered to be substantially properly aligned. Printed substrate  10 , semiconductor device  30 , and metal plate  50  begin to contract as they get cooler. Also, in this cooling process, beccause semiconductor device  30  contracts according to the thermal expansion coefficient which is equal to, or substantially equal to, that of printed substrate  10 , the respective pads and the corresponding input/output terminals  31  are not misaligned but are aligned. Or, the misalignment is so small that input/output terminals  31  and pads  11  may be considered to be substantially properly aligned. When printed substrate  10 , solder  20 , semiconductor device  30 , and metal plate  50  return to their ordinary temperature and the connection between input/output terminals  31  and pads  11  is completed, no stress or almost no stress is applied to solder  20 . 
     When heat generated from semiconductor device  30  causes thermal expansion of semiconductor device  30  and mounting substrate  10 , because the thermal expansion of semiconductor device  30  and mounting substrate  10  is equal, or substantially equal, stress applied to the connecting members can be reduced. 
     In this embodiment, an organic substrate may be used instead of metal plate  50 . When printed substrate  10  is used as the mounting substrate, a printed substrate is used as the organic substrate. Because the thermal expansion coefficient of the printed substrate coupled with semiconductor device  30  is equal to the thermal expansion coefficient of printed substrate  10 , which is the mounting substrate, the reliability of the connection between semiconductor device  30  and printed substrate  10  can be further improved. 
     Further, a ceramic substrate may be used instead of printed substrate  10  in this embodiment. Because the thermal expansion coefficient of the ceramic substrate is about 6 to 7×10 −6 /° C., aluminum nitride, alumina ceramic, or tungsten is used as metal plate  50 . This is because the thermal expansion coefficients of aluminum nitride, alumina ceramic, and tungsten are 3 to 4×10 −6 /° C., 4 to 6×10 −6 /° C., and 4 to 6×10 −6 /° C., respectively, which are substantially equal to that of the ceramic substrate. In this case, the reliability can be further improved. 
     In this embodiment, a conductive adhesive may be provided to connect pads  11  and input/output terminals  31 , in stead of solder  20 . 
     Next, a second embodiment of the present invention will be described below. A feature of the second embodiment is that, instead of the metal plate, a cap is provided. The other structures are the same as those in the first embodiment. 
     Referring to FIG. 3, a cap  51  is provided on the upper surface of semiconductor device  30 . Cap  51  has a concave portion, and the cross section thereof is shaped to be concave. The upper surface of semiconductor device  30  is adhered to the bottom surface of the concave portion by adhesive  40 . The brim of cap  51  is adhered to the upper surface of printed substrate  10  by an adhesive  60 . Semiconductor device  30  is completely encapsulated by cap  51 , the adhesive  60 , and printed substrate  10 . The thermal expansion coefficient of cap  51  is equal to, or substantially equal to, the thermal expansion coefficient of printed substrate  10 . The other structures of cap  51  are similar to those of metal plate  50  of the first embodiment. 
     In this embodiment, because cap  51 , which has a thermal expansion coefficient equal to, or substantially equal to, that of printed substrate  10 , is provided, a structure encapsulating semiconductor device  30  as well as preventing stress to solder  20  can be formed with a small number of parts. In addition, no stress is caused to the connecting portion between cap  51  and printed substrate  10 . 
     In this embodiment, the connection between the brim of cap  51  and printed substrate  10  is not limited to only adhesive  60 . A ring or other encapsulating members may be used. 
     Next, a third embodiment of the present invention will be described below. A feature of the third embodiment is that a metal plate includes a structure for attaching a cooling member. The other structures are the same as those in the first embodiment. 
     Referring to FIG. 4, a mounting structure for a semiconductor device of the third embodiment includes a metal plate  52  and a heat sink  70 . 
     Heat sink  70  is attached to the upper surface of metal plate  52  by screws  80 . Screw holes  520  are provided in the upper surface of metal plate  52 . The other structures of metal plate  52  are similar to those of metal plate  50  of the first embodiment. Holes  700  are provided in heat sink  70 . Holes  700  are through holes and are provided at locations corresponding to the screw holes in metal plate  52 . Screws  80  go through holes  700  in heat sink  70 , respectively, and are screwed into screw holes  520  in metal plate  52 , respectively. 
     As described above, in this embodiment, because the screw holes  520  are formed for attaching heat sink  70  to metal plate  52 , it is not necessary to separately provide any attaching member for the heat sink. 
     Though holes  700  in heat sink  70  are through holes in this embodiment, however, they may also be screw holes. 
     Next, a fourth embodiment of the present invention will be described below. A feature of the fourth embodiment is that a metal plate is provided with a structure for attaching the cooling member thereto. The other structures are similar to those in the first embodiment. 
     Referring to FIG. 5, a mounting structure for a semiconductor device of the fourth embodiment includes a metal plate  53  and a heat sink  71 . Heat sink  71  is attached to the upper surface of metal plate  53 . 
     A stud  530  is provided on the upper surface of metal plate  53 . A screw thread is provided on stud  530 . A screw hole  710  is provided on the lower surface of heat sink  71 . Screw hole  710  is provided at a location corresponding to stud  530  of metal plate  53  and stud  530  is screwed into the screw hole  710 . 
     The effect of this embodiment is similar to that of the third embodiment. 
     Next, a fifth embodiment of the present invention will be described below. A feature of fifth embodiment is that, instead of a metal plate, a heat sink is used. The other structures are the same as those of the first embodiment. 
     Referring to FIG. 6, a heat sink  72  is attached to the upper surface of semiconductor device  30  by adhesive  40 . Heat sink  72  is made of a metal which has excellent heat radiation. The thermal expansion coefficient of heat sink  72  is equal to, or substantially equal to, that of printed substrate  10 . The other structures of heat sink  72  are similar to those of metal plate  50  of the first embodiment. 
     This embodiment is not limited to heat sink  72 , and a metal member having at least one main surface may be applied. It is sufficient that the size of the at least one main surface of the metal member is larger than, or equal to, the size of the upper surface of semiconductor device  30 . In this case, one main surface of the metal member is coupled with the upper surface of semiconductor device  30  by adhesive  40 . 
     In this manner, because heat sink  72 , which has a thermal expansion coefficient which is equal to, or substantially equal to, that of printed substrate  10 , is coupled with the upper surface of semiconductor device  30 , a structure radiating heat generated from semiconductor device  30 , as well as preventing stress from being created to solder  20 , can be achieved with a small number of parts. 
     The present invention can be applied to various kinds of mounting structures and methods for mounting a semiconductor device. For example, when the present invention is applied to a chip carrier, the metal plate or the organic plate is provided in a region corresponding to a region where connecting members are provided in the upper surface of a carrier substrate mounting a semiconductor thereon. Specifically, the metal plate or the organic plate is provided in a region of the upper surface of the chip carrier, except a region on which the semiconductor device is mounted. 
     While this invention has been described in conjunction with the preferred embodiments described above, it will now be possible for those skilled in the art to put this invention into practice in various other manners.