Patent Abstract:
A stackable FBGA package is configured such that conductive elements are placed along the outside perimeter of an integrated circuit (IC) device mounted to the FBGA. The conductive elements also are of sufficient size so that they extend beyond the bottom or top surface of the IC device, including the wiring interconnect and encapsulate material, as the conductive elements make contact with the FBGA positioned below or above to form a stack. The IC device, such as a memory chip, is mounted upon a first surface of a printed circuit board substrate forming part of the FBGA. Lead wires are used to attach the IC device to the printed board substrate and encapsulant is used to contain the IC device and wires within and below the matrix and profile of the conductive elements.

Full Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of application Ser. No. 09/537,879, filed Mar. 29, 2000, now U.S. Pat. No. 6,268,649, issued Jul. 31, 2001, which is a continuation of application Ser. No. 09/072,101, filed May 4, 1998, now U.S. Pat. No. 6,072,233, issued Jun. 6, 2000. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates generally to packaging semiconductor devices and, more particularly, the present invention relates to fine ball grid array packages that can be stacked to form highly dense components. 
     Ball grid array (BGA) semiconductor packages are well known in the art. BGA packages typically comprise a substrate, such as a printed circuit board, with a semiconductor die mounted on the top side of the substrate. The semiconductor die has a multitude of bond pads electrically connected to a series of metal traces on the top side of the printed circuit board. The connection between the bond pads and the metal traces is provided by wire bonds electrically and mechanically connected between the two. This series of metal traces is connected to a second series of metal traces on the underside of the printed circuit board through a series of vias. The second series of metal traces each terminate with a connect contact pad where a conductive element is attached. The conductive elements can be solder balls or conductive filled epoxy. 
     The conductive elements are arranged in an array pattern and the semiconductor die and wire bonds are encapsulated with a molding compound. 
     As chip and grid array densities increase, the desire in packaging semiconductor chips has been to reduce the overall height or profile of the semiconductor package. The use of BGAs has allowed for this reduction of profile as well as increased package density. Density reduction has been achieved by utilizing lead frames, such as lead-over chips, in order to increase the densities as well as to branch out into being able to stack units one on top another. 
     One example of a lead chip design in a BGA package is shown in U.S. Pat. No. 5,668,405, issued Sep. 16, 1997. This patent discloses a semiconductor device that has a lead frame attached to the semiconductor chip. Through holes are provided that allow for solder bumps to connect via the lead frame to the semiconductor device. This particular reference requires several steps of attaching the semiconductor device to the lead frame, then providing sealing resin, and then adding a base film and forming through holes in the base film. A cover resin is added before solder bumps are added in the through holes to connect to the lead frame. This particular structure lacks the ability to stack devices one on top another. 
     U.S. Pat. No. 5,677,566, issued Oct. 14, 1997, and commonly assigned to the assignee of the present invention, discloses a semiconductor chip package that includes discrete conductive leads with electrical contact bond pads on a semiconductor chip. The lead assembly is encapsulated with a typical encapsulating material and electrode bumps are formed through the encapsulating material to contact the conductive leads. The electrode bumps protrude from the encapsulating material for connection to an external circuit. The semiconductor chip has the bond leads located in the center of the die, thus allowing the conductive leads to be more readily protected once encapsulated in the encapsulating material. Unfortunately, this particular assembly taught in the &#39;566 patent reference also lacks the ability to stack one semiconductor device on top another. 
     Attempts have been made to stack semiconductor devices in three dimensional integrated circuit packages. One such design is disclosed in U.S. Pat. No. 5,625,221, issued Apr. 29, 1997. This patent discloses a semiconductor package assembly that has recessed edge portions that extend along at least one edge portion of the assembly. An upper surface lead is exposed therefrom and a top recess portion is disposed on a top surface of the assembly. A bottom recess portion is disposed on the bottom surface of the assembly such that when the assembly is used in fabricating a three-dimensional integrated circuit module, the recessed edge portion accommodates leads belonging to an upper semiconductor assembly to provide electrical interconnection therebetween. Unfortunately, the assembly requires long lead wires from the semiconductor chip to the outer edges. These lead wires add harmful inductance and unnecessary signal delay and can form a weak link in the electrical interconnection between the semiconductor device and the outer edges. Further, the device profile is a sum of the height of the semiconductor die, the printed circuit board to which it is bonded, the conductive elements, such as the solder balls, and the encapsulant that must cover the die and any wire bonds used to connect the die to the printed circuit board. So, reducing the overall profile is difficult because of the geometries required in having the lead pads on the semiconductor chip along the outer periphery with extended lead wires reaching from the chip to the outer edges. 
     Another stacked arrangement of semiconductor devices on a substrate interconnected by pins is illustrated in U.S. Pat. Nos. 5,266,912 and 5,400,003. However, the height of the stacked package is limited by the length of the pin connections between the individual multi-chip modules or printed circuit boards. 
     Accordingly, what is needed is a ball grid array package that allows stacking of packages on one another. This stackable package would have a lower profile than otherwise provided in the prior art and would reduce the number of steps in the assembly of the package. 
     SUMMARY OF THE INVENTION 
     According to the present invention, a stackable fine ball grid array (FBGA) package is disclosed that allows the stacking of one array upon another. This stackable FBGA package is configured such that conductive elements are placed along the outside perimeter of a semiconductor device (integrated circuit (IC) device) mounted to the FBGA. The conductive elements also are of sufficient size so that they extend beyond the bottom or top surface of the IC device. Wire interconnect connects the IC device in a way that does not increase the overall profile of the package. Encapsulating material protects both the IC device and the wire interconnect as the conductive elements make contact with the FBGA positioned below or above to form a stack. The IC device, such as a memory chip, is mounted upon a first surface of a printed circuit board substrate forming part of the FBGA. Lead wires, or wire interconnect, are used to attach the IC device to the printed circuit board substrate and an encapsulant is used to contain the IC device and wires within and below the matrix and profile of the conductive elements. 
     Additionally, certain pins on the FBGA in the stack require an isolated connection to the PC board. An example of such a requirement is when an activation signal for a particular IC device within the stack must be sent solely to that device and not to any of the other devices within the stack. This isolated connection connects to an adjacent ball on a different FBGA stack above or below that particular isolated connection since in common pin layouts of the devices are stacked together, and each device requires an isolated connection to the PC board. This provides for a stair step connection from the bottom of the FBGA stacked array to the top that allows each device, from the bottom one to the top one, to have an isolated connection from each other. This allows IC devices to be stacked one upon the other while maintaining a unique pin out for each pin required in the stack. 
     Further, the FBGA of the present invention keeps the wire lengths between the IC device and the conductors of the PC board to a minimum for the control of the impedance of the conductors. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
     FIG. 1 depicts a schematic cross-sectional representation of a stacked array of FBGAs according to the present invention; 
     FIG. 2 depicts a top plan view of a representative circuit board as used in the array of FIG. 1; 
     FIG. 3 depicts a perspective view of a printed circuit board having traces connected one to another with vias and contact through holes; 
     FIG. 4 depicts a perspective view of a pair of different printed circuit boards having an electrical connection extending from one location on one board to another location on the second board; 
     FIG. 5 depicts a perspective view of multiple PC boards interconnected in a manner according to the present invention; 
     FIG. 6 is an alternative embodiment of a stackable array according to the present invention; 
     FIG. 7 depicts another embodiment where the ball grid array matrix extends below the semiconductor device; 
     FIG. 8 depicts a bottom plan view of an FBGA device found in FIG. 1; 
     FIG. 9 is a schematic diagram of a view of a printed circuit board having a mounted IC with wire leads attaching the bond pads of the IC to the bond pads of the printed circuit board; 
     FIG. 10 is a cross-sectional view of a portion of a printed circuit board illustrating the pin and connection therebetween; 
     FIG. 11 is a cross-sectional view of portions of printed circuit boards illustrating the pins and connections therebetween; and 
     FIG. 12 is a block diagram of an electronic system incorporating the FBGA module of FIG.  1 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to drawing FIG. 1, illustrated in a cross-sectional view is a plurality of fine ball grid array (FBGA) packages  10  in a stacked arrangement. Each FBGA package  10  is stacked one upon another via a matrix of conductive elements or solder balls  28  having a first height. Each FBGA package  10  includes a substrate  12  that has conductive traces formed both on the top surface and the bottom surface. Substrate  12  may be formed from an organic epoxy-glass resin base material, such as bismaleimide-triazin (BT) resin or FR- 4  board, but is not limited thereto. Other carrier substrate materials well known to those skilled in the art may also be utilized instead, such as, for example, either a ceramic or silicon substrate. 
     FBGA package  10  further comprises an integrated circuit or semiconductor die  14  attached to a die attach pad  16  formed on the upper surface of substrate  12 . 
     Semiconductor die  14  is attached to die attach pad  16  using a dielectric adhesive that is nonconductive and has a thermal coefficient of expansion (TCE) that closely matches that of the semiconductor die  14 . The adhesive can be any type of epoxy resin or other polymer adhesives typically used for such purposes. Alternately, the die attach pad  16  may be formed of double sided, adhesively coated tape, such as an adhesively coated Kapton™ tape or the like. The semiconductor die  14  is formed having a plurality of bond pads  18  that is formed on the active surface thereof which mates with die attach pad  16  of the substrate  12 . Each bond pad of the plurality of bond pads  18  aligns with a corresponding aperture  24  in substrate  12 . Each bond pad of the plurality of bond pads  18  is electrically connected to terminal pads  20  that are on the surface of substrate  12 . Wire bonds  22  are used to form the connections between the plurality of bond pads  18  on the semiconductor die  14  and the terminal pads  20  of the substrate  12  wherein the wire bonds  22  pass through an aperture  24  formed in the substrate  12 . A portion of semiconductor die  14  where the bond pads  18  are located, along with the cavity formed by aperture  24 , is covered by an encapsulating material  26 . Encapsulating material  26  covers or seals bond pads  18 , terminal pads  20 , and wire bonds  22  to protect them from dust, moisture, and any incidental contact. The encapsulating material  26  has a second height, the second height being less than the first height of the conductive elements  28 . 
     Conductive elements  28  are attached or bonded to conductive traces  30  (see FIG. 2) of substrate  12 . Conductive elements  28  may be selected from acceptable bonding substances such as solder balls, conductive or conductor-filled epoxy, and other substances known to those skilled in the art. The conductive elements  28 , which, for example, are solder balls, may be attached, as is known in the art, by coating the solder balls or bond areas or both with flux, placing the solder balls  28  on the conductive traces  30  with conventional ball placing equipment and reflowing the balls in place using an infrared or hot air reflow process. The excess flux is then removed with an appropriate cleaning agent. In this way, the solder balls  28  are electrically and mechanically connected to the conductive leads to form the external electrodes. Other processes may also be used to form external electrodes. For example, the electrodes may be “plated up” using conventional plating techniques rather than using solder balls as described above. The completed FBGA packages  10  can then be attached to a printed circuit board or the like using conventional surface mount processes and equipment. Likewise, each FBGA package  10  can be mounted one on top another, stacked, as is illustrated in drawing FIG.  1 . Solder balls  28  may have a diameter of approximately 0.6 mm with a pitch P that is 0.80 mm. The profile for each FBGA package  10 , as measured from the bottom of solder balls  28  to the top of the semiconductor die, may range from 1.0 mm to 1.22 mm. 
     Next, as illustrated in drawing FIG. 2, is a top plan view of the bottom surface of substrate  12 . This bottom surface includes pass-through aperture  24  where the wire bonds (not shown) are attached to terminal pads  20 . Each terminal pad  20  is connected to a metal conductive trace  30 , which further connects to a conductive element pad  32 . Conductive element pads  32  are placed on either side of substrate  12  and are located where the conductive elements  28  of drawing FIG. 1 are mounted. Additionally, as conductive element pads  32  are placed on the opposite side of substrate  12 , they provide a pass-through connection for the stacking of FBGA packages  10  as shown in drawing FIG.  1 . Conductive traces  30  are electrically connected to conductive traces on the opposite side (not shown) using vias  34 . Conductive traces  30  may be comprised of electrically conductive material such as copper or copper plated with gold. While conductive traces  30  are illustrated in drawing FIG. 2 on the top and bottom of the substrate  12 , other conductive traces  30  (not shown) may be located in the substrate  12  along with other vias  34  therein and conductive element pads  32  in addition to those illustrated. Depicted in drawing FIG. 3 is a perspective view of a three dimensional drawing of how conductive traces  30  may be laid out on both the top surface and bottom surface of substrate  12 . Additionally, the conductive element pads  32  are also shown to provide connection on either side of substrate  12 . Conductive traces  30  are on both sides connected using vias  34  as well as the conductive elements pads  32 . The conductive traces  30  are also connected to terminal pads  20 . The aperture  24  through substrate  12  may be any desired size in relation to the semiconductor die  14  as may be necessary. Also, the substrate  12  may have portions thereof removed after the mounting of the semiconductor die  14  thereon. 
     Depicted in drawing FIG. 4 is an expanded view of the three-dimensional arrangement of substrates  12  achieved using the pass-through holes or vias  34  in conjunction with conductive traces  30  of the substrates  12  to form a stacked arrangement. A first substrate  12  is provided to connect to a second substrate  42 . The connection occurs at conductive element pad  32  on substrate  12  and a like conductive element pad  44  on second substrate  42 . Next, conductive element pad  44  on second substrate  42  connects to a conductive trace  30  on the surface of second substrate  42 , which then passes from one side of second substrate  42  using via  34  to connect to a bond pad on the opposite side of second substrate  42 . Referring to drawing FIG. 5, depicted is the manner in which the stepping of conductive traces can continue to yet another level. Referring to drawing FIG. 5, depicted is a third conductive substrate  52  placed below substrate  12  having additional conductive element pads  32  on either side thereof that provide connection to the adjacent substrate  12 , which then, in turn, provides connection to second substrate  42 . The arrows represent the plane connection on semiconductor packages yet to be added. 
     Referring to drawing FIG. 6, depicted is an alternative embodiment of the invention where a semiconductor die  14  is mounted on the upper surface of substrate  12 . Wire bonds  22  are then used to connect the bond pads  18  on the active surface of the semiconductor die  14  to the terminal pads  20  of substrate  12 . Encapsulating material  26  is then provided to cover the semiconductor die  14 , wire bonds  22 , bond pads  18  and terminal pads  20 . Next, conductive elements  28  are then mounted on the upper surface of substrate  12  around the perimeter of semiconductor die  14 . As illustrated, this arrangement allows the stacking of multiple die packages  60 . It is understood that the substrate  12  includes circuitry and vias (not shown) as described hereinbefore in drawing FIGS. 2 through 5. 
     A third embodiment of the present invention is depicted in drawing FIG.  7 . Referring to drawing FIG. 7, shown in a cross-sectional diagram is the manner in which a semiconductor die  14  can extend near to the peripheral edges of substrate  12 . In this case, conductive elements  28  are no longer outside the perimeter of semiconductor die  14 . Again, wire bonds  22  interconnect bond pads  18  of the semiconductor die  14  to terminal pads  20  on substrate  12 . Encapsulating material  26  is utilized to cover the aperture  24 , the bond pads  18 , terminal pads  20 , and wire bonds  22 . This particular arrangement of the substrate  12  and semiconductor die  14  may be used as either a bottom level or as a top level in a stacked array, typically, with the use of an interposer. 
     Referring to drawing FIG. 8, depicted is a bottom plan view of a semiconductor package  10  as illustrated in drawing FIG.  1 . In this example, substrate  12  has a plurality of solder balls  28  mounted along the perimeter of semiconductor die  14 , which is shown in outline form. The conductive elements  28  form a connective matrix for connecting to the top surface of another substrate  12  or to the top surface of a carrier substrate that provides external electrical connectivity for the module. Encapsulating material  26  covers the wire leads and bonding pads on either substrate  12  or semiconductor die  14 . 
     Referring to drawing FIG. 9, illustrated is a schematic diagram of a sample pin and trace layout having isolated connection pads used to connect to the conductive elements  28 . As shown, semiconductor die bond pads  18  are aligned in a row down the center of the semiconductor die  14 . Wire bonds  22  interconnect bond pads  18  of the semiconductor die  14  to the terminal pads  20  of the substrate  12 . From terminal pads  20 , conductive traces  30  interconnect conductive elements  28 . As can be seen, selected conductive elements  28  have no connection to any of the conductive traces  30  or terminal pads  20  on the substrate  12 . These conductive element areas, grouped as  29  and  31 , illustrate how certain connections are isolated from that particular semiconductor die  14  mounted on that particular substrate  12 . These isolated conductive element areas  29  and  31  allow interconnection among other packages  10  (not shown) stacked one on top of the other within the stacked package arrangement of drawing FIG.  1 . The use of selected isolated pins allows for each semiconductor die  14  within the stacked array of packages  10  to have a unique pin out for selected pins on each layer of packages  10 . For example, in a memory package of like semiconductor dies  14  stacked in an array, each semiconductor die  14  requires a select pin that is separate from all other select pins of the other semiconductor dies  14  within the array and that connects to a unique pin in the final pin out configuration. The stackable BGA packages are useful in many types of electronic systems including SDRAM, EDO RAM, video RAM, cache memory, and Read-Only Memory (ROM), as well as microprocessors, application specific integrated circuits (ASIC), digital signal processors, flash memories, electrically erasable programmable read only memory (EEPROM), among others. 
     Referring to drawing FIG. 10, a connection terminal  100  is illustrated of substrate  12  having conductive traces  30  thereon and therein. The substrate  12  includes conductive traces  30  and an insulator material therebetween, thereby providing the ability of controlling the impedance of the conductive traces  30  having semiconductor die  14  connected thereto by wire bonds  22 . The connection terminals  127  include a connection pin  141  which is connected to one of the conductive traces  30 . Circuitry in intermediate layers of the substrate  12  extend through apertures  24  in order to permit all connections of the connection pins  141  to be effected through the top of the substrate  12 . The terminals include a shield  143 , which is separated from the connection pin  141  by an isolation spacer  145 . The isolation spacer  145  may be of any material, preferably a dielectric, provided that the isolation spacer  145  permits impedance matched connection through the connection terminals  127 . Impedance matching is commonly used for signal transfer applications in which the impedance between signal carrying conductors is a predetermined value per unit length. Changes in length will result in proportional (inverse) changes in impedance, but not changes in the impedance expressed per unit length. The consistent impedance per unit length, colloquially referred to as “impedance value”, results in signal matching. This is of interest as operating frequencies exceed those at which unmatched circuits are effective. The use of impedance matched conductors in the present invention of the conductive traces  30 , wire bonds  22 , and connection terminals  127  therefore facilitates the fabrication of circuits which are inherently impedance matched as desired. Matched impedance is thereby able to reduce spurious signals between semiconductor dies  14 , reduce circuit discontinuities, and allow connection circuitry to be designed while controlling the establishment of critical timing paths between components, such as semiconductor dies  14 . 
     Referring to drawing FIG. 11, the connection terminals  127  permit the stacking of the substrate  12  with connections formed by connection pins  141 . 
     Referring to drawing FIG. 12, depicted is an electronic system  130  that includes an input device  132  and an output device  134  coupled to a processor device  136 , which, in turn, is coupled to a memory module  138  incorporating the exemplary stackable FBGA package  10  and various embodiments thereof as illustrated in drawing FIGS. 1 through 9. Likewise, even processor device  136  may be embodied in a stackable array package  10  comprising a microprocessor, a first level cache memory, and additional ICs, such as a video processor, an audio processor, or a memory management processor, but not limited thereto. 
     There has been shown and described a novel semiconductor chip package that is stackable and has a lower profile over that of the prior art. The particular embodiments shown in the drawings and described herein are for purposes of example and are not to be construed to limit the invention as set forth in the pending claims. Those skilled in the art may know numerous uses and modifications of the specific embodiments described without departing from the scope of the invention. The process steps described may, in some instances, be formed in a different order or equivalent structures and processes may be substituted for various structures and processes described.

Technology Classification (CPC): 7