Abstract:
A thermally enhanced wirebond BGA package having a laminate substrate, an IC device mounted on the substrate, and a metal cap defining a cavity inside the package between the IC device and the metal cap. A substantial portion of the cavity is filled with a thermally enhanced epoxy encapsulant establishing a thermal conduction path between the IC device and the metal cap. The BGA package may be further enhanced by bonding a metal heat slug on the laminate substrate and mounting the IC device on the slug.

Description:
BRIEF DESCRIPTION OF THE INVENTION 
     This invention relates to glob top ball-grid array (“BGA”) electronic packages that are thermally enhanced to accommodate higher powered integrated circuit (“IC”) devices, a.k.a. chips. 
     BACKGROUND OF THE INVENTION 
     Among the variety of electronic packages used in the electronics industry, glob top BGA packages are one of the popular packages for low-end performance applications. Glob top BGA packages are part of a large family of wirebond BGA packages. These wirebond BGA packages utilize low-cost organic material based substrates (a.k.a. laminates) such as Bismaleimide triazine epoxy (“BT”), polyimide, and polytetrafluoroethylene, and are well-suited for the low-end performance applications because of their relatively low cost. 
       FIG. 1  illustrates a cross-sectional view of such a prior art glob top BGA package. The package consists of a laminate substrate  10  having on one side BGA solder balls  20  and on the opposite side a die attach pad (“DAP”)  14  for receiving an IC device (a.k.a. a die)  30 . DAP  14  is typically surrounded by metallized wirebond pads  12  on laminate substrate  10  to which wirebond wires  34  can be bonded. 
     An IC device  30  is attached to DAP  14  using thermally conductive epoxy  32  with its active side facing away from laminate substrate  10 . Wirebond wires  34  make the electrical interconnection between IC device  30  and metallized pads  12  on laminate substrate  10 . One end of each wirebond wire  34  is bonded to a wirebond pad (not shown) on the IC device and the other end of the wire is bonded to a pad  12 . The side of laminate substrate  10  where IC device  30  is attached is then encapsulated with glob top epoxy  40  which is dispensed in sufficient amount to cover IC device  30  and wirebond wires  34 . Glob top epoxy  40  is dispensed in a liquid state and then cured. Glob top epoxy  40  protects IC device  30  and wirebond wires  34  against corrosion and mechanical damage. 
     Although these prior art wirebond BGA packages have relatively poor cooling capability, they have been adequate because IC devices used in low-end performance applications have been relatively low powered. Typical power dissipation of IC devices used in this performance segment up to now has been about 3 Watts. 
     With ever-increasing integration of the IC devices driven by the demand for higher performance, IC devices at all levels of application have been incorporating more circuits per unit area. With every new generation of IC devices, this development has resulted in increases in both the performance and the power output of each device. This trend has also resulted in the utilization of higher performing and higher powered IC devices in the low-cost application segment where glob top BGA packages are commonly used. Therefore, there is a need for low-cost wirebond BGA packages with improved cooling capabilities that can accommodate IC devices with greater than 3 Watts of power dissipation. 
     SUMMARY OF THE INVENTION 
     The present invention provides a thermally enhanced wirebond BGA package. A substrate having two sides is provided on which an IC device is attached to one side. A metal cap having a side wall portion and a top portion is attached to the substrate along peripheral portion of substrate so that the cap forms an internal cavity enclosing the IC device. The cap has one or more holes in the top portion. The internal cavity is filled with an epoxy encapsulant material filling a substantial portion of the internal cavity, wherein the epoxy encapsulant material is in contact with both said IC device and the top portion of the metal cap. 
     The epoxy encapsulant material provides a thermal conduction path between the IC device and the metal cap to improve the package&#39;s ability to cool the IC device. Also in a preferred embodiment, the thermal conductivity of the epoxy encapsulant material is enhanced by dispersing high thermal conductivity particles in the epoxy encapsulant material. The high thermal conductivity particles are preferably also electrical insulators so that a separate electrical insulation between the IC device and the metal cap is not required. 
     Another embodiment of the present invention may incorporate a metal heat slug that is provided between the IC device and the laminate substrate to further enhance the thermal performance of the wirebond BGA package. The metal heat slug has a DAP portion, preferably at least one wirebond pad window portion, and a peripheral rim portion. The metal heat slug is bonded to the top surface of the laminate substrate and the IC device is attached to the DAP portion of the metal heat slug. The metal heat slug may extend out to the peripheral edge portion of the laminate substrate so that the metal cap is attached to the peripheral rim portion of the metal heat slug. In this configuration, the metal heat slug functions as a heat spreader for the IC device and further enhances the thermal performance of the wirebond BGA package by providing a second thermal conduction path between the IC device and the metal cap through the metal heat slug. The metal heat slug is preferably made of a metal or metal alloy having relatively high thermal conductivity such as copper, aluminum or alloys thereof. The metal heat slug may be made of a material with sufficient stiffness so that substrate warping problem sometimes observed after the epoxy encapsulant cure process is minimized. 
     The present invention also includes a method of forming a wirebond BGA package whose structures are disclosed herein. The involved process steps may include: providing a substrate having two sides; attaching a metal heat slug to one of the two sides where the metal heat slug is provided with a die attach pad portion, at least one wirebond pad window portion, and peripheral rim portions; attaching an integrated circuit device to the die attach pad portion; attaching a metal cap to the metal heat slug along the peripheral rim portions forming an internal cavity between the metal cap and the substrate; and filling a substantial portion of the internal cavity with an epoxy encapsulant material. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       For a better understanding of the invention, reference should be made to the following detailed description taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a schematic cross-section of a prior art glob top BGA package; 
         FIG. 2  is a cross-sectional view of a metal cap according to the present invention; 
         FIG. 3  is a top view of the metal cap of  FIG. 2 ; 
         FIG. 4  is an illustration of a metal heat slug that may be used in conjunction with the metal cap of  FIG. 2  to further enhance the thermal performance of a BGA electronic package according to the present invention; and 
         FIG. 5  is a schematic cross-section of an embodiment of a wirebond BGA package according to the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 2  illustrates a cross-sectional view of a metal cap  100  according to the present invention. Metal cap  100  has a sidewall portion  104  and a top portion  102  that form an internal cavity  106 . Metal cap  100  also has a plurality of holes  110  in top portion  102 . Metal cap  100  may be made from metal or metal alloys having relatively high thermal conductivity such as copper, aluminum, or alloys thereof. 
       FIG. 3  illustrates a top view of metal cap  100  showing a plurality of holes  110 . Metal cap  100  is shown as having a square outline. This particular shape is for illustrative purposes only and the particular outline shape and dimension of a given metal cap would be determined by the particular shape of the laminate substrate to which the given metal cap will be attached. 
       FIG. 4  illustrates a top view of a metal heat slug  200  according to an embodiment of the present invention. Metal heat slug  200  has a DAP portion  202  where an IC device would be attached. Along the edge of metal heat slug  200  are peripheral rim portions  207 . Between DAP portion  202  and peripheral rim portions  207  is at least one wirebond pad window portion  205  that provides clearance for wirebond pads  312  (see  FIG. 3 ) when metal heat slug  200  is attached to the top surface of a laminate substrate. Metal heat slug  200  may be made of metal or metal alloy sheets having relatively high thermal conductivity such as copper, aluminum, or alloys thereof. 
     An embodiment of a wirebond BGA package fully assembled according to an embodiment of the present invention is illustrated in  FIG. 5. A  laminate substrate  310  is illustrated as having BGA solder balls  320  on its bottom side. A metal heat slug  200  is bonded to the top surface of the laminate substrate. An epoxy adhesive may be used to bond the metal heat slug to the laminate substrate. As illustrated, wirebond pads  312  sit within wirebond finger window  205  (see  FIG. 4 ) without interfering with metal heat slug  200 . An IC device  330  is attached to DAP portion  202  (see  FIG. 4 ) of metal heat slug  200 . The IC device is typically and preferably attached to DAP portion  202  using a thermally conductive epoxy  332 . Wirebond wires  334  provide the electrical interconnection between the IC device and wirebond pads  312 . 
     Metal cap  100  having a side wall portion  104  and a top portion  102  is then attached to metal heat slug  200  along peripheral rim portions  207 . The resulting internal cavity formed between metal cap  100  and laminate substrate  310  is then filled with epoxy encapsulant material  340 . Epoxy encapsulant material  340  may be dispensed into the internal cavity through one or more holes  110  provided in top portion  102  of metal cap  100 . Epoxy encapsulant material  340  is dispensed in an uncured liquid form and then cured by heating the whole package assembly to a cure temperature of about 150-175 deg. C. Other holes  110  provide escape paths for gases produced by outgassing of epoxy encapsulant material  340  during the curing process. Epoxy encapsulant material  340  may be the same material as glob top epoxy  40  (see  FIG. 1 ) typically used in prior art % wirebond BGA packages. After the epoxy encapsulant is cured, warping of the laminate substrate may be observed in some BGA packages but this problem may be minimized by selecting a metal heat slug  200  having a sufficient stiffness. In another embodiment of the invention, metal heat slug  200  may not be included and metal cap  100  is attached directly onto the laminate substrate  310 . 
     In another embodiment, the thermal conductivity of epoxy encapsulant material  340  may be enhanced by dispersing high thermal conductivity particles in the epoxy encapsulant material. A preferred material for the high thermal conductivity particles is diamond powder, cubic boron nitride, oxides such as alumina, or other materials having high thermal conductivity. Preferably, these high thermal conductivity particles are also electrical insulators so that a separate electrical insulation is not required between the IC device and the metal cap. An example of such thermally enhanced epoxy encapsulant material is Hysol FP4450 encapsulant marketed by Dexter Corporation of Industry, California Hysol FP4450 is enhanced with diamond powder (15% by weight). The thermal conductivity of this enhanced encapsulant is about 2.8 W/MK. In comparison, the thermal conductivity of a conventional glob top epoxy found in prior art wirebond BGA packages is about 0.8−0.7 W/MK. 
     In this configuration of the wirebond BGA package, at least two primary thermal conduction paths are established between IC device  330  and metal cap  100 . A first thermal conduction path is established through epoxy encapsulant material  340  and a second thermal conduction path is established through metal heat slug  200 . In this configuration, metal cap  100  functions as a heat sink that dissipates the heat from IC device  330  that has been conducted to metal cap  100  via the thermal conduction paths described above. 
     Additionally,  FIG. 5  also illustrates a feature of another embodiment of the invention, where a retaining ring  400  may be attached to metal heat slug  200  along peripheral rim portion of the metal heat slug. In this embodiment, retainer ring  400  is attached to metal heat slug  200  before the metal cap attachment. Retainer ring  400  may be attached to metal heat slug  200  using a suitable adhesive such as an epoxy adhesive or other adhesive materials or means. Next, a first dose of an epoxy encapsulant material  340  is dispensed into the center of retainer ring  400  covering IC device  300  until the encapsulant material reaches the top edge of the retainer ring. After epoxy encapsulant material  340  is cured, a metal cap  100  is attached to laminate substrate  310 . Retainer ring  400  acts as a darn around IC device  300  to control the height of the first dose of epoxy encapsulant material  340  so that when metal cap  100  is attached, the top surface of the epoxy encapsulant material  340  comes in close proximity to the inside surface of metal cap  100  forming a small gap between the metal cap and the first dose of the epoxy encapsulant material. 
     Next, a second dose of epoxy encapsulant material  340  is applied through the one or more holes  110  in the top portion of metal cap  100  to fill the gap between the metal cap and the first dose of, now cured, epoxy encapsulant material  340 . The BGA package then goes through a second epoxy cure process to cure the second dose of epoxy encapsulant material  340 . The result is that the space between IC device  330  and metal cap  100  is substantially filled with epoxy encapsulant material  340  providing a thermal conduction path between IC device  330  and metal cap  100 . 
     A wirebond BGA package configured as illustrated in  FIG. 5 , and discussed above, is able to accommodate IC devices dissipating greater than 3 Watts of power. The applicants have successfully assembled a wirebond BGA package according to an embodiment of the invention and demonstrated that a 5 Watt IC device can be maintained at a maximum IC device junction temperature of 125 deg. C in a natural convection environment having maximum ambient temperature of 70 deg. C. These are the same thermal performance parameters met by the prior art glob top BGA package of  FIG. 1 , carrying a 3 Watt IC device. In this embodiment of the invention, the wirebond BGA package was provided with a laminate substrate having X-Y dimensions of 23 mm×23 mm and a thickness of 0.56 mm. The metal cap was made of anodized aluminum having a thickness of 0.25 mm and its dimensions were 23 mm×23 mm×0.9 mm. The metal heat slug was made of a copper sheet having a thickness of 0.25 mm and had X-Y dimensions of 23 mm×23 mm. The IC device had X-Y dimensions of 1.0 cm×1.0 cm. 
     It will be appreciated to one skilled in the art that the illustrative discussions above are not intended to be exhaustive or to limit the invention to the precise embodiments disclosed. Many modifications and variations are possible in view of the above teachings. The embodiments were chosen and described in order to best explain the principles of the invention and its practical applications, to thereby enable others skilled in the art to best utilize the invention and various embodiments with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the following claims and their equivalents.