Abstract:
A board-on-chip (BOC) semiconductor package includes a multisegmented, longitudinally slotted interposer substrate through which an elongate row of die bond pads is accessed for electrical attachment, as by wire bonding, to conductive traces on the opposite side of the interposer substrate. One or more reinforcements in the form of crosspieces or bridges span and segment intermediate portions of the interposer substrate slot to resist bending stresses acting in the slot region proximate the centerline of the interposer substrate tending to crack or delaminate a polymer wire bond mold cap filling and covering the slot and the wire bonds. Various interposer substrate configurations are also disclosed, as are methods of fabrication.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a divisional of application Ser. No. 10/409,804, filed Apr. 9, 2003, pending. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to fabrication of semiconductor devices and, more specifically, to the packaging of semiconductor dice. 
     2. State of the Art 
     In the field of semiconductor device manufacture, testing and packaging of various types of semiconductor dice are conducted in a similar manner. Thus, conventional dynamic random access memory dice (DRAMs), static random access memory dice (SRAMs), programmable memory dice (PROMs, EPROMs, EEPROMs, flash memories), logic dice, and microprocessor dice are fabricated, tested and packaged in a generally similar manner. 
     Following fabrication of a plurality of dice on a wafer or other bulk substrate of semiconductor material, a preliminary test for functionality is conducted on each die, such as by probe testing. The dice are then singulated, and those passing the functionality test are picked from the wafer, typically for packaging, such as by transfer molding, and subsequent incorporation into a higher-level assembly. Typically, each die is formed with one or more rows of bond pads on the active surface. The bond pad row or rows may be formed along a central axis of the die or along one or more peripheral portions thereof. Transfer molded packages may comprise bond wires which electrically couple bond pads on the die to leads of a lead frame, the outer ends of the leads extending beyond the protective encapsulant, which is typically a silicone-filled, thermoplastic polymer. The lead frame leads are used to achieve mechanical and electric connection of the die to a carrier substrate such as a printed circuit board (PCB), for example. 
     Following packaging, the packaged die may be characterized for compliance with selected electrical parameters under various environmental conditions. Those which fail the testing are scrapped or, in some instances, may be reworked for compliance using redundant circuitry incorporated into the die. 
     The foregoing traditional approach to packaging has a number of shortcomings. For example, the resulting package may be much larger than the enclosed die, requiring an undue amount of space or “real estate” on a carrier substrate. In addition, the insulative value of the large mass of encapsulant material of the package inhibits heat transfer from the die, which may cause die malfunction or failure over time. The packaging process is very materials-intensive and requires a substantial number of steps, including adherence of the die to a lead frame of a lead frame strip, wire bonding, placement in a transfer mold cavity and removal therefrom, followed by a trim and form operation to sever the package leads from the lead frame and deform the outer lead ends for connection to higher-level packaging. Furthermore, this type of packaging is somewhat limiting (absent somewhat exotic lead frame design approaches) in terms of the number of available I/O connections, presenting a problem as the number of required connections per die increases. 
     A board-on-chip (BOC) semiconductor package has also been developed, in which an interposer substrate such as a relatively small, slightly larger than die-size interposer substrate is formed with a centrally placed, elongate through-slot sized and configured for alignment with a row or rows of bond pads on the die. The through-slot is also known as an “interconnect slot” or “wire bond slot.” The die is adhesively joined by its active surface to one side of the interposer substrate such that the bond pads are accessible through the interconnect slot. The bond pads are connected to conductive traces on the opposite side of the interposer substrate, by bond wires, for example, which pass through the interconnect slot. The interconnect slot is then filled with a silicone-filled, thermoplastic polymer encapsulant to encase and seal the bond wires and surrounding, exposed portion of the die&#39;s active surface. Conventionally, a transfer molding process is used to form this wire bond mold cap while simultaneously encapsulating the back side and sides of the die on the opposite side of the interposer substrate. A ball grid array (BGA) or other type of array of discrete conductive elements electrically connected to the conductive traces and projecting from the side of the interposer substrate with the wire bond cap may be used to mechanically and electrically connect the package to a carrier substrate or other higher-level packaging. Various examples of this type of package construction are shown in U.S. Pat. Nos. 5,723,907 to Akram and 5,739,585 to Akram et al., and U.S. Pat. No. 5,818,698 to Corisis, all of which patents are assigned to the assignee of the present application and the disclosure of each of which is incorporated by reference herein. The resulting package has a much reduced size, which may be termed “chip scale” or “near chip scale,” and is generally capable of establishing robust, high-quality mechanical and electrical connections using conventional bonding techniques. 
     Various aspects of the general concept of the above-described type of package are also shown in U.S. Pat. No. 5,313,096 to Eide, U.S. Pat. No. 5,384,689 to Shen, U.S. Pat. No. 5,661,336 to Phelps, Jr. et al., and U.S. Pat. No. 6,190,943 B1 to Lee et al. 
     Despite the obvious advantages for this BOC-type of semiconductor package, a problem has been repeatedly noted relative to the integrity of the wire bond mold cap. Stress cracking of the wire bond mold cap has been found to occur at an unacceptably high frequency in certain package configurations. The undesirable stress cracking has been found to be attributable to tensile stresses induced in the interconnect or wire bond slot region during temperature cycling, thermal shock, curing in the mold during the encapsulation process, etc., as the package interposer substrate is cycled between compressive and tensile stress states. This cycling is due in large part to the disparity of encapsulant volume on opposing sides of the interposer substrate and the associated stress cracking to reduced rigidity against bending of the interposer substrate due to the presence of the interconnect or wire bond slot extending along a majority of the centerline or longitudinal axis of the interposer substrate. 
     The aforementioned stress cracking in conventional BOC-type packages has been found to be largely concentrated at the interface between the interconnect slot edge and the mold cap itself. It has been recognized by those skilled in the art that the frequency of occurrence and magnitude of this problem is greater where the length of the elongated, central interconnect slot is a major portion of the corresponding substrate length. In BOC-type packages having an interposer substrate with an interconnect slot, the interconnect slot length is typically about 70 to 80% of the corresponding substrate length. It has been estimated that an unacceptably high failure rate generally occurs where the slot length is about 67% or more of the substrate length (for a bismaleimide triazine (BT) resin substrate). Thus, the problem may be very pervasive, as such relative slot lengths are quite common and necessary to accommodate the large number of bond pads required for operation of state of the art dice. Where the interposer substrate comprises another material, such as a ceramic for example, the critical ratio of slot length to substrate length may be somewhat different. 
     The present invention is directed to effectively resolving the foregoing problem in an economic manner using conventional components and packaging techniques. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention, in one embodiment, comprises a reinforced interposer substrate configuration for a board-on-chip (BOC) and other similar semiconductor packages. In such packages, a segmented, elongate interconnect aperture (i.e., slot) is formed in the interposer substrate and permits interconnection of die bond pads with conductors on the opposite side of the substrate by bond wires or other intermediate conductive elements. The present invention provides resistance to thermally induced cracking of the package adjacent the interconnect apertures during temperature cycling, thermal shock, mold cure, and other thermal initiators of stress state cycling which subject the interposer substrate to unacceptably high levels of tensile stress. 
     In another embodiment, the present invention comprises interposer substrates for BOC-type packaging incorporating one or more crosspieces or bridges transversely spanning an elongate interconnect slot therein, forming a segmented interconnect slot reinforced against bending stresses. Each crosspiece or bridge joins the opposed slot walls or edges to provide added strength and rigidity against bending to the interposer substrate. As a result, tensile stresses applied under thermal cycling of the package by the encapsulant material to the interposer substrate acting between and adjacent the interconnect slot walls are resisted, preventing excessive yielding of the interposer substrate and preventing cracking or delamination of the polymer wire bond mold cap adjacent the interconnect slot. Thus, thermal stress cracking in the polymer wire bond mold cap is greatly reduced or eliminated. 
     The present invention, in yet another embodiment, also comprises BOC-type semiconductor die packages incorporating an interposer substrate having one or more crosspieces or bridges extending across an interconnect slot to form a segmented interconnect slot, semiconductor dice configured for use with the interposer substrate of the present invention and a method of fabrication of interposer substrates according to the present invention. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       The nature of the present invention as well as other embodiments thereof may be more clearly understood by reference to the following detailed description of the invention in conjunction with the several drawings herein, wherein elements and features depicted therein are identified with similar reference numerals. 
         FIG. 1  is a cross-sectional end view of an exemplary BOC semiconductor package of the prior art having an elongate interconnect slot; 
         FIG. 2  is a perspective view of an exemplary prior art interposer substrate of the BOC semiconductor package of  FIG. 1 ; 
         FIG. 3  is a schematic view of tensile bending stresses exerted on a BOC semiconductor package such as the prior art semiconductor package of  FIG. 1  as a function of lateral distance from the centerline thereof; 
         FIG. 4  is an exploded perspective view of an exemplary die and one embodiment of a matching reinforced interposer substrate in a BOC package in accordance with the present invention; 
         FIG. 5  is a perspective view of another embodiment of a reinforced interposer substrate for a BOC package in accordance with the present invention; 
         FIG. 5A  is a top elevational view of a variation of the reinforced interposer substrate of  FIG. 5 ; 
         FIG. 6  is a perspective view of a further embodiment of a reinforced interposer substrate for a BOC package in accordance with the present invention; 
         FIG. 7  is a side-sectional view of yet another embodiment of a reinforced interposer substrate in accordance with the present invention; 
         FIG. 8  is a cross-sectional end view of a BOC semiconductor package having a reinforced interposer substrate according to the present invention, taken slightly longitudinally offset from the midpoint of the interconnect slot; 
         FIG. 9  is a top view of a semiconductor die having a plurality of groups of bond pads on the active surface thereof, the groups being longitudinally spaced by a distance greater than individual bond pad spacing within a group; and 
         FIG. 9A  is a top view of an interposer substrate configured with a plurality of crosspieces or bridges for use with the semiconductor die of  FIG. 9 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In use and operation, and referring to  FIG. 1 , an exemplary BOC semiconductor package  10  of the prior art is depicted. The semiconductor chip or die  12  has an active surface  14  and a back surface  16 . The die  12  is shown with a plurality of bond pads  34  disposed on the active surface  14 , in one or more mutually parallel, generally linear rows  36  (see  FIG. 4 ) along a centerline bisecting the active surface  14 . Generally, the bond pads  34  have a uniform interpad spacing or pitch  8  (see  FIG. 4 ). 
     As shown in  FIG. 1 , the package  10  includes an interposer substrate  20  such as may be formed of a sheet of circuit board material such as a BT resin or epoxy-glass composite, or may comprise silicon (with a passivated surface), ceramic or other suitable dielectric material. The interposer substrate  20  has an interconnect or wire bond slot  40  corresponding to the position of the central row or rows  36  of bond pads  34  ( FIG. 4 ) and exposing bond pads  34  therethrough. In addition, conductive traces  30  are typically formed on the surface  24  of interposer substrate  20  and extend from locations adjacent interconnect slot  40  to other, more remote locations on surface  24 . The conductive traces  30  are connected through interconnect slot  40  to the bond pads  34  by elongated conductive elements in the form of bond wires  38 . Conductive traces  30  are also connected to discrete conductive elements  68  of a ball-grid-array (BGA) comprising solder balls or conductive or conductor-filled or coated columns, pillars or studs, enabling attachment of the package  10  to a carrier substrate (not shown) such as a circuit board of an electronic system such as a computer. 
     In  FIG. 1 , die  12  is shown as having its active surface  14  mounted on a die attachment area  18  on the surface  22  of interposer substrate  20  by adhesive material  32 . The adhesive material  32  may comprise one of many suitable adhesives such as thermoplastic adhesive, a thermoset adhesive or one or more tape or film segments such as a polyimide (e.g., Kapton tape®) having a pressure-sensitive adhesive on both sides thereof. 
     As shown, the package  10  includes a molded filled polymer body  54  extending over the back surface  16  and lateral edges  19  of the die  12  to surface  22  of interposer substrate  20 . As shown, the molded filled polymer body  54  may extend to the peripheral edges  26  of interposer substrate  20 , but this is not required. In addition, a filled polymer wire bond mold cap  56  is formed to fill the interconnect slot  40  and cover the bond wires  38 , including the bond sites to conductive traces  30 . Typically, the molded filled polymer body  54  and filled polymer wire bond mold cap  56  are formed substantially simultaneously by conventional transfer molding techniques which are well-known in the electronic industry. Alternatively, other packaging methods may be used, including pot molding and injection molding, for example. 
       FIG. 1  also illustrates bending stresses  72  which occur when the package  10  is subjected to temperature cycling and thermal shock. The interposer substrate  20  is thus cycled back and forth between compressive and tensile stress conditions. When in a tensile state, the bending stresses  72  act on the wire bond mold cap  56  and the edges  46  of the interconnect slot  40 , tending to separate them. Cracks  58  propagate at the interface  74  between the mold cap  56  and edges  46 , or within the mold cap  56  itself, to relieve the applied tensile stress. Breakage of bond wires  38  lying in the path of a crack  58  may also occur. As depicted in the generalized graph of  FIG. 3 , the stress values  61  (whether tensile or compressive) increase as shown at  62  toward the center of interposer substrate  20  and attain peak values  64  generally along the centerline  42  of the interposer substrate  20 . Conversely, stress levels decrease with distance  66  from the centerline  42  of interposer substrate  20 . Of course, it is the occurrence of peak values  64  for stress, which causes the aforementioned damage in the interconnect slot  40  region of interposer substrate  20 . 
       FIG. 2  depicts the exemplary interposer substrate  20  of  FIG. 1 . Interposer substrate  20  is shown in this embodiment as a planar member with a surface  22  and an opposed surface  24 . The interposer substrate  20  has a length  52 . A die  12  (not shown in  FIG. 2 ) with a central row  36  of bond pads  34  will be attached to die attachment area  18  on the surface  22  such that the bond pads  34  will be exposed through the interconnect slot  40 . Conductive traces  30  (not shown in  FIG. 2 ) are formed on the surface  24 , as already discussed. As shown, the interconnect slot  40  has a length  48  which, in many instances, is about 70–80% of the interposer substrate  20  length  52  so as to extend a length at least slightly greater than the row or rows  36  of centrally placed bond pads  34  of the die  12  with which interposer substrate  20  is assembled. The slot width  50  is typically made as narrow as possible because of the required space for conductive traces  30  on the outer surface  24  of interposer substrate  20 , but is required to be of sufficient width to accommodate a wire bond capillary used to place bond wires  38  and therefore, form bonds with bond pads  34  and the ends of conductive traces  30  adjacent interconnect slot  40 . Also shown are vertical axis  28  oriented perpendicular to the plane of interposer substrate  20  through the interconnect slot  40  and longitudinal axis or centerline  42  extending through the interconnect slot  40  in the plane of interposer substrate  20 . The interconnect slot ends  44  are typically rounded or filleted, a natural consequence of slot formation in the interposer substrate  20  by milling. Rounded slot ends as illustrated in  FIG. 2 , therefore, have a greater strength than, e.g., squared ends, the corners of which are subject to crack initiation and propagation. 
     In the present invention, one or more crosspieces or bridges  70  ( FIG. 4 ) are formed between the slot ends  44  of the elongate interconnect slot  40 . These crosspieces or bridges provide a multisegmented interconnect slot  40  and reinforce the interposer substrate  20  between the opposing edges  46  of the interconnect slot  40  at intermediate locations along the interconnect slot  40  against bending attributable to stresses applied thereto. Turning now to  FIG. 4 , one exemplary embodiment of the interposer substrate  20  of the invention is shown, together with a die  12  with a single central row  36  of bond pads  34 . A crosspiece or bridge  70  comprises a filleted portion of the interposer substrate  20  which is left uncut during manufacture, i.e:, two longitudinally adjacent interconnect slots or slot segments  40 A,  40 B are formed in interposer substrate  20  instead of a single interconnect slot  40  (as depicted in  FIGS. 1 and 2 ), leaving crosspiece or bridge  70  in place. The interconnect slot segments  40 A,  40 B of the present invention are shown with a combined length of  48 A plus  48 B, which is slightly less than the length  48  of a single prior art interconnect slot  40  for a similarly sized interposer substrate  20 . However, the longitudinal distal end-to-distal end length of the two interconnect slot segments  40 A,  40 B may be equivalent to, or even longer than, that of a single prior art interconnect slot  40 . The width  76  of the crosspiece or bridge  70  in the direction of centerline  42  is small, generally about 0.5 mm or more for a BT resin interposer substrate  20  given manufacturing tolerances, yet sufficient to extend between longitudinally adjacent bond pads  34 . It may be desirable to space bond pads  34  into two or more longitudinally adjacent groups with increased interpad spacing or pitch  8  between groups of the plurality of bond pads  34 , as depicted in  FIG. 9 , to enable the use of larger-width crosspieces or bridges  70 . If necessary, more than one crosspiece or bridge  70  may be used, generally evenly spaced along the interconnect slot  40  (see slot segments  40 A,  40 B and  40 C in  FIG. 9A ), to divide the interconnect slot  40  into three or even more segments to provide a required resistance to bending. Generally, however, for overall length  84  of the row  36  of bond pads  34  for dice  12  of about 3 to 15 mm in length, a single, substantially centrally placed crosspiece or bridge  70  is sufficient to avoid stress cracking or delamination of the wire bond mold cap  56 . For longer dice  12 , two or more longitudinally spaced crosspieces or bridges  70  may be desirable to thereby avoid stress cracking or delamination of the wire bond mold cap  56 . 
     Referring now to  FIG. 8 , a cross-sectional end view of a BOC semiconductor package  100  according to the present invention is illustrated. Elements and features of semiconductor package  100  are substantially the same as those of BOC semiconductor package  10 , however, a notable addition to semiconductor package  100  is the transverse extension of crosspiece or bridge  70  across interconnect slot  40 , thereby dividing interconnect slot  40  into slot segments  40 A and  40 B (see  FIG. 4 ). 
       FIG. 5  illustrates another embodiment of a crosspiece or bridge  70 . In this version, the crosspiece or bridge  70  comprises a narrow segment of material which is adhered by its underside  86  to surface  24  of interposer substrate  20  with a high-strength adhesive. This crosspiece or bridge  70  may be formed of a high-strength material with a coefficient of thermal expansion (CTE) approximating the CTE of the interposer substrate  20 . For example, a reinforced polymer (such as a glass-reinforced polymer) may be used to form a thin crosspiece or bridge  70  having a minimum width  76  of about 0.5 mm. Other reinforced materials such as a polyimide tape, a ceramic element or a silicon-type element may be used. 
     It is also contemplated, as illustrated in  FIG. 5A , that a laterally elongated “I”-shaped segment  70 A bearing adhesive material  32  on both sides thereof and used for mounting a die  12  to interposer substrate  20  may be formed such as by die-cutting from a larger sheet of reinforced polymer, for example, and placed on surface  22  of interposer substrate  20  with the head  70 H and foot  70 F of the “I”-shaped segment  70 A lying on opposing sides of an interconnect slot  40  and the body  70 B of the “I”-shaped segment  70 A forming the reinforcing crosspiece or bridge  70  thereacross. Of course, segment  70 A may also be formed with two or more crosspieces to extend at intervals across interconnect slot  40 , or two or more “I”-shaped segments  70 A employed. Segment  70 A may comprise, for example, a tape segment or a relatively stiff plastic segment. 
       FIG. 6  depicts a further embodiment of a crosspiece or bridge  70  which comprises a narrow plug or bar of material joined to each of the opposed slot edges  46 A and  46 B, preferably by a high-strength adhesive. This narrow plug or bar of material is preferably a dielectric material with sufficient strength to accommodate the compressive and tensile stresses applied along the opposed slot edges  46 A and  46 B, respectively. The various types of materials which may be used to form the plug or bar include, for example, glass, rigid plastic and ceramic. 
       FIG. 7  depicts yet another embodiment of the present invention, in which a “T”-shaped crosspiece or bridge  70 T is placed with its body  70 B snugly placed in interconnect slot  40  and the legs of cap  70 C extending over surface  22  transversely to centerline  42 , both body  70 B and cap  70 C being adhesively bonded to interposer substrate  20 . 
     As noted above, for dice  12  which may normally have an interpad spacing or pitch  8  (see  FIG. 4 ) less than about 0.5 mm, the die design to accommodate any of the foregoing embodiments of the present invention may require a slightly larger bond pad spacing at one or several locations along the row  36  of bond pads  34 . Thus, for example, a die  12  may be formed with a bond pad spacing of 0.4 mm along 95% of the row  36  of bond pads  34 , while the spacing between two adjacent centrally located bond pads  34  is increased to 0.6 mm. Thus, a crosspiece or bridge  70  may be accommodated without significantly changing the overall length  84  of the row  36  of bond pads  34 . Such an arrangement of bond pads  34  on a die  12  in the form of three groups of bond pads  34 , each group of bonding pads  34  comprising two parallel rows flanking the centerline of the die  12 , is illustrated in  FIG. 9 . However, in the embodiments of  FIGS. 5 ,  5 A,  6  and  7 , it should be noted that use of a crosspiece or bridge  70  of a higher strength against bending than the material of interposer substrate  20  may enable the use of a thinner crosspiece or bridge  70  which may accommodate existing bond pad spacing or pitch  8 . Similarly, if an appropriate material is selected for interposer substrate  20  and stringent manufacturing tolerances may be held, a thin yet effective crosspiece or bridge  70  may provide adequate resistance to bending stresses while still accommodating existing bond pad spacing or pitch  8 . 
     While not specifically illustrated, it should be noted that the invention encompasses various combinations of the embodiments discussed and illustrated above, including stacked packages thereof. 
     In the discussion thus far, it is noted that the dice  12  are disposed on a planar surface  22  of the interposer substrate  20 . However, the present invention is applicable to semiconductor packages in which the interposer substrate  20  or a base comprising the interposer substrate  20  has a die-receiving cavity and/or a conductor-carrying cavity on a surface  22  or an opposed surface  24  thereof. 
     It will be recognized from the above description that the segmentation for reinforcement of interconnect slots  40  in BOC semiconductor packages through the use of crosspieces or bridges  70  enhances the functionality and reliability of such semiconductor packages. 
     While the present invention has been disclosed herein in terms of certain exemplary embodiments, those of ordinary skill in the art will recognize and appreciate that it is not so limited. Many additions, deletions and modifications to the disclosed embodiments may be effected without departing from the scope of the invention. Moreover, elements and features from one embodiment may be combined with features from other embodiments. The scope of the present invention is defined by the claims which follow herein.