Abstract:
Provided is a lead frame having an improved wire bonding property of inner leads and an improved soldering property of outer leads and preventing defects with high producing yield, and a method of manufacturing the lead frame. The lead frame includes a plurality of inner leads formed with predetermined intervals between them; and a plurality of outer leads extended from the inner leads in length directions of the inner leads, each of which has an end portion overlapped with the inner lead and coupled thereto and the other end connected to neighboring outer lead by a supporting portion.

Description:
[0001]     This application claims priority of Korean Patent Application No. 10-2004-0070614, filed on Sep. 4, 2004, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.  
       FIELD OF THE INVENTION  
       [0002]     The present invention relates to a semiconductor device and a lead frame for the semiconductor device, and a method of manufacturing the lead frame and the semiconductor device.  
       DESCRIPTION OF THE RELATED ART  
       [0003]     A lead frame is an element of a semiconductor device, which supports a semiconductor chip thereon and electrically connects the semiconductor chip to an external circuit such as a circuit board.  FIG. 1  shows a lead frame  10  according to a conventional art. The lead frame  10  includes a die pad  11  and leads  15  arranged about the die pad  11 . The die pad  11  is typically disposed at a central location of the lead frame  10  and connected to an outer frame or side rail member  18  by tie bars  12 . The leads  15  include inner leads  13  and outer leads  16 , and a dam bar  17  is formed between inner leads  13  and outer leads  16  while supporting the leads to maintain a designed shape. The outer frame  18  and dam bar  17  are to be removed after the semiconductor packaging is completed. The lead frame  10  is formed typically by a stamping or an etching process.  
         [0004]      FIG. 2  illustrates a structure of a semiconductor device  20  having a semiconductor chip  50  mounted on a lead frame such as shown in  FIG. 1 . According to a known semiconductor package process, the semiconductor chip  50  is attached on the die pad  11  using an adhesive  51 , the inner leads  13  and corresponding bonding pads (electrodes)  52  of the semiconductor chip  50  are wire-bonded by a wire  60 , and the outer leads  16  are electrically connected to the external circuit. The semiconductor chip  50  and the inner leads  13  are molded by a mold resin  70  to form the semiconductor package  20 .  
         [0005]     In order to improve the wire bonding capability between the semiconductor chip  50  and the inner leads  13 , a silver (Ag) plating layer  14  is typically applied on an end portion of the each of the inner leads  13 . In addition, in order to improve the soldering attachment between the external circuit (not shown) and the outer leads  16 , a tin (Sn) plating layer  19  is applied on an outer region of the outer lead  16  after the semiconductor packaging is performed with the resin mold material. However, this process should be performed through a wet process after the resin molding process, which is often a troublesome process and may result in reduction of the product qualities and reliabilities.  
         [0006]     In order to solve the problems discussed above, it has been suggested to use a pre-plated frame (PPF). This pre-plating method is typically performed by pre-plating a material having good solder wettability onto a surface of the lead frame. U.S. Pat. No. 6,518,508 assigned to the coassignee of present application discloses an example of the PPF. In this example, Ni plating layer, Pd plating layer, and Ag plating layer are sequentially formed on a base material for the lead frame. The PPF method may be advantageous in that one plating process can prepare the lead frame for both wire-bonding and connecting the lead frame to the external circuit. However, the cost of manufacturing the lead frame becomes relatively high because noble metals are used in the process, and cracks may be easily generated during a subsequent bending of the leads (to form a desired shape requiring in the manufacturing process) due to a so-called Galvanic corrosion effect particularly in the case where the base lead frame is formed of Fe—Ni alloy.  
         [0007]     In order to solve these problems, it has been suggested to use a two-tone pre-plated frame, in which two different kinds of metal are plated one on the inner lead portions and the other on the outer lead portions of the base lead frame. In the two-tone pre-plated frame method, for example, silver (Ag) is plated on the portion of the base frame corresponding to the inner leads and tin (Sn) is plated on the portion of the base frame corresponding to the outer leads. This method can be effectively applied when the alloy of Fe—Ni is used for the base frame material. As such, the two-tone pre-plated frame method may be advantageous because of its applicability to different base materials, low costs of material, and enhanced quality of assembly. However, in the two-tone pre-plated frame process, a whisker is often generated regardless of the plating solution condition or the plating time. Due to this whisker phenomenon, the lead frame may be short-circuited. In order to remove the whisker, a reflow process is typically performed, in which the lead frame with the tin (Sn) plating layer disposed thereon is subjected to heat at a temperature higher than a re-crystallization temperature. In this case, however, the entire processing time increases, and as a result, the productivity becomes lowered and manufacturing costs increase by the additional processes required therein. In addition, the Ag plating layer formed on the inner leads becomes easily damaged by the heat in the reflow process, which causes degradation of bonding quality between the bonding wire and the inner leads.  
       SUMMARY OF THE INVENTION  
       [0008]     The present invention provides a lead frame having an improved wire-bonding of inner leads and an improved soldering of outer leads, and a method of manufacturing the lead frame. The present invention also provides a lead frame having an improved yield and reduced defects, and a method of manufacturing the lead frame. The present invention also provides a lead frame having an improved mechanical strength and workability of manufacturing processes thereof, and a method of manufacturing the lead frame.  
         [0009]     The present invention further provides a method of forming a semiconductor device, and the semiconductor device formed with the lead frame of the invention.  
         [0010]     According to one aspect of the present invention, a lead frame for a semiconductor device comprises: a plurality of inner leads arranged in an inward direction of the lead frame with predetermined intervals between them; and a plurality of outer leads, each of the outer leads extending outwardly from a corresponding one of the inner leads, each of the outer leads having an inner end overlapped and connected with the corresponding one of the inner leads and an outer end connected to a supporting portion disposed at an outer area of the outer leads.  
         [0011]     The lead frame preferably includes a dam bar formed along the outer leads in a transverse direction thereof and connecting the outer leads to one another, and a die pad disposed at an inward location from the inner leads. The inner leads of the lead frame preferably includes a first plating layer formed on an inner surface of the inner leads, wherein the first plating layer contains at least one material selected from a group consisting of Ag, Au and Pd. The outer leads of the lead frame preferably include a second plating layer formed thereon, wherein the second plating layer contains Sn as a main material. The overlapped portions of the inner leads and the outer leads can be connected together by welding, bonding, or other metal connection methods.  
         [0012]     According to another aspect of the present invention, a method of producing a lead frame comprises: providing a first lead frame; forming the first lead frame into a predetermined shape, the shape including a plurality of inner leads and a first coupling portion disposed around the inner leads; forming a first plating layer at least on some surface area of the inner leads, the first plating layer containing at least one material selected from Ag, Au and Pd; providing a second lead frame; forming the second lead frame into a predetermined shape, the shape including a plurality of outer leads and a second coupling portion disposed around the outer leads; forming a second plating layer at least on some surface area of the outer leads, the second plating layer containing Sn as a main material; heat treating the second plating layer at a temperature higher than a re-crystallization temperature of the second plating layer; and coupling the first and second lead frames with the first and second coupling portions of the first and second lead frames joined together.  
         [0013]     The method of producing a lead frame preferably includes a further step of removing predetermined portions of the first and second coupling portions for separating connection between each lead of the lead frame, which is performed after coupling the first and second lead frames.  
         [0014]     According to another aspect of the present invention, a semiconductor device comprises: a lead frame, the lead frame including a plurality of inner leads arranged in an inward direction of the lead frame, and a plurality of outer leads each of the outer leads extending outwardly from a corresponding one of the inner leads, each of the outer leads having one end portion overlapped and connected with the corresponding one of the inner leads; a semiconductor chip disposed adjacent to a central location of the lead frame in a manner supported by the lead frame; and an encapsulation material covering at least some portions of the semiconductor chip and the lead frame including the overlapped portion of the inner and outer leads.  
         [0015]     According to another aspect of the present invention, a method of producing a semiconductor device comprises: forming a lead frame, the lead frame including a plurality of inner leads arranged in an inward direction of the lead frame, and a plurality of outer leads each of the outer leads extending outwardly from a corresponding one of the inner leads, each of the outer leads having an inner end overlapped and connected with the corresponding one of the inner leads and an outer end connected to an outer supporting structure of the lead frame; mounting a semiconductor chip to the lead frame in a manner supported by the lead frame; encapsulating at least some portions of the semiconductor chip and the lead frame with an encapsulation material except at least the outer supporting structure of the lead frame; and removing at least some of the outer supporting structure of the lead frame to separate the leads of the lead frame with one another.  
         [0016]     The encapsulating of the semiconductor device is performed in a manner preferably covering the overlapped portion of the inner and outer leads of the lead frame. 
     
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0017]     The above and other features and advantages of the present invention will become more apparent by describing in detail exemplary embodiments thereof with reference to the attached drawings in which:  
         [0018]      FIG. 1  is a plan view of a lead frame according to a conventional art;  
         [0019]      FIG. 2  is a cross-sectional view of a semiconductor package including the lead frame of  FIG. 1 ;  
         [0020]      FIG. 3  is an exploded perspective view of a lead frame in a pre-assembled state and constructed according to one embodiment of the present invention;  
         [0021]      FIG. 4  is a cross-sectional view of a semiconductor package including the lead frame of  FIG. 3 ;  
         [0022]      FIG. 5  is an exploded perspective view of a lead frame in a pre-assembled state and constructed according to another embodiment of the present invention;  
         [0023]      FIG. 6  is an exploded perspective view of a lead frame in a pre-assembled state and constructed according to a further embodiment of the present invention;  
         [0024]      FIG. 7  is a cross-sectional view of a semiconductor package including the lead frame of  FIG. 6 ;  
         [0025]      FIG. 8  is a cross-sectional view of a semiconductor package similar to that of  FIG. 7  and including a lead frame constructed according to still a further embodiment of the present invention;  
         [0026]      FIG. 9  is a flow chart of a method for manufacturing a lead frame according to one preferred embodiment of the present invention;  
         [0027]      FIG. 10  is a schematic view illustrating the lead frame coupling process according to one embodiment of the invention; and  
         [0028]      FIG. 11  is a plan view of a lead frame constructed according to one embodiment of the invention.  
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0029]      FIG. 3  is an exploded perspective view of a lead frame constructed according to the present invention, and  FIG. 4  is a cross-sectional view of a semiconductor package including the lead frame of  FIG. 3 . The lead frame shown in  FIG. 3  is in a state before performing a coupling portion trimming process that will be described later, and the lead frame in the final form has a different shape with the two lead frames of  FIG. 3  coupled with each other and some of the outer portions removed by the trimming process.  
         [0030]     Referring to  FIG. 3 , a lead frame  100  includes a first lead frame  110  and a second lead frame  120  that are to be coupled or joined to each other in a face-to-face relation by a subsequent process as will be described later. The first lead frame  110  includes a first side rail (i.e., outer frame)  118  forming an outer boundary of the frame, a plurality of inner leads  113  extending generally inwardly (or radially) from the first side rail  118  at predetermined intervals, and a first coupling portion  115  connecting the inner leads  113  to each other at outer portion of the inner leads  113 . In addition, outer penetration holes  116  are formed on the outer portion of the first coupling portion  115 .  
         [0031]      FIG. 4  illustrates an assembled state of a semiconductor device of the invention, in which each of the inner leads  113  is electrically connected to a bonding pad (electrode)  152  of the semiconductor chip  150  by a bonding wire  160 . In this embodiment, a first plating layer  114  is formed on an inside area of the inner leads  113  to enhance the wire-bonding capacity for bonding with the wire  160  in the wire-bonding process. The first plating layer  114  is formed of a metal, such as Ag, Au, and Pd, having such a characteristic for facilitating the wire-bonding.  
         [0032]     Referring to  FIGS. 3 and 4 , the first and second lead frames  110  and  120  can be coupled together in a manner such that the first coupling portion  115  of the first lead frame  110  joins with a second coupling portion  125  of the second lead frame  120 . The first coupling portion  115  connects and supports the inner leads  113  and maintains the gaps between the inner leads  113 . After coupling the first and second coupling portions  115  and  125 , a predetermined area of the coupling portions  115  and  125  of the lead frame is to be cut in the coupling portion trimming process in order to separate each of the inner leads  113  as independent leads for the semiconductor device, which will be described herein after in further details.  
         [0033]     The first lead frame  110  includes outer penetration holes  116  formed in a manner corresponding to the outer leads  126  of the second lead frame  120 , thus the outer leads  126  are exposed from the upper portion of the lead frame through the outer penetration hole  116  after coupling the first and second lead frames  110  and  120 . With the provision of the penetration holes  116 , electric short-circuits between the outer leads  126  can be prevented.  
         [0034]     The die pad  111  is connected to the side rail  118  by the tie bars  112 , and supports the semiconductor chip thereon. As shown in  FIG. 4 , the semiconductor chip  150  is attached on the die pad  111  via an adhesive  151  on the lower surface thereof. The die pad  111  can be disposed on a plane that is lower than that of the inner leads  113  by a down-set process.  
         [0035]     Referring to  FIG. 3 , guide holes  117  are formed at suitable corner areas of the side rail  118  to prevent a misalignment when the first and second lead frames  110  and  120  are coupled to each other. The first lead frame  110  can be formed typically by a stamping or etching method.  
         [0036]     The second lead frame  120  includes a second side rail  128  forming an outer boundary of the second frame, a plurality of outer leads  126  formed integrally on the inside of the second side rail  128  at predetermined intervals, and second coupling portion  125  which connects the outer leads  126  to each other at the inside of the outer leads  126 . In addition, an inner penetration hole  121  is formed on the inner portion of the second coupling portion  125 .  
         [0037]     An end portion of the each outer lead  126  is supported by the second coupling portion  125 , and the other end portion is supported by the side rail  128 , more specifically, by a supporting portion  128   a  of the side rail  128 . As shown in  FIG. 4 , the outer leads  126  extend to the outside of the semiconductor package  105 , and exposed portions of the outer leads are to be connected to the external circuit such as a circuit board. A second plating layer  129  is formed on the outer leads  126  exposed out of the semiconductor package  105 . The second plating layer  129  is for enhancing the solder-wettability of the outer leads  126  during mounting of the semiconductor package on the circuit board, and also can reduce corrosions in the outer leads  126 . The second plating layer  129  is generally formed of Sn or Sn alloy.  
         [0038]     Referring again to  FIG. 3 , the second coupling portion  125  provides a surface for coupling with the first lead frame  110 , and also supports the outer leads  126  to maintain the gaps between the outer leads  126 . Predetermined areas of the second coupling portion  125  (along with the corresponding area in the first coupling portion  115 ) are cut in the coupling portion trimming process after the first and second lead frames  110  and  120  are coupled together.  
         [0039]     The inner penetration hole  121  is formed in the central area including the area corresponding to the inner leads  113  of the first lead frame  110 , thus the inner leads  113  can be exposed from the lower portion of the lead frame through the inner penetration hole  121  after the first and second lead frames  110  and  120  are connected together, and the electric shorts between the inner leads  113  can be prevented.  
         [0040]     Guide holes  127  are formed on side areas of the second side rail  128  to align the first and second lead frames  110  and  120  for the connection and thus preventing a misalignment thereof.  
         [0041]     As mentioned above, after the first lead frame  110  and the second lead frame  120  each fabricated in independent processes are coupled or connected to each other, predetermined areas of the first and second coupling portions  115  and  125  are removed through the coupling portion trimming process. The removing areas include the areas in the coupling portions  115  and  125  disposed between individual leads, as will be described in association with the coupling portion trimming operation (i.e., S 43  in  FIG. 9 ).  
         [0042]     As shown in  FIG. 4 , which illustrates a semiconductor package having a lead frame of the invention in a final form, the die pad  111  is formed at the central portion, the inner leads  113  are formed around the die pad  111  and aligned toward the die pad  111 , and the outer end portions of the inner leads  113  are connected with the inner end portions of the outer leads  126  while some portions in the coupling portions  115  and  125  (i.e., the portions remaining after the coupling portion trimming operation) are overlapping with each other as shown. Here, the outer leads  126  are directly supported by the supporting portion  128   a  that connects the outer leads  126  as a unit at the outer portion of the outer leads  126 , and the inner leads  113  are supported by the outer leads  126 . It is desirable that the coupling portions, on which the end portions of the inner leads  113  and the outer leads  126  are overlapped, are sealed by an encapsulation material (e.g., molding resin) in the subsequent semiconductor packaging process as shown in  FIG. 4 .  
         [0043]      FIG. 5  is an exploded (i.e., pre-assembled) perspective view of another embodiment of the lead frame according to the present invention. Referring to  FIG. 5 , the lead frame  200  includes a first lead frame  210  and a second lead frame  220  that are coupled to each other. For simplicity and clarity purposes, only the lead frame before performing the coupling portion trimming process is shown in  FIG. 5 .  
         [0044]     A dam bar  230  that connects outer leads  226  to each other is formed between a second side rail  228  and a second coupling portion  225  of the second lead frame  220 . The outer leads  226  can be secured as a unit by the dam bar  230  on the second lead frame  220 , and a predetermined portion of the outer lead  226  is exposed to outside from the mold resin as described above.  
         [0045]     As is similar to the lead frame of the first embodiment as described above, the lead frame  200  includes a die pad  211 , tie bars  212 , inner leads  213 , a first plating layer  214 , outer penetration holes  216 , guide holes  217 , a supporting portion  218   a,  and a first side rail  218 , respectively in the first lead frame  210 , and an inner penetration hole  221 , guide holes  227 , a supporting portion  228   a,  a second side rail  228 , and a second plating layer  229 , respectively in the second lead frame.  
         [0046]      FIG. 6  is an exploded perspective view of a lead on chip (LOC) type lead frame constructed according to the present invention, and  FIG. 7  is a cross-sectional view of a semiconductor package including the lead frame shown in  FIG. 6 . The LOC type semiconductor package has a structure in which the semiconductor chip is located under the inner leads. In  FIG. 6 , only the lead frame before performing the coupling portion trimming process is shown for simplicity.  
         [0047]     The lead frame  300  includes a first lead frame  310  and a second lead frame  320  that are coupled to each other in a face-to-face relation. The first lead frame  310  includes a first side rail  318  generally forming the outer boundary of the lead frame, inner leads  313  arranged inwardly from the inside of the side rail  318  and having predetermined intervals between the leads, and a first coupling portion  315  connecting the inner leads  313  at the outer portion of the inner leads  313 . In addition, an outer penetration hole  316  is formed at the outer portion of the first coupling portion  315 . The second lead frame  320  includes a second side rail  328 , a plurality of outer leads  326  supported by a supporting portion  328   a  of the second side rail  328 , and a second coupling portion  325  connecting the outer leads  326  to one another at the inner portion of the outer leads  326 . In addition, an inner penetration hole  321  is formed at the inner portion of the second coupling portion  325 . The first and second lead frames  310  and  320  are coupled or joined to each other by connecting the first coupling portion  315  and second coupling portion  325  together, and the inner leads  313  of the first lead frame  310  and the outer leads  326  of the second lead frame  320  are exposed to the outside of the lead frame  300 , respectively, through the inner penetration hole  321  of the second lead frame  320  and the outer penetration holes  316  of the first lead frame  310 .  
         [0048]      FIG. 7  shows a semiconductor package structure including the lead frame of  FIG. 6 . Referring to  FIG. 7 , the semiconductor chip  350  is attached on a lower surface of the inner leads  313  by an adhesive material, such as an adhesive tape  351  attached onto the lower surface of the inner leads  313 , and a bonding pad (electrode)  352  formed on the central portion of the semiconductor chip  350  is electrically connected to the end portions of the inner leads  313  by a bonding wire  360 . Here, a first plating layer  314  is formed on the end portion of each inner lead  313 , and a second plating layer  329  is formed on each outer lead  326  that is exposed to the outside of the semiconductor package  350 . Reference numeral  370  denotes a mold resin that seals the semiconductor chip  350 , the first coupling portion  315 , and the second coupling portion  325 , respectively.  
         [0049]      FIG. 8  shows a semiconductor package structure including a lead frame of the invention, constructed according to another embodiment of the invention. Similar to the lead frame as shown in  FIG. 6 , this lead frame also includes inner leads  413  and outer leads  426 , and the inner leads  413  and the outer leads  426  are also connected to each other via coupling portions  415  and  425 . In addition, a second plating layer  429  is formed on the outer leads  426  which are exposed out of the semiconductor package  405 . However, in this embodiment, the semiconductor chip  450  is electrically connected to the inner leads  413  by conductive bumps  460  formed on the bonding pads. Here, because the semiconductor chip  450  is directly connected to the inner leads  413  by the conductive bumps  460 , there is no need to form the first plating layer on the end portions of the inner leads  413  as the prior embodiments described above. Reference numeral  470  denotes a mold resin that seals the semiconductor chip  450 , the first coupling portion  415 , and the second coupling portion  425 , respectively.  
         [0050]     Hereinafter, a method of manufacturing the lead frame according to the present invention is described with reference to  FIG. 9 .  
         [0051]     According to one preferred method of the invention, a process of forming the first lead frame (S 10 ) and a process of forming the second lead frame (S 20 ) can be performed simultaneously or at least in a non-serial manner, and the fabricated first and second lead frames are coupled as one lead frame through a lead frame coupling process (S 30 ). After that, a post-coupling process (S 40 ) is performed.  
         [0052]     The process of forming the first lead frame (S 10 ) typically includes the operations of preparing (or providing) a first base substrate (S 11 ), shaping the first base substrate into a designed shape (S 12 ), performing a thermal treatment (S 13 ), and forming the first plating layer (S 14 ).  
         [0053]     An elongate base lead frame strip is first formed of a suitable lead-frame material known in the art, for example, of a metal alloy composed of Fe and Ni. In the operation of shaping (or forming) the first base substrate (S 12 ), a plurality of inner leads, outer penetration holes, and a first coupling portion such as described above are integrally formed on the base substrate according to the particular design of the lead frame. Here, a die pad, tie-bars, dam-bars, or the like known in the art can also be formed in the first lead frame depending on the particular design of the lead frame. This shaping process can be performed typically by a stamping method or an etching method. When using the stamping method, the base substrate (in the form of elongate strip) is sequentially moved on a progressive-moving type pressing machine (e.g., a progressive punching machine), and a plurality of holes corresponding to the shape of the lead frame are formed progressively in the base strip by the punching machine. When using the etching method, a photosensitive layer is applied on the base substrate, and suitable light rays are radiated onto the substrate through a photo mask having a predetermined pattern, and then, the substrate is subject to a development process to form the predetermined patterns on the base substrate.  
         [0054]     After performing the operation S 12 , the substrate is optionally subject to a thermal treatment (S 13 ) for removing residual stresses in the first lead frame, if necessary. In the operation S 14 , the first plating layer, such as an Ag, Au, or Pd layer is formed on an upper tip portion of the inner leads. By forming the first plating layer, the bonding wire can be readily attached on the inner leads.  
         [0055]     The operation of forming the second lead frame (S 20 ) includes the operations of preparing a second base substrate (S 21 ), shaping the second base substrate (S 22 ) into a designed configuration, forming a second plating layer (S 23 ), and performing a thermal treatment (S 24 ). In the operation S 21 , the second base substrate for producing the second lead frame is provided. The base substrate is typically in the form of an elongate strip for producing a plurality of lead frames from the strip. As is the first lead frame, this second lead frame can also be formed of suitable materials for forming the lead frame, such as an alloy of Fe and Ni. In the operation S 22 , a plurality of outer leads, an inner penetration hole, and a second coupling portion are integrally formed on the second base substrate using the stamping method or the etching method as described above.  
         [0056]     Next, in the operation S 23 , the second plating layer is formed on the outer leads. The second plating layer is formed preferably of Sn or alloy of Sn. Therefore, the soldering characteristic of the outer leads can be improved, and the semiconductor package can be easily attached on the circuit board with strong bond. In addition, the corrosion resistance property of the outer leads that are exposed out of the semiconductor package can be improved.  
         [0057]     After performing the operation S 23 , the thermal treatment process (S 24 ) such as a reflow process is performed. Here, the second plating layer is heated at a temperature higher than a re-crystallization temperature in order to restrict the growth of whiskers in the second plating layer formed typically of Sn or Sn alloy, in particular. The operation S 24  can be performed in the conventional reflow methods known in the art.  
         [0058]     Now, after providing the first and second lead frames, the operation of coupling lead frame (S 30 ) is performed. Here, the coupling of the first and second lead frames can be performed by known coupling or joining methods, for example, by a spot welding method, metal bonding method, or other electric resistance welding methods. That is, referring to  FIG. 10 , the first lead frame  510  and the second lead frame  520  are overlapped in a manner described above and positioned between welding electrodes  600  as shown, and an electrode supporting bar  610  of the upper electrode  600  is pressed toward a lower electrode supporting board  620  to connect the first and second coupling portions  515  and  525  together while applying suitable electric currents. The first and the second lead frames can be joined together by various other bonding technologies such as a laser welding method or the like.  
         [0059]     After coupling the first and second lead frames, the post-coupling process S 40  is performed. The post-coupling process S 40  can include a strip cutting operation (S 41 ), a lead-lock tape attaching operation (S 42 ), a coupling portion trimming operation (S 43 ), a down-set operation (S 44 ), and an inspection and packing operation (S 45 ).  
         [0060]     In the strip cutting operation S 41 , the elongate lead frame strip is cut into a plurality of unit lead frames. Next, in the lead-lock tape attaching operation S 42 , a lead-lock tape  750  is attached on a surface of the inner leads  713  around the circumference of the inner leads to securely support the inner leads  713 , as shown in  FIG. 11 , for example. Even when a predetermined portion of the coupling portion is removed by the subsequent coupling portion trimming operation S 43 , the inner leads  713  can be secured by the lead-lock tape  750  without deformation, while maintaining the designed intervals of the leads.  
         [0061]     Referring to  FIG. 11 , in the coupling portion trimming operation S 43 , predetermined portions of the coupling portion  725  (that is, the portions where the first and second coupling portions of the first and second lead frame are connected) are removed to separate each lead from its neighbouring leads. Here, a plurality of connection portions (in the coupling portion  725 ) disposed between two neighbouring leads are cut along cutting lines C, thus providing a plurality of separate leads with the ends of the inner and outer leads  713  and  726  overlapping (as shown in  FIGS. 4, 7  and  8 ). As such, when the coupling portion  725  is cut along the cutting lines C, each of the unit lead (with each inner lead  713  and each outer lead  726  connected) is physically separated from its neighbouring leads, and thus is usable as a terminal for the semiconductor device.  
         [0062]     More specifically, in the coupling portion trimming operation S 43 , the lead frame is typically positioned between an upper punching die and a lower die having suitable configurations, and the upper die is compressed to punch the lead frame at the coupling portion  725  of the lead frame. It is desirable that certain cavities (not shown) connecting the terminal ends of the inner leads  713  are also cut in this process. The particular cutting positions can be modified by changing the shape of the punch in the upper die. The cutting operation can be performed preferably by a single punching stroke.  
         [0063]     In the down-set operation S 44 , the tie bars connecting the die pad to the side rail is down-set to dispose the die pad at a plane that is lower than that of the inner leads. Then, the inspection process and packing process S 45  are performed, which completes the manufacturing process of the lead frame of the invention.  
         [0064]     After the lead frame is produced, it now subjects to a semiconductor packaging process to produce a semiconductor device such as described above in connection with  FIGS. 4, 7  and  8 , in which the packaging process typically includes a die-bonding process for mounting a semiconductor chip on the die pad, a wire-bonding process for electrically connecting the semiconductor chip with the inner leads of the lead frame, and an encapsulation process for covering the designed area of the semiconductor chip and the lead frame with an encapsulating material such as a resin mold. These packaging processes are described above, and conventional methods known in the semiconductor manufacturing field can be applied.  
         [0065]     After the semiconductor package is produced as described above, the semiconductor device is now subject to a further trimming process to remove some of the structural elements of the lead frame (such as  100 ,  200  and  300  in  FIGS. 3, 5  and  6 ) that are not necessary for the ultimate semiconductor device. For example, supporting portions (such as  128   a,    228   a  and  328   a  in  FIGS. 3, 5  and  6 ) are only necessary to secure the lead frame in shape during the manufacturing process of the semiconductor device, and are removed typically by stamping, punching, etching, or other known cutting methods. The resultant semiconductor has a plurality of outer leads (such as  126 ,  226  and  326  in  FIGS. 3, 5  and  6 ) that are separated from one another for functioning as multiple terminals for the semiconductor device.  
         [0066]     As described above, according to the conventional art, after forming the pattern of the lead frame, the process of forming the first plating layer (e.g., Ag plating layer) on the inner area of the inner leads, and the process of forming Sn plating layer on the outer leads should be performed sequentially. However, according to the present invention, the first lead frame having the inner leads and the second lead frame having the outer leads are fabricated separately, preferably in simultaneous or parallel forming processes (e.g., S 10  and S 20 ). Thus, when the first and second lead frames are formed simultaneously, the total processing time can be reduced substantially. That is, during forming the first plating layer by the first lead frame forming operation S 10 , the second plating layer can be formed simultaneously by the second lead frame forming operation S 20 .  
         [0067]     Generally, the reflow process is performed in order to restrain a whisker generation in the Sn plating layer disposed on the outer leads. In the conventional art, where the inner leads and the outer leads are formed in a single sheet of lead frame, the first plating layer (e.g., Ag layer) formed on the inner leads can be affected or disordered by the reflow process. Thus, because the first plating layer is disordered by the reflow process, the wire-bonding capability of the inner leads becomes degraded. However, according to the present invention, the first lead frame having the inner leads and the second lead frame having the outer leads are subject to different processes from each other, thus the above problem of the conventional art can be solved effectively.  
         [0068]     In addition, utilizing the lead frame of the invention which is formed by coupling two (i.e., first and second) lead frames together, the strength of the lead frame can be improved because the coupling portion has a dual-layered (overlapped) structure. Therefore, deformation and vibration of the lead frame in the post-coupling process S 40  can be reduced.  
         [0069]     The present invention is not limited to the particular structures of the lead frame and/or the semiconductor devices as shown in the accompanying drawings, but it can be applied to any lead frames and/or semiconductor devices having different structures, formats or configurations.  
         [0070]     According to the lead frame and the method of manufacturing the lead frame and the semiconductor device of the present invention, the yield of production can be improved because the total processing time is reduced. In the present invention, since the first lead frame having the inner leads and the second lead frame having the outer leads are separately fabricated in different forming processes, the plating layer forming operations of the two lead frames can be performed at the same time. Therefore, the processing time can be reduced greatly.  
         [0071]     In addition, since the processes of forming the first and second lead frames are separately performed, the conventional degradation problems causable from a sequential production of the lead frame can be reduced substantially. In particular, while in the conventional art the first plating layer can be degraded or damaged by the reflow process performed after forming the Sn plating layer, this thermal damage problem can be prevented according to the present invention as described above.  
         [0072]     Moreover, having the coupling portion in dual-layered structure, the strength and rigidity of the lead frame is increased, and the deformation and vibration of the lead frame can be effectively prevented, which make it easy to handle the lead frame in the post-coupling process.  
         [0073]     While the present invention has been particularly illustrated and described with reference to exemplary embodiments thereof, it will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present invention as defined by the following claims.