Abstract:
Provided is a semiconductor device having an element covered with a resin mold and a metal lead protruding from the resin mold in which a lead-tip portion thereof is entirely covered by solder plating and in which a lead-tip end surface, which is not covered by solder plating, has an area less than half of a cross-sectional area of the metal lead, whereby solder wettability of the metal lead is improved and a bonding strength to a circuit board is also improved.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a semiconductor device manufactured by molding an element such as a semiconductor integrated circuit mounted on a lead frame with resin, and to a manufacturing method for the lead frame. 
         [0003]    2. Description of the Related Art 
         [0004]    A semiconductor device in which an element such as a semiconductor integrated circuit mounted on a lead frame is molded with resin is generally mounted onto a circuit board for use. In order to ensure sufficient mounting strength between the semiconductor device and the circuit board, leads of the semiconductor device are elongated to enlarge soldering area. Though solder wettability in a cut surface of leads of the semiconductor device, conventionally, has not been kept in good condition, large soldering area and Z-shaped forming of the leads make the solder wettability not only in both ends of the lead but also in heel portions of the leads good, permitting the solder to go up along the leads. Accordingly, a sufficient mounting strength can be ensured. 
         [0005]    Large soldering area ensures mounting strength. However, along with the miniaturization of a semiconductor device, the mounting strength against a circuit board tends to be lower. Because increase in mounting density requires reduction both in lead area with which the semiconductor device is bonded to the circuit board, and in electrode area locating on a circuit board side. Solder is mainly used to mount a semiconductor device to a circuit board, and the mounting strength changes widely depending on whether or not the lead of the semiconductor device becomes wet easily with the solder. When the temperature of the solder reaches the melting point by reflow or the like, the solder melts together with plating covering the lead of the semiconductor device. At this time, formation of the plating on the entire lead surface of the semiconductor device is desirable, because a portion where plating is absent does not become wet by the solder and will have low strength. The smaller the semiconductor device is, the less the strength between the lead of the semiconductor device and the electrode of the circuit board becomes when bonded only with the solder. It is, thus, necessary to ensure an area which becomes wet with the solder as much as possible. In particular, the solder wettability is important in a lead-tip portion of the semiconductor device. This is because the lead-tip portion is apt to be affected by influence such as warpage of the circuit board when the semiconductor device is mounted onto the circuit board. 
         [0006]      FIG. 6  is a schematic cross-sectional view showing a structure of a conventional semiconductor device. As shown in  FIG. 6 , in the conventional semiconductor device, a formed lead  2  protrudes from a resin  21 . The lead  2  is covered by a plating layer  3  except a lead-tip end surface  12  in a lead-tip cut portion  10  made by cutting the lead  2 . The presence of the lead-tip end surface  12  deteriorates the wettability to a soldering material, which serves as a bonding agent at the time of mounting onto a circuit board or the like. 
         [0007]      FIG. 7  is a simplified side view showing a structure of a conventional semiconductor device in a manufacturing process step. As shown in  FIG. 7 , a lead  2 , which is also a portion of a lead frame connecting two resins  21  and  31 , has a uniform cross-section. When the lead frame having such a structure is cut and the semiconductor devices are separated from one another, the lead-tip end surface  12  as shown in  FIG. 6  is formed. 
       SUMMARY OF THE INVENTION 
       [0008]    In order to firmly bond a lead of a semiconductor device with an electrode of a circuit board, there is provided a semiconductor device having a lead in which plating covers entirely a lead-tip portion of the semiconductor device as means for improving solder wettalibity in the lead of the semiconductor device. Further, as another means, there is provided a semiconductor device having a lead in which an area of a lead-tip cut portion on which plating is not formed is less than half of a cross-sectional area of the lead of the semiconductor device. 
         [0009]    According to the present invention, a molten soldering material melted by heat generated in reflow goes up the lead to the upper surface thereof, whereby mounting strength to a circuit board can be improved without enlarging the lead area of the semiconductor device. Further, it is possible to improve strength with respect to warpage or the like of the circuit board on which the semiconductor device is mounted. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    In the accompanying drawings: 
           [0011]      FIG. 1  is a schematic cross-sectional view showing a first embodiment of a semiconductor device according to the present invention; 
           [0012]      FIG. 2  is a schematic cross-sectional view showing a second embodiment of the semiconductor device according to the present invention; 
           [0013]      FIG. 3  is a schematic plan view showing the first embodiment of a lead frame according to the present invention; 
           [0014]      FIG. 4  is a schematic plan view showing the second embodiment of the lead frame according to the present invention; 
           [0015]      FIG. 5  is a schematic side view partially showing the lead frame according to the present invention; 
           [0016]      FIG. 6  is a schematic cross-sectional view showing a structure of a conventional semiconductor device; 
           [0017]      FIG. 7  is a schematic side view partially showing a conventional lead frame; 
           [0018]      FIG. 8  is a schematic side view showing a third embodiment of the semiconductor device according to the present invention; and 
           [0019]      FIG. 9  is a schematic cross-sectional view showing the third embodiment of the semiconductor device according to the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0020]    Hereinafter, embodiments of a semiconductor device according to the present invention will be described with reference to the drawings. 
         [0021]      FIG. 1  is a schematic cross-sectional view showing a structure of a semiconductor device of a first embodiment of the present invention. The semiconductor device includes a resin  1 , a lead  2 , and a metal plating layer  3  covering the surface of the lead  2 . Since elements including a semiconductor integrated circuit (IC chip) are covered by the resin  1 , the elements can not be seen from outside thereof generally. One end of the lead  2  is electrically connected to the semiconductor integrated circuit inside the resin  1 , and another end thereof protrudes from the resin  1 . The protruding portion of the lead  2  is formed into a shape suitable for mounting onto a substrate by the use of a die or the like. The plating layer  3  is formed on the entire surface of a portion of the lead  2  which is exposed to the outside of the resin  1 . In the embodiment shown in  FIG. 1 , the plating layer  3  covers a lead-tip portion  4  of the lead  2  at an end portion thereof so that no surface of the lead is exposed to the outside. 
         [0022]      FIG. 2  is a schematic cross-sectional view showing a structure of a semiconductor device of a second embodiment of the present invention. In the embodiment shown in  FIG. 2 , the lead-tip portion of the lead  2  has a portion covered by the plating layer  3 , and a lead-tip end surface  5  not covered by the plating layer  3 . A cross-sectional area of the lead-tip end surface  5  is made smaller than a half of the cross-sectional area of the lead  2 . Consequently, at the time of mounting the semiconductor device onto a circuit board or the like, it is easy for a soldering material to wet the plating layer, permitting the solder to go up the lead-tip portion to form a rigid mounting state. 
         [0023]      FIG. 3  shows a first embodiment of a lead frame according to the present invention, that is, shows an aggregate in which a plurality of semiconductor devices are arranged on a single lead frame  20 . A lead  8  of each of the semiconductor devices is connected to another lead  8  by a plating bar  7 , and the plating bar  7  is joined to a side surface of the lead  8  and to a flame  6 . The plating bar  7  is a path for electric current at a time of performing electrolytic plating on a lead frame  20 . With the structure in which the plating bar  7  is joined to the side surface of the lead  8 , the semiconductor device whose lead-tip portion  4  is entirely covered by the plating layer  3  can be manufactured as shown in  FIG. 1 . 
         [0024]      FIG. 4  shows a second embodiment of the lead frame according to the present invention, that is, shows an aggregate in which a plurality of semiconductor devices are arranged on a single lead frame  20 . In this embodiment, a lead  8  of each of the semiconductor devices is extended to form a plating bar  9  and joined to a frame  6 . The plating bar  9  is a path for electrical current at a time of performing electrolytic plating on the lead frame  6 . A portion where the plating bar  9  is joined to the lead  8  becomes a cut surface after cutting the lead  8  later, and plating is not applied to the portion. Accordingly, it is desirable that the cross-sectional area of the plating bar  9  is smaller than a half of the cross-sectional area of lead  8 , and is even finer and thinner as much as possible. 
         [0025]      FIG. 5  is a side view of a semiconductor device according to the second embodiment of the present invention and shows an intermediate state of a manufacturing process step in which two adjacent semiconductor devices  21  and  31  are connected to each other by a lead frame. As shown in  FIG. 5 , in this embodiment, a lead frame thin portion  11  is formed in a lead frame  2 . Cutting the lead frame thin portion  11  between the semiconductor devices to separate from one another, the semiconductor device having the structure of the embodiment shown in  FIG. 2  can be obtained. 
         [0026]      FIG. 8  is a side view of the semiconductor device according to a third embodiment of the present invention and shows an intermediate state of a manufacturing process step in which two adjacent semiconductor devices are connected to each other by a lead frame. As shown in  FIG. 8 , in this embodiment, a lead frame thin portion  11  is formed in a lead frame  2 . The lead frame thin portion  11  locates on a lower surface side where the semiconductor device is mounted onto a circuit board. Cutting the lead frame thin portion  11  between the semiconductor devices to separate from one another, a semiconductor device having the structure of the third embodiment shown in  FIG. 9  can be obtained. With this structure, the upper surface of the lead is easy to become wet with the solder printed on the circuit board. 
         [0027]    In a forming method of the lead frame thin portion  11 , at first a lead frame is cut into a desired shape by using a press die and then partial processing continues. As the partial processing, there are a method of forming a thin portion through etching by using chemicals, a method of performing a local crushing processing through press working, and other methods. 
         [0028]    Finally, manufacturing process steps are schematically described. A semiconductor integrated circuit (IC chip) is bonded to a lead frame to make connection between the semiconductor integrated circuit and each lead by a wire. Next, the semiconductor integrated circuit is covered by a resin. Up to this process step, embodiments shown in  FIGS. 3 and 4  are obtained. Then, plating is performed and a portion other than the resin is covered by film of plating. Further, the lead frame is cut by using a die or the like to separate the semiconductor devices from each other. 
         [0029]    The semiconductor device according to the present invention can be widely used for products that are required to be small and light, such as mobile phones, laptop personal computers, and mobile electronic equipment.