Abstract:
The present invention thermally enhanced package with embedded metal slug and patterned circuitry discloses a thermal enhanced package with an embedded metal slug that can be easy directly assembled to the printed circuit board to significantly improve package&#39;s thermal dissipation efficiency through the assistance of metal traces in the application board.

Description:
TECHNICAL FIELD OF THE INVENTION 
       [0001]    The present invention is directed, in general to semiconductor packaging, and more specifically, to a thermally enhanced semiconductor package with an embedded solid metal slug and patterned circuitry. 
       BACKGROUND OF THE INVENTION 
       [0002]    High voltage and high frequency applications normally require substantial amount of power to perform specific functions. As power increases, the semiconductor chip&#39;s temperature would increase accordingly if the thermal management of the device were not properly designed. Drawbacks of this high temperature operation includes performing at lower speeds, exhibits non-ideal operating characteristics and relatively shorter operating life span. Furthermore, the less than desirable performances can be aggravated by the trend of miniaturization as there is less surface area to dissipate the heat away since chips and passives are placed closely together in the package or module for accommodating a smallest possible profile. 
         [0003]    In order to achieve the desired performances for high power IC devices, the designer needs to ensure that the semiconductor package is capable of dissipating a large amount of heat and this would largely depend upon the heat-carrying characteristics of the package and thermal management strategies. 
         [0004]    Plastic packages such as ball grid array packages (PBGA) are built using laminate-based substrates and the heat dissipation of these packages are mainly through the fiber glass and dielectric material in the laminate substrate to the solder balls and then to the attached printed circuit board (PCB). However, since fiber glass and plastics have very low thermal conductivity and provide poor characteristics in both heat conduction and heat spreading and hence plastic BGA have relatively poor thermal performances. 
         [0005]      FIG. 1  describes a typical Quad-Flat-No Lead (QFN) package in which the semiconductor chip is attached to the die pad, which in turn is soldered to the PCB directly to enhance its thermal spreading function. Since the heat conduction includes copper die pad and the attached printed circuit board, QFN packages in general exhibit better thermal characteristics than that of the PBGA. However, due to the limited routing capabilities of lead-frame type interposer, QFN is unable to accommodate high I/O devices (for example, more than 100) and suffers from many assembly difficulties when trying to accommodate passive elements therein. 
         [0006]    To achieve similar thermal characteristics for plastic laminate package, U.S. Pat. No. 6,670,219 discloses a thermal enhanced package wherein a heat sink and a ground plate are adhered together to form the thermal dissipating substrate. As shown in  FIG. 2 , a cavity is formed in a central portion of the substrate to allow chip contacts the heat spreader of the package for a better thermal dissipation. However, this “cavity-down” ball grid array (CDBGA) configuration suffers a major drawback in that the heat spreader doesn&#39;t contact the printed circuit board directly when assembled. As such, the primary heat dissipation mechanism becomes thermal convection instead of thermal conduction and that mechanism greatly limits the heat spreading and dissipation efficiency. Furthermore, passive assembly is also being constrained due to the heat spreader spanning across one side of the package and solder balls from the other side. 
         [0007]    U.S. Pat. No. 6,528,882 discloses a thermal enhanced ball grid array package wherein a metal core layer is enclosed in the substrate to enhance the thermal performance. As shown in  FIG. 3 , even though the internal thermal pathway can be improved due to direct attachment of the chip to the metal core, the thermal insulation layer deposited on the bottom surface of the metal core causes considerable heat resistance that reduces the thermal performance of the package. 
         [0008]    U.S. Pat. No. 7,038,311 disclose a thermal enhanced ball grid array package wherein a plastic substrate having an opening therethrough is covered by a metal slug. As shown in  FIG. 4 , this metal slug serves as the die paddle and can be soldered to the PCB directly for the improvement of thermal performance. Since the generated heat can be directly conducted to the PCB through the high thermal conductivity metal slug and solder balls, this construction displays a desired thermal performance equivalent to that of a QFN. However, the drop-in metal slug normally induces an un-even substrate surfaces, and this non-planar issue often cause tremendous difficulties in chip assembly especially die bonding, wire bonding and molding, and therefore suffers reliability, yield loss and significant higher cost problems. 
         [0009]    Prior arts disclosed in U.S. Pat. No. 6,900,535, U.S. Pat. No. 6,541,832, and U.S. Pat. No. 6,507,102 etc., provides solutions wherein the drop-in metal slug is adjusted to essentially the same level as the terminal leads. As shown in  FIG. 5 , this approach still suffers from many challenges in terms of assembly cost and package reliability. For example, during metal slug attachment, it is not easy to achieve a consistent bond line for good reliability as this requires void free bonding with very tight lateral (x-y) placement tolerance. The reason for the poor reliability is that the inserted metal slug does not provide rigidity support for the cavity-opened substrate, and the partial attachment makes the substrate becomes fragmental. Furthermore, the differences in the thermal expansion coefficients among metal slug, laminate substrate, and molding compound can result in potential de-lamination at the interfaces. This in turns results in ingression of moisture into the molded plastic package that leads to corrosion and posing a serious threat to reliability of integrated circuits. 
         [0010]    Considering the deficiencies of the above-mentioned prior arts, it would be desirable for a plastic laminate package to perform equivalent or better thermal characteristics to QFN, can accommodate high I/O devices or module, having high package reliability, low cost and does not require expensive tooling of the substrate and heat spreader. 
       SUMMARY OF THE INVENTION 
       [0011]    The present invention discloses a thermal enhanced package with an embedded metal slug that can be directly assembled to the printed circuit board to significantly improves package&#39;s thermal dissipation efficiency through the assistance of metal traces in the application board. 
         [0012]    It is another object of the present invention to provide a thermal enhanced package whereby the embedded metal slug and terminal leads are portion of a single piece of metal. This single metal structure ensures high package reliability and enables a planar bottom surface for high assembling yield. 
         [0013]    It is yet another object of the present invention to provide a thermal enhanced package wherein the multiple routing layers enclosed in the substrate allow multiple chips to be packaged in conjunction with multiple passive elements. 
         [0014]    One aspect of the invention is the flexibility of the package interface with the application board; the options include package designs as land grid array, ball grid array, or pin grid array. 
         [0015]    The technical advances represented by the invention, as well as the aspects thereof, will become apparent from the following description of the preferred embodiments of the invention, when considered in conjunction with the accompanying drawings, thermal performance and the novel features set forth in the appended claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]      FIG. 1  illustrates schematic cross-sectional views of the conventional Quad-Flat-No Lead (QFN) packages 
           [0017]      FIG. 2-5  illustrates schematic cross-sectional views of the conventional thermal enhanced ball grid array packages 
           [0018]      FIG. 6  is a schematic, cross-sectional view of a thermal enhanced package according to the preferred embodiment of the present invention. As the semiconductor chip is depicted with its integrated circuit (IC) facing upwards relative to the connection to the application board, it is therefore referred to as a “cavity up” package. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0019]    As shown in  FIG. 6 , the thermal enhanced package designated  600 , comprises a substrate  610  that includes an embedded metal slug  620 , a patterned circuitry  630 , and a plurality of terminal leads  640 . 
         [0020]    A pivotal part of the package of the present invention is the embedded metal slug  620  in the substrate  610 . In the preferred embodiment, the embedded metal slug  620  consists of a die pad portion  620 A and a thermal pad portion  620 B. The die pad portion  620 A exposed from the upper surface of the substrate  610  and the thermal pad portion  620 B exposed from the bottom surface of the substrate  610 . An essential feature of the embedded metal slug  620  is that the exposed surface area of the thermal pad portion  620 B is larger than that of the die paddle portion  620 A. This configuration allows the heat generated from the semiconductor chip can be transferred to the die pad and quickly spread out to a much larger area for effective thermal dissipation. The exposed surface of the die pad portion  620 A is typically deposited with a combination of metal layers such as nickel/palladium/gold for a better die attachment interface. Likewise, the exposed surface of thermal pad portion  620 B is deposited with a similar combination of metal for solderable finishing purpose. 
         [0021]    As shown in  FIG. 6 , the patterned circuitry  630 , consisting of at least one conductor layer  631  and at least one dielectric layer  632  alternatively stacked on one another, is provided in a region adjacent to the die pad portion  620 A and on the upper surface of the thermal pad portion  620 B and terminal leads  640 . The patterned circuitry  630  is adhered to the die pad portion  620 A through the dielectrics  632  to ensure embedded metal slug  620  is securely bonded to the substrate  610  vertically and horizontally. A plurality of metallized via holes  633  in the dielectric layer  632  is provided to electrically connect the patterned circuitry  630  to the terminal leads  640 . The metallized via holes  633  can also connect the patterned circuitry  630  to the embedded metal slug  620  through thermal pad portion  620 B when electrical grounding or power is needed. 
         [0022]    The dielectric layer  632  can include epoxy resin, glass epoxy resin, Ajinomoto build-up film (ABF) or bismaleimide-triazine (BT) resin. A commercially available substrate such as FR-4 substrate, FR-5 substrate and BT substrate can be used as the dielectric layer, if desired. The via hole  633  can be formed by laser ablating or through hole drilling. The laser used typically includes gas laser, solid laser, such as carbon dioxide laser, yttrium-aluminum-garnet laser (YAG laser). 
         [0023]    A plurality of terminal leads  640 , which is made of the same material as embedded metal slug  620  is formed on the lower portion of the substrate  610  for signal input/output purpose. It is essential for the present invention that the terminal leads  640  are horizontally aligned with the thermal pad  620 B disposed on the bottom surface of the substrate  610 . This co-planarity feature is naturally formed since they are made of a single piece of metal. Thus, no additional concerns would add to the production process. The co-planarity of the thermal pad  620 B with the terminal leads  640  is essential not only for package reliability and proper board level assembly, but also to ensure that when under operation, the heat generated from IC can freely flow through the die pad portion  620 A to the larger thermal pad portion  620 B before dispersed to the metal traces in the application board (not shown). This configuration provides an extremely short thermal path and the largest possible contact area therefore ensures an excellent heat dissipating efficiency of the package. 
         [0024]    As used herein, the term “terminal lead” is to serve as connection to other parts or to printed circuit board and does not imply that the contacts are necessarily in a specific shape. They may have various forms, such as land, ball, pillar, pin, post, semispherical, truncated cone, or generally bump. The exact shape is a function of the formation technique (such as etching, plating) and soldering technique (such as infrared or radiant heat). 
         [0025]    The attachment of chip  601  is typically performed with a conductive paste, heat conductive tape or soft solder, which is standard in semiconductor technology. 
         [0026]    The semiconductor chip  601  includes the plurality of bonding pads (not shown) are wire bonded  602  to the conductor layer  631  integral with the patterned circuitry  630 . Wire bonding  602  is the preferred method of using coupling members to create electrical interconnections between the plurality of chip bonding pads and the conductor layer  631 . Other methods such as flip chip bonding and ribbon bonding can be applied as well. 
         [0027]    In this package configuration, each terminal lead  640  is electrically connected with one specific via hole  633 , and one specific conductor line  631 , which is in turn connected to one specific bond pad of the integrated circuit die  601  through one specific wire bond  602 , and thus functions as an input/output for the packaged device. 
         [0028]    The chip  601 , the wire bond  602 , and the substrate  610  are encapsulated with a molding compound  650 . If needed, a heat sink can be further provided on the surface of the chip  601  or molding compound  650  to further increase the heat dissipation and performance of a package. 
         [0029]    While this invention has been described in reference to illustrative embodiments, this description is not intended to be construed in a limiting sense. Various modifications changes, substitutions, variations, enhancements, gradations, lesser forms, alterations, revisions and improvements of the invention disclosed herein may be made without departing from the spirit and scope of the invention in its broadest form. As an example, the finishing of bottom surface of die pad and terminal leads may comprise gold, nickel, silver, palladium, tin, solders or any other soldering material used in manufacturing. As another example, the number of patterned conductor layer  631  in the patterned circuitry  630  used for signal routing may include multiple layers, thus provides a multi-level substrate for a flexible design in package.