Patent Publication Number: US-2004046267-A1

Title: Semiconductor package having a die pad with downward extended tabs

Description:
FIELD OF THE INVENTION  
       [0001] This invention relates to semiconductor packages, and more particularly to a semiconductor package having a die pad for a semiconductor chip to be mounted thereon.  
       BACKGROUND OF THE INVENTION  
       [0002] Semiconductor devices are required to be miniaturized in profile in order to cope with the recent tendency of reduction in size and weight of electronic equipments. As a result, thin type semiconductor devices, such as TSOP (Thin Small Outline Package), SSOP (Shrink Small Outline Package), and TQFP (Thin Quad Flat Package), are developed to conform to industrial needs.  
       [0003] Illustrated in FIG. 1A is a cross-sectional view of a conventional thin type semiconductor package in the mold injection process. The laminated structure of a die  1  and a die pad  3  on which the die  1  is mounted, is received within a mold cavity  71  of a conventional encapsulating mold  7  composed of an upper mold  4  and a lower mold  5 . Melted encapsulating resin  2  is introduced via a gate  6  of the encapsulating mold  7  into the mold cavity  71  and is thereby divided into an upper resin flow  24  passing through the space above the die  1  and a lower resin flow  25  passing through the space below the die pad  3 . When the upper resin flow  24  moves at a speed substantially equivalent to the lower resin flow  25 , the laminated structure of the die  1  and die pad  3  is kept horizontally stable in the mold cavity  71  until the, encapsulating resin  2  completely fills the mold cavity  71 , as shown in FIG. 1B.  
       [0004] However, as the size of the chip changes, the velocity of the upper resin flow  24  would differ from that of the lower resin flow  25 . This tends to incline the laminated structure of the die  1  and die pad  3  and thereby results in an adverse effect on the reliability of a semiconductor device. Referring to FIG. 2A, while chip  1 ′ is of a size smaller than the chip  1  of regular size as shown in FIG. 1A, the upper resin flow  24  moves faster than the lower resin flow  25  and incurs pressure against the chip  1 ′ by the upper resin flow  24  greater than that against the die pad  3  by the lower resin flow  25 . As a result, the laminated structure of the die  1 ′ and die pad  3  downwardly inclines toward the inner surface  51  of the lower mold  5 , so that the die pad  3  will be exposed to the outside of the encapsulant formed by the encapsulating resin  2  at the time the transfer molding process is completed, as shown in FIG. 2B.  
       [0005] Referring to FIG. 3A, while chip  1 ″ is of a size larger than the chip  1  of regular size as shown in FIG. 1A, contrary to what has been described above, the upper resin flow  24  moves slower than the lower resin flow  25  and incurs a pressure against the chip  1 ″ by the upper resin flow  24  smaller than that against the die pad  3  by the lower resin flow  25 . As a result, the laminated structure of the chip  1 ″ and die pad  3  inclines upwardly toward the inner surface  41  of the upper mold  4 , so that the die  1 ″ and/or gold wires (not shown) for electrically connecting the chip  1 ″ and inner leads (not shown) are exposed to the outside of the encapsulant formed by the encapsulating resin  2  at the time the transfer molding process is completed, as shown in FIG. 3B.  
       [0006] Therefore, to avoid the aforesaid exposures of the die and die pad to the encapsulant from taking place, U.S. Pat. No. 5,371,044 discloses a process for preparing a semiconductor device in which a control plate is used in the molding process to prevent the resin flows above the die and below the die pad from being different in velocity. As shown in FIG. 4, in the U.S. Pat. No. 5,371,044, a control plate  8 , which projects inwardly toward the die pad  3  and horizontally extends from the inner edge of a lead frame (not shown), is formed near the gate  6  of the encapsulating mold  7  to balance the velocity of the upper resin flow  24  and that of the lower resin flow  25 . However, the control plate  8  is only designed to decrease the velocity of the upper resin flow  24  so that it is substantially equal to that of the lower resin flow  25  where the upper resin flow  24  moves faster than the lower resin flow  25  in transfer molding. Thus, while the size of the chip  1  is increased to cause the lower resin flow  25  to move faster than the upper resin flow  24 , the control plate is useless for balancing the upper resin flow  24  and the lower resin flow  25  such that in this case the chip  1  and die pad  3  tend to upwardly incline and are exposed to the encapsulant. Therefore, the use of the control plate of the U.S. Pat. No. 5,371,044 can not effectively solve the problem caused by the velocity unbalance between the upper resin flow and the lower resin flow in the transfer molding process.  
       [0007] Accordingly, as shown in FIG. 5, the U.S. Pat. No.  5 , 623 , 162  discloses a die pad  3 ′ having a pair of wing leads  8 ′ which outwardly extend from a pair of opposing sides of the die pad  3 ′ to the spacing between a pair of adjacent leads  89 ′. The wing leads  8 ′ are slightly projected from the side surface of the encapsulant  9 ′, as shown in FIG. 6, after the transfer molding process is completed. In the transfer molding process, the outer end of the wing lead  8 ′ is sandwiched between the upper mold and the lower mold (not shown) such that the die pad  3 ′ is held in position by means of the wing leads  8 ′. As a result, the die pad  3 ′ does not have upward or downward slant problem caused by the velocity unbalance between the upper resin flow and the lower resin flow as described above.  
       [0008] Although a die pad constructed with wing leads as disclosed in the U.S. Pat. No. 5,623,162 is able to solve the positioning problem of the die pad, it is primarily designed to provide the moisture in the encapsulant with a path to expel the moisture from the encapsulant to the atmosphere while avoiding the crack of the encapsulant caused by the “popcorn” effect, but not to solve the velocity unbalance between the upper resin flow and the lower resin flow in the transfer molding process. Meanwhile, the trimming process for cutting the wing leads is different from the conventional lead cutting/trimming process. As a result, it becomes necessary to incorporate an additional wing lead trimming step to the packaging process and to use equipment different from conventional ones, whereby the manufacturing cost is undesirably increased. Further, the die pad formed with wing leads increases the material cost for the lead frame, resulting in the wing-lead design not being widely accepted in the industry.  
       SUMMARY OF THE INVENTION  
       [0009] The present invention was made in accordance with the circumstances of the prior art, and therefore the object of the present invention is to provide a semiconductor package having a die pad with downward-extended tabs which allows the die pad to be maintained in position and not to be exposed to the encapsulant in the transfer molding process without increasing the material cost and packaging cost and using additional treatment process and equipment.  
       [0010] The above and other objects of the present invention are achieved by a semiconductor package comprising a semiconductor chip, a die pad for bearing the semiconductor chip, a plurality of leads each having an inner lead electrically connected with electrode pads disposed on the semiconductor chip by bonding wires such as gold wires, and an encapsulant formed by a molding resin for encapsulating the semiconductor chip, the die pad, and the inner leads of the leads. The die pad is formed with a plurality of tabs extending downwardly from the plane where the die pad is positioned, allowing a lower resin flow of the molding resin flowing below the die pad to move slower than an upper resin flow of the molding resin flowing above the die in the molding process. Thus, a downward pressure is produced to press the die pad with the die mounted thereon until the tab of the die pad reaches the inner surface of a mold cavity of a mold for forming the encapsulant.  
       [0011] The above and other objects, features and advantages of the present invention will be apparent from the following description of preferred embodiments of the invention with reference to the accompanying drawings. It is important to point out that the illustrations may not necessarily be drawn to scale, and that there may be other embodiments of the present invention that are not specifically illustrated.  
     
    
    
     BRIEF DESCRIPTION OF THE DRAWING  
     [0012]FIG. 1A is a cross-sectional view which illustrates during a transfer molding process a molding resin introduced from a gate into the cavity of a mold within which a conventional die pad with a die of standard size mounted thereon is disposed;  
     [0013]FIG. 1B is a cross-sectional view showing that the molding process of FIG. 1A is completed and the die pad is kept in position in the cavity of the mold;  
     [0014]FIG. 2A is a cross-sectional view which illustrates during a transfer molding process a molding resin introduced from a gate into the cavity of a mold within which a conventional die pad with a die of small size mounted thereon is disposed;  
     [0015]FIG. 2B is a cross-sectional view showing that the molding process of FIG. 2A is completed and the die pad downwardly inclines and is exposed to the encapsulant;  
     [0016]FIG. 3A is a cross-sectional view which illustrates during a transfer molding process a molding resin introduced from a gate into the cavity of a mold within which a conventional die pad with a die of large size mounted thereon is disposed;  
     [0017]FIG. 3B is a cross-sectional view showing that the molding process of FIG. 3A is completed and the die pad upwardly inclines and is exposed to the encapsulant;  
     [0018]FIG. 4 is a cross-sectional view showing another conventional semiconductor package in the transfer molding process of which the lead frame is formed with a control plate;  
     [0019]FIG. 5 is a top view which illustrates a lead frame for a further conventional semiconductor package having wing leads;  
     [0020]FIG. 6 is a partial perspective view which illustrates the portion of the wing lead of the lead frame of FIG. 5 projected from the encapsulant;  
     [0021]FIG. 7 is a top view of a lead frame suitable for use in a semiconductor package in accordance with the first embodiment of the present invention;  
     [0022]FIG. 8 is a cross-sectional view of the die pad of FIG. 7 having a die mounted thereon and disposed in the cavity of a mold in the transfer molding process;  
     [0023]FIG. 9 is a top view of a lead frame suitable for use in a semiconductor package in accordance with the second embodiment of the present invention;  
     [0024]FIG. 10 is a cross-sectional view of the die pad of FIG. 9 having a die mounted thereon and disposed in the cavity of a mold in the transfer molding process; and  
     [0025]FIG. 11A to  11 I are top views in accordance with other embodiments of the present invention. 
    
    
     [0026]FIG. 7 is a top view illustrating a lead frame used for a first embodiment of the present invention. As shown therein, a lead frame  30  is formed with a die pad  31  in a rectangular shape for bearing a semiconductor die (not shown in FIG. 7) and a plurality of conductive leads  32  surrounding the die pad  31 . A pair of hanger bars  33  outwardly extending from a pair of opposing sides of the die pad  31  to both sides of the lead frame  30  are provided for supporting the die pad  31 . The die pad  31  is also formed with a plurality of tabs  34  downwardly extending from the plane of the die pad  31  and disposed along another pair of opposite sides of the die pad  31  orthogonal to the pair of sides from which the hanger bars  33  extend. The tabs  34  can be formed by a conventional punching method or the like at the time the lead frame  31  is made. By punching, etching, stamping, or similar methods, the tab  34  is to be formed in the peripheral sides of the die pad  31  and/or in the edges of an opening of the die pad  31  (not shown in FIG. 7) and in a triangular, plate shape so that the planarity of the die pad  31  can be maintained after the die pad  31  is formed with the tabs  34 . The tab  34  which is in a triangular, plate shape rather than in a protrusion or recess form can also prevent voids from incurring in the encapsulant for encapsulating the die pad  31 , as will be depicted in the following, during transfer molding. In addition, the die pad  31  can be either in or downset from the plane of the leads  32 .  
     [0027] Referring to FIG. 8, it is illustrated that a transfer molding process is performed to encapsulate the die pad  31  of the lead frame  30 . As seen in the drawing, a die  10  is mounted on the die pad  31  by die attaching adhesive such as silver paste (not shown) and electrically connected with the leads  32  by gold wires  20 . When the wire bonding process is completed, the lead frame  30  on which the die  10  is mounted is positioned between an upper mold  61  and a lower mold  62  of an encapsulating mold  60 . A mold cavity  63  formed by the combination of a recess on the upper mold  61  and that on the lower mold  62  is used to receive the die pad  31  and the die  10  attached thereto. During the transfer molding process, an encapsulating resin  70  is introduced via a gate  64  formed on the upper mold  61  into the mold cavity  63  so as to completely encapsulate the die  10 , gold wires  20 , die pad  31 , and inner portions of the leads  32 . As aforementioned, when the resin flow introduced from the gate  64  into the mold cavity  63  reaches the die pad  31 , the encapsulating resin  70  is diverted by the die  10  and die pad  31  so that a portion of the encapsulating resin  70  flows above the die  10  and a portion thereof flows below the die pad  31 . Upon passing the tabs  34  of the die pad  31 , the resin flow below the die pad  31  is impeded by the downwardly extended tabs  34  such that the resin flow above the die  10  moves at a speed faster than that below the die pad  31 , resulting in a downward pressure against the die pad  31  and die  10  downwardly pressing the die pad  31  and die  10 . The tabs  34  of the die pad  31  abut against the inner surface of the lower mold  62 , as shown in FIG. 8; the die pad  31  and die  10  are kept in position in the mold cavity  63  whereby the die pad  31  is prevented from being exposed to the encapsulant formed by the encapsulating resin  70  by the abutting of the tabs  34  against the lower mold  62 .  
     [0028] The tab  34  has a height substantially corresponding to or slightly lower or higher than the distance between the bottom surface of the die pad  31  and that of the mold cavity  63 . Also, the tab  34  is preferably formed in a triangular shape with a corner end pointing toward the bottom surface of the mold cavity  63 . Therefore, as the transfer molding process is completed, merely the corner ends of the tabs  34  may be exposed to the encapsulant formed by the encapsulating resin  70  such that the reliability of the semiconductor package thus-obtained is secured. Furthermore, in the case that the corner ends of the tabs  34  are exposed to the encapsulant, the tabs  34  also provide the semiconductor package of the present invention with a path for expelling the moisture within the encapsulant to the atmosphere so that the occurrence of crack or “popcorn” in the package can be eliminated. Also by way of the tabs  34 , the heat generated by the die  10  can be directly dissipated to the atmosphere or to an externally attached heatspreader or heat block so that the heat dissipation of the semiconductor package of the present invention will be enhanced.  
     [0029] Referring to FIG. 9, there is shown a lead frame for use in a semiconductor package of a second embodiment of the present invention. In FIG. 9, elements corresponding to those shown in FIG. 7 are given the same reference numerals, while their explanation is here omitted for the-sake of simplifying the description.  
     [0030] As seen from FIG. 9, the lead frame  30 ′ in accordance with the second embodiment of the present invention has a die pad  31 ′ with a rectangular opening  35 ′ formed in the center thereof and a plurality of tabs  34  formed in a pair of opposing edges of the opening  35 ′. The formation of the opening  35 ′ on the die pad  31 ′ is to decrease the adhesion area of a die  10  adhered to the die pad  31 ′ whereby delamination between the die  10  and die pad  31 ′ is eliminated. The tab  34  also has a height substantially corresponding to or slightly lower than the distance between the bottom surface of the die pad  31 ′ and that of the mold cavity  63 , as shown in FIG. 10. Accordingly, when the transfer molding process is completed, merely the tip ends of the tabs  34  instead of the die pad  31 ′ are exposed to the encapsulant formed by the encapsulating resin  70 .  
     [0031] As seen from the above, the present invention has been described hitherto with exemplary preferred embodiments. However, it is to be understood that the scope of the present invention need not be limited to the disclosed preferred embodiments. It should also be noted that the present invention is in no way limited to the details of the illustrated structures and shapes, but changes and modifications, such as those shown in FIG. 11A to  11 I, can be made within the scope of the appended claims.