Abstract:
The package of the present invention includes a chip located on a substrate with signal transferring device electrically connected between them. Solder balls connect the substrate and thus electrically connect the substrate to external circuits. Molding compound is covered to protect the chip and signal transferring means. The heat-slug is capped over the molding compound through a conductive glue. The entire area of the upper surface of the heat-slug is exposed to the ambient to improve the ability to spread heat.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a semiconductor package, more particularly, to a semiconductor package which can improve the efficiency of spreading heat. 
     BACKGROUND OF THE INVENTION 
     In light of the trend to smaller and smaller-sized electronics devices, conventional discrete circuits are narrowly fabricated into a silicon-based or GaAs-based wafer using integrated circuits technology. The advent of the ULSI era in chip density has forced a radical upgrading of semiconductor processing technology. An integrated circuit chip itself is fragile and very tiny in dimension. There is a desire, for practical applications, to “wrap it up” thus isulating it from external force or environmental factors, which may cause the chip to be electrically or physically damaged. Meanwhile, it is needed to connect the chip to other external circuits to make the circuits combined with on-chip and off-chip circuits, forming a device to perform a specific function. This technology, “electronic packaging”, puts these two needs into realization, which not only makes the chip properly located and well-connected to the external circuits, but also forms connections between the chip and external circuits. 
     Objectives of the IC chips packaging designs are not only to provide a substantial lead system, physical protection, environmental protection, but also to provide a heatsink for the chip. 
     For the present, electronic products have a trend to be becoming slighter and smaller. To satisfy the above requirements, integrated circuit technologies have been progressing in producing high-density, high-speed, high-capacity and light chips. The increasing power dissipation from on-chip circuits arouses an issue the IC is over-heated, which would make some electronic components walk off the range which they may always feature at normal temperature. Some components have changes in their properties, and some, even, get damaged perpetually. There is an expectation to deal with such a heat-spreading problem in packaging technology as the increase on speed of the current IC packages. 
       FIG. 1  shows a conventional package which includes a substrate  2 , a die  4  formed on the substrate via die attach epoxy  6 . The die is electrically connected to the substrate  2  through gold wire bonds  8 . Solder balls  10  for signal transfer are formed on the bottom surface of the substrate  2 . Molding compound  12  is used to cap the die  4  and gold wire bonds  8  for protection purpose. Heat from the die  4  is spread by using thermal vias  14  in the substrate  2  and thermal balls  16  connected to the thermal vias  14 . However, the heat generated by components in the die is increased due to the increasing packaging density. This also causes the conventional package to fail to satisfy the future demands. 
     As the semiconductor production continuously grows, many structures of packages are suggested. Among them, a plastic molded package can be found, as described in U.S. Pat. No. 5,586,010. Another structure of package is disclosed in U.S. Pat. No. 5,629,835 to Mahulikar et al., entitled “METAL BALL GRID ARRAY PACKAGE WITH IMPROVED THERMAL CONDUCTIVITY”. 
     For the present, conventional packages such as SOP and PQFP-type packages are not able to further increase the number of the lead frames around them. For the sake of more lead frames, the current packaging technology has turned to BGA-type packages. The BGA package is featurized by its spherical I/O-functioned leads, which are shorter, and hence operate with higher speed, and are not apt to become deformed. Therefore, the BGA packaging is well-suited for the future packaging topology. The spherical leads of the BGA are arranged as an array, but not circumferentially about the package as conventional lead frames are. Consequently, the BGA can readily increase the spherical balls on it. Coupled with the larger pitches, the BGA are a rather competitive candidate as considered a current and future packaging type. 
     Many proposals for an improved heat-spreader equipped BGA package are put forth. For example, kinds of heat slugs, heat sinks in any shape are attached to packages or packaging structures to improve the efficiency of spreading heat from packages. 
       FIG. 2  shows the package in the prior art  2 . Two wings of the heat spreader  32 ′ is fixed on the substrate  20 ′ by a soft material, and thus the heat spreader  32 ′ is well supported. Then the substrate  20 ′ and the heat spreader  32 ′ is sealed with molding compound  30 ′ but the top side of the heat spreader  32 ′ exposed. Actually, there are many holes (not shown) through the heat spreader  32 ′, and molding compound  30 ′ is driven into the heat spreader  32 ′ therethrough. Unfortunately, there may be defects on the molding compound  30 ′ (e.g., some air may be left in the molding compound) which would make the thermally conductive paths between the molding compound  30 ′ and the heat spreader  32 ′ discontinuous. The heat resistance between them is thus raised making the efficiency of heat spreading poor. Additionally, two wings of the heat spreader  32 ′ are sealed within the molding compound  30 ′ but only the top surface at the center of the heat spreader  32 ′ is exposed to the ambient, which further worsens efficiency. 
     With the increase in speed of current and future integrated circuits, the current heat-spreading mechanisms are expected to be further improved. To settle the problem of heat spread in a package, the present invention suggests two structures with high-performance capability of spreading heat from a package, which are different from those in prior art as to their fabrication processes and structures. 
     SUMMARY OF THE INVENTION 
     It is an objective of the present invention to provide heat slug equipped packaging structures, which are able to sink more heat from a package as compared to those used in prior arts. 
     There are two structures proposed to enhance the efficiency of spreading heat in accordance with this invention. Different from the process used in the prior art of attaching a heat slug atop the substrate before molding process, the present process is to mold on the die to protect it and to proceed with heat slug attachment in this invention. By enlarging the area of the heat slug contacted with ambient air, the efficiency of spreading heat is apparently risen. However, in the prior art, large parts of the area of the heat sink embedded in the molding compound layer, and hence less area is contacted with the ambient. Obviously, the evidence tells that the proposed structures show better performance in reducing thermal resistance between the junctions of transistors in the chip and the ambient, and hence achieve improved heat sink equipped packaging structure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing aspects and many of the attendant advantages of this invention will become more readily appreciated as the same becomes better understood by reference to the following detailed description, when taken in conjunction with the accompanying drawings, wherein: 
         FIG. 1  is a cross-sectional diagram of a heat sink equipped package in accordance with the prior art  1 . 
         FIG. 2  is a cross-sectional diagram of a heat sink equipped package in accordance with the prior art  2 . 
         FIG. 3  is a cross-sectional diagram of the heat sink equipped package in the first embodiment in accordance with the present invention. 
         FIG. 4  is a cross-sectional diagram of the heat sink equipped package in the second embodiment of the present invention. 
         FIG. 5   a  is a diagram representative of a die bonding process representative of the manufacturing process flow of the heat sink equipped package in the preferred embodiment of the present invention. 
         FIG. 5   b  is a diagram representative of a wire bonding process in the manufacturing process flow diagram of the heat sink equipped package in the preferred embodiment of the present invention. 
         FIG. 5   c  is a diagram representative of a molding process in the manufacturing process flow diagram of the heat sink equipped package in the preferred embodiment of the present invention. 
         FIG. 5   d  is a diagram representative of the assembly process of the heat-spreader in the manufacturing process flow of the heat sink equipped package in the preferred embodiment of the present invention. 
         FIG. 5   e  is a diagram representative of the ball placement process in the manufacturing process flow of the heat sink equipped package in the preferred embodiment of the present invention. 
         FIG. 5   f  is a diagram representative of a singulation process in the manufacturing process flow diagram of the heat sink equipped package in the preferred embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The present invention discloses two packaging structures each equipped with a heat slug, and their manufacturing processes. 
     Turning to  FIG. 3 , the cross section view of the package in the first embodiment is illustrated. As shown therein, the package  10   a  includes a substrate  20 . A semiconductor chip or die  22  is fixedly adhered to the substrate  20  by means of a die attaching material such as die attaching epoxy  24 . The substrate  20  has a first major surface and a second major surface. The first major surface is referred to as the upper-sided surface of the substrate and the second major surface is referred to as the lower-sided surface of the substrate. The substrate  20  includes a plurality of conductive traces (not shown), such as flexible printed circuits formed therein. The conductive traces of the substrate  20  are used to provide electrical conductive paths for signal transfer. The material used for the substrate can be a dielectric material, for example, polyimide, phenolic resin or bismaleimidetriazine (BT). Of course, any other suitable materials can be used for the substrate. The conductive traces can be made of gold, copper or conductive metal or alloy. 
     Again referring to  FIG. 3 , the chip (die)  22  and the substrate  20  are interconnected by means of signal transferring means such as bonding wires  26 , which can be, for example, gold wires. Actually, the die  22  is connected to the conductive traces of the substrate  20 . Using conventional wire bonding or some other techniques, the chip  22  is coupled to the conductive traces. As aforesaid, the conductive traces are on the substrate for providing electrical connective paths. One end of the bonding wire  26  is connected to the chip  22  via a conductive pad formed thereon, the other end of the bonding wire  26  is connected to a solder ball of a BGA array  28  formed on the lower-sided surface (second major surface) of the substrate  20  via the conductive traces. 
     Molding compound  30  is covered on the die  22  to protect the die  22  and the signal transferring device  26 . A thermally conductive material is layered on the top surface of the molding compound. The heat-spreading device is capped atop the thermally conductive material and the molding compound, wherein the thermally conductive material acts as a material for thermal transfer from the molding compound to the heat-spreading device. The heat-spreading device can be made of highly conductive material, such as copper, silver, metal, or metal alloy. 
     Ball grid array (BGA), preferably a solder bump array  28 , is formed on the lower-sided surface of the substrate  20  by conventional positioning technique and using a solder re-flow operation. The solder bumps  28  are used for electrically coupling to the chip  22 . It is appreciated that metal alloy can be used to act for the solder bumps  28 . Typically, at an end of each conductive trace on the substrate  20  is connected to one of the solder bumps  28 . Solder bumps  28  are terminals of the foregoing electrical conductive paths which permit electrical signals to transfer to the chip  22 . In general, the solder bumps  28  are arranged in a matrix array configuration. 
     In the preferred embodiment of the invention shown in  FIG. 4 , there exists a downward bump  31  at the center of the heat slug which is geometrically different from the heat slug shown in FIG.  3 . The compound  30  has a receessed portion to receive the downward bump  31 . The bump  31  makes the heat slug near the top side of the die  22 , and thus conducts heat from the die  22  more efficiently. It is noted that the bump  31  should not attach tightly to the die  22 , but should have some spacing from the die  22  to prevent the die  22  from rubbing against the bump  31  caused by the different thermal expansion coefficient. A thermally conductive glue can be added between the bump  31  and the die  22 . The glue also isolates the bump  31  with respect to the die  22 . 
     Referring to  FIGS. 5   a - 5   f , which depict the manufacturing process of the structure in the second embodiment of the invention. The process starts with die bonding as shown in  FIG. 5   a , and is then succeeded by wire bonding as shown in  FIG. 5   b , molding as shown in  FIG. 5   c , and then by assembly of a heat-spreading device as shown in  FIG. 5   d . The assembly process begins with priming a thermally conductive glue  48  on the concave of the molding compound  30 . Next the heat-spreading device  32  is fixed onto the molding compound  30  and the glue  48  by a vacuum pick head  52 . The glue  48  acts as an intermediate layer for conducting heat from the die to the heat-spreading device, which then conducts heat away from the package to the ambient. 
     The prototype of the structure appears with the finishing of the assembly for the heat-spreading device. Then ball placement is undertaken to connect external circuits by implanting solder balls onto the conductive plate below the substrate as shown in  FIG. 5   e . Finally there is singulation to obtain individual packages from batches of packages in the manufacturing flow, which is shown in  FIG. 5   f . The method for formation of the structure in the first embodiment is similar to the method for formation of the structure in the second embodiment. 
     Finally, the comparison of thermal performances among the three packages is shown in Table 1. As set forth therein, 5.0W power is applied to the tree packages respectively with the ambient temperature of 22° C., and with their heat spreaders  32 ′ made of aluminum and copper. The package in the prior art  2  shows a thermal resistance of 16.72° C./W, and 18.83° C./W for aluminum and copper-made heat spreader respectively. The package in the first embodiment is 16.53° C./W and 16.28° C./W. The package in the second embodiment is 15.71° C./W and 15.34° C./W. By data measured and shown above, the two packaging structures put forth in the present invention are obviously superior to the packaging structure used in the prior art  2 . 
     
       
         
               
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                   
                 Material for the heat- 
                   
                   
                   
               
               
                   
                 spreading device/ 
                   
                   
                   
               
               
                 Structure 
                 Parameters concerned 
                 P 
                 θ ja   
                 T j (° C.) 
               
               
                   
               
             
             
               
                 Prior Art 2 
                 Aluminum 
                 5.0 
                 16.72 
                 105.6 
               
               
                   
                 Copper 
                 5.0 
                 18.83 
               
               
                 The First 
                 Aluminum 
                 5.0 
                 16.53 
                 104.65 
               
               
                 Embodiment 
                 Copper 
                 5.0 
                 16.28 
                 103.38 
               
               
                 The Second 
                 Aluminum 
                 5.0 
                 15.71 
                 100.56 
               
               
                 Embodiment 
                 Copper 
                 5.0 
                 15.34 
                  98.69 
               
               
                   
               
             
          
         
       
     
     As is understood by a person skilled in the art, the foregoing preferred embodiments of the present invention are illustrative of the present invention rather than limiting of the present invention. They are intended to cover various modifications and similar arrangements included within the spirit and scope of the appended claims, the scope of which should be accorded the broadest interpretation so as to encompass all such modifications and similar structures.