Semiconductor memory control device and method of mounting same in high density

Abstract: A semiconductor memory control device comprises a flexible board 7 which is folded in such a manner as to form a stack; a plurality of semiconductor elements 10 mounted on the flexible board; and a package main body 6, 8, 9 which encloses and seals the flexible board and the plurality of semiconductor elements. A method of mounting semiconductor memory control devices in high density comprises the steps of: mounting a plurality of semiconductor elements on a flexible board; forming an internally mounted module by bending the flexible board in such a manner as to be stacked in multiple layers; and sealing the internally mounted module in a package's main body.

Description: BACKGROUND OF THE INVENTION FIELD OF THE INVENTION: The present invention relates to a semiconductor memory control device and a high-density mounting method therefor. More particularly, the present invention relates to a large-capacity memory control device and to a method of mounting large-capacity memory control devices in high density in a small package. DESCRIPTION OF THE RELATED ART: Fig. 11 shows the construction of a conventional semiconductor memory control device. A plurality of semiconductor elements 19 are mounted on both sides of a printed board 18. An electrode 14 of a connector 13 is electrically connected to the printed board 18. A semiconductor memory control device is connected to an external device through this connector 13. In Fig.11, reference numeral 15 denotes a sheathing frame; and reference numerals 16 and 17 denote an obverse panel and a reverse panel, respectively. Next, a method of mounting this semiconductor memory control device will be explained. First, semiconductor elements 19 are mounted on both sides of the printed board 18, and the electrode 14 of the connector 13 is soldered to the printed board 18. Thus, an internally mounted module is formed, and is thereafter buried inside the sheathing frame 15. The obverse panel 16 and the reverse panel 17 are mounted on the sheathing frame 15. Thus, mounting of the semiconductor memory control device is completed. In the conventional semiconductor memory control device, as shown in Fig. 11, there is a limitation on the number of semiconductor elements 19 which can be mounted because they are mounted on one or both sides of one printed board 18. For example, if it is assumed that a maximum of 12 semiconductor elements 19 can be mounted on one side of the printed board 18, the maximum number of semiconductor elements is 24 even if both sides of the printed board 18 are used. Therefore, there is a problem in that it is difficult to make a small semiconductor memory control device with a large capacity. WO 90/13989 discloses an arrangement in which a printed circuit board is flexible and bent back upon itself. SUMMARY OF THE INVENTION The present invention has been accomplished to solve the above-mentioned problems of the prior art. An object of the present invention is to provide a large-capacity semiconductor memory control device in spite of its small size. Another object of the present invention is to provide a method of mounting large-capacity semiconductor memory control devices in high density in small packages. In accordance with the present invention, there is provided a semiconductor memory control device as set out in Claim 1. Preferred features are set out in Claims 2 to 10. The invention also provides a method of manufacturing such a device, as set out in Claim 11. Preferred features are set out in Claims 12 to 14. In the semiconductor memory control device of the present invention, the bending of the flexible board in such a manner as to be stacked in multiple layers and electrical connection in the bent region enables multi-stage mounting of a plurality of semiconductor elements. In the high-density mounting method for semiconductor memory control devices of the present invention, a multi-stage mounting of a plurality of semiconductor elements is performed by bending the flexible board in such a manner as to be stacked in multiple layers and forming an electrical connection in the bent region. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a cross-sectional view which illustrates a semiconductor memory control device of a first embodiment of the present invention; Fig. 2 is a plan view, partly in cross section, which illustrates the semiconductor memory control device of the first embodiment; Fig. 3 is a cross-sectional view taken in the direction of the arrows along the line A-A of Fig. 2; Fig. 4 is a plan view which illustrates a flexible board used in the the semiconductor memory control device of the first embodiment; Fig. 5 is a plan view which illustrates a flexible board used in a modification of the first embodiment; Fig. 6 is a cross-sectional view which illustrates a semiconductor memory control device of a second embodiment of the present invention; Fig. 7 is a cross-sectional view which illustrates a semiconductor memory control device of a third embodiment of the present invention; Fig. 8 is a plan view which illustrates a flexible board used in the third embodiment; Fig. 9 is a cross-sectional view which illustrates an internally mounted module of a semiconductor memory control device of a fourth embodiment; Fig. 10 is a plan view which illustrates a flexible board used in the fourth embodiment; and Fig. 11 is a cross-sectional view which illustrates a conventional semiconductor memory control device. DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of the present invention will be explained below with reference to the accompanying drawings. Fig. 1 is a cross-sectional view which illustrates a semiconductor memory control device of the first embodiment of the present invention. One continuous flexible board 7 is bent at three bending portions 71 to 73 in such a manner as to be stacked on one another. Thus, first to fourth layers 7a to 7d of the flexible board 7 are formed. A plurality of semiconductor elements 10 are mounted on each of the first to fourth layers 7a to 7d of the flexible board 7. An electrode 4 of a connector 3 is soldered to the second-stage bending portion 72 of the flexible board 7. The flexible board 7, the semiconductor elements 10 and the connector 3 constitute an internally mounted module 2. The internally mounted module 2 is buried inside a sheathing frame 6, on which frame an obverse panel 8 and a reverse panel 9 are mounted. The sheathing frame 6, the obverse panel 8 and the reverse panel 9 constitute the package's main body. As shown in Fig. 2, the flexible board 7 is fixed by a plurality of thickness mold members 11 which are securedly fixed to the sheathing frame 6. A detailed cross section of this thickness mold member 11 is shown in Fig. 3. The thickness mold member 11 has four formed slits 11a spaced apart from each other by a space L. The side edge portion of each of the layers 7a to 7d of the flexible board 7 is inserted into a corresponding slit 11a. The space between each adjacent two layers 7a to 7d of the flexible board 7 can be kept at a predetermined value L by using such thickness mold member 11. As a result, deformation of each of the layers 7a to 7d can be prevented. The space L is determined by the thickness of the semiconductor elements 10 mounted on the flexible board 7 or other factors. Next, a method of mounting semiconductor memory control devices according to this embodiment will be explained. First, one continuous flexible board 7 shown in Fig. 4 is prepared. The flexible board 7 has first to fourth circuit regions 7A to 7D having substantially the same size, which become first to fourth layers 7a to 7d, respectively. A first bending region 71 is formed between the first circuit region 7A and the second circuit region 7B; a second bending region 72 is formed between the second circuit region 7B and the third circuit region 7C; and a third bending region 73 is formed between the third circuit region 7C and a fourth circuit region 7D. An electrode pattern 74, which is soldered to the electrode 4 of the connector 3, is formed in the vicinity of the second bending region 72. After parts, such as a plurality of semiconductor elements 10, are mounted on both sides of the fourth circuit regions 7A to 7D of the flexible board 7, the flexible board 7 is bent at the bending portions 71 to 73 in such a way that these circuit regions 7A to 7D are stacked in multiple layers. Furthermore, the electrode pattern 74 formed in the vicinity of of the second bending portion 72 is soldered to the electrode 4 of the connector 3, thus an internally mounted module 2 is formed. Thereafter, the internally mounted module 2 is buried inside the sheathing frame 6, and the obverse panel 8 and the reverse panel 9 are mounted on the sheathing frame 6. Thus, the mounting of the semiconductor memory control device is completed. In the semiconductor memory control device mounted as described above, since semiconductor elements 10 are mounted on both sides of the first to fourth layers 7a to 7d of the flexible board 7, high-density mounting four times that of the conventional semiconductor memory control devices shown in Fig. 10 is made possible. The obverse panel 8 and the reverse panel 9 may be constructed with one of either an electrically conductive member or insulating member. In addition, if conduction wiring is formed by printing or the like on the thickness mold member 11, an electrical connection between the layers 7a to 7d of the flexible board 7 can be obtained by using the thickness mold member 11. A flexible board 27 shown in Fig. 5 may be used as a flexible board. In the flexible board 27, a first circuit region 27A and a second circuit region 27B, both of which are in the shape of a rectangle, are in contact with each other on their long sides. The connection portion between them becomes a first bending portion 271. In like manner, a third circuit region 27C and a fourth circuit region 27D, both of which are in the shape of a rectangle, are in contact with each other on their long sides. The connection portion between them becomes a third bending portion 273. The second circuit region 27B and the third circuit region 27C are in contact with each other on their short sides. The connection portion between them becomes a second bending portion 272, and an electrode pattern 274 is formed in this portion. Since such a flexible board 27 is used, the length of the wiring from the electrode pattern 274 to each circuit region, and in particular, to the parts on the first and fourth circuit regions 27A and 27D, is short, and thus electrical characteristics of the semiconductor memory control device can be improved. A semiconductor memory control device of a second embodiment of the present invention is shown in Fig. 6. In this embodiment, semiconductor elements are not mounted on the outside surfaces of the outermost layers 37a and 37d of a flexible board 37 bent in such a manner as to be stacked on one another. The entire surfaces of these surfaces are ground planes 30a and 30b, respectively. The ground planes 30a and 30b become signal ground planes by being electrically connected to a signal ground pin inside an electrode 34 of a connector 33. When a frame ground pin is provided inside the electrode 34, the ground planes 30a and 30b are connected to a frame ground pin, thus these planes can be made frame ground planes. If the ground planes 30a and 30b are provided as in this embodiment, when noise from outside, such as electrostatic charges, are applied to an obverse panel 38 and a reverse panel 39, the noise can be made to escape to the outside from the ground planes 30a and 30b through the signal or frame ground pin. As a result, deterioration/breakdown of the semiconductor elements 10 resulting from external noise can be prevented. A semiconductor memory control device of a third embodiment of the present invention is shown in Fig. 7. In this embodiment, a shield layer 40 formed from a part of the flexible board 47 is inserted between a first layer 47a and a second layer 47b of the flexible board 47 bent in such a manner as to be stacked on one another. The shield layer 40 is electrically connected to a signal ground pin or a frame ground pin inside an electrode 44 of a connector 43. This semiconductor memory control device can be mounted by using one continuous flexible board 47 shown in Fig. 8. The flexible board 47 has first and second circuit regions 47A and 47B, which become the first layer 47a and the second layer 47b respectively, and a shield layer 47C, which is connected to the second circuit region 47B and whose entire surface is made into a ground plane. Since the flexible board 47 is bent so that the shield layer 47C is sandwiched between the first circuit regions 47A and the second circuit region 47B, the shield layer 40 shown in Fig. 7 is formed. According to this embodiment, even if semiconductor elements 10 which operate at high speed are mounted in high density, electromagnetic or electrostatic induction between the semiconductor elements 10 resulting from a high-speed electric current or large electric current can be prevented. As a result, noise can be prevented and thus malfunctions can be prevented also. As a consequence, a highly reliable semiconductor memory control device can be realized. Although in each of the above-described embodiments a flexible board, which is bent in a maximum of four layers, with a connector having a group of two-row electrodes, is shown, the number of the layers of the flexible board is not limited to this number. For example, as shown in Fig. 9, a flexible board 57 is bent in six layers 57a to 57f, and the flexible board 57 may be connected to a connector 53 having a group of four-row electrodes. An expansion view of the flexible board 57 is shown in Fig. 10. The flexible board 57 has first to sixth circuit regions 57A to 57F, which become first to sixth layers 57a to 57f respectively. A first electrode pattern 571 is formed in the vicinity of a bending portion between the second circuit region 7B and the third circuit region 7C. A second electrode pattern 572 is formed in the vicinity of a bending portion between the fourth circuit region 7D and a fifth circuit region 7E. After parts, such as a plurality of semiconductor elements 10, are mounted on both sides of the six circuit regions 57A to 57F of such flexible board 57, the flexible board 57 is bent, and further, the first and second electrode patterns 571 to 572 are soldered to a group of four-row electrodes of the connector 53. Thus, an internally mounted module shown in Fig. 9 is formed.

Claims: A semiconductor memory control device comprising: a circuit board (7); a plurality of semiconductor elements (10) mounted on the board (7); a connector (3), electrically connected to the semiconductor elements (10); and a main body (6,8,9) which seals the board (7) and the plurality of semiconductor elements (10); characterised in that the board (7) is flexible and folded back upon itself to define a plurality of layers (7a-7d); and the connector is connected to a bent region (72) of the board (7). A semiconductor memory control device according to Claim 1 wherein each of the layers (7a-7d) of the flexible board (7) has substantially the same size. A semiconductor memory control device according to Claim 1 or 2 wherein the outermost layer of the flexible board (7) forms a ground plane (30a,30b). A semiconductor memory control device according to any preceding claim wherein some layers of the flexible board (7) form a shield layer (40) for shielding the semiconductor elements (10). A semiconductor memory control device according to any preceding claim wherein the package's main body comprises a sheathing frame (6) for holding the flexible board (7) having the plurality of semiconductor elements (10) mounted thereon, and obverse and reverse panels (8,9) mounted on the sheathing frame (6). A semiconductor memory control device according to any preceding claim further comprising fixing means (11) for fixing the end portions of each of the layers (7a-7d) of the flexible board (7) so that the spacing between each two adjacent layers (7a-7d) of the flexible board (7) is uniform. A semiconductor memory control device according to any preceding claim, wherein each of the layers (7a-7d) of the flexible board (7) is in the shape of a rectangle, and each of the layers (7a-7d) is connected to an adjacent layer by a short side of its respective rectangle. A semiconductor memory control device according to any preceding claim wherein each of the layers (7a-7d) of the flexible board (7) is in the shape of a rectangle, and at least one layer (7a-7d) is connected to adjacent layers (7a-7d) by the short and long sides of its respective rectangle. A semiconductor memory control device according to any preceding claim wherein the connector (3) has a dual row of electrodes. A semiconductor memory control device according to any preceding claim wherein the connector (3) has a quadruple row of electrodes. A method of mounting semiconductor memory control devices in high density comprises the steps of: mounting a plurality of semiconductor elements (10) on a circuit board (7); providing a connector (3) and electrically connecting it with the semiconductor elements (10); and sealing the arrangement in a main body; characterised in that the board (7) is flexible and is bent back upon itself to define a plurality of layers (7a-7d), and the connector (3) is connected to a bent region (72) of the board (7). A mounting method according to Claim 11 wherein the flexible board (7) is bent in such a manner that the layers (7a-7d) thereof have substantially the same size. A mounting method according to Claim 11 or 12 wherein a ground plane (30a,30b) is formed in the outermost layer of the flexible board (7). A mounting method according to Claim 11, 12 or 13 wherein shield layers (40) for shielding semiconductor elements are formed on some layers of the flexible board (7).

Inventors: KIMURA MASATOSHI C O MITSUBISH, KIMURA, MASATOSHI, C/O MITSUBISHI DENKI K. K.
Applicants: MITSUBISHI ELECTRIC CORP, MITSUBISHI DENKI KABUSHIKI KAISHA
Application year: 1992
Main classification: H05K3
IPC codes: H05K7, H01L25, H05K1, H01L23, H05K5