1.  Field of the Invention
The present invention relates to a semiconductor chip package, and more particularly to a package with multiple stacked semiconductor chips and a manufacturing method of the package. 
2.  Description of the Related Art
As higher performance and integrated improvement of a semiconductor device are required, the size of the semiconductor chips increases, and higher surface-mounting density of a semiconductor device is demanded.  A semiconductor package including a number of semiconductor chips inside, which is called a chip-stack package, can satisfy such demand.  For example, semiconductor chips can be stacked in the package.  Alternatively, a number of packages, each of which includes a single chip, can be stacked to increase the surface-mounting density. 
The stacking of the packages may cause the increased height of the stacked packages.  Further, for interconnecting the external leads (or terminals) of the packages often requires customized shaping of the leads, which is an additional process.  For instance, such additional process may include reforming of the leads so that the leads of a upper package can contact the leads of a lower package, or connecting the leads using additional pins.  In addition, the additional stacking process may reduce the yield of the stacked device. 
However, the chip-stack package can be effective than the stacked packages in accomplishing the reduction of the total height of the stacked semiconductor chips.  FIG.  1 shows a conventional chip-stack package 10.  A lower semiconductor chip 13 is attached to the lower surface of a lead frame die pad 11 via an adhesive 12, and an upper semiconductor chip 15 is attached to the upper surface of the die pad 11 via an adhesive 14.  The active surface of the lower chip 13 faces downward, and the active surface of the upper chip 15 faces upward.  Bonding pads (or electrode pads) 2 of the upper and lower semiconductor chips 13 and 15 are electrically connected to leads (or external terminals) 16 via bonding wires 17 and 18, respectively.  The upper and lower semiconductor chips 13 and 15 and the bonding wires 17 and 18 are encapsulated by a package body 19 formed of a molding compound.  In the package 10, the upper and lower semiconductor chips 13 and 15 are mirror chips to each other, and bonding pads 2 are formed along the edges of the chips 13 and 15. 
FIG.  2 shows another known chip-stack package 20, in which the active surfaces of chips 23 and 25 face toward the same direction.  Chip 23 is attached via an adhesive 22 on a die pad 21, and chip 25 is attached via an adhesive 24 on the active surface of the chip 23.  The lower chip 23 is larger than the upper chip 25.  Bonding pads 2 of the upper and lower chips 23 and 25 are electrically connected to leads 26 via bonding wires 27 and 28, and are protected by a package body 29.  The upper and lower chips 23 and 25 may be different from each other, and bonding pads 2 are formed along the edges of the chips 23 and 25. 
FIG.  3 shows still another known chip-stack package 30.  A lower chip 33, on which bonding pads 2 are formed along the edges, is attached to the lower surface of a lead frame 31 via an adhesive 32, and an upper chip 35, on which bonding pads 2 are formed along the center line, is attached to the upper surface of the lead frame 31 via an adhesive 34.  For electrical connection of the central pad chip 35, an opening is formed in the central portion of the lead frame 31.  The edge pad chip 33 and the central pad chip 35 are electrically connected to the lead frame 31 via bonding wires 37 and 38, respectively.  The end portions of the lead frames are exposed through the lower surface of a package body 39, and the end portions are connected to land type connections 36. 
In memory chips, use of the central-pad chips is preferred because a signal skew for a number of memory shell blocks can be decreased when the bonding pads are formed along the center line of the semiconductor chip.  Further, as memory capacity and speed of the memory chips increase, the central-pad memory chips are preferred to the edge-pad chips.  Accordingly, technology for stacking a number of central-pad memory chips is in demand.