Patent Publication Number: US-2006006517-A1

Title: Multi-chip package having heat dissipating path

Description:
BACKGROUND OF THE INVENTION  
      This application claims the priority of Korean Patent Application No. 2004-52984, filed on Jul. 8, 2004, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.  
      1. Field of the Invention  
      The present invention relates to an integrated circuit (IC) chip package, and more particularly, to a multi-chip package (MCP) in which at least two IC chips are stacked.  
      2. Description of the Related Art  
      Many methods have been suggested in semiconductor manufacturing technology to package semiconductor integrated circuits (ICs), or IC chips. Some IC chip packaging technology demands very thin chips of tens of μm to achieve high density integration. To meet the demand, very thin chips or packages are suggested to be stacked. For example, there is an approach to a multi-chip package where semiconductor chips or IC chips are stacked to achieve high density integration.  
       FIG. 1  is a schematic sectional view illustrating a conventional MCP.  
      Referring to  FIG. 1 , in the conventional MCP, at least two IC chips  31  and  35  are embedded in one package. As shown in  FIG. 1 , the MCP contains the IC chips  31  and  35  stacked on a substrate  10 , such as a printed circuit board (PCB). Metal lines, such as connection pads  21 , are disposed on the substrate  10 . The connection pads  21  are electrically connected to solder balls  27 , which may be attached under the substrate  10 , through ball pads  28 . Gold wires or bonding wires  23  and  25  may be connected to the connection pads  21  to electrically connect bonding pads  29  of the IC chips  31  and  35  to the connection pads  21 .  
      The lower chip, that is, the first chip  31 , is adhered to the substrate  10  by a first adhesive layer  41 , and the second chip  35  is adhered to the first chip  31  by a second adhesive layer  45 . The second adhesive layer  45  may function as a spacer for keeping the first chip  31  and the second chip  35  spaced apart from each other. An encapsulating part  50  for protecting the stacked chips  31  and  35  and the bonding wires  23  and  25  is formed by a molding process using an encapsulating material, such as an epoxy molding compound (EMC).  
      The conventional MCP may have a problem with heat that may be trapped between the first chip  31  and the second chip  35 . Heat generated during the operation of the first chip  31  and the second chip  35  should be dissipated through the solder balls  27  that are outwardly exposed. It is not easy for heat to be dissipated from a portion between the first chip  31  and the second chip  35 , that is, the region of the second adhesive layer  45 . Thus, heat may be trapped in the second adhesive layer  45 .  
      Such heat trapping may occur because the conventional MCP shown in  FIG. 1  has a limitation in dissipating heat. Heat trapped between the chips  31  and  35  should be transferred or dissipated through a heat dissipating path composed of the encapsulating part  50 , the PCB substrate  10 , and the solder balls  27 . However, the heat dissipating path in the conventional MCP shown in  FIG. 1  is very poor at heat transfer ability.  
      The heat trapped between the chips in the conventional MCP may result in a temperature rise of the package and an unwanted failure. In particular, when the conventional MCP including a high speed and high density chip product is applied to a mobile system, the temperature of the package increases during operation and the temperature rise leads to a decrease in the stability of junctions in chips. Consequently, chip product characteristics, for example, refresh characteristics, operating speed, and life time, may deteriorate.  
      Therefore, to secure a heat dissipating path that can effectively transfer and dissipate heat trapped between chips in an MCP is considered important in using the MCP.  
     SUMMARY OF THE INVENTION  
      The present invention provides a thermally enhanced multi-chip package (MCP) having a heat dissipating path, which effectively transfers or dissipates heat generated during the operation of at least two stacked chips.  
      According to an aspect of the present invention, there is provided a multi-chip package comprising: a stack of integrated circuit chips; a heat sink part interposed between the integrated circuit chips so that at least one end portion can be exposed from at least a side of the stack of integrated circuit chips; a substrate on which the stack of integrated circuit chips is mounted; and a thermally connecting part to thermally connect the exposed end portion of the heat sink part to the substrate to dissipate heat collected in the heat sink part through the substrate.  
      Similarly, the heat sink part may be made of one selected from the group consisting of a copper plate, a metal plate, a silicon plate, a metal foil, a copper foil, a silicon plate coated with a metal layer, and a silicon plate coated with a copper layer.  
      The substrate may further comprise: a heat transfer pad connected to the thermally connecting parts; and a connecting solder ball thermally connected to the heat transfer pad and attached to the substrate to be connected to an external circuit.  
      A plurality of thermally connecting parts may be arranged along a side of the stack of integrated circuit chips on the substrate.  
      The thermally connecting parts may comprise solder balls attached to the exposed end portions of the heat sink part and attached to the heat transfer pads. The end portions of the heat sink part may have ball lands selectively opened so that the solder balls can be self-aligned and attached to the end portions. The ball lands may be open copper areas surrounded by an aluminum layer.  
      The heat sink part may comprise: a copper plate; and a printed solder resist film formed on the end portions of the copper plate and opening the ball lands on a surface of the copper plate so that the solder balls can be self-aligned and selectively attached to the end portions.  
      The heat sink part may comprise: a silicon plate; an aluminum layer deposited on the silicon plate; and a copper layer deposited on the end portions of the silicon plate and including ball lands in which the solder balls are self-aligned and selectively attached to the end portions.  
      Additionally, the heat sink part may comprise: a silicon plate; a copper layer deposited on the silicon plate; and an aluminum layer selectively deposited on the copper layer at the end portions of the silicon plate and opening the ball lands on a surface of the copper layer so that the solder balls can be self-aligned and selectively attached to the end portions.  
      The thickness of the heat sink part may range from 50 to 120 μm.  
      The thermally connecting parts may comprise solder parts formed by injecting solder paste between the exposed end portions of the heat sink part and the heat transfer pads and performing a reflow process.  
      The substrate may further comprise connecting solder balls attached to the substrate to be connected to an external circuit, wherein the heat transfer pads are electrically and thermally connected to grounding solder balls among the connecting solder balls. The thermally connecting parts may be arranged in two rows along side surfaces of the stack of integrated circuit chips on the substrate near to the sides of the stack of integrated circuit chips.  
      The multi-chip package has a heat dissipating path, which can effectively transfer and dissipate heat generated during the operation of the stacked at least two chips to the outside of the package, to enhance thermal performance. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The above and other features and advantages of the present invention will become more apparent by describing in detail exemplary embodiments thereof with reference to the attached drawings.  
       FIG. 1  is a schematic sectional view illustrating a conventional multi-chip package (MCP).  
       FIG. 2  is a schematic perspective view illustrating an MCP according to an embodiment of the present invention.  
       FIG. 3  is a schematic sectional view illustrating the MCP shown in  FIG. 2 .  
       FIG. 4  is a schematic sectional view illustrating a ball-shaped thermally connecting part employed in an MCP according to another embodiment of the present invention.  
       FIG. 5A  and  FIG. 5B  are schematic sectional views illustrating a method of forming thermally connecting parts by solder reflow, which are employed in an MCP according to still another embodiment of the present invention.  
       FIGS. 6 through 8  are schematic perspective views illustrating examples of heat sink parts employed in the MCP according to embodiments of the present invention.  
       FIG. 9  is a schematic sectional view illustrating thermally connecting parts employed in an MCP according to yet another embodiment of the present invention.  
       FIG. 10  is a schematic sectional view illustrating an MCP having three stacked chips according to a further embodiment of the present invention.  
       FIG. 11  is a schematic sectional view illustrating an MCP having three stacked chips according to another embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      The present invention will now be described more fully with reference to the accompanying drawings, in which preferred embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as being limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete and will fully convey the scope of the invention to those skilled in the art.  
      In the embodiments of the present invention, a heat sink part made of a thermally conductive plate or foil is interposed between stacked integrated circuit (IC) chips of a multi-chip package (MCP). In one embodiment, an end portion of the heat sink part protrudes from a side of the stack of IC chips. The protrusion is thermally connected to a heat transfer pad, which is formed on a substrate supporting thereon the IC chips, through thermally connecting parts arranged along the side of the stack of IC chips. The heat transfer pad is thermally connected to solder balls that are attached to a rear surface of the substrate and function as heat dissipating parts.  
      Accordingly, a heat dissipating path is composed of the heat sink part, the thermally connecting parts, the heat transfer pad, and the solder balls. Heat generated in the chips, particularly, heat trapped between the chips, is effectively transferred and dissipated through the heat dissipating path. As a result, the temperature of the chips is prevented from increasing due to the trapped heat during the operation of the chips. Also, product characteristics, such as chip operating speed, refresh characteristics, life time, or resistance to wrong operation, are effectively prevented from deteriorating due to the temperature rise of the chips.  
       FIG. 2  is a schematic perspective view illustrating an MCP according to an embodiment of the present invention.  FIG. 3  is a schematic sectional view illustrating the MCP shown in  FIG. 2 .  
      Referring to  FIGS. 2 and 3 , an MCP basically has at least two IC chips  310  and  330  embedded in one package. The MCP is basically provided with a substrate  100 , such as a printed circuit board (PCB), as a carrier on which the chips are mounted. Substrates other than the PCB may be used as the carrier on which the chips are mounted.  
      Connection pads  210  for electrical connection, such as a metal line connection, are provided on the substrate  100 . Heat transfer pads  650  that constitute a heat dissipating path may be formed on the substrate  100 . The connection pads  210  may be electrically connected to a plurality of connecting solder balls  270 , which are attached under the substrate  100 , by ball pads or the like. Although not shown, the connection pads  210  and the connecting solder balls  270  may be connected using vias passing through the substrate  100 .  
      The heat transfer pads  650  may be thermally or electrically connected to the connecting solder balls  270  and substantially corresponding grounding solder balls  271 , which are attached under the substrate  100 . When the heat transfer pads  650  are electrically or thermally connected to the grounding solder balls  271 , the grounding solder balls  271  act as ground terminals for preventing noise generated in the chips  310  and  330  and constitute the heat dissipating path. Heat inside the package is transferred and dissipated through the heat transfer pads  650  and the grounding solder balls  271  connected to the heat transfer pads  650 .  
      Referring to  FIG. 3 , the lower chip, that is, the first chip  310 , is adhered to the substrate  100  by a first adhesive layer  410 , and the second chip  330  is adhered to the first chip  310  by a second adhesive layer  431  and a third adhesive layer  435 . A heat sink part  610  is interposed between the first chip  310  and the second chip  330 . The second adhesive layer  431  and the third adhesive layer  435  are disposed over and under the heat sink part  610 , respectively, so that the heat sink part  610  is attached between the chips  310  and  330 .  
      In the meantime, bonding wires  230 , such as gold wires, including first and second bonding wires  231  and  233  may be connected to the connection pads  210  to electrically connect the chips  310  and  330  to an external circuit. The first chip  310  may be electrically connected to the external circuit by the first bonding wires  231  through the connection pads  210 , and the second chip  330  may be electrically connected to the external circuit by the second boding wires  233  through the connection pads  210 .  
      A sealing part  500  to protect the stacked chips  310  and  330  and the bonding wires  230  is formed by a molding process using a sealing material, such as an epoxy molding compound (EMC).  
      The heat sink part  610  may be formed of a high thermally conductive plate or foil, such as a copper plate, a metal plate, a silicon plate, a metal foil, a copper foil, a silicon plate coated with a metal layer, or a silicon plate coated with a copper layer. Here, it is preferable that the heat sink part  610  is made of a flat plate or a flexible foil to prevent the chips from being cracked due to irregular pressure distribution during the chip stacking process or during the molding process.  
      The heat sink part  610 , as shown in the embodiment of  FIGS. 2 and 3 , is interposed between the IC chips so that end portions of the heat sink part  610  protrude from both sides of the stack of IC chips  310  and  330 . Heat collected in the heat sink part  610  is dissipated through the substrate  100 , substantially through the heat transfer pads  650  on the substrate  100  and the grounding solder balls  271  connected to the heat transfer pads  650 . Although not shown, the heat transfer pads  650  and the grounding solder balls  271  may be connected using vias passing through the substrate  100 .  
      To complete the heat dissipating path, the heat sink part  610  and the heat transfer pads  650  must be thermally connected to each other. To this end, thermally connecting parts  630  are introduced to thermally connect the protruding end portions of the heat sink part  610  and the substrate  100 .  
      The thermally connecting parts  630  may be solder balls attached on the substrate  100  and the exposed end portions of the heat sink part  610 . If the thermally connecting parts  630  are solder balls, the thermally connecting parts  630  can be easily formed using a solder ball attaching system and method that are currently used in a semiconductor chip or an IC chip package.  
      Further, if the thermally connecting parts  630  are solder balls, when a sealing material EMC for the sealing part  500  is injected, the sealing material EMC can be sufficiently injected under the heat sink part  610 . In addition, although the end portions of the heat sink part  610  protrude beyond the stack of IC chips  310  and  330 , the heat sink part  610  can be supported at a uniform height by the plurality of solder balls that are the thermally connecting parts  630 . As a consequence, a pressure is prevented from being irregularly applied to the end portions of the heat sink part  610 , and the sealing part  500  is effectively prevented from being cracked due to the irregular pressure.  
      The heat dissipating path of the MCP, as shown by an arrow in the embodiment of  FIG. 3 , is composed of the heat sink part  610 , the solder balls as the thermally connecting parts  630 , the heat transfer pads  650 , and the grounding solder balls  271  connected to the heat transfer pads  650 . The heat dissipating path can effectively transfer and dissipate heat generated in the chips  310  and  330  since the parts constituting the heat dissipating path are all made of high thermally conductive materials.  
      Accordingly, heat generated during the operation of the chips  310  and  330  is prevented from accumulating or collecting between the chips  310  and  330 . Therefore, a decrease in operating speed, refresh characteristics, life time, and a danger of wrong operation, which may result from the temperature rise of the chips  310  and  330  due to the heat generated during the operation of the chips  310  and  330 , can be effectively prevented.  
      Meanwhile, the heat dissipating path as shown in  FIG. 3  may be formed in various shapes when the heat sink part is made from a plate material and the thermally connecting parts are solder balls.  
       FIG. 4  is a schematic sectional view illustrating ball-shaped thermally connecting parts employed in an MCP according to another embodiment of the present invention.  
      Referring to  FIG. 4 , solder balls  634  may be formed as thermally connecting parts contacting tips of end portions of a heat sink part  614 . The solder balls  634  as the thermally connecting parts are differently formed from the solder balls  630  as the thermally connecting parts shown in  FIG. 3  that are interposed between and attached to the heat sink part  610  and the heat transfer pads  650 .  
      Specifically, the solder balls  630  of the thermally connecting parts as shown in  FIG. 3  are formed by attaching the solder balls  630  to the exposed end portions of the heat sink part  610 , attaching the heat sink part  610  between the chips  310  and  330 , which is performed while the stacked chips  310  and  330  are mounted on the substrate  100 , and attaching the solder balls  630  on the heat transfer pads  650 . That is, after the solder balls  630  are attached to the heat sink part  610 , the heat sink part  610  is attached between the chips  310  and  330 . In this case, it may be a little bit harder to maintain a uniform height of the heat sink part  610  when the solder balls  630  are attached to the heat sink part  610 .  
      On the other hand, the solder balls  634  of the thermally connecting part as shown in  FIG. 4  are formed by attaching the heat sink part  614  between the chips  310  and  330 , which is performed while the chips  310  and  330  are stacked on the substrate  100 , and attaching the solder balls  634  to the heat transfer pads  650  and the end portions of the heat sink part  614 . In this case, since the heat sink part  614  is attached between the chips  310  and  330  before the solder balls  634  are attached to the heat sink part  614  and the heat transfer pads  650 , it may be a little bit easier to maintain a uniform height of the heat sink part  614 .  
      In the meantime, when the thermally connecting parts are solder balls  630  and  634 , it is preferable that solder paste or flux used for solder ball mounting does not remain in the package. In this case, it is preferable that water-soluble flux is used for a user to remove remaining flux with a flux cleaner without damaging bonding pads (not shown) provided on the chips  310  and  330 .  
      Although  FIGS. 3 and 4  show that the thermally connecting parts are solder balls  630  and  634 , the thermally connecting parts may also be formed by solder paste reflow instead of the solder balls.  
       FIGS. 5A and 5B  are schematic sectional views illustrating a method of forming thermally connecting parts by solder paste reflow, which are employed in an MCP according to still another embodiment of the present invention.  
      The thermally connecting parts may be formed by solder paste reflow rather than by solder balls. For example, as shown in  FIG. 5A , a solder paste  640  is injected between the exposed end portion of the heat sink part  610  and the substrate  100 , substantially between the exposed end portion of the heat sink part  610  and the heat transfer pad of the substrate  100 . As shown in  FIG. 5B , an infrared (IR) reflow process is performed to form solder parts  645 . The solder parts  645  thermally or/and electrically connect the heat sink part  610  and the heat transfer pad of the substrate  100 .  
      Although the thermally connecting parts may be the solder parts  645  formed by the solder paste reflow, it may be more advantageous in productivity to attach solder balls to the heat sink part  610  using a solder ball mounting device and use the attached solder balls as the thermally connecting parts. It is preferable that the heat sink part  610  has ball lands in which the solder balls are self-aligned so that the solder balls can be mounted well on the heat sink part  610 .  
       FIG. 6  is a schematic perspective view illustrating a first example of a heat sink part employed in the MCP according to an embodiment of the present invention. Referring to  FIG. 6 , a heat sink part  660  may have ball lands  665  formed on exposed end portions so that solder balls can be easily self-aligned when being attached to the heat sink part  660 . The ball lands  665  may be made of a solder-wettable layer, for example, a copper layer.  
      For example, when the heat sink part  660  is made of a copper plate, solder resist films  663  are printed on the exposed end portions to open the ball lands  665 . Since the solder resist films  663  do not permit solder to be attached thereto, the solder balls are self-aligned in the ball lands  665  made of copper that are opened by the solder resist films  663 .  
       FIG. 7  is a schematic perspective view illustrating a second example of the heat sink part employed in the MCP according to another embodiment of the present invention.  
      Referring to  FIG. 7 , when a heat sink part  670  is made of a silicon plate or the like, ball lands  673  may be formed by depositing an aluminium layer  671  on the entire surface of the silicon plate and selectively depositing a copper layer on exposed end portions. The aluminium layer  671  is basically a non-wettable layer and functions as a metal layer for isolating the ball lands  673  from one another.  
       FIG. 8  is a schematic perspective view illustrating a third example of a heat sink part employed in the MCP according to yet another embodiment of the present invention.  
      Referring to  FIG. 8 , when a heat sink part  680  is formed of a silicon plate or the like, a copper layer  681  is deposited on the entire surface of the silicon plate and band-shaped aluminium layers  683  are formed on exposed end portions to selectively open ball lands made of copper.  
      Since the heat sink part  610  is a layer formed of metal (e.g., aluminium or copper) or silicon, and can be grounded to the grounding solder balls  271  through the thermally connecting parts  630  and the heat transfer pads  650 , which are also used as ground pads, as described with reference to  FIG. 3 , the heat sink part  610  can prevent signal interference between the IC chips  310  and  330  that are disposed over and under the heat sink part  610 . That is, the heat sink part  610  can effectively prevent noise in the IC chips  310  and  330 .  
      In the meantime, the solder balls or the thermally connecting parts constituting the heat dissipating path of the MCP according to the present invention may be arranged in one, two, or more rows along a side surface of the stack of IC chips on the substrate  100 .  
       FIG. 9  is a schematic sectional view illustrating thermally connecting parts employed in an MCP according to yet another embodiment of the present invention.  
      Referring to  FIG. 9 , the thermally connecting parts that thermally connect a heat sink part  619  to the substrate  100  may be arranged in two rows. That is, as shown in  FIG. 9 , a first heat transfer pad  651  is formed on the substrate  100 , and a second heat transfer pad  655  is formed behind the first heat transfer pad  651 . A plurality of first solder balls  631  as first thermally connecting parts may be arranged to thermally or/and electrically connect the heat sink part  619  to the first heat transfer pad  651 . A plurality of second solder balls  632  as second thermally connecting parts may be arranged to thermally or/and electrically connect the heat sink part  619  to the second heat transfer pad  655 . The solder balls  631  and  632  as the thermally connecting parts may be arranged in one, two, or more rows along the side surface of the stacked chips on the substrate  100 .  
      The MCP according to an embodiment of the present invention may be applied to a case where three or more chips are stacked. In this case, a plurality of heat sink parts and thermally connecting parts are accordingly provided.  
       FIG. 10  is a schematic sectional view illustrating an MCP having three stacked chips according to a further embodiment of the present invention.  
      Referring to  FIG. 10 , when three IC chips  310 ,  330 , and  350  are stacked, a first heat sink part  611  is interposed between the first chip  310  and the second chip  330 , and a second heat sink part  612  is attached by third adhesive layers  451  and  453  between the second chip  330  and the third chip  350 .  
      In order to dissipate heat collected in the first heat sink part  611  and the second heat sink part  612  through the substrate  100 , first thermally connecting parts  636  that thermally connect the first heat sink part  611  and the second heat sink part  612  and second thermally connecting parts  637  that thermally connect the first heat sink part  611  to the substrate  100  are employed. The first thermally connecting parts  636  and the second thermally connecting parts  637  may be solder balls substantially vertically spaced in parallel to each other, as shown in  FIG. 10 . In the meantime, when solder balls are used as the thermally connecting parts, heat transfer pads may be positioned inside recessed portions of the substrate  100  so that the solder balls can be easily aligned in the recessed portions and easily attached to the substrate  100 . Recessed portions may also be formed on the first heat sink parts  611 , so that the solder balls as the second thermally connecting part  636  can be easily aligned.  
       FIG. 11  is a schematic sectional view illustrating an MCP having three stacked chips according to another embodiment of the present invention.  
      Referring to  FIG. 11 , when the three chips  310 ,  330 , and  350  are stacked, a first heat sink part  613  is interposed between the first chip  310  and the second chip  330 , and a second heat sink part  614  is attached by the third adhesive layers  451  and  453  between the second chip  330  and the third chip  350 .  
      To dissipate heat collected in the first heat sink part  613  and the second heat sink part  614  through the substrate  100 , first thermally connecting parts  631  that thermally connect the first heat sink part  613  to a first heat transfer pad  651  of the substrate  100  and second thermally connecting parts  638  that thermally connect the second heat sink part  614  to a second heat transfer pad  653  of the substrate  100  may be employed. At this time, the second thermally connecting parts  638  may be solder balls larger than those of the first thermally connecting parts  631 . The second heat transfer pad  653  to which the second thermally connecting parts  638  are attached is disposed behind the first heat transfer pads  651 . Accordingly, the second heat sink part  614  may protrude longer than the first heat sink part  613 .  
      The MCP according to the embodiments of the present invention may include a stack of IC chips, a plate-shaped heat sink part interposed between the IC chips so that two facing end portions of the heat sink part can protrude from both sides of the stack of IC chips, heat transfer pads formed in the vicinity of side surfaces of the stack of IC chips where the stack of IC chips is mounted and the two end portions of the heat sink part are exposed, a substrate including connection pads to which bonding wires arranged near to the other side surfaces of the stack of IC chips are connected, and a plurality of thermally connecting parts for thermally connecting the two end portions of the heat sink part to the heat transfer pads so that heat collected in the heat sink part can be dissipated through the substrate.  
      The MCP may include a stack of IC chips, a first heat sink part interposed between the IC chips so that one end portion of the first heat sink part can be exposed from a side of the stack of IC chips, a second heat sink part interposed between the first heat sink part and one of the IC chips so that an end portion of the second heat sink part can be exposed from the side of the stack of IC chips, a substrate on which the stack of IC chips is mounted, and thermally connecting parts for thermally connecting the exposed end portions of the heat sink parts to the substrate to dissipate heat collected in the first and second heat sink parts through the substrate.  
      The thermally connecting parts may include first thermally connecting parts that thermally connect the first heat sink part to the second heat sink part, and second thermally connecting parts that thermally connect the first heat sink to the substrate.  
      Alternatively, the thermally connecting parts may include first thermally connecting parts that thermally connect the first heat sink part to the substrate and second thermally connecting parts that thermally connect the second heat sink part to the substrate.  
      Further, the MCP may include a stack of IC chips, a heat sink part interposed between the IC chips so that one end portion of the heat sink part can be exposed from a side of the stack of IC chips, a substrate on which the stack of IC chips is mounted, and thermally connecting parts for thermally connecting the exposed end portion of the heat sink to ground terminals of the substrate to dissipate heat collected in the heat sink part through the ground terminals of the substrate.  
      The ground terminals may include a ground pad formed on the substrate to be thermally and electrically connected to the thermally connecting parts, and grounding solder balls attached to the substrate to be electrically connected to the ground pad and connected to an external circuit.  
      The thermally connecting parts may be solder balls attached to the exposed end portion of the heat sink part and attached on the ground pads.  
      The thermally connecting parts may be solder parts formed by injecting solder paste between the exposed end portion of the heat sink part and the ground pad and performing a reflow process.  
      As described above, since the thermally connecting parts connecting the heat sink part interposed between the stacked IC chips to the grounding solder balls attached to the rear surface of the substrate are solder parts or solder balls, a heat dissipating path through which heat between the chips is dissipated to the outside of the package can be formed.  
      As a result, heat generated in the chips, especially heat trapped in the chips, is transferred and dissipated to the outside effectively. Accordingly, the temperature rise of the chips during the operation of the chips is prevented, and product characteristics, such as operating speed, refresh characteristics, life time, or resistance against wrong operation can be effectively prevented from deteriorating.  
      While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present invention as defined by the following claims.