Semiconductor chip packages and modules formed of stacks of such packages

Abstract: A semiconductor chip package (2) comprising at least one semiconductor chip disposed in a package and a plurality of first and second pins (18,20) extending from the package, which first pins are electrically connected to the at least one semiconductor chip and are adapted to conduct signals between the at least one semiconductor chip and external circuitry, the first pins being divided into a plurality of groups, each group representing a respective signal type, and which second pins are not electrically connected to the at least one semiconductor chip, the first pins of at least one group and the second pins being asymmetrically disposed along edges (14,16) of the package and the remaining groups of first pins being symmetrically disposed along edges of the package. The invention also provides a stacked module of the semiconductor chip packages.

Description: The present invention relates to a semiconductor chip package and to a module of semiconductor chip packages. In the electronics industry there is a continuing need to reduce the size of electronic components. Currently, there is a trend towards the use of a third dimension i.e. the vertical dimension, both in the manufacture of silicon devices and in the way those devices are assembled on a printed circuit board. The Applicant currently manufactures Transputer (computer on a chip) modules (TRAMs), which modules are stackable by the employment of redundant pins on the modules that are not connected to any circuitry. A TRAM with active circuitry on those pins can then be plugged into the redundant pins and hence connected to a mother board. Random Access Memories (RAMs) conventionally come in Dual Inline Packages (DIPs) with an even number of pins. If a particular device requires an odd number of pins, one of the pins is normally left redundant or not connected. If this redundant pin is adjacent to one of the signals that selects the RAM, it is possible to build two different assemblies of the RAM, one which uses the normal pin for chip select (CS), and the other of which uses the adjacent pin for chip select. When a pair of one of each of these two different RAMs is stacked together, it appears on the printed circuit board as a single device with two chip select pins. The upper device has a chip select pin which is above the redundant pin of the lower device and the lower device has a chip select pin which is below a redundant pin of the upper device. A disadvantage of this arrangement is that two different assemblies of the RAM are required; one with a chip select pad on the silicon chip bonded to the normal chip select pin and another with the chip select pad bonded to the normally not connected (NC) pin. EP-A-0157147 (IBM) discloses a stacked double density memory module using industry standard memory chips which is a modification of the known arrangement discribed above. On the top chip the chip select pin is bent so as not to contact the corresponding pin on the bottom chip and the upper chip select pin is connected to the no-connect pin of the upper chip by a jumper in the form of a U-shaped strap. The no-connect pin contacts the underlying no-connect pin of the bottom chip. This arrangement also suffers from the disadvantage that two different chip assemblies are required, one with a jumper and one without a jumper. Also, one of the pins on the top chip must be bent or cut to retain the jumper. US-A-4632293 discloses a method of upgrading memory boards wherein a memory chip of one type is soldered over an original memory chip of another type after the power pins of the original chip has been disconnected and insulated from the original chip. Patent Abstracts of Japan, Vol. 12, No. 318 (E-650), 29 August 1988 (JP-A-63081973) discloses a rectangular semiconductor device having pins on four edges. The present invention provides a semiconductor chip package according to claim 1. Advantageous embodiments are defined by claims 2 to 6. The present invention further provides modules of semiconductor chip packages according to claims 7 to 10. Embodiments of the present invention will now be described by way of example of example only with reference to the accompanying drawings in which:- Figure 1 is a schematic plan view of the pinout arrangement of a semiconductor chip package in accordance with a first embodiment of the present invention; Figure 2 is an elevational side view of the semiconductor chip package of Figure 1; Figure 3 is a schematic diagram of the electrical circuitry of the semiconductor chip package of Figure 1; Figure 4 is an elevational side view of a module comprising two semiconductor chip packages of Figure 1 in stacked configuration; Figure 5 is a schematic diagram of the electrical circuitry of the module of Figure 4; Figure 6 is a schematic plan view of the pinout arrangement of a semiconductor chip package in accordance with a second embodiment of the present invention; Figure 7 is an elevational side view of a module comprising four semiconductor chip packages of Figure 6 in stacked configuration; Figure 8 is a schematic plan view of the pinout arrangement of a semiconductor chip package in accordance with a third embodiment of the present invention; Figure 9 is an elevational side view of a module comprising eight semiconductor chip packages of Figure 8 in stacked configuration; and Figure 10 is a schematic plan view of the bonding pad footprint on a printed circuit board for connecting the module shown in Figure 9. Referring to Figures 1 to 3 of the drawings, there is shown a semiconductor chip package 2 in accordance with a first embodiment of the present invention. The semiconductor chip package 2 comprises an outer casing 4 within which are encapsulated one or more semiconductor chips (not shown). In this embodiment, four RAM chips are encapsulated in the outer casing 4 and these chips are represented by the four blocks 6, 8, 10, 12 shown in Figure 3. On opposed edges 14, 16 of the outer casing 4 are disposed respective rows of pins 18, 20. The pins 18,20, being disposed on opposed edges of the packages, are arranged in a conventional dual inline format but, as is shown in Figure 2, they are formed as gull-wing leads. The package 2 has a memory capacity of 4x64Kx4 or 2x64Kx8. In this illustrated embodiment, the package has forty pins, with twenty pins being disposed along each of edges 14 and 16 of the package 2 respectively. The pins are numbered 1 to 40 on Figure 1. The pins comprise sixteen address inputs (A0 to A15), eight data signal pins (D0 to D7), two power supply pins (VCC), two earth pins (GND), four chip select pins (CS0 to CS3), two write enable pins (WE0 and WE1) and two output enable pins (OE0 and OE1). Each of these pins is connected to one or more chips as schematically represented in Figure 3 (the VCC and GND pins being omitted in Figure 3 for clarity). Thus these pins constitute a first type of pin 22 which is electrically connected to the at least one semiconductor chip 6,8,10,12 in the package 2 and is adapted to conduct electrical signals between the at least one semiconductor chip 6,8,10,12 and external circuitry which is not shown. The first type of pins are divided into a plurality of groups, each group representing a respective signal type, i.e. address input pins (A), data signal pins (D), power supply pins (VCC, GND) and various control pins (CS, WE, OE). The package is also provided with four redundant pins 24 which are constituted by pins numbered 24 to 27 in Figure 1. The redundant pins 24 constitute a second type of pin 24 which is not electrically connected to the at least one semiconductor chip 6,8,10,12 in the package 2. The distribution of the first type 22 and second type 24 of pins on package 2 will now be described. All of the first type of pins 22 other than one group of first type of pins 22, namely the CS pins, are symmetrically disposed on respective sides 14,16 of the package 2. Thus opposite each address A, power VCC or GND, data D, write enable WE or output enable OE pin is another pin of the same group. The chip select CS pins and the redundant NC pins are functionally asymmetrically disposed on respective sides 14,16 of the package 2 such that opposite each of the four chip select CS pins there is a redundant NC pin. For those pins of the groups of the first type of pins 22 other than the chip select CS pins, the symmetry is reflectional symmetry about a plane extending longitudinally through the centre of the package 2 parallel to the two sides 14 and 16. Thus the pinout is symmetrical with respect to all signals except the chip select signals and has redundant NC pins where symmetry of the chip select CS pins would require additional chip select CS pins. Figure 4 shows a module 26 comprising a stacked configuration of two of the semiconductor chip packages 2 of Figure 1. The semiconductor chip packages 2 are double stacked by turning one of the pair of identical semiconductor chip packages 2 upside down with respect to the other whereby the upper surfaces 28 of the two semiconductor chip packages 2 are adjacent to each other and the bottom surfaces 30 are remote from each other. The pins 18,20 on opposing sides of each package 2 are connected in the manner shown in Figure 4 to the directly adjacent pins 20,18 of the other package 2. Thus, pin number 1 (VCC) of one package 2 is connected to pin number 40 (also VCC) of the other package 2, and vice versa. Pin number 2 (A) of one package 2 is connected to pin number 39 (also A) of the other package 2, both of which pins are address inputs. Since the semiconductor devices are RAMs rather than Read Only Memories (ROMs), the way in which the address inputs are connected between different packages is not important. Similar connections are made between the data D pins, the write enable WE pins, the output enable OE pins and the ground GND pins. However, each chip select CS pin (i.e. pins numbered 14 to 17) of a respective package 2 will be connected to a respective redundant NC pin (pins numbered 27 to 24 respectively) of the other package 2 and so the stacked module 26 will have twice as many CS pins as each individual package. The circuit diagram of the resultant stacked module 26 is shown in Figure 5. It will be seen that there are sixteen address inputs in total since the thirty two address input pins of the two packages 2 have been connected together in pairs thereby to result in only sixteen address inputs A0 to A15 in the resultant module 26. The identification of the sixteen address inputs A0 to A15 in Figure 5 corresponds to that of the bottom package 2. The module also has, for a similar reason, only eight data inputs D0 to D7 and the identification of these data inputs in Figure 5 corresponds to that of the bottom packages. The module 26 similarly has two write enable inputs WE0 and WE1 and the identification of these inputs corresponds to that of the bottom package 2. Furthermore, in a similar fashion the module 26 has two output enable inputs OE0 and OE1 and the identification of these inputs corresponds to that of the bottom package 2. The module 26 has eight chip select CS inputs, the total number of chip select CS inputs corresponding to the total number in the individual separate packages 2 since the chip select CS inputs are connected to redundant NC inputs and are not connected together to each other in pairs. Four chip select inputs CS0 to CS3 are inputs for the four chips in the bottom package 2 and four chip select inputs CS0 (top) to CS3 (top) are chip select inputs for the four chips in the top package 2. The module 26 has a memory capacity of 8x64Kx4 or 4x64Kx8. In the illustrated embodiment, the functional asymmetry is present on the chip select CS pins. An advantage of putting the functional asymmetry on the chip select pins, rather than the chip select pins, is that power in the package can be reduced and fewer pins would generally be used. However, in an alternative embodiment it is possible to have, for example, chip select signals on pins numbered 14, 15, 27 and 26 and to have just four data signals on pins numbered 8, 9, 10 and 11; pins numbered 33, 32, 31 and 30 would be redundant (NC) and pins numbered 16, 17, 24 and 25 would also be unused. Optionally, pins numbered 22 and 23 would become redundant as well. A single package of this type has a memory capacity of 4x64Kx4 and a double-stacked module has a memory capacity of 4x64Kx8. It should be understood that in the device illustrated in Figures 1 to 5 the symmetry of the pinout is reflectional symmetrv about a line parallel to pins numbered 1 to 20 with reflectional functional asymmetry being present on the chip select and the redundant pins. However, in alternative arrangements rotational symmetry may be provided either with or without reflectional symmetry. For example, for a package with pins on two edges, there could be rotational symmetry through an angle of 180° and this could enable a module of four stacked packages to be provided. Such a configuration constitutes a second embodiment of the present invention which is illustrated in Figures 6 and 7. Figure 6 is a schematic plan of a semiconductor chip package 70 having an outer casing 72 within which are encapsulated one or more semiconductor chips (not shown). In this embodiment, two or four RAM chips are encapsulated in the outer casing 72, having a memory capacity of 2 x 64K x 8. On opposed edges 74, 76 of the outer casing 72 are disposed respective rows of pins 78, 80, the arrangement being similar to that of the first embodiment shown in Figures 1 and 2. In this embodiment, the package 72 has forty eight pins, with twenty four pins being disposed along each of edges 74 and 76 of the package 72 respectively. The pins are numbered 1 to 48. The pins 82 which are electrically connected to the one or more semiconductor chips comprise sixteen address inputs (A), eight data signal pins (D), four power supply pins (VCC), four earth pins (GND), two chip select pins (CS), four write enable pins (WE), and four output enable pins (OE). Each of these pins 82 is connected to one or more chips in the package 70. The pins also comprise six redundant pins 84 which are constituted by pins numbered 13, 14, 35, 36, 37 and 38 in Figure 6. The redundant pins 84 are not electrically connected to the at least one semiconductor chip in the package 70. The distribution of the pins on the package 70 is such that for all the connected pins i.e. the VCC, GND, D, A, OE and WE pins, other than the chip select CS pins, there is both a reflectional symmetry about a longitudinal central plane extending through the package and rotational symmetry about 180° about an axis extending perpendicularly from the plane of the package 70. The two chip select pins numbered 11 and 12 are opposite redundant pins numbered 37 and 38 on the opposite side of the package and both the chip select pins numbered 11 and 12 and those redundant pins numbered 37 and 38 are each adjacent a respective further pair of redundant pins on the same side of the package numbered 13 and 14 and 35 and 36 respectively. It will be seen that if a package 70 is disposed over a second package 70 in an upside down configuration, each of the electrically connected pins 82 other than the chip select CS pins will overlie a similar pin 82 of the other package 70. The two chip select pins of one package 70 will be connected to respective redundant pins 84 of the other package 70 and vice versa. Thus a module 88 of two of the packages 70 can be constructed similar to the module 88 shown in Figure 4 wherein the module 88 is provided withn four separate chip select pins, with a pair of chip select pins being disposed on each side of the module 88. The two package module 88 also has four redundant pins corresponding to pins 13, 14, 35 and 36 of each package 70. Referring to Figure 7, a composite module 90 can be constructed out of two of the modules 88, each of which modules 88 comprises a pair of the packages 70. The top module 88 is rotated by an angle of 180° relative to the bottom module 88 whereby the four chip select CS pins of the upper module 88 are located over the four redundant pins 13, 14, 35 and 36 of the lower module 88. Thus the resultant four-layer stacked module 90 has eight resultant chip select pins, constituted by positions 11, 12, 13 and 14 on one side of the module 90 and by pins 35, 36, 37 and 38 on the other side of the module 90. Since there is four-fold symmetry in each package, all of the electrically connected pins 82 of each module 88 other than the chip select CS pin are connected to similar corresponding pins 82 of the other module 88 whereby the resultant four-layer stacked module 90 has a distribution of those pins which corresponds to the distribution shown in Figure 6 of one package 70. In a further alternative embodiment a package with pins on four orthogonal edges can provide rotational symmetry through an angle of 90°, allowing an eight package high stack and such a configuration constitutes a third embodiment of the present invention which is illustrated in Figures 8 to 10. Figure 8 is a schematic plan of a semiconductor chip package 32 having an outer casing 34 which has four orthogonal edges 36, 38, 40, 42. In the illustrated embodiment the outer casing 34 of the package 32 encapsulates a single semiconductor RAM chip (not shown) and each package has a memory capacity of 64Kx8. The package 32 is provided with four rows 44, 46, 48, 50 of pins, each row 44, 46, 48, 50 being located along a respective edge 36, 38, 40, 42 of the outer casing 34. In each package 32 the chip has a capacity of 64Kx8. Each package 32 has sixteen address pins A; eight data pins D; four write enable pins WE; eight power input pins VCC and eight earth pins GND. These groups of pins are both reflectively and rotationally symmetrically disposed on the four edges 36, 38, 40 and 42 of the package 32 whereby on opposing edges of the package 32 each of those pins is opposite a similar pin and for adjacent edges of the package 32 there is rotational symmetry about an angle of 90°. For simplicity, the pinout arrangement employs only a single chip select CS pin per package. The chip select CS pin is located on one edge 38 of the package 32, and in Figure 8 is present as pin number 19. Seven redundant NC pins are additionally provided in positions numbered 6, 8, 21, 32, 34, 45 and 47. Two redundant pins are provided on each of the three edges 36, 40, 42 which are not provided with the chip select CS pin and the seventh redundant pin is provided on the same edge 38 as the chip CS select pin. The chip select CS pin is opposite to the redundant pin numbered 47 and in addition the redundant pin numbered 21 which is on the same edge 38 as the chip CS select pin numbered 19 is reflectively symmetrically disposed relative thereto about a plane passing through the central pin numered 20 on edge 38 of the package 32. On each edge 36, 40, 42 the redundant pins are located in positions corresponding to the positions of the chip select CS pin and the redundant pin on edge 38. For simplicity, the pinout does not include an output enable pin. A module 52 of eight stacked packages 32 is shown in Figure 9. The module consists of four stacked pairs 54,56,58,60 of packages 32 thus providing an eight layer stack. The bottom pair 54 of packages 32 has a top package 32 which is disposed in an upside down configuration on a bottom package 32 with the overlying rows of pins 44, 46, 48, 50 being connected together. The pins are connected together in the manner similar to that of the first embodiment in that pin number 1 is connected to pin number 39, pin number 2 is connected to pin number 38, etc. for rows 44 and 48, and pin number 14 is connected to pin number 52, pin number 15 is connected to pin number 51, etc. for rows 46 and 50. In the bottom pair 54 of packages the two packages 32 are not rotated relative to each other whereby a common edge 38 is shown in Figure 9. The chip select pin number 19 is connected to a redundant pin number 47 and the remaining redundant pins are connected to each other. A second pair 56 of packages 38 is disposed over the first pair 54. The second pair of packages 56 is connected together in the same way as the first pair 54 but the second pair 56 is rotated about an angle of 90° relative to the first pair 54 whereby the two edges 40 of the packages 32 of the second pair 56 is located above the two edges 38 of the first pair 54 as shown in Figure 9. The pins of the second pair 56 are connected to each other and also to those of the first pair whereby it will be seen that the chip select CS pins of each package 32 of the second pair 36 are each connected to a respective redundant pin which is itself not connected to any other chip select CS pin. A third pair 58 of packages 32 is similarly mounted on the second pair 56, the third pair 58 being rotated about an angle of 90° relative to the second pair whereby the edges 42 of the two packages 32 of the third pair 58 are located above the edges 40 of the second pair 56. A fourth pair 60 of packages 32 is disposed in a similar manner above the third pair 58 with the edges 36 of the fourth pair being located above the edges 42 of the third pair 58. Thus the four stacked pairs 54,56,58,60 of packages 32 form an eight layer module 52 wherein each of the eight layers has a respective chip select CS position which is different from the chip select CS position of any other layer, with the chip select position of any given layer corresponding to a redundant pin position of all the other layers. Figure 10 shows the pinout connecting pad arrangement 62 on a printed circuit board 64 for the module 52 shown in Figure 9. It will be seen that there are eight chip select pads CS0 to CS7 each of which accesses a respective chip in a respective package 32. In the drawing, CSn corresponds to the chip select CS pin of layer n of the module 52. Thus each edge of the bonding pad arrangement has two chip select CS pads, four address A pads, two data D pads, two earth GND pads, two power VCC pads and one write enable WE pad. The module offers a memory facility of 8x64Kx8. The present invention can provide a real advantage over the prior art in providing pinout arrangements which enable identical semiconductor chip packages to be stacked thereby to enable a semiconductor assembly to take up a reduced surface area. Although specific arrangements of reflectional and rotational symmetry have been described, it will be understood that various arrangments of rotational and reflectional symmetry can be employed in accordance with the present invention.

Claims: A semiconductor chip package (2;32;70) comprising at least one semiconductor chip disposed therein, the package having arranged along its edges (14,16;36,38,40,42;74,76) a plurality of pins (22,24;82,84), the pins (22,24;82,84) being symmetrically arranged with respect to at least one plane of symmetry extending through the package and/or an axis of symmetry extending perpendicularly from a plane of the package and the pins (22,24;82,84) consisting of first and second pins, each first pin (22;82) being electrically connected to the at least one semiconductor chip to conduct signals between the at least one semiconductor chip and external circuitry and each second pin (24;84) being not electrically connected to the at least one semiconductor chip, the first pins (22;82) being divided into a plurality of groups respectively comprising address input (A), data (D), power supply (VCC), earth (GND), chip select (CS) and write enable (WE) pins, the first pin or pins (22;82) of at least one group and a corresponding number of second pins (24;84) are disposed along said edges (14,16;36,38,40,42;74,76) in such a manner that each of the first pins (22;82) of the at least one group coincides with the position of a corresponding second pin (24;84) upon a symmetry operation with respect to said at least one plane of symmetry and/or said axis of symmetry, whereas each of said first pins (22;82) of the remaining groups coincides with the position of a functionally corresponding first pin (22;82) of the same group upon the same symmetry operation. A semiconductor chip package according to claim 1 wherein the package (2;70) has a rectangular shape and the first and second pins (22,82;24,84) are arranged on a pair of opposed edges (14,16;74,76) of the package (2;70), each of the first pins (22;82) of the at least one group being opposite a corresponding second pin (24;84) and each first pin (22;82) of the remaining groups being opposite a functionally corresponding first pin (22;82) of the same group. A semiconductor chip package according to claim 1 wherein the package (32) has a square shape and the first and second pins are arranged on each of the four edges (36,38,40,42) of the package (32), each of the first pins of the at least one group being opposite a corresponding second pin and each first pin of the remaining groups being opposite a functionally corresponding first pin of the same group. A semiconductor chip package according to claim 2 or 3 having further second pins (84) which are not opposite a respective first pin (82) of the at least one group, the further second pins being arranged opposite to each other and their positions are chosen such that, upon further symmetry operations with respect to said at least one plane of symmetry and/or said axis of symmetry, which operations transfer each first pin (82) of the remaining groups to the position of a functionally corresponding first pin (82) of the same group,each respective first pin (82) of the at least one group coincides with the position of one of said further second pins (84). A semiconductor chip package according to any foregoing claim wherein the first pins (22;82) of the at least one group are chip select pins. A semiconductor chip package according to any one of claims 1 to 4 wherein the first pins (22;82) of the at least one group are data pins. A module of semiconductor chip packages, the module (26) comprising two semiconductor chip packages (2;32;70) according to any one of claims 1 to 6 which have identical pin arrangements and are stacked together to form a stacked pair of packages with each first pin (22;82) of the said remaining groups of one package (2;32;70) contacting a functionally corresponding first pin (22;82) of the same group of the other package (2;32;70) and each first pin (22;82) of the at least one group of the said one package (2;32;70) contacting a respective second pin (24;84) of the said other package (2:32;70), such that each chip can be individually accessed within the module. A module according to claim 7 wherein the said other package (2;32;70) is mounted upside-down on the said one package (2;32;70). A module according to claim 8 constituted of packages with identical pin arrangement according to claim 4 when referring to claim 2 or 3, further comprising an additional stacked pair of packages with identical pin arrangement which is stacked above the first-mentioned stacked pair, the additional stacked pair being connected to the first-mentioned stacked pair with each first pin/first pin connection of the first-mentioned stacked pair being connected to a first pin/first pin connection of the same group of the additional stacked pair and each first pin/second pin connection of the first-mentioned stacked pair being connected to a second pin/second pin connection of the additional stacked pair, the symmetry and the number of the first pin/second pin connections being chosen such that each chip can be individually accessed within the module. A module according to claim 9 constituted of packages (32) with identical pin arrangement according to claim 4 when referring to claim 3, comprising two further stacked pairs of packages (32) which are identical to, stacked above and connected to the said additional stacked pair, such that each chip can be individually accessed within the module.

Inventors: WAKEFIELD ELWYN PAUL MICHAEL, WALKER CHRISTOPHER PAUL HULME, WAKEFIELD, ELWYN PAUL MICHAEL, WALKER, CHRISTOPHER PAUL HULME
Applicants: SGS THOMSON MICROELECTRONICS, STMICROELECTRONICS LIMITED
Application year: 1990
Main classification: H01L23
IPC codes: H01L25, H01L23