Abstract:
A method of mounting an integrated circuit die on an inside surface of a cover for an IC package uses a solder preform having isolatable structures that provide connections to a plurality of metalized pads while allowing selective electrical isolation therebetween.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention generally relates to packaging integrated circuit (IC) die in packages. Further, the invention relates to preformed sheets of solder for use with two-piece IC packages having a solderable lid such as, but not limited to, a metal (or ceramic), hermetically sealed package.  
         BACKGROUND OF THE INVENTION  
         [0002]    Integrated circuits are often enclosed in a package to protect the delicate IC die from damage (a die is a single piece of silicon, GaAs, etc, cut from an IC wafer and may contain a single electronic device or an electrical circuit).  
           [0003]    Many types of IC packages used in modern IC packaging systems employ a package body for housing a die, and a cover (or lid) that is attached to the body (see FIG. 1). The body  2  of the IC package contains a cavity  5  allowing a die  4  to be placed inside the package and bondwires  14  are then attached to die  4 , connecting die  4  to package leads  18  for interfacing to other circuits outside the package (not shown). The body  2  and/or the cover  8  may be made from ceramic, metal, glass, plastic, or a combination of ceramic, metal, glass, or plastic. For example, one type of IC package body is composed of ceramic forming the walls and floor, with metal leads extending from the sides and/or bottom. A metal layer  6  is deposited on the tops of walls  22  to allow a cover  8  to be attached to the body. The corresponding cover  8  may be metal, or may be ceramic having a portion thereof layered with metal  10  that is the mirror image of the metal layer  6  on the package body  2 . A sheet of solder  12 , preformed to match the shape of the metal layer  6  on the tops of walls  22 , and the metal layer  10  on cover  8  (with the center of the solder sheet removed), is placed on top of, and aligned with, the metal layer  6  on the package body  2 . The cover  8  is then placed on top of the preformed solder sheet  12  (called a solder preform). The entire stacked combination of package body  3  containing the die, solder preform  12  and cover  8  are then heated to melt the solder preform. When the solder preform melts, the molten solder wets both the metal layer  6  on the package body and the corresponding metal portion  10  of the cover  8 . After wetting, the stacked combination of package body  2  containing the die  4 , solder preform  12  and cover  8  are cooled which causes the solder to solidify and adhere to the metal surfaces thereby sealing the cover to the package body.  
           [0004]    One problem associated with the type of packages described above is they require a large amount of space (area) on a printed circuit board (PCB), especially hermetically sealed packages. A small die, when packaged as described above, occupies a relatively large space on a PCB due to the package. It is therefore desirable to fill the space within a package of a given size, with as much (or as many) die as possible.  
           [0005]    Some workers have installed die side by side in a package body. Though this approach conserves some package area, the package footprint increases nonetheless.  
           [0006]    Workers have also stacked die on top of one another. This approach, though keeping package area from increasing, suffers from thermal management issues. The thermal load of the upper die must be sunk through the lower die (in addition to the thermal load of the bottom die). Electrical isolation may also be compromised with the stacked die approach and this must be taken into account during the design of the electrical circuitry on the die. Electrical isolation (e.g. Electro-Magnetic Interference—EMI) techniques can increase die area, which in turn drives package area. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]    [0007]FIG. 1 shows a prior art integrated circuit package body with IC die installed, a solder preform, and a cover for the package body.  
         [0008]    [0008]FIG. 2 shows an integrated circuit package body with IC die installed for use with the invention herein.  
         [0009]    [0009]FIG. 3 shows an embodiment of the inventive integrated circuit package cover herein.  
         [0010]    [0010]FIG. 4 shows an embodiment of the inventive solder preform herein.  
         [0011]    [0011]FIG. 5 shows a solder preform in accordance with the invention herein, positioned on top of an embodiment of the inventive package body herein.  
         [0012]    [0012]FIG. 6 is a side view of a package body, solder preform, and cover in accordance with the inventions herein just prior to placing the cover on top of the solder preform and package body.  
         [0013]    [0013]FIG. 7 is a side view of a package body, solder preform, and cover in accordance with the inventions herein just after placing the cover on top of the solder preform and package body, and prior to melting the solder preform.  
         [0014]    [0014]FIG. 8 is a side view of a package body, solder preform, and cover in accordance with the inventions herein just after melting the solder preform. 
     
    
     SUMMARY OF THE INVENTION  
       [0015]    It is an object of the invention herein to provide a solder preform having cantilevered portions that separate from the remaining part of the solder preform upon melting of the preform.  
         [0016]    It is an object of the invention herein to provide a solder preform having portions that are isolatable from each other and from the remaining part of the solder preform upon melting of the preform, so as to connect a plurality of electrical contacts without shorting said plurality of electrical contacts together.  
         [0017]    It is a further object of the invention herein to provide a structure and method of mounting a first IC die on the underside of a cover for an IC package having a second die installed in the package body.  
         [0018]    It is another object of the invention herein to define a method for providing electrical connections between a first die mounted on a cover to be attached to a package body, and a second die mounted in said package body using a solder preform in accordance with the invention herein.  
         [0019]    It is another object of the invention herein to define a method for providing electrical connections between a first die mounted on a cover to be attached to a package body, and leads extending outside the package body using a solder preform in accordance with the invention herein, without making electrical connections to a second die mounted in said package body.  
         [0020]    The invention herein provides a solder preform having sections that are isolatable from at least the main body of the preform upon melting of the preform.  
         [0021]    The inventive solder preform is positioned between a body of an IC package and a corresponding cover for said IC package body. Some first portions of the solder preform are placed in contact with metalized portions of the package cover and/or IC package body. Other second portions of the solder preform are not in contact with metalized portions of the cover and/or body. The second portions of the preform that are not in contact with metalization, exist between two other first portions that are in contact with metalization. Upon application of sufficient heat, the solder preform melts and subsequently wets the metalized areas it is in contact with. Molten solder has the characteristic of being drawn towards the heated metalized portions that the solder is in contact with, much like a loose strand of solder is drawn onto the tip of a hot soldering iron. The first portions of molten solder that are in contact with metalization and are flanking a second portion of solder, exert a tensile force on the second portion sufficient to separate or break the molten solder into two parts that each subsequently flow to the different metalized sections.  
         [0022]    The inventive solder preform and inventive methods herein allow more IC die to be installed into a package of given size than was previously possible. The invention herein solves the problems of thermal management and growth of package area on a PCB. Further, the invention herein allows multiple die to be electrically connected together if desired. This means that die that ordinarily would be packaged separately, may now be packaged together saving precious PCB space.  
       DESCRIPTION OF THE INVENTION  
       [0023]    An integrated circuit (IC) package body  3  in accordance with the invention herein is shown in FIG. 2. Package body  3  may have an IC die  4  installed in the cavity  5  of the package as is generally known in the art. Die  4  may be connected to external leads  18  via bondwires  15  and pads  17  or may be mounted in flip-chip fashion (not shown). Body  3  has a plurality of metalized pads  20  affixed to the tops of walls  22 . Pads  20  may be connected via connections  21  to external leads  18 , or to internal pads  17  (which also may or may not be connected to external leads  18 ) for subsequent connection to die  4 . Connections  21  may be buried circuit traces as is known or may be vertical circuit traces deposited on the inside walls of body  3  (not shown).  
         [0024]    Cover  9 , shown in FIG. 3, has an IC die  24  mounted on an underside surface thereof that faces cavity  5  of body  3  when cover  9  is soldered to body  3  (see FIG. 6). Cover  9  has metalized pads  32  deposited on the underside surface such that when cover  9  is inverted and positioned on body  3 , pads  32  are aligned over pads  20 . Cover  9  also has pads  26  that are connected to die  24  via wirebonds  28 . Pads  26  are connected to pads  32  via metalized circuit traces  30  that may buried or exposed traces as is well known. Cover  9  has an area of metalization  11  that is soldered to metalization area  7  on body  3  in the inventive manner described below. Metalization area  11  maybe electrically isolated from some or all of pads  32 . Metalization area  7  on body  3  may also be electrically isolated from some or all of pads  20 .  
         [0025]    A solder preform  34  in accordance with the invention herein is shown in FIG. 4. Preform  34  comprises a main body  36  having a plurality of isolatable structures  37  protruding from main body  36  in a cantilevered fashion.  
         [0026]    Main body  36  performs at least two functions, first, as a support structure for supporting said plurality of isolatable structures  37  during handling of solder preform  34  similar to the well-known lead frame. A lead frame (not shown) is a metallic frame containing leads and a package base to which an unpackaged integrated circuit die is attached. After encapsulation, the outer part of the lead frame (similar to main body  36 ) is cut away, discarded and the leads (which are attached to the package base) are bent into the required shapes.  
         [0027]    Secondly, main body  36  is used for making a connection (sealing) between cover  9  and package body  3  when solder preform  34  is sufficiently heated to at least the melting point of the alloy used to manufacture solder preform  34 .  
         [0028]    Though isolatable structures  37  are shown on the inside edges of main body  36 , this is not to be construed as a limitation of the invention herein as isolatable structures  37  may also be positioned on an outside edge of main body  36  (not shown). Each of isolatable structures  37  consist of a distal portion  40  and a proximal portion  38 . Proximal portion  38  connects distal portion  40  to the main body  36 . In use, solder preform  34  is positioned on a substrate that is only partially patterned with metalized areas. Referring to FIG. 4, distal portion  40  and main body  36  (only a portion of which is shown) is positioned on metalized areas  20  and  7  respectively. Proximal portion  38  is positioned on a non-metalized area such as ceramic (alternatively, a metal cover may be selectively coated with a material suitable to prevent molten solder from wetting and adhering to it). When solder preform  34  (including distal portion  40  and proximal portion  38 ) and metalized areas  20 , &amp;  7  are sufficiently heated, solder preform melts and transitions to a molten state. The molten solder wets and adheres to metalized areas  7  and  20  creating opposing forces F 1  and F 2  that are exerted on proximal portion  38  in tension as shown in FIG. 4. Opposing forces F 1  and F 2  are sufficiently large to cause distal portion  40  to separate from proximal portion  38 . Surface tension draws proximal portion  38  onto one or both metalized areas  7  and/or  20  to become part of distal portion  40  and/or main body  36 . Distal portion  40  is now isolated from main body  36 .  
         [0029]    Solder preform  34  is placed onto package body  3  such that distal portions  40  are aligned with and on pads  20 , and main body  36  is aligned with and on metalized area  7  of package body  3 . Cover  9 , with die  24  mounted thereon, is then inverted and placed on solder preform  34  (and subsequently package body  3 ) such that pads  32  are aligned with, and on, distal portion  40 , and metalized area  11  (of cover  9 ) is aligned with, and on, main body  36  as shown in FIGS.  6 - 8 . Proximal portion  38  is shown in FIG. 7 as being bounded by two small gaps  46  and  48 . These gaps are due to the thickness of metalized areas  7 ,  11 ,  20 , and  32 . Gaps  46  and  48  may assist in causing distal portion  40  from separating from proximal portion  38  once solder preform  34  enters the molten state and wets metalized areas  7 ,  11 ,  20 , and  32 . After distal portion  40  separates from proximal portion  38 , gaps  46  and  48  combine to form a larger gap  50  that isolates distal portion  40  from main body  36  as shown in FIG. 8. Referring to FIGS. 6 and 8, it is clearly seen that an electrical connection between die  4  mounted in package body  3  and die  24  mounted on cover  9  is completed. Said electrical connection is made via bondwire  28 , pad  26 , trace  30 , pad  32 , distal portion  40 , pad  20 , traces  21 , pad  17 , and bondwire  15 .  
         [0030]    Solder preform  34  is described herein as being used to mount a single IC package. The invention herein is not limited to such.  
         [0031]    As is well known in the art, electrically conductive epoxy (or solder paste) is deposited on a substrate or printed circuit board in a predetermined pattern using automatic epoxy dispensers. The epoxy dispensers deposit small “dots” of conductive epoxy (or solder paste) on metalized pads. Electrical components are then placed on the epoxy/pad combination and cured (or solder paste melted) thereby creating an electrical connection between the pad and a terminal of the electrical component. The inventive solder preform  34  may be used as described supra in lieu of depositing epoxy or solder paste dots. The inventive solder preform  34  may also be used to replace silkscreening operations where solder paste is deposited on a substrate using a mask made of metal as is well known in the art. Unwanted electrical “shorts” between electrical components will not occur using the inventive solder preform  34  because isolatable structures  37  separate from main body  36  as well as other isolatable structures that may or may not be present as part of solder preform  34 . Multiple solder preforms  34  in accordance with the invention herein may be used on the same substrate, each having a different geometry.