Abstract:
A no-lead electronic package including a heat spreader and method of manufacturing the same. This method includes the steps of selecting a matrix or mapped no-lead lead frame with die receiving area and leads for interconnect; positioning an integrated circuit device within the central aperture and electrically interconnecting the integrated circuit device to the leads; positioning a heat spreader in non-contact proximity to the integrated circuit device such that the integrated circuit device is disposed between the leads and the heat spreader; and encapsulating the integrated device and at least a portion of the heat spreader and leads in a molding resin.

Description:
CROSS REFERENCE TO RELATED APPLICATION(S) 
     This application claims the benefit of U.S. Provisional Application No. 60/777,316, filed Feb. 28, 2006, which is incorporated by reference as if disclosed herein in its entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     (1) Field of the Invention 
     This invention relates to packages for encapsulating one or more semiconductor devices, and more particularly to a method for the assembly on a no-lead package having exceptional thermal performance. 
     (2) Description of the Related Art 
     In lead frame based semiconductor packages, electrical signals are transmitted between at least one semiconductor device (die) and external circuitry, such as a printed circuit board, by an electrically conductive lead frame. The lead frame includes a number of leads, each having an inner lead end and an opposing outer lead end. Inner lead ends are electrically interconnected to input/output (I/O) pads on the die and outer lead ends provide terminals outside the package body for interconnection to external circuitry. When the outer lead end terminates at the face of the package body, the package is known as a “no lead” package. If the outer leads extend beyond the package body perimeter, the package is referred to as “leaded.” Examples of well known no-lead packages include quad flat no lead (QFN) packages which have four sets of leads disposed around the perimeter of the bottom of a square package body and dual flat no lead (DFN) packages which have two sets of lead disposed along opposite sides of the bottom of a package body. Interconnection of the die to the inner lead ends is typically performed using wire bonding, tape automated bonding (TAB) or flip chip bonding. In wire bonding or TAB bonding, the inner lead ends terminate a distance from the die and are electrically interconnected to I/O pads on an electrically active face of the die by small diameter wires or conductive tape. The die may be supported by a die pad which is surrounded by the leads. In flip chip bonding, the inner lead ends of the lead frame extend beneath the die and the die is flipped so that the I/O pads on the electrically active face of the die contact the inner lead ends by a direct electrical contact, such as a solder joint. 
     A representative QFN package and its method of manufacture is more fully disclosed in commonly owned U.S. patent application Ser. No. 10/563,712 published as PCT International Application No. WO2005/017968 A2 on Feb. 24, 2005. The disclosure of U.S. patent application Ser. No. 10/563,712 is incorporated by reference in its entirety herein. 
     An ongoing objective for the designers of no lead semiconductor packages is better thermal management. That is, the ability to remove heat from the electrically active semiconductor die. The QFN is one of the best lead frame based packages in terms of thermal management and cost, but as integrated circuit devices become more complex, there is a need for improved thermal and electrical performance. Among the options available in the market are the use of heavy wires and metal ribbons to conduct heat away from the integrated circuit die. 
     The use of a heat spreader in a leaded package is disclosed in U.S. Pat. No. 5,608,267 to Mahulikar et al. The use of the heat spreader with a substrate based package is disclosed in U.S. Pat. Nos. 5,977,626 to Wang et al. and 6,432,749 to Libres. The disclosures of U.S. Pat. Nos. 5,608,267; 5,977,626 and 6,432,749 are all incorporated by reference in their entireties herein. 
     None of the prior art designs include a no-external lead, lead frame based package having a heat spreader. Such a package would have enhanced thermal performance as compared to the QFN and other no-lead type packages presently known. 
     BRIEF SUMMARY OF THE INVENTION 
     One aspect of the invention is a method for the manufacture of a no-lead electronic package. The method includes the following: providing a lead frame having desired features including a plurality of leads terminating about a central aperture; positioning an integrated circuit device within the central aperture and electrically interconnecting the integrated circuit device to the leads; positioning a heat spreader in non-contact proximity to the integrated circuit device such that the integrated circuit device is disposed between the leads and the heat spreader; and encapsulating the semiconductor device and at least a portion of the heat spreader and leads in a molding resin. 
     Another aspect of the invention is a semiconductor package, which includes the following: a plurality of leads having inner ends and outer ends disposed about a centrally disposed die pad with a plurality of die pad tie bars extending outward therefrom; an integrated circuit device having an electrically inactive face bonded to the die pad and electrically active face electrically interconnected to the inner leads by wires or TAB bonds; a heat spreader in non-contact proximity to the electrically active face whereby the integrated circuit device is disposed between the die pad and the heat spreader; and a molding resin encapsulating the integrated circuit device, at least a portion of the heat spreader and all but a planar surface of the die pad and the outer ends. 
     Yet another aspect of the invention is a semiconductor package, which includes the following: a plurality of leads having inner ends and outer ends disposed about a centrally disposed aperture; an integrated circuit device spanning the aperture and having an electrically active face directly bonded to the inner ends of the plurality of leads by a solder; a heat spreader in non-contact proximity to an electrically inactive face of aid integrated circuit device whereby the integrated circuit device is disposed between the plurality of leads and the heat spreader; and a molding resin encapsulating the integrated circuit device, at least a portion of the heat spreader, and all but a planar surface of the die pad and the outer ends. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For the purpose of illustrating the invention, the drawings show a form of the invention that is presently preferred. However, it should be understood that the present invention is not limited to the precise arrangements and instrumentalities shown in the drawings, wherein: 
         FIG. 1  is a top planar view of a lead frame matrix as known from the prior art for use in the invention; 
         FIG. 2  is a top planar view of a heat spreader matrix for use in the lead frame array and semiconductor die assembly; 
         FIG. 3  is a cross-sectional view of the heat spreader array of  FIG. 2 ; 
         FIG. 4  is a cross-sectional representation illustrating the heat spreader array bonded to the lead frame array and semiconductor die assembly; 
         FIG. 5  is a cross-sectional representation of a molded package array formed by the process of the invention; 
         FIG. 6  is a cross-sectional representation of singulated wire-bonded packages formed by the process of the invention; 
         FIG. 7  illustrates in cross-sectional representation a process sequence for a wire-bonded package in accordance with the invention; 
         FIG. 8  illustrates in cross-sectional representation a package formed according to the method of the invention; 
         FIG. 9  illustrates in cross-sectional representation a package formed according to the method of the invention; 
         FIG. 10  illustrates in cross-sectional representation a package formed according to the method of the invention; 
         FIG. 11  illustrates in cross-sectional representation a package formed according to the method of the invention; 
         FIG. 12  illustrates in cross-sectional representation a package formed according to the method of the invention; 
         FIG. 13  illustrates a method to assemble a flip-chip bonded package formed by the process of the invention; 
         FIG. 14  is a cross-sectional representation of a flip-chip bonded package formed by the process of the invention; 
         FIG. 15  is a cross-sectional representation of another flip-chip bonded package formed by the process of the invention; 
         FIG. 16  is a cross-sectional representation of another flip-chip bonded package formed by the process of the invention; 
         FIG. 17  is a cross-sectional representation of another flip-chip bonded package formed by the process of the invention; 
         FIG. 18  is a cross-sectional representation of another flip-chip bonded package formed by the process of the invention; 
         FIG. 19  is a cross-sectional representation of a heat spreader flange showing an alignment feature; 
         FIG. 20  is a cross-sectional representation of a heat spreader flange showing an alternative alignment feature; and 
         FIG. 21  is a cross-sectional representation of a lead showing an alignment feature. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  illustrates in top planer view a matrix  10  of lead frames as known from the prior art. Typically, the lead frames are formed from an electrically conductive material that is amenable to controlled chemical etching. Suitable materials include copper and copper alloys, iron-nickel alloys, and aluminum and aluminum alloys. Package features defined by the etch include die pads  14 , leads  16  and tie bars  18 . It is noted that not all features are required for every type package. For example, the die pad  14  is optional in a flip chip package. The matrix forms a repetitive array of package features such that on completion of the assembly process, the array is singulated to a plurality of individual packages. 
     A heat spreader that improves the thermal capability of the QFN package is illustrated in top planer view in  FIG. 2  and cross-sectional representation in  FIG. 3 . A metallic sheet is chemically etched or mechanically punched to form a matrix  32  of heat spreaders  34  interconnected one to another by heat spreader tie bars  36 . The heat spreader  34  and heat spreader tie bars  36  have a typical thickness in the range of 0.1 millimeter to 1.0 millimeter. The heat spreader  34  is formed from a ductile, high thermal conductivity metal such as copper, aluminum and alloys thereof. The heat spreader may be coated to impart a color to enhance contrast for package marking or to impart enhanced and resistance to environmental corrosion. For example, when the heat spreader is formed from copper or a copper-base alloy, it may be coated with nickel by an electrolytic or electroless process. When formed from aluminum or an aluminum alloy, it may be anodized, such as a black anodization process. As shown in  FIG. 3 , subsequent to etching or punching, portions  38  of the tie bars  36  may be mechanically formed to elevate the heat spreaders  34  relative to tie bars  36 . This upset elevates the heat spreader  34  for an amount effective to provide clearance from the wires of a wire bonded package and to enable top most surface  40  to be exposed following package molding if desired. A typical amount of upset, u, is between 0.25 mm and 0.7 mm. 
     With reference to the insert expanding a portion of  FIG. 2 , portion  60  of heat spreader tie bars  36  may imparted with a reduced thickness during etching to facilitate singulation. Such partial etching may also be used on that portion of lead tie bars that is cut during singulation. 
     With reference to  FIG. 4 , the array  32  of heat spreaders  34  is then attached to a feature, such as leads  16  or tie bars to be in non-contact proximity to the die. The array  32  may be attached by an adhesive  42  such as an epoxy or conductive tape. Adhesive  42  is optional and the array  32  may be simply placed in position and held firm with a molding resin. 
       FIG. 5  shows in cross-sectional representation an array  44  after a molding resin  46  has encapsulated the package. Encapsulated components and features include the die, at least a portion of the heat spreader and all but an out lead end  47 . A typical molding resin is a dielectric polymer. The assembly of  FIG. 4  is placed in a suitable mold and molding resin at an elevated temperature is introduced into the mold forming the array of packages  44  shown in  FIG. 5 . After encapsulation, the array of packages is singulated such as by sawing or punching to form individual packages  48  as illustrated in  FIG. 6 . 
     The die  28  is disposed between two metallic plates, the die pad  14  and heat spreader  34 . This provides shielding from both electric and magnetic fields for electrically sensitive devices. 
       FIG. 7  illustrates a process flow to manufacture a wire bonded package  70  in accordance with the invention. A lead frame  72  that may be a member of a matrix or a single lead frame is etched to possess desired features such as leads  74  and a die pad  76 . To support the features following etching, a backing strip  78 , such as an adhesive tape is applied. 
     An integrated circuit device  80  is bonded to an interior surface  82  of die pad  76  by a die attach  84 . Typical die attach material include gold/tin alloy eutectics, gold/silver alloy eutectics, various silver-base alloys and metal filled polymers. Wire bonds  86  or TAB tape then electrically interconnect leads  74  to I/O pads on an electrically active face of the integrated circuit device  80 . The electrically active face of the integrated circuit device  80  includes circuitry and I/O pads while the opposing electrically inactive face is devoid of these features. 
     Heat spreader  88  is next positioned on the leads  74 . Optionally, the heat spreader  88  is affixed to the leads  74  or lead frame tie bars by an adhesive  90  such as an epoxy or conductive tape. Such as an epoxy or conductive tape. A molding resin  91  then encapsulates the integrated circuit device  80 , at least a portion of the heat spreader  88  and a portion of the leads  74 . At least one outer lead surface  92 ,  92 ′ is exposed and forms a planar surface with the sidewalls  94 ,  94 ′ of the molding resin. An outermost surface  96  of the heat spreader  88  may also be exposed and forms a planar surface with sidewall  94 ″ of the molding resin. 
     If the lead frame and heat spreader were provided as members of a matrix, the final step is singulation. If single unit lead frame and heat spreader were used, then singulation is not required. 
     An enlarged view of the package  70  is illustrated in cross-sectional representation in  FIG. 8 . The package includes a thinned portion  60  of the heat spreader tie bars to facilitate singulation by sawing or punching. A second thinned portion  96  mechanically locks the heat spreader  88  in molding resin  91 . 
     A first alternative package  100  is illustrated in  FIG. 9 . In this package, the heat spreader  102  includes a plurality of apertures  104  such that molding resin  91  projects through the apertures to mechanically lock the head spreader in the molding resin. The plurality of apertures  104  may be used in combination with any of the package configurations described herein. 
     A second alternative package  110  is illustrated in  FIG. 10 . In this package, the heat spreader  112  has a recessed central portion  114 . A thermally conductive grease or adhesive such as an epoxy  116  or conductive tape provides good thermal conduction. The thermally conductive grease or epoxy may be a dielectric or electrically conductive depending on the application. When used as a wire bond replacement, it is selected to be electrically conductive. If only for thermal dissipation and not intended to electrically interconnect to I/O pads, then it is selected to be a dielectric to prevent shorting. Peripheral portions  118  of the heat spreader form a planar surface with a sidewall  94 ″ of the package  110  to facilitate the removal of heat by forced air, thermal fluid or contact with an external heat sink. 
     A third alternative package  120  illustrated in  FIG. 11  is similar to the package of  FIG. 10  except that peripheral portions  118  of heat spreader  112  do not form a portion of the sidewall  94 ″ of the package. 
     A fourth alternative package  130  is illustrated in  FIG. 12 . The package  130  has a die pad  132  with a recessed central portion  134 . An electrically conductive, thermally conductive adhesive such as a thermal grease  136 , epoxy, or conductive tape provides both electrical and thermal connectivity between an electrically active face of the integrated circuit device  80  and heat spreader  138 . An exemplary electrically conductive, thermally conductive thermal grease is an emulsion of ceramic or metal particles, such as silver, copper and/or aluminum based, in an organic or silicone fluid. Alternatively, the thermal grease  136  may be replaced with an electrically conductive, thermally conductive epoxy such as a silver filled epoxy or a dispensable solder paste. 
       FIG. 13  illustrates a method for the assembly of a flip chip package  150  in accordance with another embodiment of the invention. Most of the assembly steps for the package  150  are similar to the previously described steps. However, the electrically active face of the integrated circuit device  80  is directly bonded to the leads  74 , and optionally to a central die pad  182  ( FIG. 18 ), by solder bumps  152 . Referring back to  FIG. 13 , solder bumps  152  typically have the height of 0.07 mm and are formed from a suitable solder such as lead-base eutectic, high lead content and pillar bump. Projections  154 ,  154 ′ extend into the molding resin  91  mechanically locking leads  74  and heat spreader  88  in place. 
     Alternative flip chip packages,  150 ,  160 ,  170 ,  180  embodiments of the packages of the invention are shown in  FIGS. 14 through 17 . Most of the features have been previously described. For the flip chip version, the thermal grease  136  is electrically and thermally conductive and electrically and thermally interconnects the heat spreader  102 ,  112  and electrically inactive face of integrated circuit device  80 . As above, thermal epoxies, solder pastes, and conductive tape may substitute for the thermal grease. One suitable thermal epoxy is filled with in excess of 60 weight percent of silver powder. 
     In both the flip chip version and the wire bonded/TAB bonded version, a surface  158  of the heat spreader of any of the heat spreaders  88  may be exposed to the environment forming a planar surface with a sidewall  94 ″ surface of the molding resin  91 . In addition to providing a marking surface, the surface  158  may be exposed to forced air, a thermally conductive fluid or a heat sink to improve thermal management. The shape of the exposed surface may be square, rectangular, circular or any other shape. 
     Referring now to  FIG. 19 , heat spreader tie bars  190  may have bumps  192  to enhance standoff clearance from the wires used for wire bonding. Bumps  192  are also useful to align and lock the heat spreader in position on leads  194 . Apertures  196  may be formed in the leads  194  to further enhance alignment and locking. 
     Alternatively, as shown in  FIG. 20 , bumps  192  may be formed in the package leads  194  or lead frame tie bars. Apertures  196  may be formed in heat spreader tie bars  190 . The bumps  192  again function as alignment and locking features. The bumps are typically formed during the chemical etching process or by coining/punching during the upset process. 
     Referring now to  FIG. 21 , in another embodiment, package leads  194  may include a bump  192  and heat spreader tie bar  190  may include an aperture  196 . Aperture  196  and bump  192  are configured to function as an alignment and locking feature. 
     While the assembly process describes the array of leads and array of heat spreaders being molded together and subsequently singulated, it is within the scope of the invention for the heat spreaders and leads to be singulated prior to encapsulation with the molding resin and a pick and place process used to place individual lead frame assemblies and individual heat spreaders in individual mold cavities for encapsulation. 
     One or more embodiments of the present invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. For example, the process may be used for the manufacture of a DFN package or to encapsulate one or more semiconductor devices and passive electrical devices such as in a hybrid package. Accordingly, other embodiments are within the scope of the following claims.