HIGH DENSITY SEMICONDUCTOR MEMORY MODULE

Abstract: A method and a memory module are provided which allow the duplication of the density of a memory module with a minimum of increasing of the module size and with low manufacturing costs. The method uses well-known techniques which are used by the manufacturing of DIP modules containing only one chip inside the moulded plastic or ceramic housing. Contrary to with the common methods the inner bond leads are punched so that they have a greater width than the common inner bond leads. Subsequently or together with the punching step the inner bond leads are slotted in order to allow the bending of at least one of each of the smaller inner bond leads obtained before for providing a space between the two inner bond leads. Next, two semiconductor memory chips are glued together back-to-back and inserted in the gap between the upper inner bond leads and the lower inner bond leads so that the upper and lower bond leads embrace the two chips. The chips can be equal with the pad occupation, or mirrored. The method is applicable to dual inline package (DIP) as well as to ZIG-ZAG package. The semiconductor memory module comprises a housing (4) of plastic or ceramic in which two chips (8, 10) are stacked together back-to-back. The pads (20) of the chips are electrically connected by wire-bonding to beam leads (14) which comprise outer bond leads (6), generally arranged outside the housing to form the ocntact pins or contact leads of the module to a printed circuit board, and inner bond leads (16) in the housing. The inner bond leads are spread in the area of the inner lead bond ends into upper (16a) and lower (16b) bond leads forming a gap (22) for receiving and embracing the stacked chips.

Description: The present invention relates to high density semiconductor memory modules, and more particularly to a method for forming semiconductor memory modules, and a semiconductor memory module. A substantial part of the cost of manufacturing computer systems directly depends upon the space occupied by the integrated circuit chips used to fabricate the systems. Thus it is important to minimize the size of semiconductor memory modules in dual inline packages which increasingly are used in modern computer systems. One structure to increase the chip packaging density is formed by mounting or stacking one chip on top of another one. Prior art shows examples of stacking chips to form double-chip structures in which chips are contained within a rigid housing, with the back of one chip facing the back of the second chip, there being a rigid support between the chips on which the back of the chips rests. The active faces are directed away from each other to permit easy electrical connection to the exposed input/output (I/O) terminals on each chip. JP-A-58 130 553 describes a device where two chips are attached on both surfaces of a frame bed. JP-A-56-137665 describes a dual inline package module (DIP) containing two pairs of semiconductor chips, each chip having solder mounds on its active face for electrical connection to the circuits within the chips. The active faces of the chips of one pair are mounted facing each other with solder mounds on one chip aligned with corresponding solder mounds of the other chip. The two pairs are stacked by facing the back of the upper chip of the first pair with the back of the lower chip of the second pair. The invention as claimed therefore solves the object of providing a method and a memory module which allow the duplication of the memory module density with a minimum increase of the module size and with low manufacturing costs. Accordingly, the method of this invention uses well-known techniques to provide a double density memory module. These techniques are used in the manufacturing of DIP modules containing only one chip inside the moulded plastic housing or ceramic housing. Commonly these chips are produced by employing a thin metal carrier tape which is punched for providing lead frames with spaced series of beam lead patterns which comprise inner bond leads to be bonded to the chip pads and outer bond leads to form the contact pins. Subsequently the chips are connected to the inner bond leads. This is usually realized by first gluing the chip to the inner bond leads and then bonding the chip pads to the inner bond leads, but other ways of connecting are also possible. Next, the housing is formed and the outer leads are punched to correct the length of the contact pins or contact leads. Several tests of the modules are usually carried out during the manufacturing process. Contrary to the common methods for manufacturing memory modules, the inner bond leads are punched in the method according to the invention so that they have a greater width than the common inner bond leads. Subsequently, or together with the punching step the inner bond leads are slotted or treated in any other appropriate manner in order to allow the bending of at least one of each of the smaller inner bond leads obtained before for providing a space between the two inner bond leads. The inner bond leads are preferably slotted only in the area of the inner lead bond ends as far as necessary to obtain a sufficient gap for inserting the chips. Next, two semiconductor memory chips having an active and a back face are glued together back-to-back. Gluing can be accomplished by any appropriate material which is well known to a person skilled in the art. Afterwards the chips stacked together back-to-back are inserted in the gap between the upper inner bond leads and the lower inner bond leads so that the upper and lower bond leads embrace the two chips. The chips can be equal with the pad occupation or mirrored. In the first case the respective inner lead bond ends and the chip pads have to be bonded, e.g. by wire-bonding with intersections for accomplishing a corresponding connection. In the second case the inner lead bond ends can be bonded directly to the respective chip pads. Then, the top chip must be shifted relative to the bottom chip for adjusting the chip pads to the respective inner bond leads and for compensating the displacement of the upper smaller inner bond leads and the lower smaller inner bond leads if the displacement is not compensated by buckling of the upper and/or lower smaller inner bond leads. The method according to the invention is applicable to dual inline package (DIP) as well as to ZIG-ZAG package, where the two chips are arranged in a vertical position and the clamplike bond leads embrace the memory chips only in the lower part. The handling of the module after fixing the chips to the bond leads corresponds to the common methods for the manufacturing of memory modules. The method according to the invention has the advantage that only few additional manufacturing steps are necessary to provide a memory module with double density. Thus known techniques can be use of thus allowing a low cost manufacturing of memory modules. Another advantage is the height of the modules which is in the range of the common single chip modules. So the density of printed circuit boards on which the memory modules are arranged can be doubled or the assembly of the printed circuit board can be reduced, which in turn leads to a reduction of manufacturing costs. The semiconductor memory module according to this invention comprises a housing of plastic or ceramic in which two chips are stacked together back-to-back. The pads of the chips are electrically connected to bond leads which comprise outer bond leads generally arranged outside the housing to form the contact pins or contact leads of the module to a printed circuit board, and inner bond leads in the housing. The inner bond leads are spread in the area of the inner lead bond ends into upper and lower bond leads forming a gap for receiving and embracing the stacked chips. The upper and lower lead beams may run into a lead beam part which is also arranged inside the housing and subsequently passes over to the outer lead bond arranged outside the housing. Preferably, the upper and lower inner bond leads have a smaller width than the lead beam part before leaving the housing. The inner bond leads can be formed by using a metal carrier tape having at least in the area of the inner bond leads a double sheet material so that after punching the lead beams an upper and a lower inner lead beam end arranged one above the other is provided. Another method for forming the clamplike inner bond leads is the method according to the invention and as described above, for example. For better understanding of the present invention, together with further objects, advantages and features thereof, a preferred embodiment of the invention is described in the following with reference to the accompanying drawings, in which: Fig. 1 is a view of a dual inline package module with a part of the housing broken away and showing the top chip; Fig. 2 is a top view of Fig. 1; Fig. 3 is a top view of inner and outer bond leads with lead frame; and Fig. 4 is a cross-sectional view of a double-chip stack. Fig. 1 shows a semiconductor memory module 2 of the DIP-type comprising a plastic housing 4, and outer bond leads 6 usually on both sides outside the housing 4 and formed as J-leads for surface mounting of the memory module on a printed circuit board (not shown). Part of the top portion of the housing is broken away and shows the top chip 8. The pads 20 of the chip 8 are wire-bonded with inner bond lead ends 18a of the upper inner bond leads 16a. The lower inner bond leads 16b which are connected to the pads of the bottom chip 10, and the upper inner bond lead 16a join in the inner bond lead part 24 which is located inside the housing. The bottom chip 10 is not visible in this depiction. The power supply 26 and the ground supply 28 both comprise upper and lower inner bond leads 16a and 16b, respectively, but the inner bond lead part 24 is not connected to outer bond leads. In the preferred embodiment three inner bond leads are interconnected to provide appropriate power and ground supply. Fig. 2 is a top view of Fig. 1. Not visible parts are depicted with broken lines. Beam leads 14 comprising inner bond leads 16 and outer beam leads 6 (Fig. 3) are fixed with the inner bond leads 16 to the chips 8, 10 (chip 10 is shown in Fig. 4) and with the outer bond leads 6 to a lead frame 12. The lead frame 12 (Fig. 3) usually serves for positioning the beam leads and the chips and will be removed for providing the J-leads in a further punching and bending step. The inner bond leads 16 comprise an upper bond lead 16a and lower bond lead 16b, at least one of which is bent so that they embrace the top chip 8 and the bottom chip 10. The inner bond lead ends 18a and 18b of the respective upper and lower bond leads are wire-bonded with chip pads 20. The upper inner bond lead 16a and the lower inner bond lead 16b have a smaller width than the part 24 of the inner bond lead 16. Fig. 3 shows the lead frame 12 with the outer bond leads 6 which are later punched and formed to contact pins. The chips 8 and 10 stacked together back-to-back with the inner bond lead 16 and the inner bond leads 16a and 16b whose ends 18a and 18b, respectively, are connected by wire-bonding to the pads 20 of the top chip 8 and the bottom chip 10, respectively, are shown in Fig. 4 in a cross-sectional view. The memory module of the preferred embodiment as shown in the drawings is manufactured in accordance with the inventive method as described above. Thus the inner bond leads 16 are slotted at least in the area of the inner bond lead ends 18 after or during the punching step. Subsequent to the punching step the slotted ends are spread to form a gap 22 in which the stacked chips 8 and 10 are inserted after having been glued together back-to-back. Then the ends 18a and 18b are wire-bonded to the chip pads 20. The further steps of the method correspond to common steps for manufacturing modules of the DIP or ZIG ZAG type. Both memory chips should be accessed exclusively by seperate chip select pins and all other pins like data lines, address lines, etc. are commonly used by the top chip and the bottom chip.

Claims: A method for forming high density semiconductor memory modules (2) comprising the following steps: providing a thin sheet metal carrier; punching of said thin sheet metal carrier for providing a lead frame (12) with a spaced series of beam lead patterns, which beam leads (14) comprise inner bond leads (16) to be bonded to chip pads and at least some outer bond leads (6) to form contacts; slotting at least some of said inner bond leads for providing a pair of smaller inner bond leads (16a, 16b) per beam lead; bending at least one of each of said pair of smaller inner bond leads for providing a space (22) between the top of the lower one of said pair of smaller inner bond leads and the bottom of the upper one of said smaller inner bond leads sufficient to receive two chips (8, 10) which are stacked together with their flat surfaces; gluing said chips together back-to-back forming a stack with a bottom and a top chip; inserting said chips stacked together back-to-back into said space (22) between the lower and the upper smaller inner bond leads; connecting said stacked chips to said bond leads by bonding the pads of said bottom chip (10) to said lower smaller inner bond leads (16b) and the pads of said top chip (8) to said upper smaller inner bond leads (16a); molding the housing (4) round the chips and the inner bond leads (16); and preparing the outer bond leads (6) to provide contact pins or leads. The method as claimed in claim 1, further comprising shifting said top chip (8) relative to said bottom chip (10) for adjusting the chip pads to the respective inner bond leads. The method as claimed in claim 1 or 2, wherein the bonding of the pads of said bottom chip to said lower smaller inner bond leads and the bonding of the pads of said top chip to said upper smaller inner bond leads are accomplished by wire-bonding. The method as claimed in any one of the preceding claims wherein prior to bonding said pads to said inner bond leads said stacked chips are fixed to said bond leads. The method as claimed in any one of the preceding claims wherein preparing the outer bond leads to provide contact pins or leads includes punching the outer bond leads to correct the length and to remove said lead frame; and bending said outer bond leads to form the contact pins or leads. A semiconductor memory module comprising a housing (4); two chips (8, 10) stacked together back-to back in said housing; beam leads (14) electrically connected to the pads (20) of said stacked chips by wire-bonding, which beam leads comprise outer bond leads (6) substantially outside said housing and inner bond leads (16) in said housing, which inner bond leads are spread into upper inner bond leads (16a) and lower inner bond leads (16b) forming a gap (22) for receiving said stacked chips. The semiconductor memory module as claimed in claim 6 wherein said upper (16a) and said lower (16b) inner bond leads run into an inner bond lead part (24) having a greater width than said upper and lower inner bond leads.

Inventors: KLINK, ERICH, DIPL.-ING., KOHLER, HELMUT, DIPL.-ING., PROSS, HARALD, DIPL.-ING.
Applicants: INTERNATIONAL BUSINESS MACHINES CORPORATION
Application year: 1989
Main classification: H01L25
IPC codes: H01L23