Patent Document

CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application is a continuation of application Ser. No. 10/118,365, filed Apr. 8, 2002, pending, which is a continuation of application Ser. No. 09/832,539, filed Apr. 11, 2001, now U.S. Pat. No. 6,406,943 B2, issued Jun. 18, 2002, which is a continuation of application Ser. No. 09/614,403, filed Jul. 12, 2000, now U.S. Pat. No. 6,259,153 B1, issued Jul. 10, 2001, which is a continuation of application Ser. No. 09/302,196, filed Apr. 29, 1999, now U.S. Pat. No. 6,150,710 issued Nov. 21, 2000, which is a divisional of application Ser. No. 09/137,782, filed Aug. 20, 1998, now U.S. Pat. No. 6,124,150 issued on Sep. 26, 2000. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The present invention relates to semiconductor devices in general and, more particularly, to the configuration of Leads-Over-Chip (LOC) semiconductor devices.  
           [0004]    2. State of the Art  
           [0005]    Modern packaged integrated circuits (IC) comprise one or more encapsulated semiconductor devices, dice or chips within a protective “package” of plastic, ceramic or other moldable material. Typically, a large number of dice are formed from a wafer made from a semiconductor material such as silicon, germanium or gallium arsenide. Microscopic circuits are formed on a surface of each semiconductor die by photolithographic techniques and typically attached to a lead frame with conductive wires. More particularly, a plurality of leads of the lead frame is connected to bond pads on the semiconductor die or dice, enabling the dice to be electrically interconnected to an external electrical host apparatus, typically mounted on a circuit board.  
           [0006]    Early semiconductor devices used relatively large semiconductor dice with peripheral bond pads. Off-die leads were wire-bonded to the peripheral bond pads. With the later introduction of Leads-Over-Chip (LOC) technology, the package size using large semiconductor die could be reduced. This was accomplished by using centrally positioned conductive bond pads on the semiconductor dice and insulatively bonding the inner leads to the semiconductor die surface close to the bond pads for wire connection. Thus, the semiconductor die and lead frame were more intimately joined, and the outer leads could be formed close to or adjacent the semiconductor die.  
           [0007]    In early LOC devices, the semiconductor dice were relatively large, consuming most of the package space. The numbers of leads attached to the semiconductor dice were also limited. Thus, wide and short leads which closely approached the bond pads on the active surface of the semiconductor die were used. The resulting wirebonds were short, and the inductance between the semiconductor die and the host apparatus was low. Examples of such are found in U.S. Pat. No. 5,227,661 of Heinen, U.S. Pat. No. 5,233,220 of Lamson et al., U.S. Pat. No. 5,252,853 of Michii, U.S. Pat. No. 5,331,200 of Teo et al., U.S. Pat. No. 5,418,189 of Heinen, and U.S. Pat. No. 5,466,888 of Beng et al.  
           [0008]    In later generation IC devices, the semiconductor dice have become progressively smaller while the numbers of leads of the lead frame have typically increased. As a result, the inner leads of the lead frame of such devices must, of necessity, be reduced in lead width and pitch, both of which increase the lead inductance and slow the speed of the device. In addition, a minimum lead width is required for high-quality wire bonding. The high density of wire connections typically makes wire bonding more difficult and increases the frequency of bond failures. Furthermore, with very small semiconductor dice, the very fine wires may be very long, resulting in “wire sweep,” sagging, short circuiting and bond failure during encapsulation of the semiconductor die and lead frame. For a very small semiconductor die, fitting all of the inner leads of the lead frame onto the active surface of the semiconductor die is generally not possible, given the present size and space limitations. Even conventional off-die wire bonding is very difficult or not possible in production scale.  
           [0009]    High inductance and reduced speed limit the usefulness of packaged semiconductor dice with long, narrow leads, and shorting or destruction of the wire bonds will make the device useless.  
           [0010]    The required spacing, width and length of leads and wires have become serious limitations in further miniaturization of semiconductor dice and their packages. While complex integrated circuits may be formed in very small semiconductor die, connecting such a die or dice to leads for interconnection to a host apparatus while maintaining the semiconductor die characterization is very difficult.  
           [0011]    There have been various attempts at overcoming the high inductance or interactive conductance effects of small semiconductor die devices. For example, in U.S. Pat. No. 5,521,426 of Russell is disclosed a lead-on-chip (LOC) device with long, narrow leads. In order to decrease the capacitance between the leads and the die and increase lead strength, the leads are stamped or rolled to have a non-rectangular cross-section such as a “U” configuration. Thus, the strength of the lead and its cross-sectional area are increased, resulting in less lead sag and lowered capacitative interaction. However, the cost of producing such leads is considerable, and the package thickness is increased. Furthermore, the method does not increase the size of wire bonding areas on the lead fingers, and the wire bonding operation is no easier.  
           [0012]    In U.S. Pat. No. 4,984,059 of Kubota et al., a semiconductor device is shown with the long sides of the die parallel to the rows of outer lead ends, i.e. in a non-transverse configuration. The device has a very limited number of pins.  
           [0013]    U.S. Pat. No. 5,218,229 of Farnworth discloses a lead frame design in which a semiconductor die with opposing rows of peripheral bond pads on the active surface of the die is positioned for off-die wire attachment. The rows of bond pads are perpendicular to the two rows of outer lead ends.  
           [0014]    U.S. Pat. No. 4,989,068 of Yasuhara et al. shows a semiconductor device in which all leads are LOC leads between two rows of peripheral bond pads.  
           [0015]    None of the above prior art documents discloses a high-speed semiconductor device having a large number of bond pads on a small die, whereby the lead inductance is minimized and wire bonding operations are enhanced. The need exists for such a device.  
         SUMMARY OF THE INVENTION  
         [0016]    In the invention, an improved device uses a hybrid lead frame/semiconductor die configuration wherein a semiconductor die having peripheral or near-peripheral bond pads is positioned in a transverse direction relative to the lead frame. The inner leads, i.e. lead fingers, include a set of lead fingers configured to be wire-bonded off-die to peripheral bond pads and another set of lead fingers configured for lead-over-chip (LOC) attachment inside of the row(s) of wire bond pads. The resulting device has lead fingers of increased width and pitch.  
           [0017]    As a result of this hybrid lead frame/semiconductor die configuration, (a) lead inductance is decreased to ensure signal integrity, (b) wire bonding is faster, easier, and more accurate, (c) wire bond integrity and reliability are enhanced, (d) the shorter wires avoid problems with “wire sweep,” (e) the lead frame is stronger and less subject to damage in handling, (f) signal integrity is increased, (g) the speed grade of the device is increased because of the reduced lead/wire inductance, and (h) a higher value product may be manufactured at lower cost. 
       
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS  
       [0018]    The drawings presented herein illustrate the prior art and the advances of the present invention, in which the figures are not necessarily drawn to scale, whereby:  
         [0019]    [0019]FIG. 1 is a plan view of a semiconductor die and attached 54-pin lead frame of an exemplary early generation prior art semiconductor device;  
         [0020]    [0020]FIG. 1A is a perspective, partial view of a semiconductor lead frame of an early generation prior art semiconductor device;  
         [0021]    [0021]FIG. 2 is a plan view of a semiconductor die and attached lead frame of an exemplary later generation prior art semiconductor device;  
         [0022]    [0022]FIG. 3 is a plan view of a semiconductor die and attached lead frame of an exemplary hybrid semiconductor device of the present invention; and  
         [0023]    [0023]FIG. 4 is a cross-sectional end view of a semiconductor die and attached lead frame of an exemplary hybrid semiconductor device of the invention, as taken along line  4 - 4  of FIG. 3.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0024]    The invention is illustrated and compared with prior art devices, which for purposes of illustration are examples with the same number of pins, i.e. 54.  
         [0025]    A die and lead frame configuration of an early generation prior art leads-over-chip (LOC) semiconductor device  10  is illustrated in drawing FIGS. 1 and 1A. As shown, the relatively large semiconductor die  12  is mounted on a substrate  14  with outlined edges  15 . A row  22  of electrically conductive bond pads  24  with spacing  54  is located on the active surface of the semiconductor die  12  and aligned generally along the longitudinal centerline  30 , parallel to the opposing long sides  26  of the die  12 , and extending generally between the opposing short sides  28 . Two insulative layers  32  of Kapton polymer or similar material are adhesively joined to the active surface  34  of die  12 , one on each side of the bond pad row  22 . A conductive lead frame  16  is shown with inner leads  18  adhesively joined to the insulative layers  32 . The lead frame  16  comprises inner leads  18  and outer leads  20  for connecting the bond pads  24  to an electrical apparatus, not shown. As shown, the outer leads  20  are directed outward from the device  10  on opposite sides  36 , i.e the long sides, of the lead frame  16 . Thus, the single central row  22  of bond pads  24  is parallel to the two opposing sides  36  of outer lead ends  38 ,  40 .  
         [0026]    As depicted in drawing FIG. 1, the inner ends  50  of the inner leads  18  comprise wire bonding areas for attachment of conductive wires  48  leading to specific bond pads  24  on the semiconductor die  12 .  
         [0027]    Following wire bonding, the semiconductor die  12  and attached lead frame  16  are typically encapsulated with a polymer or ceramic material to form a packaged device. The dam bars  52  between the outer leads  20  are cut away, and the outer leads are thus singulated, enabling electrical connection of the bond pads  24  to an electronic apparatus, not shown, with minimum lead inductance. The outer leads  20  may be left as straight projections, or bent to a J-shape, L-shape or other shape, depending upon the apparatus to which the device  10  is to be connected.  
         [0028]    A typical prior art lead frame  16  is shown in drawing FIG. 1A as having a recurring pattern  42  of inner leads  18  and outer leads or pins  20  for accommodating a plurality of single semiconductor dice having longitudinal centerline  30 . The leads  18 ,  20  are temporarily interconnected to each other and to the supportive lead frame rails  46  by dam bars  52 . Index holes  44  in the lead frame rails  46  permit sequential positioning of the lead frame  16  in a wire bonding machine for joining the semiconductor die to the leads  18 ,  20 . The lead frame  16  has a width  58  typically ranging from less than about one inch (2.54 cm.) to several inches or more.  
         [0029]    In this early version of a LOC device, the large semiconductor die  12  enabled the inner leads  18  to be of sufficient width  56  (FIG. 1) to avoid significant resistance and/or inductance effects, particularly at the design speeds typical of that period. The current need for much higher speeds with smaller dice has limited the usefulness of these early devices.  
         [0030]    An exemplary LOC semiconductor device  10  of a later generation is shown in drawing FIG. 2 following wire bonding. The semiconductor die  12  and lead frame  16  are configured the same as die  12  and lead frame  16  of drawing FIG. 1. For purposes of comparison, the overall lead frame width  58  may be assumed to be the same as the lead frame width of FIG. 1A. The semiconductor die  12  is similar to the die of FIG. 1 with respect to its central bond pad location along the centerline  30 . However, the reduced size of the semiconductor die  12  provides about one third of the surface area of the earlier die  12  of drawing FIG. 1, and the bond pad spacing or pitch  54  is considerably reduced, i.e. by nearly 50 percent. In drawing FIG. 2, the semiconductor die  12  is shown adhesively attached to a substrate  14  and has two insulative layers  32  on its active surface  34  upon which inner leads  18  of the lead frame  16  are adhesively attached. The inner lead widths  56  are reduced by about 50 percent to accommodate the smaller semiconductor die  12 . In addition, many of the inner leads  18  have an increased length.  
         [0031]    Thus, the smaller semiconductor die  12  as depicted in drawing FIG. 2 has an increased susceptibility to resistance and inductance effects which severely limit usefulness of the device. In addition, manufacture of the device is made more difficult by the limited room for wire bonding the crowded bond pads to the narrow inner leads  18 .  
         [0032]    It should be noted that the devices  10  may be formed without a permanent substrate  14 . The semiconductor die  12  may be separately supported during attachment of the LOC lead frame  16 , and the final encapsulated package outline may be represented by the edges  15 .  
         [0033]    Turning now to drawing FIGS. 3 and 4, a device  70  having a semiconductor die  72 /lead frame  76  configuration of the invention is depicted. The semiconductor die  72  is positioned transversely relative to the lead frame  76 , i.e. such that its long sides  86  are perpendicular to the opposing rows  96  of outer lead ends. The semiconductor die  72  is shown as having peripheral rows  82 A,  82 B of bond pads  84  along opposing long sides  86 , parallel to the longitudinal centerline  90  of the semiconductor die  72 . The rows  82 A,  82 B of bond pads  84  are shown as generally extending between the opposed short sides  88  of the semiconductor die  72 .  
         [0034]    The lead frame  76  is shown with three sets  100 ,  102 ,  104  of inner leads  78  and outer leads  80 . A first set  100  has inner leads  78  which are positioned off-die for wire-bonding with wires  98  to bond pads  84  of row  82 A. A second set  102  has inner leads  78  which are also positioned off-die for wire-bonding with wires  98  to bond pads  84  of row  82 B.  
         [0035]    A third set  104  has inner leads  78  which are adhesively joined to the active surface  94  of the die  72  with an intervening insulative layer  92 , i.e. as leads-over-chip (LOC) leads. The third set  104  is positioned between the two rows  82 A,  82 B of bond pads  84  and includes leads wire-bonded to both rows.  
         [0036]    In the example shown, the minimum width  106  of the critical function non-LOC inner leads  78  of lead set  100  in device  70  is about 30-60 percent greater than the minimum width  56  of the comparable leads in device  10  of drawing FIG. 2. The twelve LOC leads  78  of lead set  104  are shown as having a width  110  nearly double that of width  56  of the bulk of the LOC leads  18  of the prior art device of drawing FIG. 2.  
         [0037]    The sixteen non-LOC leads  78  of lead set  102  are shown to have a width  108  which is about 30 to 100 percent greater than the width  56  of nearly all LOC leads  18  of the device of drawing FIG. 2.  
         [0038]    The twenty eight non-LOC leads  78  of lead set  100  are shown as having a width  106  roughly comparable to the width  56  of nearly all LOC leads of the device of drawing FIG. 2. Thus, in this example, critical leads subject to inductance have a greater width while non-critical leads are formed with a reduced width. The lead widths may be adjusted as needed for the particular use of the device.  
         [0039]    The invention presents, on average, inner leads having a shorter length of the minimum width portions than the prior art device of drawing FIG. 2. Moreover, the range of lead lengths is much greater. Thus, in the particular example of drawing FIG. 3, twelve leads with very abbreviated lengths are positioned near the semiconductor die corners to carry critical signals subject to inductance.  
         [0040]    As shown in drawing FIG. 4, the device  70  is formed by adhesively joining a semiconductor die  72  to a substrate  74  with an intervening insulative layer  112 . The hybrid lead frame  76  includes a set  102  of inner leads  78  which are attached by conductive wires  98  to a row  82 B of bond pads  84  (FIG. 3). Another set  104  of inner leads  78  overlies the active surface  94  of semiconductor die  72  in a LOC configuration and is adhesively joined to the die with an intervening insulative layer  92 . In this embodiment, the LOC lead set  104  is thus at a different level than the non-LOC lead sets  102  (and  100 , not visible in FIG. 4). The outer leads  80  terminate in lead ends  114  which may be straight or formed as J-leads or L-leads, etc., as known in the art. The lead widths  108  and  110  are illustrated in the figure.  
         [0041]    Following the wire bonding operation, the lead frame  76  and attached semiconductor die  72  are encapsulated and extraneous lead frame portions excised to form a device package.  
         [0042]    The device  70  may be formed without a permanent substrate  74 . The semiconductor die  72  may be separately supported during attachment of the LOC lead frame  76 , and the final encapsulated package outline may be represented by the edges  75 . In this embodiment, the non-LOC leads and LOC leads may be in the same horizontal plane.  
         [0043]    As explained in the foregoing description, the invention provides wider and generally shorter inner leads  78  for small dice  72 . This obviates problems with inductance at high speed operation, making the design extremely useful for state-of-the-art applications.  
         [0044]    The LOC leads overlying a large portion of the active surface also result in enhanced heat transfer, improving the overall operation of the device.  
         [0045]    In addition, the larger leads and greater pitch enable a much improved wire-bonding operation in terms of speed and integrity.  
         [0046]    While the invention has been described using a semiconductor die  72  with a pronounced difference in lengths of the long sides  86  and short sides  88 , the term “long side” includes sides having a length equal to or greater than that of the “short side.” The semiconductor die  72  is shown as rectangular in surface shape, but may be of other shapes provided space is provided between two sets of bond pads for entry of a significant number of LOC leads.  
         [0047]    It is apparent to those skilled in the art that there is provided herein according to the invention a transverse hybrid LOC semiconductor package particularly useful with small dice and in enhancing the construction and operability of a semiconductor package. Although the device has been described and illustrated with reference to a specific embodiment thereof, it is not intended that the invention be limited by the illustrated embodiment. Those skilled in the art will recognize that various modifications can be made without departing from the spirit and intent of the invention. For example, the invention is not limited to devices having a specific number or type of leads, bond pads, or dice, nor to a device with a permanent substrate supporting the die. Thus, it is intended that this invention encompass all such modifications and variations which fall within the scope of the appended claims.

Technology Category: 5