Abstract:
Provided is a semiconductor flat package with improved mountability. In a semiconductor device, an end surface of a lead, which is exposed from an encapsulation resin, is covered with a plated layer, and a side end surface of the plated layer and a side end surface of the encapsulation resin are flush with each other. A material with good solder wettability is formed at a lead cut portion of the semiconductor flat package, to thereby improve solder connection strength with a circuit board. A solder fillet is formed from the lead cut portion of the semiconductor package, to thereby enable adaptation of solder automatic visual inspection after mounting.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a resin-encapsulated semiconductor device, and more particularly, to a so-called flat package semiconductor device having a flat lead so that a bottom surface of a semiconductor package and a bottom surface of the lead are flush with each other. 
         [0003]    2. Description of the Related Art 
         [0004]    Various electronic devices, including portable devices, are becoming thinner, smaller, and lighter. Semiconductor packages to be mounted on the electronic devices are also required to be thinner and smaller. Thinning and downsizing of the semiconductor package cannot be achieved by a related-art gullwing type semiconductor package. It is thus effective to employ a so-called flat package, which has a flat lead so that a bottom surface of the semiconductor package and a bottom surface of the lead are flush with each other. 
         [0005]    In the flat package, a lead for connection to a circuit board is exposed from a rear surface of the package (surface to be mounted on the circuit board). Further, an island that is a portion for mounting a semiconductor chip thereon is generally formed of a lead frame, a plated layer, or the like, and the island is exposed from the rear surface of the package in some cases and not in other cases. In addition, in some semiconductor packages, no island is formed but a semiconductor chip is directly mounted on an encapsulation resin. When the flat package is mounted on a circuit board with solder, a bottom surface of an outer lead and a pattern of the circuit board are bonded to each other with solder. 
         [0006]    A method of manufacturing a related-art flat package is described with reference to the drawings. 
         [0007]      FIGS. 10A to 10G  are cross-sectional views illustrating the method of manufacturing the related-art semiconductor package in the order of steps. Referring to  FIG. 10A , a photo resist  2  is applied and patterned on a conductive base plate  3 , and a plated layer  1  is formed in an opening of the photo resist  2  to form an electrode of the semiconductor package. The plated layer  1  is formed of three layers in many cases. A gold or silver plated layer is formed on the conductive base plate  3 , a nickel or copper plated layer is formed thereon, and further a gold or silver plated layer is formed thereon. Referring to  FIG. 10B , the photo resist  2  is removed. Referring to  FIG. 10C , a semiconductor chip  7  is die-bonded on an island  36  formed of the plated layer  1 , and a wire  8  is electrically connected to an outer lead  30  similarly formed of the plated layer  1 . Referring to  FIG. 10D , an encapsulation resin layer  9  is formed in order to protect the semiconductor chip  7 , the wire  8 , and the like. Referring to  FIG. 10E , the conductive base plate  3  is removed.  FIG. 10F  illustrates dicing with a dicing blade  10  for singulating the semiconductor package. In dicing, the outer lead  30  formed of the plated layer  1  is cut.  FIG. 10G  illustrates the final cross section of the semiconductor package. An end surface  31  of the outer lead  30  is exposed. 
         [0008]    The dicing step involves cutting the outer lead  30  formed of the plated layer  1 , and hence nickel or copper is exposed from the end surface  31  as a cutting surface.  FIG. 11  is a cross-sectional view illustrating a state in which the semiconductor package manufactured by the related-art manufacturing method is bonded on a circuit board with solder. Because a gold plated layer or a silver plated layer is exposed from a bottom surface  32  of the outer lead  30 , solder wettability is good and a satisfactory bonded state with solder is obtained (see, for example, Japanese Patent Application Laid-open No. 2002-9196). 
         [0009]    Further, a technology of forming a material with good solderability on a cutting surface of an outer lead in a related-art gullwing type semiconductor package is disclosed in Japanese Patent Application Laid-open Nos. Hei 8-213540 and Hei 7-030043. 
         [0010]    As described above, more and more packaged semiconductor devices have employed the flat type due to the thinning, downsizing, and lightening of electronic devices. 
         [0011]    In the structure illustrated in  FIG. 11 , however, the main material of the plated layer  1  such as nickel or copper is exposed on the end surface  31  of the outer lead  30 , and hence solder wettability is poor and it is difficult to bond the semiconductor package with solder. Accordingly, as illustrated in  FIG. 11 , a good solder fillet cannot be obtained from the end surface  31  and hence a sufficient bonding area cannot be obtained in bonding with the circuit board, thus resulting in a problem of low bonding strength. Further, automatic visual inspection using image inspection, which is performed after the semiconductor package is bonded on the circuit board with solder, determines a pass/fail of a connection state between the outer lead  30  and solder  11  by observing the shape of the fillet. However, because the solder fillet is not bonded on the end surface  31  of the outer lead  30 , a pass/fail of the connection state cannot be determined simply by observing the fillet. Thus, there is another problem in that automatic visual inspection cannot be applied to the flat package. 
       SUMMARY OF THE INVENTION 
       [0012]    The present invention aims to solve the problems inherent in the related-art flat package as described above, and it is an object thereof to provide a packaged semiconductor device that is capable of forming a good solder fillet at a solder bonding portion to have high bond strength with a circuit board and is compatible with automatic visual inspection. 
         [0013]    In order to solve the above-mentioned problems, the following measures are used. 
         [0014]    First, there is provided a semiconductor device, including: a semiconductor chip mounted on an island; an encapsulation resin for covering the semiconductor chip; and a lead that is partially covered with the encapsulation resin and is electrically connected to the semiconductor chip, in which an end surface of the lead, which is exposed from the encapsulation resin, is covered with a plated layer, and a side end surface of the plated layer and a side end surface of the encapsulation resin are flush with each other. 
         [0015]    Further, there is provided a method of manufacturing a semiconductor device, including: forming a first resist pattern having a predetermined pattern on a conductive base plate; forming an island and a lead by precipitating a first plated layer by an electrolytic plating method on an opening surface of the conductive base plate except for a region of the first resist pattern; applying a photo resist on a surface of the first resist pattern and a surface of the first plated layer; forming a second resist pattern having an air gap at an end surface of the lead by removing a part of the first resist pattern and a part of the photo resist; precipitating a second plated layer in the air gap to form a plated layer on the end surface of the lead; removing the second resist pattern; mounting a semiconductor chip on the island, and connecting the semiconductor chip and the lead to each other via a wire; covering the semiconductor chip, the wire, the lead, and the second plated layer, which are formed on the conductive base plate, with an encapsulation resin having insulating property; separating the conductive base plate from the encapsulation resin, the island, the lead, and the second plated layer; and singulating the semiconductor device by cutting the encapsulation resin and a center part of the second plated layer. 
         [0016]    Further, there is provided a method of manufacturing a semiconductor device, including: forming a first resist pattern having a predetermined pattern on a conductive base plate; forming leads by precipitating first plated layers by an electrolytic plating method on opening surfaces of the conductive base plate except for a region of the first resist pattern; forming a first encapsulation resin having insulating property so as to fill a space between the leads; separating the conductive base plate from the first encapsulation resin and the first plated layer; forming photo resists on both surfaces of a member formed of the first encapsulation resin and the first plated layer, and removing the photo resist on a first surface of the first plated layer on one side, to thereby form a photo resist removed portion; forming an air gap by partially removing the photo resists and the first encapsulation resin so as to expose an end surface of the first plated layer; precipitating a second plated layer in the air gap and on the photo resist removed portion to form a plated layer on an end surface of each of the leads; removing a remaining part of the photo resists; reversing a member formed of the first encapsulation resin, the first plated layer, and the second plated layer, mounting a semiconductor chip on a surface of the first encapsulation resin serving as an island portion, and connecting the semiconductor chip and the each of the leads to each other via a wire; covering the semiconductor chip, the wire, the leads, and the second plated layer, which are formed on the first encapsulation resin, with a second encapsulation resin having insulating property; and singulating the semiconductor device by cutting the second encapsulation resin and a center part of the second plated layer. 
         [0017]    By using the above-mentioned measures, a good solder fillet can be formed at the end surface of the outer lead of the flat package when the semiconductor package is mounted on a circuit board, and hence the bonding strength with the circuit board is improved. Besides, a pass/fail determination on bonding between the semiconductor package and the circuit board can be performed even by an automatic visual inspection based on observation of the solder fillet. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]      FIG. 1  is a cross-sectional view illustrating a semiconductor device according to a first embodiment of the present invention. 
           [0019]      FIGS. 2A to 2L  are cross-sectional views illustrating a manufacturing process for the semiconductor device according to the first embodiment of the present invention. 
           [0020]      FIGS. 3A to 3J  are cross-sectional views illustrating a manufacturing process for a semiconductor device according to a second embodiment of the present invention. 
           [0021]      FIGS. 4A to 4E  are cross-sectional views illustrating a manufacturing process for a semiconductor device according to a third embodiment of the present invention. 
           [0022]      FIGS. 5A to 5D  are top views illustrating the manufacturing process for the semiconductor device according to the third embodiment of the present invention. 
           [0023]      FIGS. 6A to 6C  are top views illustrating the manufacturing process for the semiconductor device according to the third embodiment of the present invention followed by  FIGS. 5A to 5D . 
           [0024]      FIG. 7  is a cross-sectional view illustrating a state in which the semiconductor device according to the first embodiment of the present invention is mounted on a circuit board. 
           [0025]      FIG. 8  is a cross-sectional view illustrating a state in which the semiconductor device according to the second embodiment of the present invention is mounted on a circuit board. 
           [0026]      FIG. 9  is a cross-sectional view illustrating a state in which the semiconductor device according to the second embodiment of the present invention is mounted on a circuit board. 
           [0027]      FIGS. 10A to 10G  are cross-sectional views illustrating an example of a manufacturing process for a related-art semiconductor device. 
           [0028]      FIG. 11  is a cross-sectional view illustrating a state in which the related-art semiconductor device is mounted on a circuit board. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0029]    Now, embodiments of the present invention are described in detail with reference to the accompanying drawings. 
         [0030]      FIG. 1  is a cross-sectional view of a semiconductor device according to a first embodiment of the present invention. 
         [0031]    The semiconductor device according to the present invention includes a semiconductor chip  7 , an outer lead  30 , a wire  8  for electrically connecting the semiconductor chip  7  and the outer lead  30  to each other, an island portion encapsulation resin  37  for mounting the semiconductor chip  7  thereon, a plated layer  6  formed to cover an end surface  31  of the outer lead  30 , and an encapsulation resin  19  for protecting the whole semiconductor device. 
         [0032]    The feature of the present invention resides in that the plated layer  6  is formed in contact with the end surface  31  of the outer lead  30  and the plated layer  6  does not substantially protrude from external dimensions of the semiconductor device. In other words, a side end surface of the plated layer  6  and a side end surface of the encapsulation resin  19  are flush with each other. 
         [0033]    Further, in  FIG. 1 , a plated layer  26  is subsequently formed also on a bottom surface  32  of the outer lead  30 , to thereby form a standoff on a lower surface of the island. It is, however, not always necessary to subsequently form the plated layer  26  on the outer lead bottom surface. If a plurality of plated layers are formed in advance at a stage of forming the outer lead  30  so that a plated layer with good solder wettability such as gold is formed on the lead bottom surface  32  that is to be brought into contact with solder, the semiconductor device can be soldered to a circuit board only with this plated layer. In this case, it is not necessary to subsequently form the plated layer  26 . 
         [0034]      FIGS. 2A to 2L  are explanatory cross-sectional views of a manufacturing process for the semiconductor device according to the first embodiment of the present invention. 
         [0035]    The known art is used in  FIGS. 2A to 2D , and the feature of the method of manufacturing the semiconductor device according to the first embodiment is illustrated in  FIGS. 2E to 2L . 
         [0036]      FIG. 2A  is a view in which a photo resist  2  is patterned on a conductive base plate  3 , and a plated layer  1  is precipitated in an opening of the photo resist  2 . The plated layer  1  has a double layer structure in which a nickel or copper plated layer is formed directly on a surface to be brought into contact with the conductive base plate  3 , and a gold or silver plated layer is formed thereon. Alternatively, the plated layer  1  may have a triple layer structure in which a gold or silver plated layer with good solder wettability is formed on the conductive base plate  3 , a nickel or copper plated layer is precipitated thereon, and a gold or silver plated layer or the like is formed thereon. 
         [0037]    Referring to  FIG. 2B , the photo resist  2  is removed. Referring to  FIG. 2C , a component obtained by forming the plated layer  1  on the conductive base plate  3  is encapsulated with a resin to form an encapsulation resin layer  9 . The encapsulation resin layer  9  fills an air gap around the plated layer  1 . Referring to  FIG. 2D , the conductive base plate  3  on the lower surface is removed. 
         [0038]    Referring to  FIG. 2E , photo resists  14  are applied on the front and rear surfaces of a component obtained by integrating the encapsulation resin layer  9  and the plated layer  1 , and the photo resist applied on one-side surface of the plated layer  1  is removed. A photo resist removed portion is represented by reference numeral  16 . Referring to  FIG. 2F , the photo resist  14  and the encapsulation resin layer  9  are partially removed so as to expose an end surface of the plated layer  1 , to thereby form an air gap  5 . 
         [0039]    Available methods for removing the photo resist  14  and the encapsulation resin layer  9  in this step include laser irradiation, punching with a die, and dicing. Referring to  FIG. 2G , plating is performed to form a plated layer  6  in the air gap  5  formed in the previous step. Available plating performed in this step includes tin-based plating and gold plating, which exhibit good solderability. 
         [0040]    Referring to  FIG. 2H , the photo resists  14  are removed. Referring to  FIG. 2I , a component completed in the previous step is reversed (vertically), the semiconductor chip  7  is die-bonded on the encapsulation resin layer  9 , and further the semiconductor chip  7  and the outer lead  30  formed of the plated layer  1  are electrically connected to each other via the wire  8 . In the die-bonding step, the semiconductor chip can be bonded with use of insulating paste or conductive paste. 
         [0041]    Referring to  FIG. 2J , the whole semiconductor package is covered with an encapsulation resin layer  19  in order to protect the semiconductor chip  7  and the wire  8 .  FIG. 2K  illustrates a step of singulating the semiconductor package by dicing with a dicing plate  10 . It is important to cut a center part of the plated layer  6  so that the plated layer  6  may remain on the end surface  31  of the outer lead  30 . Referring to  FIG. 2L , the semiconductor packages are finally singulated. Through the process described above, the semiconductor device illustrated in  FIG. 1  is completed. 
         [0042]      FIGS. 3A to 3J  illustrate a manufacturing method according to a second embodiment of the present invention. 
         [0043]    Referring to  FIG. 3A , a photo resist  2  is patterned on a conductive base plate  3 , and a plated layer  1  is formed in an opening of the photo resist  2 . In many cases, the plated layer  1  is formed as follows. A gold or silver plated layer with good solder wettability is formed on the conductive base plate  3 , a nickel or copper plated layer serving as a main material of an electrode of the semiconductor package is formed thereon, and further a gold or silver plated layer is formed thereon for ensuring good wire bonding. In other words, the plated layer  1  has a triple layer structure in which the gold plated layer is formed on the conductive base plate  3  as a first plated layer, the nickel or copper plated layer is formed thereon, and the gold or silver plated layer is formed on the uppermost surface. 
         [0044]    Referring to  FIG. 3B , a photo resist  21  is further applied on a surface of the plated layer  1  and a surface of the photo resist  2  that are formed on the conductive base plate  3  in the previous step. Referring to  FIG. 3C , the photo resist  2  and the photo resist  21  are partially removed so as to expose an end surface  31  of a region of the plated layer  1  serving as an outer lead  30 . The photo resists can be removed by etching with a chemical solution, laser irradiation, or other such methods. Referring to  FIG. 3D , the region from which the photo resists are removed in the previous step is subjected to plating to from a plated layer  6 . A material with good solder wettability is used for the plated layer  6 . For example, gold plating or tin-based solder plating is suitable. Referring to  FIG. 3E , the photo resists  2  and  21  that have served as a mask in the previous plating step are removed. The structure of the semiconductor package is defined at the end of this step, and an island  36  for mounting a semiconductor chip thereon in a subsequent step, the outer lead  30 , and the plated layer  6  formed on the end surface thereof are formed on the conductive base plate  3 . 
         [0045]    Referring to  FIG. 3F , a semiconductor chip  7  is mounted on the island  36 , and is electrically connected to the outer lead  30  via a wire  8 . Referring to  FIG. 3G , the whole semiconductor package is covered with an encapsulation resin layer  9  in order to protect the semiconductor chip  7  and the wire  8 . Referring to  FIG. 3H , the conductive base plate  3  is separated.  FIG. 3I  illustrates a dicing step of singulating the semiconductor package by a blade  10 . What is important in this step is to divide a center part of the plated layer  6  so that the plated layer  6  may remain on the end surface of the outer lead  30 .  FIG. 3J  illustrates the final form of the singulated semiconductor package. Through the process described above, the plated layer  6  made of a tin-based or gold material can be formed on the end surface  31  of the outer lead  30 , and further a bottom surface  32  of the outer lead  30  is a gold or silver plated layer with good solder wettability. 
         [0046]      FIGS. 4A to 4E  illustrate a manufacturing method according to a third embodiment of the present invention. After the common process of  FIGS. 3A to 3H , a material with good solder wettability is formed also on a bottom surface of the outer lead  30  and a bottom surface of the island  36 . However, in the case where a semiconductor package is manufactured by the process illustrated in  FIGS. 4A to 4E , the plated layer  1  to be formed in the step of  FIG. 3A  is not required to have a triple layer structure. It is only necessary to form a nickel or copper plated layer directly on the conductive base plate  3  and to form a gold or silver plated layer thereon. In other words, the plated layer  1  may have a double structure in which a gold or silver plated layer is formed on a bonding surface with a wire in order to bond the wire  8  and the outer lead  30  to each other. 
         [0047]    Referring to  FIG. 4A , a photo resist  33  is patterned in order to form a plated layer on the bottom surface  32  of the outer lead  30  and a bottom surface  35  of the island  36 . Referring to  FIG. 4B , a plated layer  26  is formed in an opening of the patterned photo resist  33 . Referring to  FIG. 4C , the photo resist  33  is removed. The plated layer  26  is shaped to protrude from a bottom surface of the encapsulation resin layer  9 , and can serve also as a standoff of the semiconductor package. The formation of the standoff facilitates self-alignment when the semiconductor package is mounted on a circuit board.  FIG. 4D  illustrates a step of singulating the semiconductor package by the blade  10  for the dicing step similarly to  FIG. 3I . What is important in this step is to divide an inner side of the plated layer  6  so that the plated layer  6  may remain on the end surface  31  of the outer lead  30 .  FIG. 4E  is a cross-sectional view of the semiconductor package finally completed through the process of the present invention. The feature of this structure resides in that the plated layer  6  and the plated layer  26  each formed of the material with good solder wettability are formed on the end surface  31  and the bottom surface  32  of the outer lead  30 , respectively. 
         [0048]      FIGS. 5A to 5D  and  FIGS. 6E to 6G  are top views for facilitating the understanding of a part of the process of  FIGS. 3A to 3J . 
         [0049]      FIG. 5A  is a top view of  FIG. 3A . The plated layer  1  is formed in accordance with the shape of the patterned photo resist  2 . The plated layer  1  serves as the island portion for mounting the semiconductor chip thereon and the outer lead when the semiconductor package is completed.  FIG. 5B  is a top view of  FIG. 3C . In order to expose the end surface  31  of the outer lead  30 , the photo resist and the encapsulation resin layer are removed to form the air gap  5 . It is desired that the air gap  5  be formed only in a region in contact with the end surface  31  of the outer lead  30 , but it is not desired that the end surfaces of adjacent outer leads be connected to each other via the air gap  5 .  FIG. 5C  is a top view of  FIG. 3D . Plating is performed to form the plated layer  6  in the air gap  5 . The plated layer  6  needs to be formed of a material with good solder wettability.  FIG. 5D  illustrates a state in which the photo resist  2  is removed and the semiconductor chip  7  is mounted on the island  30  and connected to the outer lead  30  via the wire  8 . 
         [0050]    Referring to  FIG. 6A , the whole semiconductor package is covered with the encapsulation resin layer  9 . Referring to  FIG. 6B , the semiconductor package is singulated by the dicing blade  10  similarly to  FIG. 3I . What is important in this step is to cut the plated layer  6  formed of the material with good solder wettability so that the plated layer  6  may remain on the end surface  31  of the outer lead  30 . Through the process described above, the singulated semiconductor packages as illustrated in  FIG. 6C  are completed. 
         [0051]      FIG. 7  is a cross-sectional view illustrating a state in which the semiconductor package manufactured through the process of  FIGS. 4A to 4E  is bonded on a circuit board with solder. Solder  11  wets well on the plated layer  6  formed on the end surface  31  of the outer lead  30 , and a good fillet is formed. The plated layer  26  is formed also on the bottom surface  32  of the outer lead  30 , thus facilitating bonding with the solder  11 . Automatic visual inspection using image inspection, which is performed after the semiconductor package is bonded on the circuit board with solder, determines a pass/fail of a connection state between the outer lead  30  and the solder  11  by observing the shape of the fillet. Because the solder fillet  11  is formed on the end surface  31  of the outer lead  30 , the connection state can be easily inspected by the automatic visual inspection. 
         [0052]      FIG. 8  is a cross-sectional view illustrating a state in which the semiconductor package manufactured through the process of  FIGS. 3A to 3J  is bonded on a circuit board with solder. Similarly to  FIG. 7 , solder  11  wets well on the plated layer  6  formed on the end surface  31  of the outer lead  30 , and a good fillet is formed. Further, in the case of this embodiment, the post-formed plated layer  26  is not formed on the bottom surface  32  of the outer lead  30 , but as described above with reference to  FIG. 3A , solder bondability is good because the plated layer with good solder wettability is formed in advance. 
         [0053]      FIG. 9  illustrates a state in which the semiconductor package according to the second embodiment of the present invention is mounted on a circuit board. So-called flip chip bonding is performed, in which a bump formed on the semiconductor chip  7  is bonded on the outer lead  30 . In this manner, the semiconductor chip  7  and the outer lead  30  in the semiconductor package can be connected to each other via the bump as well as the wire.