Abstract:
An integrated circuit package including a substrate, an integrated circuit, and an encapsulant. The substrate has two opposing surfaces and an opening that extends between the two surfaces. The integrated circuit is mounted to the substrate substantially centered over the opening such that a portion of the opening is left uncovered by the integrated circuit. The encapsulant encapsulates the integrated circuit with a portion of the encapsulant extending between the two surfaces of the substrate and attached to the lower surface of the integrated circuit.

Description:
RELATED APPLICATIONS 
     This application is a continuation-in-part of application Ser. No. 09/088,469, filed Jun. 1, 1998, now abandoned. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to an integrated circuit package. 
     BACKGROUND OF THE INVENTION 
     Integrated circuits are typically assembled into packages that are mounted to a printed circuit board. A plastic ball grid array (PBGA) package includes an integrated circuit that is mounted to a substrate. The substrate is typically a printed circuit board which has routing traces, power/ground planes and vias that interconnect the integrated circuit to a plurality of solder balls located on a bottom surface of the substrate. The solder balls are reflowed to attach the package to an external printed circuit board such as the motherboard of a computer. 
     The integrated circuit is electrically connected to the substrate by bond wires. The bond wires are typically attached to the integrated circuit and substrate with a wire bond machine. To prevent movement during the wire bond process the integrated circuit is attached to the substrate with an adhesive. The adhesive is typically a silver filled epoxy which tends to absorb moisture during the assembly process. After the wire bonding process is completed, the integrated circuit, bond wires and substrate are placed in a mold that is injected with an encapsulant which encapsulates the circuit. The solder balls are then attached to the substrate. 
     The solder balls are typically reflowed by applying heat to the entire package. The heat may convert the moisture absorbed by the epoxy into steam. The creation of steam may crack the package and separate the integrated circuit from the substrate. For this reason the integrated circuit/substrate subassembly is handled in a way to minimize moisture absorption before, during and after the wire bond process. This may require humidity controlled storage and assembly areas. Controlling the humidity during the assembly process increases the complexity and cost of mass producing the package. It would be desirable to provide a PBGA package which is more reliable and less sensitive to moisture in a manufacturing environment. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a top view of an the integrated circuit package of the present invention. 
     FIG. 2 is a side sectional view of the integrated circuit package of FIG. 1 along line  2 — 2 . 
     FIG. 3 is a side sectional view of the integrated circuit package of FIG. 1 along line  3 — 3 . 
     FIGS. 4 a-c  show a process for assembling the integrated circuit package. 
     FIG. 5 is a top view of an alternate embodiment of an integrated circuit package. 
     FIG. 6 is a top view of an alternate embodiment of an integrated circuit package. 
     FIG. 7 is a top view of an alternate embodiment of an integrated circuit package. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     One embodiment of the present invention is an integrated circuit package which has a portion of an encapsulant that extends through an opening in the substrate and is attached to a lower surface of an integrated circuit mounted to the substrate. The encapsulant also encapsulates a top surface and side surfaces of the integrated circuit. The encapsulant that extends through the substrate provides a relatively robust mechanical interconnect between the integrated circuit and the substrate to prevent separation during a subsequent solder reflow process. The opening reduces the amount of adhesive that is used to attach the integrated circuit to the substrate for a wire bond process. The reduction in adhesive lowers the amount of moisture that is absorbed by the package. 
     Referring to the drawings more particularly by reference numbers, FIGS. 1 to  3  show an embodiment of an integrated circuit package  10  of the present invention. The package  10  may include a substrate  16  having a first surface  14 , an opposing second surface  20 , and an opening  22  that extends from the first surface to the second surface. The package  10  may include an integrated circuit  12  having a third surface  11  and an opposing fourth surface  13  that is mounted to the first surface  14  of the substrate  16 . The package  10  may further have a plurality of solder balls  18  that are attached to the second surface  20  of the substrate  16 . The substrate  16  may be a printed circuit board which has surfaces pads, routing traces (not shown) and vias  23  that electrically connect the first surface  14  to the second surface  20 . 
     As shown in FIGS. 1 to  3 , the center opening  22  may have a size and shape such that the substrate  16  will support the integrated circuit  12  while a substantial portion of the fourth surface  13  of the integrated circuit  12  is adjacent the opening  22  and, therefore, uncovered by the substrate  16 . The size and shape of the opening  22  is also such that an open passage  25  is left outside the periphery of the integrated circuit  12 . The open passage  25  allows encapsulant  30  flow around the integrated circuit and attach to the fourth surface  13  of the integrated circuit  12 . For clarity, the top view of FIG. 1 shows only an outline of the encapsulant  30  to allow the substrate  16 , the integrated circuit  12 , and the related structures and spatial relationships to be shown. 
     The substrate  16  may further have a plurality of tabs  24  that extend into the opening  22 . The tabs  24  may support the integrated circuit  12  during the assembly process of the package  10 . The package  10  may have a plurality of bond wires  26  that electrically connect the integrated circuit  10  to the substrate  16 . The package  10  may further include an adhesive  28  that attaches the integrated circuit  12  to the substrate  16  during the wire bond process. The adhesive may be a silver filled epoxy. 
     The integrated circuit  12  may be encapsulated by an encapsulant  30 . A portion of the encapsulant  30  may extend through the open passage  25  between the edge of the center opening  22  and the integrated circuit  12  and into the opening to form a continuous body of encapsulant and become attached to the integrated circuit  12 . This portion of the encapsulant  30  may provide a robust mechanical interconnect between the integrated circuit  12  and the substrate  16 . The opening  22  also reduces the amount of adhesive  28  under the integrated circuit  12  and the amount of moisture that is absorbed by the package  10 . 
     FIGS. 4 a-c  show a process for assembling the integrated circuit package  10 . FIGS. 4 a-c  show a cross section along line  2 — 2  in FIG.  1 . The integrated circuit  12  is initially attached to the substrate  16 . The adhesive  28  may attach the integrated circuit  12  to the tabs  24  of the substrate  16 . The bond wires  26  may then be attached to the integrated circuit  12  and the substrate  16 . 
     As shown in FIG. 4 c , the substrate  16 , integrated circuit  12  and bond wires  26  are then placed into a cavity  32  of a mold  34 . The cavity  32  is then filled with the encapsulant  30 . A portion of the encapsulant  30  is allowed to flow through the open passage  25  between the edge of the center opening and the integrated circuit and into the center opening  22  of the substrate  16  to form a continuous body of encapsulant. After the encapsulant  30  is cured the encapsulated integrated circuit  12  is removed from the mold  34  so that the solder balls  18  can be attached to the substrate  16 . FIG. 5 shows an alternate embodiment of an integrated circuit package of the present invention. The substrate  16  may have a plurality of center tabs  26   a  and corner tabs  26   b  that extend into the opening  22   a . The tabs may support the integrated circuit  12  during the assembly process of the package  10 . 
     FIG. 6 shows an alternate embodiment of an integrated circuit package of the present invention. The substrate  16  may have support segments  38  that separate the opening  22   b  into areas  40 . In the exemplary embodiment shown in FIG. 6, the support segments separate the opening  22   b  into four areas  40 . The substrate  16  may have a center die paddle  36  connected to the support segments  38 . The substrate  16  may have a plurality of corner tabs  24   c  that extend into the opening  22   b . The die paddle  36  and/or corner tabs may provide greater attach area and support for the integrated circuit  12  during the wire bond process than the single opening embodiment shown in FIGS. 1 and 5. The size and shape of the opening  22   b  leaves an open passage outside the periphery of the integrated circuit  12 . The open passage allows encapsulant  30  flow around the integrated circuit and into the areas  40  and become attached to the integrated circuit  12  during the mold process to form a continuous body of encapsulant. The encapsulant  30  provides a relatively robust mechanical interconnect between the integrated circuit  12  and the substrate  16 . 
     FIG. 7 shows an alternate embodiment of an integrated circuit package of the present invention. The opening  22   d  is circular with a diameter that is less than the diagonal of the rectangular integrated circuit  12 . The substrate  16  may support the corners of the integrated circuit. The size and circular shape of the opening  22   d  leaves an open passage  25   a  outside the periphery of the integrated circuit  12 . The open passage  25   a  allows encapsulant  30  flow around the integrated circuit and attach to the lower surface of the integrated circuit  12 . 
     While certain exemplary embodiments have been described and shown in the accompanying drawings, it is to be understood that such embodiments are merely illustrative of and not restrictive on the broad invention, and that this invention not be limited to the specific constructions and arrangements shown and described, since various other modifications may occur to those ordinarily skilled in the art. In particular, the opening in the substrate may be of any form that provides support for the integrated circuit during the assembly process while leaving a substantial portion of the lower surface of the integrated circuit uncovered by the substrate and leaving an open passage outside the periphery of the integrated circuit that allows encapsulant to flow around the integrated circuit and attach to the lower surface of the integrated circuit.