Patent Publication Number: US-2006012031-A1

Title: Heat dissipation device for integrated circuits

Description:
FIELD OF THE INVENTION  
      The present invention relates to a method of forming semiconductor packages, and to packages which are the result of the method.  
     BACKGROUND OF THE INVENTION  
      Several ways are known of mounting a semiconductor integrated circuit (die) onto a surface of a substrate. This process is known as “packaging”. The substrate has electrical connections leading out of the substrate (e.g. through the material of the substrate, through “via holes”) for connection to other components.  
      For example, in the case of an integrated circuit having input/output die pads, it is well known to mount the integrated circuit onto a substrate having corresponding electrical pads which are electrically connected out of the substrate (e.g. by via holes). Wire bonding is used to connect the pads of the integrated circuit to respective pads of the substrate, and then the die and wire bonds are encased in resin. Optionally, a number of integrated circuits can be mounted on a single substrate in this way, and then the substrate “singulated”, i.e. cut to provide a number of individual packaged devices each containing one (or more) of the integrated circuits.  
      In a second example, a “flipchip” is an integrated circuit where the input/output connections are provided as electrically conductive protrusions on one of its surfaces. The flipchip is mounted in a cavity formed on the upper surface of the integrated circuit, with the protrusions facing downwardly. The protrusions are received into openings in the substrate (i.e. in the surface at the bottom of the cavity). Each opening includes electrically conductive material which contacts the protrusions, and the openings are in turn are electrically connected out of the substrate (e.g. by via holes). Again, once the flipchip is in position, it is encased in protective resin, which may fill the cavity  
      In some arrangements, it is known to provide a flipchip encased as described in the preceding paragraph, and a second integrated circuit mounted directly above it. The second integrated circuit is connected by wire bonding to pads on the upper surface of the substrate laterally outward of the cavity. Then the second integrated circuit is encased in resin.  
      One of the main limitations on integrated circuit design is heat generation within the integrated circuit, since if the integrated circuit overheats, it may fail operate properly. It would therefore be advantageous to provide ways of mounting integrated circuits on substrates such that heat is more easily transmitted from them.  
     SUMMARY OF THE INVENTION  
      The present invention aims to provide a new and useful semiconductor packages (that is, substrates incorporating at least one integrated circuit mounted thereon), and methods for mounting integrated circuits on substrates.  
      In general terms, the present invention proposes that an integrated circuit is mounted on a substrate via a heat conductive plate interposed between the integrated circuit and the substrate and having at least one portion extending laterally out from under the integrated circuit.  
      The integrated circuit is generally of the type having pads for connection to the substrate by wire bonding. Following the wire bonding, the integrated circuit and wire bonds are encased in resin, but the plate preferably extends out of the resin, so that heat generated in the integrated circuit is conducted out of the resin.  
      The plate is preferably shaped so as to not to block the areas at which the pads of the integrated circuit are connected to the substrate. For example, the plate may extend out from under the integrated circuit in directions which are diagonal relative to the overall square or rectangular circumference of the integrated circuit, since the integrated circuit will not generally require wire bonding to the substrate in these directions.  
      Preferably, the plate is grounded. In this case, it may supplement or even replace the ground ring (that is, the device which in many known arrangements is provided electrically connected to ground and also to the pads of the integrated circuit which are to be grounded). Some or all of these ground pads may instead be connected to the plate. If any ground ring is provided, it may be electrically connected to the plate. In the case that certain pads of the integrated circuit are to be electrically connected to ground, then it is desirable that the plate should extend out from under the integrated circuit in the direction towards these pads.  
      The plate may have portions of increased thickness laterally outward from the integrated circuit. For example, there may be a rim extending in transversely to the substrate surface. Optionally, a further heat-transmissive element may be connected to the plate after the application of the resin, for example to the rim.  
      The present device may be used in arrangements which include a flipchip. In this case, the plate may be mounted over a flipchip (preferably directly onto the upper surface of a flipchip which has not been encased in resin, or in alternative arrangements onto the upper surface of resin encasing the flipchip).  
      In cases when a plurality of integrated circuits are mounted onto the same substrate, a single heat conductive plate is preferably provided extending under more than one of the integrated circuits (e.g. preferably under all the integrated circuits), and this plate too is cut when the substrate is singulated. 
    
    
     BRIEF DESCRIPTION OF THE FIGURES  
      Two embodiments of the invention will now be described in detail for the sake of example only, with reference to the following figures in which:  
       FIG. 1  shows in top view a heatspreader plate used in a first embodiment;  
       FIG. 2  shows in an assembled structure which is the first embodiment of the invention and includes the plate of  FIG. 1 ;  
       FIG. 3  is an exploded perspective view of the arrangement of  FIG. 2 ;  
       FIG. 4 , which is composed of FIGS.  4 ( a ) and  4 ( b ), illustrates the mounting of a heatspreader plate in the second embodiment of the invention;  
       FIG. 5  shows in an assembled structure which is the second embodiment of the invention and includes the plate of  FIG. 4 ( a ); and  
       FIG. 6  is an exploded perspective view of the arrangement of  FIG. 5 . 
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS  
      A first embodiment of the invention is shown in  FIGS. 1-3 . Referring to  FIG. 1 , the heatspreader plate  1  is composed of a central portion  3 , four lateral arms  5 , four diagonal arms  7  and a rim portion  9 . As seen (for example in  FIG. 3 ) the rim portion  9  includes a rim extending upwardly, so that the rim portion  9  is thicker in the height direction than the other portions of the plate  1 . The plate  1  is preferably formed of metal, such as an aluminium/copper alloy.  
      Turning to  FIG. 2 , a structure according to the invention is shown in cross section. The plate  1  is mounted on a substrate  11  having three layers  13 ,  15 ,  17 . The upper layer  13  contains a square central aperture so that the substrate  11  includes a cavity  21 . A flipchip  22  is located in the cavity  21  and connected to the bottom surface of the cavity  21  by protrusions  23 . These protrusions are surrounded by an underfill layer  25 , which may be of resin. The central portion  3  of the device  1  is sandwiched between the flipchip  22  and the die  27 , and preferably connected to each by a heat-conductive glue. The pads on the die  27  are connected by wire bonds  29  to corresponding pads on the upper surface of layer  13  laterally outward from the cavity  21 . The die  27  is encased in resin  31 . The undersurface of the substrate  11  is provided with eutectic solder balls  33 . In this cross section two of the diagonal arms  7  are visible laterally outward from the resin  31 . Since the plate  1  contacts both the die  27  and the flipchip  22 , it is able to receive heat generated within each and transmit it out of the structure laterally (i.e. in the sideways direction in  FIG. 2 ). Note that the plate  1  preferably extends laterally outside the resin  31  in all four lateral directions.  
      Turning to  FIG. 3  the structure of  FIG. 2  is shown in an exploded view, with the plate  1  taking the form shown in  FIG. 1 . In this view the pads  35  on the upper surface of the layer  13  are visible, with their corresponding via holes  36 . Note that when the plate  1  rests on the substrate  11 , the diagonal arms  7  tend not to cover any of these pads  35 . The lateral arms  5  do however cover some of the pads  5 . For this reason the lateral arms  5  may be omitted. Alternatively, if the lateral arms  5  are included, the die  27  may be designed such that its pads which correspond in position to the position of these arms (i.e. its pads at the centre of its sides) are the pads which are to be connected to ground. In this case, these pads may be connected directly to the plate  1  rather than to pads on the substrate  11 .  
      Note that it is preferred that the rim portion  9  of the device  1  (i.e. the portion of the device  1  which entirely encircles the die  27 ) is laterally outward of the edge of the substrate  11 . This is because the upper surface of the substrate  11  may include a number of areas (such as via holes) having a function which would be disrupted if they were connected to ground. Since the rim  9  is laterally outward of the substrate, the area at which the substrate  11  and plate  1  contact each other is minimised.  
      The order of steps used to form the arrangement of  FIG. 3  is as follows. Firstly, after bumping, the flipchip  22  is located on the substrate  11 . Then, the underfill layer  25  is applied. Then the plate  1  is attached to the flipchip  22  by heat-conductive glue. Then the die  27  is attached to the plate  1  by heat-conductive glue. To avoid pressure of the die  27  upon the flipchip  22  the flipchip  22  should be larger in area than the die  27 , and this feature also has advantages in terms of the IO count of the two devices. Then the wire-bonding is done to connect the substrate  11  and the die  27 . Once wirebonding is completed, the resin  31  is applied. As shown in  FIGS. 2 and 3  the resin  31  is only applied to a central region of the substrate  11  (using a mould, not shown), however the plate  1  can itself constitute the mould and in this case the resin might extend laterally as far as the rim  9 . Alternatively, another rim might be formed on the plate  1  laterally inward of the rim  9  to provide the sides of a mould in which the resin  31  could be formed. Curing of the resin  31  is performed only once to avoid die crack. The marking is done to complete the packaging.  
      Optionally, further heat dissipative devices may be attached to the plate  1  (at this stage, or earlier) to aid the transmission of head out of the plate  1 .  
      The second embodiment of the invention is shown with reference to FIGS.  4  to  6 . The second embodiment relates to a LFBGAS (low profile ball grid array package) with a fine ball pitch (0.5, 0.65 or 1.00 mm). Such BGA packages, delivering higher performance and thermal dissipation, are shrinking in size, so that packaging such silicon dies in an increasing challenge.  
      The headspreader plate shown in  FIG. 4 ( a ) is an aluminium/copper alloy. It is provided as a matrix  41  having central portions  43  for location under integrated circuits and diagonal arm portions  47 . It is provided with a strip  49  including holes  51  for location onto holes  53  provided on a substrate strip. The substrate  55  is shown in top view in  FIG. 4 ( b ). It has slots  57 , and is provided with a heat-conductive adhesive  59  located in the pattern shown in  FIG. 4 ( b ), having regions  61  onto which the central portions  43  of the matrix  41  are located. It further has regions  63  onto which the arm portions  47  of the matrix  41  are located.  
      The structure of a portion of the arrangement after the matrix  41  is attached to the substrate  55  is shown in cross-section in  FIG. 5 . The substrate  55  is provided with via holes  63  and eutetic solder balls  65  connected to the substrate  55  by a copper connection. The substrate  55  is connected to the matrix  41  by the heat-conductive adhesive  59 . Subsequently, the die  67  is connected to the central portion  43  of the matrix  41  using the same heat conductive adhesive.  
      Wire-bonds  69  are produced. Optionally, ground pads at the corners of the die can be directly connected to the diagonal arms  47 . Then a resin  71  is formed encasing the die  67  and the wire bonds  69 .  
      Singulation is now performed, separating the structure of  FIG. 5  into separate units  73  each including a single die  67 . The result is shown, in an exploded view, in  FIG. 6 . Note that due to the singulation process, the matrix  41  has been sliced into a section  75  within each unit  73  which includes a single central region  43  and four diagonal arms  47  (each of which is half as long as the diagonal arms of  FIG. 4 ( a )). By the section  75  of the matrix  41  heat generated by the die  67  is transmitted out of the package at its corners. The section  75  may be connected to ground.  
      Although only two embodiments of the invention have been described in detail, many variations of them are possible within the scope of the invention as will be clear to a skilled reader.