Abstract:
A semiconductor package with thermally enhanced properties is described. The semiconductor package includes a substrate upon which a die is affixed. The die and the substrate each have contacts which are respectively connected with each other. A heat sink is affixed to a surface of the die by way of a thermally compliant material. The compliant material reduces the stresses caused by temperature fluctuations which cause the heat sink and the die to expand and contract at different rates. A first molding material is deposited around the periphery of the die, compliant material and heat sink, thereby leaving exposed substantially an entire surface of the heat sink.

Description:
This application is a divisional of application Ser. No. 09/386,319, filed on Aug. 31, 1999 now U.S. Pat. No. 6,208,519, which is hereby incorporated by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to the packaging of semiconductor devices. More particularly, the present invention relates to a packaged semiconductor device and a method for making it. 
     BACKGROUND OF THE INVENTION 
     Semiconductor device packaging techniques are well known. In conventional packaged devices, a die is attached to a substrate, and contacts of each are electrically connected. A heat sink may also be affixed to the die. The die and heat sink are then completely encapsulated, using an overmold (a heated container with a cavity), with a mold material. An example of such a conventional packaged device may be found in U.S. Pat. No. 5,901,041 (Davies et al). 
     Other conventional methodologies include packaging the die and then adding the heat sink, leaving it exposed. Yet other conventional approaches include taping a heat sink to internal leads of the die and encapsulating the die and heat sink. 
     Referring now to FIGS. 1 and 2, there is shown a conventional packaged semiconductor die assembly  10  including a substrate  22 , a die  12 , a heat sink  40 , and a package molding  50 . The substrate  22  has a first surface  24  and a second surface  26 . An opening  28  extends between the surfaces  24 ,  26 . 
     The die  12  has a first surface  14 , a second surface  16 , and one or more sides  18 . The second surface  16  abuts the first surface  24  of the substrate  22 . Electrical contacts  20  are located on the second surface  16  and are connected to electrical contacts  30  on the second surface  26  of the substrate  22 . The contacts  20 ,  30  arc connected by wiring  32  which may be printed or bonded. 
     The heat sink  40  has a first surface  42 , a second surface  44 , and one or more sides  46 . The second surface  44  abuts the first surface  14  of the die  12 . The package molding  50  completely encapsulates the die  12  and the heat sink  40 . Specifically, the sides  18 ,  46  and the first surface  42  are covered by the package molding  50 . 
     One problem with such conventional methodologies is that current overmold techniques generally completely encapsulate the head sink with no surface of the heat sink directly exposed as shown in FIG.  2 . This reduces efficiency of the heat transfer process. 
     There thus exists a need for a packaged semiconductor device having a heat sink which allows greater heat transfer properties, particularly as the density of components within a die increases and heat build up becomes more of a problem. 
     SUMMARY OF THE INVENTION 
     The present invention provides a packaged semiconductor device which includes a substrate, a die connected to said substrate, a heat sink affixed to said die, and a first molding material encapsulating said die and said heat sink, said first molding material leaving exposed substantially an entire upper surface of the said heat sink. 
     The present invention further provides a molded packaged semiconductor device including a die having first contacts, a substrate connected to the die and having second contacts, the first contacts being connected to respective second contacts, a heat sink, a thermally compliant material adhering the heat sink with the die, and a molding material encapsulating the die, the thermally compliant material, and the substrate, the molding material leaving exposed substantially the entire upper surface of the heat sink. 
     The present invention further provides a method for packaging a semiconductor device. The method includes affixing a heat sink to a die located on a substrate, and encapsulating the die and the heat sink with a first molding material such that substantially an entire upper surface of the heat sink remains exposed. 
    
    
     These and other advantages and features of the invention will be more readily understood from the following detailed description of the invention which is provided in connection with the accompanying drawings. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a top view of a conventionally molded semiconductor die assembly; 
     FIG. 2 is a cross-sectional view taken along line II—II of the semiconductor die assembly of FIG. 1; 
     FIG. 3 is a top view of a semiconductor die assembly constructed in accordance with an embodiment of the invention; 
     FIG. 4 is a cross-sectional view taken along line IV—IV of the semiconductor die assembly of FIG. 3; 
     FIG. 5 is a flow chart of steps used in manufacturing the semiconductor die assembly of FIG. 3; 
     FIG. 6 is a cross-sectional view showing injection of a mold around a die in accordance with an embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to FIGS. 3 and 4, where like numerals designate like elements, a semiconductor die assembly  100  is shown having a die  112  and an insulator substrate  122 . The substrate  122  includes a opening  128  to allow electrical connection between contacts  120  on a first surface  116  of the die  112  and contacts  130  on a first surface  126  of the substrate  122 . Preferably, the assembly  100  is a Rambus ball grid array, which is differentiated from other semiconductor packages in that it is a high frequency, high speed and high power dissipation package. 
     The die  112  is generally rectangular having, in addition to the first surface  116 , a second surface  114  and a plurality of sides  118 . The substrate  122  further includes a second surface  124  upon which the die first surface  116  rests. Preferably, the die  112  contains a memory circuit, such as, for example, a DRAM, SRAM, SDRAM or other suitable memory circuit. 
     A heat sink  140  is located above the die  112 . The heat sink  140  includes a first surface  144 , a second surface  142 , and a plurality of sides  146 . The heat sink  140  is formed of a heat conductive material. An example of suitable materials include metals, such as, for example, copper or aluminum, or the oxides thereof. 
     The heat sink  140  need not, however, be formed completely of a metal. Instead, the heat sink  140  may be formed of a thermally conductive material and be metal plated. Alternatively, the heat sink  140  may be formed of a film or hardened paste of polymeric material with thermally conductive components embedded therein. The components may be metallic or inorganic, such as silicon nitride. One commercially available material is Thermaxx™ 2600K adhesive made by National Starch and Chemical Company of Bridgewater, N.J., which has a thermal conductivity factor of about twenty Watts/meter-Kelvin (W/mK) to about forty W/mK. 
     Sandwiched between the heat sink  140  and the die  112  is a thermally conductive compliant material  152 . The compliant material  152  has a first surface  156  which contacts the die second surface  114  and a second surface  154  which contacts the heat sink first surface  144 . The material  152  further has a plurality of sides  158 . The material  152  is preferably an adhesive which serves to dissipate thermal stress caused by differing thermal expansions for two adhered materials, in this case the die  112  and the heat sink  140 . That is, it allows for some relative movement between the two during use when they become heated. An example of the material  152  is QMI 506, a bismaleimide material, which is a silver-filled adhesive manufactured by Quantum Material, Inc. of San Diego, Calif. 
     For example, a heat sink  140  formed of copper has an expansion factor of 15-17×10 −6 ° C., while the expansion factor of a die  112  is about 3×10 −6 ° C. The thermally conductive compliant material  152  assists in dissipating stresses caused by the relative expansions between the die  112  and the heat sink  140 . The thermally conductive compliant material  152  is particularly useful in applications where the die  112  is a large semiconductor die, such as, for example, a die in the range of about 500 mils. 
     The die  112 , material  152  and heat sink  140  are encapsulated within a molding material  150 . The molding material  150  encapsulates the sides  118 ,  158  and  146  of, respectively, the die  112 , the thermally compliant material  152  and the heat sink  140 . While the molding material  150  may encroach slightly above the sides  146  of the heat sink  140 , in general the molding material  150  does not cover the heat sink second (upper) surface  142 . 
     An advantage in leaving exposed the second surface  142  of the heat sink  140  is that the exposed surface  142  provides enhanced thermal dissipation to the assembly  100 . For example, a conventional Rambus ball grid array package operates at a temperature of about 100° C. By leaving exposed the outer heat sink surface  142 , such a package would operate about 10° C. cooler. 
     Another advantage is that the semi-exposed heat sink  140  provides greater protection against mechanical and chemical stress and against moisture. Further, the exposed surface  142  provides a surface upon which identifying marks and/or alphanumeric symbols may be formed. 
     The contacts  120  and the wiring  132  are encapsulated by a molding material  160  which is preferably the same as molding material  150 . Encapsulating the contacts  120  and the wiring  132  protects them against damage from corrosion and/or shock. 
     The molding material  150  may be placed around the die  112  by locating a heated cavity around the die  112  and injecting the molding material  150  in a semi-liquid or gel state (FIG. 6) into the cavity. The molding material  150  then cures and hardens. FIG. 6 shows a first mold cavity  300  and a second mold cavity  350  used for the injection of, respectively, the molding materials  150  and  160 . The cavity  300  includes a plurality of openings  302  extending into a cavity area  304 . The cavity area  304  may be heated by a suitable heating source (not shown) which is well known in the industry. Molding material  150  is injected in the direction of arrows A through the openings  302  into the cavity area  304 . 
     Likewise, the mold cavity  350  has one or more openings  352 . The molding material  160  is injected in the direction of arrow B into the cavity  350 . The contacts  120 ,  130  and the wiring  132  are not shown in FIG. 6 for simplicity of illustration. 
     Alternatively, the molding material  150 , as well as the molding material  160 , may be positioned about the die  112  by dispensing a liquid epoxy, also known as a glob top, where desired. The glob top cures to form the molding materials  150 ,  160  as shown in FIGS. 3,  4  and  6 . 
     An additional advantage of the thermally compliant material  152  is that it prevents any of the molding material  150  being injected into the heated cavity  304  from entering between the heat sink first surface  144  and the die second surface  114 . Thus, the compliant material  152  not only insures good contact between the heat sink  140  and the die  112 , because it is compliant it also ensures that there is a no gap in the area between the upper surface of the heat sink  140  and the mold cavity  300 . To further insure that no molding material  150  gets between the upper surface of the heat sink  140  and the mold cavity  300 , a biasing force may be exerted on the heat sink  140  in the direction of the die  112  during the molding process. 
     Referring now to FIG. 5, next will be described a method of packaging a semiconductor device. First, at step  200 , the die  112  is affixed to the substrate  122  and the wiring  132  is applied between the contacts  120  and respective contacts  130 . As noted, this may be through a wire bonding or a printed contact. 
     The thermally conductive compliant material  152  is dispensed at step  210 . The material  152  may be an adhesive, such as the silver-filled bismaleimide material described above. At step  220 , the heat sink  140  is attached to the die  114  through the compliant material  152 . The material  152  is then cured at step  230 . 
     Step  240 , is the application of the molding material  150  to the sides  118 ,  158 , and  146  of, respectively, the die  114 , the compliant material  152 , and the heat sink  140 . The application of the molding material  150  may be through cavity injection, as described above, or by another suitable method, such as through glob top dispensing. Optionally, at step  250 , the exposed surface  142  of the heat sink  140  may be marked, either by laser or by other suitable marking structure. 
     The present invention provides a packaged semiconductor device with enhanced thermal stress reduction, environmental protection and enhanced thermal dissipation properties stemming from an exposed heat sink. The present invention further provides a method for packaging a semiconductor package leaving an exposed surface of the heat sink without encountering the problem of having the molding material leak onto an upper surface of the heat sink during the molding operation. 
     While the invention has been described in detail in connection with the preferred embodiments known at the time, it should be readily understood that the invention is not limited to such disclosed embodiments. Rather, the invention can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described, but which are commensurate with the spirit and scope of the invention. Accordingly, the invention is not to be seen as limited by the foregoing description, but is only limited by the scope of the appended claims.