Abstract:
A semiconductor module includes a substrate having a pad electrode on a surface, a lower layer semiconductor package mounted on the substrate, and an upper layer semiconductor package mounted on the substrate while arranged in a position substantially overlying the former. The pad electrodes connected to the leads of these semiconductor packages are arranged alternately. The lead has a dambar residual portion. An inner surface of a lead downward portion of the upper layer semiconductor package is positioned outside an outer surface of a lead downward portion of the lower layer semiconductor package.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a semiconductor module such as a memory module. A “semiconductor module” herein refers to a module having one or more parts including a semiconductor package mounted on one substrate. 
     2. Description of the Background Art 
     Information equipment such as a personal computer has a memory module mounted as a semiconductor module. A common and conventional memory module will now be described. First, in FIG. 9, a semiconductor package  1  mounted on a memory module is shown. Semiconductor package  1  includes a package body  2  and a plurality of leads  3  protruding in parallel, respectively from opposing side portions. A dimension of semiconductor package  1  is determined by an organization for standardizing a semiconductor package, JEDEC (Joint Electron Device Engineering Council), and a “TSOP” (Thin Small Out-line Package) of “400 mil” is one example. When semiconductor package  1  is an SDRAM (Synchronous Dynamic Random Access Memory), 54 pins are provided, pitch A between leads  3  is set to 0.8 mm, and width B per one lead  3  is set to 0.3 mm. 
     As shown in FIG. 10, a memory module  100  has semiconductor package  1  mounted on a surface of substrate  4  in a prescribed arrangement. On the surface of substrate  4 , in addition to semiconductor package  1 , a packaged parts  5   a ,  5   b  such as a resistance, and a buffer IC (Integrated Circuit)  6  for amplifying and timing a signal of the memory are also mounted. In order to make effective use of a limited area on substrate  4 , packages are often mounted on opposing surfaces of substrate  4 , as shown in FIG.  11 . On both surfaces of substrate  4 , pads  7  are formed in positions corresponding respectively to leads  3 , which are electrically connected to pads  7  respectively. In an example shown in FIGS. 10 and 11, nine semiconductor packages  1  are mounted on one surface of substrate  4  of 133.35 mm long and 31.75 mm wide, which is a dimension determined in accordance with JEDEC standard. This means that, in total, eighteen semiconductor packages  1  are mounted on both surfaces. 
     As personal computers and the like are more sophisticated, an increase of memory capacity has been demanded. Accordingly, more semiconductor packages need to be mounted per one substrate. In an effort to achieve this, in Japanese Patent Laying-Open No. 4-276649, a technique to stack and mount a semiconductor package is proposed. According to the technique, as shown in FIG. 12, a semiconductor package  1   e  having a longer lead is prepared in addition to semiconductor package  1 . As shown in FIGS. 13 and 14, a two-layered structure is provided on one surface of substrate  4 . That is, an inner pad  7  having a conventional arrangement and a pad  7   e  arranged outside the former together form pads on the surface of substrate  4 . In the two-layered structure of the semiconductor packages, lead  3  of semiconductor package  1  located on a side close to substrate  4  (hereinafter, referred to as a “lower layer”) is connected to pad  7 , while a lead  3   e  of semiconductor package  1   e  overlying the former on a side far from substrate  4  (hereinafter, referred to as an “upper layer”) relative to semiconductor package  1  is connected to pad  7   e , going around the outside of lead  3 . In this case, however, a row of pad  7   e  for upper layer semiconductor package  1 e should be arranged parallel to, and outside, a row of pad  7  for lower layer semiconductor package  1 . Therefore, width of the area occupied on substrate  4  will be larger. Consequently, for example, though nine semiconductor packages could conventionally be arranged per one layer on one surface of substrate  4 , only eight semiconductor packages per one layer on one surface can be arranged, as can be seen in a memory module  101  shown in FIG.  15 . 
     Further improved techniques are possible as described below. As shown in FIG. 16, a semiconductor package if is prepared, which is a 400 mil package having 54 pins in accordance with a conventional standard. Though pitch between leads  3   f  is the same as a conventional example, width C per one lead  3   f  is made smaller to 0.16 mm. This semiconductor package  1   f  is provided as a lower layer. Separately, a semiconductor package  1   g  is prepared having a lead  3   g  that has the same length as lead  3   f  when viewed from the top and has longer length than the same when viewed from the side. This package is provided as an upper layer. Width C per one lead  3   g  of semiconductor package  1   g  is also made smaller to 0.16 mm. Both packages are mounted, with one overlying the other, as shown in FIGS. 17 and 18. The pad of upper layer semiconductor package  1   g  and the pad of lower layer semiconductor package  1   f  are alternately arranged, and lead  3   g  of semiconductor package  1   g  is interposed between leads  3   f  of semiconductor package  1   f  respectively. Consequently, as can be seen in a memory module  102  shown in FIG. 19, nine packages can be arranged per one layer on one surface of substrate  4 , as in a conventional example. 
     In FIG. 20, an enlarged view of the vicinity of a root portion of the lead is shown. Generally, a plurality of leads protruding in parallel from a side portion of a package body of the semiconductor package are manufactured in the following manner. A package body  2  portion is formed with resin mold so as to partially cover a leadframe  14  integrally formed. Thereafter, as shown in FIG. 21, a punch region  13  set on a dambar  12  linking each lead in a portion protruding from the side portion of package body  2  is punched through, and thus each lead is separated. In an attempt to punch the region to completely remove dambar  12  linking each lead, a puncher may strike a lead portion and damage the lead, or useful life of the puncher may be shortened. Therefore, usually, punch region  13  is set to a size covering only a main portion of dambar  12  with a small clearance from the lead portion, not exactly covering both full ends of dambar  12 . Accordingly, as shown in FIG. 22, after punching, a dambar residual portion  8  will remain in the middle of lead  3 . Lead  3  is folded thereafter, to have a shape shown in FIG.  23 . In FIG. 23, the semiconductor package is shown, disposed on substrate  4 . Here, the lead can be divided in three parts: a lead drawn-out portion  31  horizontally drawn from the side portion of package body  2 ; a lead extending-downward portion  32  hereinafter, referred to as a “lead downward portion”) extending down to the surface of substrate  4 ; and a lead foot portion  33  for contacting pad electrode  7 . 
     A side view of the techniques described with reference to FIGS. 16 to  19  is shown in FIG.  24 . Width of the lead is made smaller in both upper and lower layers so that lead  3   g  of upper layer semiconductor package  1   g  passes a gap between leads  3   f  of lower layer semiconductor package  1   f . In practice, however, as dambar residual portion  8  is present, the gap where lead  3   g  can pass is narrow. Therefore, only a slight displacement of a position of either the upper or lower semiconductor package may cause a contact of lead  3   f  with lead  3   g.    
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a semiconductor module capable of increasing the mountable number of semiconductor packages per one layer on one surface of a substrate as well as avoiding contacts between leads due to a dambar residual portion. 
     In order to achieve the object above, a semiconductor module according to the present invention includes a substrate having a pad electrode on a surface, a lower layer semiconductor package mounted on the substrate, and an upper layer semiconductor package mounted on the substrate while arranged in a position substantially overlying the lower layer semiconductor package. The lower layer semiconductor package and the upper layer semiconductor package include a package body and a plurality of leads protruding in parallel respectively from opposing side portions of the package body and electrically connected to the pad electrode. The pad electrode having the lead of the upper layer semiconductor package connected and the pad electrode having the lead of the lower layer semiconductor package connected are alternately arranged. The lead includes a lead drawn-out portion horizontally drawn from a side portion of the package body, a lead downward portion extending from the lead drawn-out portion down to a surface of the substrate and a lead foot portion continuing to a tip end of the lead downward portion and contacting the pad electrode. The lead has a dambar residual portion protruding toward the lead adjacently protruding from the same package body in any position the middle between the lead drawn-out portion and the lead downward portion. An inner surface of the lead downward portion of the upper layer semiconductor package is positioned outside an outer surface of the lead downward portion of the lower layer semiconductor package. By adopting this structure, even if slight displacement of the relative positions of upper and lower layer semiconductor packages, with one overlying the other, may occur, contact of the lead of the upper layer semiconductor package with the dambar residual portion of the lower layer semiconductor package can be prevented. 
     In the present invention, preferably, when viewed two-dimensionally, the pad electrode is arranged in a staggered manner so that the pad electrode connected to the upper layer semiconductor package is located outside and the pad electrode connected to the lower layer semiconductor package is located inside, with a projection region onto the substrate of the package body serving as a center. By adopting this structure, while minimizing a material for the pad electrode, a connection portion to the lead can efficiently be arranged in a limited area. 
     In the present invention, preferably, a horizontal distance from the package body to the dambar residual portion in the upper layer semiconductor package is substantially equal to a horizontal distance from the package body to the dambar residual portion in the lower layer semiconductor package, and the lead downward portions of the upper layer semiconductor package and the lower layer semiconductor package extend diagonally relative to the substrate. By adopting this structure, contact between the lead downward portions can be prevented, even if the lead drawn-out portions are of the same length. 
     In the present invention, preferably, the lead has a section including the dambar residual portion, wider than other sections. By adopting this structure, a conventional punching apparatus can be used, obviating the need of a new punching apparatus. 
     The present invention preferably includes a structure in which a plurality of combinations of the upper layer semiconductor package and the lower layer semiconductor package are vertically stacked. By adopting this structure, larger number of semiconductor packages can be mounted in unit area of the substrate, and a semiconductor module of high density and high performance can be obtained. 
     The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a plan view of a semiconductor package mounted on a semiconductor module in a first embodiment according to the present invention. 
     FIG. 2 is a side view of the semiconductor module in the first embodiment according to the present invention. 
     FIG. 3 is a partially enlarged plan view of the semiconductor module in the first embodiment according to the present invention. 
     FIG. 4 is a plan view of the semiconductor module in the first embodiment according to the present invention. 
     FIG. 5 is a partially enlarged side view of the semiconductor module in the first embodiment according to the present invention. 
     FIG. 6 is a partially enlarged side view of a semiconductor module in a second embodiment according to the present invention. 
     FIG. 7 shows a manufacturing process of a semiconductor package used in the semiconductor module in the second embodiment according to the present invention. 
     FIG. 8 is a side view of a semiconductor module in a third embodiment according to the present invention. 
     FIG. 9 is a plan view of a common and conventional semiconductor package. 
     FIG. 10 is a plan view of a first semiconductor module according to a conventional art. 
     FIG. 11 is a side view of the first semiconductor module according to the conventional art. 
     FIG. 12 is a plan view of a semiconductor package used in a second semiconductor module according to the conventional art. 
     FIG. 13 is a side view of the second semiconductor module according to the conventional art. 
     FIG. 14 is a partially enlarged plan view of the second semiconductor module according to the conventional art. 
     FIG. 15 is a plan view of the second semiconductor module according to the conventional art. 
     FIG. 16 is a plan view of a semiconductor package used in a third semiconductor module according to the conventional art. 
     FIG. 17 is a side view of the third semiconductor module according to the conventional art. 
     FIG. 18 is a partially enlarged plan view of the third semiconductor module according to the conventional art. 
     FIG. 19 is a plan view of the third semiconductor module according to the conventional art. 
     FIG. 20 is a first illustration representing a manufacturing process of a common and conventional semiconductor package. 
     FIG. 21 is a second illustration representing the manufacturing process of the common and conventional semiconductor package. 
     FIG. 22 is a third illustration representing the manufacturing process of the common and conventional semiconductor package. 
     FIG. 23 is a partially enlarged side view from a first direction, of the common and conventional semiconductor package. 
     FIG. 24 is a partially enlarged side view from a second direction, of the common and conventional semiconductor package. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     First Embodiment 
     Referring to FIGS. 1 to  4 , a structure of a semiconductor module in a first embodiment according to the present invention will be described. In the semiconductor module, a semiconductor package  1   f  shown in FIG. 16 is provided as a lower layer while a semiconductor package  1   h  shown in FIG. 1 is provided as an upper layer. Semiconductor package  1   h  has a lead  3   h , of which width C is 0.16 mm. When viewed from the top, the lead of semiconductor package  1   h  appears to be slightly longer than that of semiconductor package  1   f . Both of the above packages are mounted on a substrate  4 , as shown in FIGS. 2 and 3. When viewed in a direction of FIG. 2, lead  3   h  appears to run outside lead  3   f . Note that a dambar residual portion is not illustrated in FIG.  2 . 
     As shown in FIG. 3, semiconductor package if and semiconductor package  1   h  are stacked with a displacement by 0.4 mm, which is half the lead pitch A=0.8 mm. A pad  7   f  for semiconductor package If and a pad  7   h  for semiconductor package  1   h  are arranged alternately, and at the same time, are staggered so that pad  7   h  is located outside pad  7   f , when viewed from the package body. A memory module  110  is shown in FIG. 4 in its entirety. 
     Pad  7   h  and pad  7   f  corresponding respectively to the upper and lower semiconductor packages are not arranged in parallel in two distant rows as shown in FIGS. 13 and 14 but arranged in an alternate combination in a staggered manner. Therefore, horizontal width of a region occupied on substrate  4  by a set of vertically stacked semiconductor packages is not as large as that shown in FIGS. 13 and 14. Thus, as shown in FIG. 4, as in a conventional example, nine semiconductor packages can be arranged per one layer on one side of one substrate  4  with a dimension determined in accordance with the conventional standard. 
     In addition, as shown in FIG. 5, assume that lead  3   h  of upper layer semiconductor package  1   h  has 3 portions, that is, a lead drawn-out portion  31   h , a lead downward portion  32   h  and a lead foot portion  33   h , and that lead  3   f  of lower layer semiconductor package if has 3 portions, that is, a lead drawn-out portion  31   f , a lead downward portion  32   f  and a lead foot portion  33   f . Here, an inner surface  35   h  of lead downward portion  32   h  of lead  3   h  is positioned outside an outer surface of lead downward  32   f  of lead  3   f . Therefore, even if slight displacement of the relative positions of upper and lower layer semiconductor packages, with one overlying the other, may occur, contact of lead downward portion  32   h  of lead  3   h  with dambar residual portion  8   f  of lead  3   f  can be prevented. 
     Second Embodiment 
     A structure of a semiconductor module in a second embodiment according to the present invention will be described. The semiconductor module has semiconductor package if mounted as a lower layer and semiconductor package  1   h  mounted as an upper layer on substrate  4 , basically in a similar manner to the first embodiment, except for the shape of a lead of each semiconductor package as shown in FIG.  6 . Leads  3   f ,  3   h  have wide portions  10   f ,  10   h  respectively in the vicinity of the root when viewed from package body  2 . Dambar residual portions  8   f ,  8   h  are located in the middle of wide portions  10   f ,  10   h  respectively. Sides far from package body  2  of leads  3   f ,  3   h  are provided as narrow portions  11   f ,  11   h . Width of the wide portion is 0.3 mm, which is the same as conventional lead width B, while width C of the narrow portion is 0.16 mm. 
     In addition to the effect described in the first embodiment, a portion of the lead is provided as a wide portion having the same width as the conventional lead width, whereby, the size of a region to be punched will be the same as in a conventional example (see FIG.  21 ), as shown in FIG.  7 . Thus, a conventional punching apparatus can be used, obviating the need for a new punching apparatus. Moreover, large width at the root portion will increase the strength of the lead itself. 
     In the present embodiment, leads of each semiconductor package in both upper and lower layers are provided with wide portions and narrow portions. Meanwhile, only the lead of each semiconductor package in the upper layer may be provided with the wide and narrow portions while the lead of each semiconductor package in the lower layer may have the conventional width, that is, the same width as the wide portion. 
     Third Embodiment 
     Referring to FIG. 8, a semiconductor module in a third embodiment according to the present invention will be described. In the semiconductor module, based on the concept in the first and second embodiments, the number of combinations of upper and lower layer semiconductor packages are increased, and a plurality of those combinations are stacked vertically (a top-to-bottom direction in the drawing) to the main surface of the substrate. In an example of a combination mounted on one surface of substrate  4  as shown in FIG. 8, though two combinations, that is, a combination of semiconductor packages  1   h ,  1   f  and a combination of semiconductor packages  1   j ,  1   i  are stacked, three or more combinations may be stacked. Further, the semiconductor packages stacked on one surface do not always have to be a combination of upper and lower layers. For example, a stack in which semiconductor package  1   j  of FIG. 8 is absent is possible. 
     Thus, larger number of semiconductor packages are mounted per unit area of a substrate, and a semiconductor module of high density and high performance can be obtained. For example, if the module is a memory module, the one with a large capacity can be obtained. 
     According to the present invention, even if slight displacement of the relative positions of upper and lower layer semiconductor packages, with one overlying the other, may occur, contact of the lead of the upper layer semiconductor package with the dambar residual portion of the lower layer semiconductor package can be prevented. 
     Although the present invention has been described and illustrated in detail, it is clearly understood that the same is by way of illustration and example only and is not to be taken by way of limitation, the spirit and scope of the present invention being limited only by the terms of the appended claims.