STACKED MODULE USING STANDARD INTEGRATED CIRCUIT CHIPS

Abstract: A module (30) formed of stacked integrated circuit chips (20, 25) having similar functional pin assignments includes a connector strap (22) which contacts the select or enable pin (4) of the upper chip and forms an extra pin (24) so that each chip in the module can be separately selected. The remaining overlapping pins of the stacked chips are connected in parallel to form a module occupying the same area on a printed circuit board or other second level package as that of one of the individual chips. According to one embodiment of the invention two industry standard memory chips are stacked to form a double density memory module. The connector strap (22) extends from the chip enable pin (4) of the upper chip (20), between the chips, and down one side of the lower chip to form a pin (24). According to another aspect of the invention the connector strap extends the length of the upper surface of the lower chip. One end (26) of the strap (22) is bent over one end of the lower chip, the other end (24) of the strap being bent over the other end of the chip and extending downward to form the upper chip select pin. A portion (27) of the connector strap near the chip select pin (4) of the upper chip is bent upwards to form a connection thereto. The strap is thus urged against the lower chip and held in place while the module is assembled.

Description: Background of the Invention The present invention relates generally to packaging structures for integrated circuit devices or arrays and more particularly to a module configuration employing stacked industry standard chips. Semiconductor integrated circuit memory chips are widely used in data processing systems ranging from personal computers to large mainframe systems. The memory chips typically comprise a packaged or encapsulated semiconductor memory array provided with a plurality of input/output pins, for example, in a dual-in-line package (DIP) arrangement of two parallel rows of pins. Perhaps the single most widely used chip is the read/write memory, also referred to as a dynamic random access memory (DRAM). The package dimensions and pin assignments for all memory chips having the same bit storage capacity are standardized so that the chips are interchangeable, regardless of the manufacturer. To accommodate the need for increased storage density, it is known in the art to stack memory chips one atop another to double the storage density on a given printed circuit board. Within the required electrical, timing and thermal constraints, the corresponding address, power supply and data lines may be connected in parallel. Separate chip enable pin connections must be provided for each chip in order to select either the top or bottom chip. Examples of stacking arrangements for chips and other electronic packages are described, for example, in U.S. Patent Nos. 3,746,934; 4,018,494; 4,080,026; 4,364,620; and 4,406,508. Two basic approaches have been used to achieve the advantage offered by stacking chips. The first approach involves the use of an interposer connector having a larger "footprint" than that of an individual chip, i.e., the interposer connector occupies a larger area on a printed circuit board or other structure on which the assembly is mounted. Since two chip select paths are required at every stacked chip location on the printed circuit board, the chip select pins from the upper and lower chips are routed to two separate locations on the interposer connector. For example, two standard 16 pin DRAMS may be mounted in an 18 pin socket, with the extra pin locations being used for the chip enable pins. A disadvantage of this approach is that it requires extra board space for the larger socket. In addition, a housing is typically required to provide the wiring paths from 16 to 18 pins, which increases cost and decreases the reliability of the stacked module arrangement. In the second approach the pin assignments of one or both of the chips are rearranged from the industry standard to permit stacking. For example, in the case of a 64K DRAM, pin 1 is normally a no-connect or unused pin and pin 4 is a row address select or chip enable pin. One of the chips to be stacked may be customized so that pin 1 is the chip enable pin and pin 4 is the no-connect pin. The pins of the chips can then be directly paralleled. While this arrangement provides a 16 pin footprint on the printed circuit board, a customized chip is typically significantly more expensive than a standard chip, which makes the stacked arrangement less advantageous. A more recent approach is described in the U.S. Patent application Serial No. 594,184, which was filed March 28, 1984. This document corresponds to EP-A-0 157 147. The disclosed structure includes a connector strap from the chip enable pin of the upper chip to the no-connect pin of the lower chip, the connector strap in one embodiment being placed between the two chips. The remaining pins are paralleled and soldered together. This arrangement, using industry standard chips, occupies the standard footprint and does not require a separate housing for the chips. A memory chip currently in use is the 256K DRAM, which stores approximately 256,000 bits of data. The industry standard 256K chip is a 16 pin DIP made of plastic, ceramic, or plastic/ceramic. In the standard pin assignment all of the 16 pins are used, so that only the first known approach described above would be feasible for stacking two chips. Summary of the Invention Accordingly, the present invention, as claimed, provides an improved module for stacking DIP integrated circuit chips and a memory board comprising a circuit board having a plurality of memory modules mounted thereon. In one embodiment two industry standard memory chips having the same storage capacity and pin assignment are stacked to form a double density memory module. A connector strap extends from the chip enable pin of the upper chip, between the chips, and down one side of the lower chip to form a pin. The upper chip enable pin is electrically connected to the connector strap and the remaining pins on the upper chip are electrically connected to the corresponding pins on the lower chip, thus forming a stacked module having the same footprint as one of the standard chips, plus one additional pin. According to another aspect of the invention the connector strap extends the length of the upper surface of the lower chip. One end of the connector strap is bent over one end of the lower chip, the other end of the strap being bent over the other end of the chip and extending downward to form the chip enable pin for the upper chip in the stacked module. A portion of the connector strap protrudes to form a connection to the chip enable pin of the upper chip. The strap is thus urged against the lower chip and held in place while the module is assembled. It is therefore an object of the present invention to provide a stacked integrated circuit chip module which occupies the same area on a printed circuit board as that of one of the chips. Another object of the invention is to provide a stacked module mountable on a printed circuit board, multilayer ceramic substrate or other second level package without a separate housing. Still another object of the invention is to provide a stacked module which employs industry standard integrated circuit chips. Brief Description of the Drawings Other objects and many of the attendant advantages of the present invention will be better understood by reference to the following detailed description of preferred embodiments in conjunction with the accompanying drawings, wherein: FIG. 1 is a diagrammatic view in perspective of the upper chip of a stacked module according to one embodiment of the present invention; FIG. 2 is a diagrammatic view of an assembled stacked module; FIG. 3 is a perspective view of a connector strap used in the stacked module of FIG. 2 according to another embodiment of the invention; and FIG. 4 is a diagram of the pin assignment of a double density memory module according to FIG. 2. Detailed Description of the Invention Referring now to the drawings, there is shown in FIG. 1 an upper chip 20 of a stacked module according to the present invention. By way of example to illustrate the principles of the present invention, upper chip 20 is shown with pins 1-16 which are labelled with reference to a notch 21 according to standard industry practice. It will be apparent to those skilled in the art that the present invention may be applied to chips having any number of pins in any configuration in order to stack chips having similar pin assignments. For an industry standard 256K DRAM chip, pin 4 of chip 20 is the chip select or enable pin. The remaining pins 1-3 and 5-16 have the functions as shown in FIG. 4, described below. A connector strap 22 positioned along the lower surface of chip 20 includes at one end a hole 23 aligned with pin 4, which is inserted therein and bent toward the body of chip 20 to form a mechanical and electrical connection to strap 22. This ensures that pin 4 does not make contact with the corresponding pin 4 of the lower chip when the chips are stacked, and that strap 22 is held in place. The other end of connector strap 22 is bent downward to form a section 24 which is the chip enable pin for upper chip 20. Strap 22 may be formed of any suitable metal having the required electrical and mechanical characteristics. In particular, a tin plated brass configuration is compatible with standard tin plated chip pins and with conventional lead-tin solders. FIG. 2 shows the assembled stacked module 30 with a lower chip 25 in place. Section 24 of connector strap 22 is advantageously twisted, aligned with notch 21 of chip 25, and bent under the lower surface of the chip to hold it in place during assembly of the module. The lower end of section 24 is shown substantially parallel with pins 1-16 and coplanar with pins 1 and of chip 25, but it can be formed in various positions to be aligned with the desired location in a printed circuit board or the like to which module 30 is attached. To complete the assembly, chip 20 is positioned so that the pins of chip 20 contact the corresponding pins of chip 25, except for the chip enable (row address select) pin 4. Module 30 may then be dip soldered or wave soldered using a conventional process, which also solders strap 22 to pin 4 of chip 20. If a strap 22 without a hole 23 is abutted against pin 4, a high temperature solder process may first be used to solder the strap to pin 4. Chips 20 and 25 are then stacked and soldered together using a low temperature process to ensure that strap 22 is not disturbed. It will be understood by those having skill in the art that other configurations of connector strap 22 are possible. For example, strap 22 can be located along the side or the top of chip 20 and section 24 of strap 22 can be located at either end of the chip. Further, instead of extending through a hole 23 in strap 22, the end of pin 4 can be cut off and bent over to contact the strap. An adhesive may also be employed to retain strap 22 in place against chip 20 prior to assembly of module 30. In yet another embodiment a conductive wire of the appropriate diameter and stiffness can be wrapped around pin 4, routed underneath or on top of chip 20, and extend downward over an end of chip 25 in the same manner as section 24 of strap 22. Referring now to FIG. 3, there is shown another embodiment of the invention wherein connector strap 22 extends the full length of lower chip 25. The end of strap 22 opposite section 24 is bent to form a section 26 so that the strap is held in place when positioned over chip 25. A protrusion 27 intermediate end sections 24 and 26 of strap may be bent upwards and makes contact with pin 4 of chip 20 when the module is assembled. FIG. 4 illustrates the pin assignments for a module 30 having a storage capacity of 512K when chips 20 and 25 are industry standard 256K DRAM chips. The pin labelled TOP CHIP ROW ADDRESS SELECT is the chip 20 enable pin, which is formed by section 24 of connector strap 22. The assembled module 30 may be mounted on a printed circuit board or other second level package in a conventional manner, with separate signal lines being routed to the upper chip 20 and lower chip 25 row address select pins. While the illustrated embodiment shows a double density memory module, the present invention is not limited thereto. The term chip or integrated circuit chip as employed herein is intended to include such devices as read-write memories, static, programmable, or erasable programmable read-only memories, logic chips, programmable logic arrays and the like. It will be understood that the stacked chips need not be identical, as long as both include a chip select pin and the function of each pin position is the same on both chips, i.e., the same pin positions are used for address, power supply, data, and enable. Thus, for example, a read-only memory (ROM) or a programmable read-only memory (PROM) may be stacked according to the present invention. The stacked chips need not even have the same number of pins or footprint size, as long as both chips include a chip select or enable pin and the function of each overlapping pin is the same on both chips. While the invention has been shown and described with reference to particular embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made without departing from the scope of the invention as set forth in the appended claims.

Claims: A module (30) for stacking DIP integrated circuit chips comprising: a first DIP integrated circuit chip (20) mounted on top of a second DIP integrated circuit chip (25), each said chip including a chip select pin (4) and a plurality of other pins in the same pin positions on said first and second chips, characterized in that the pins on said first chip (20) are affixed to the corresponding pins on said second chip (25), except for said chip select pins (4); and connector means (22) are provided contacting said chip select pin (4) of said first chip (20) and extending downwards to form an additional output pin (24) from said module (30) so that said first and second chips (20, 25) are independently selectable. The module of claim 1 wherein said connector means (22) comprises: a conductive strap (22) located between the lower surface of said first chip (20) and the upper surface of said second chip (25), said strap (22) extending from said chip select pin (4) of said first chip (20), over one end of said second chip (25), and downward to form said additional output pin (24) of said module (30). The module of claim 2 wherein said first chip select pin (4) is bent under said first chip (20) to contact said conductive strap (22, 23, 27) without contacting said second chip select pin (4). The module of claim 2 or 3 wherein said conductive strap (22) extends the full length of said second chip (25) and the end (26) of said strap (22) opposite said additional output pin (24) of said module (30) is bent over the end of said second chip (25). The module of claim 1 wherein said connector means (22) comprises: a conductive strap (22) running along the upper surface of said first chip (20) and over one end thereof, then extending downward to a point below the lower surface of said second chip (25) to form said additional output pin (4) of said module. The module of claim 1 wherein wherein said connector means (22) comprises: a wire connected to said first chip select pin (4) and extending downward to form said additional output pin (24) of said module. The module of claim 6 wherein said wire runs between said first and second chips (20, 25) and extends over one end of said second chip (25). The module of any preceding claim wherein said first and second chips (20, 25) comprise memory devices. The module of claim 8 wherein said memory devices are industry standard 256K dynamic random access memory chips and said module has a storage capacity of 512K. The module of claim 8 wherein said memory devices are read-only memory chips. The module of claim 8 wherein said memory devices are programmable array chips. The module of claim 8 wherein said first chip (20) is a different type of memory device from said second chip (25). A double density memory board comprising: a circuit board having a plurality of mounting positions thereon; and a plurality of memory modules (30) mounted in at least some of said mounting positions, each of said memory modules (30) comprising: a first DIP integrated circuit memory chip (20) mounted on top of a second DIP integrated circuit memory chip (25), each said chip (20, 25) including a chip select pin (4) and a plurality of other pins in the same pin positions on said first and second chips (20, 25), characterised in that the pins on said first chip (20) are affixed to the corresponding pins on said second chip (25), except for said chip select pins (4); and connector means (22) are provided contacting said chip select pin (4) of said first memory chip (20) and extending downwards to form an additional output pin (24) from said memory module (30) so that said first and second memory chips (20, 25) are independently selectable.

Inventors: MUELLER, WOLFGANG F., SPENCER II, GWYNNE W.
Applicants: INTERNATIONAL BUSINESS MACHINES CORPORATION
Application year: 1986
Main classification: H01L25
IPC codes: H01L25