Moulded plastic power semiconductor device

Abstract: According to this invention, a semiconductor chip (11) incorporating a power control element is mounted on a heat dissipation member (12) by a conductive solder (13), and lead members (14, 15) electrically connected to terminals extending from the semiconductor chip are arranged to have a surface level almost equal to that of the heat dissipation member. A main body structure including the semiconductor chip and the members is sealed by a sealing plastic body (18). A plastic table (19) integrated with the sealing plastic body projects and is formed at a position corresponding to a lower side of outer leads (122, 141, 151) which extends from the heat dissipation and lead members and is extracted from the sealing plastic body, and the outer leads are bend upward at a position corresponding to a position of the plastic table. The sealing plastic body is mounted on an external mounting member (21) having a heat dissipation function.

Description: The present invention relates to a moulded plastic semiconductor device having the features indicated in the preamble clause of claim 1 and disclosed in PATENT ABSTRACTS OF JAPAN 13(346), (E-798) and JP-A-1106448, or in PATENT ABSTRACTS OF JAPAN 13(347),(P-910) and JP-A-1106391. The problem of heat dissipation is not mentioned in either one of these prior art documents. PATENT ABSTRACTS OF JAPAN 12(174), (E-612) and JP-A-62282451 discloses a resin sealed semiconductor device where, in order to improve the dielectric breakdown strength, protrusions made of resin and surrounding the external leads of the semiconductor device are provided. The resin body of the semiconductor device is mounted on an external member. For facilitating the comprehension of the present invention the technical background thereof will be briefly outlined hereinafter. Since a power control semiconductor device generates heat during operation of the device, an important condition in its arrangement is that heat dissipation characteristics of the device are set to be sufficient. In a plastic molded power semiconductor device, a heat dissipation section incorporated in the semiconductor device which is tightly mounted on an external mounting member constituting a heat dissipation unit having, e.g., a cooling fin or the like, is used. In this case, the external mounting member must be made of a material having good heat conductivity. For example, as the material, a metal material such as aluminum generally having electric conductivity is used. Fig. 6 is a conventional plastic molded power semiconductor device. The semiconductor device comprises a heat dissipation member 41 made of a metal material such as copper, and a power control semiconductor chip 43 is mounted on the heat dissipation member 41 by a Pb-Sn solder 42. In this case, the heat dissipation member 41 also serves as a heat dissipation plate for conducting heat generated by the semiconductor chip 43. In addition, a lead member 44 made of the same material as that of the heat dissipation member 41 is set at a position spaced apart from the heat dissipation member 41. The lead member 44 is connected to an extending terminal of the semiconductor chip 43 on the heat dissipation member 41 by a bonding wire 45. Thus, a main body of the semiconductor device is constituted. The main body constituted by the heat dissipation member 41, the semiconductor chip 43, the lead member 44, and the bonding wire 45 is sealed by a transfer-molded plastic body 46. In this case, an insulated type structure in which the heat dissipation member 41 is covered with the plastic body 46 so as to also cover its lower surface portion is used, and the heat dissipation member 41 is mounted by screws or the like on an external mounting member 47 made of a metal material through the plastic layer on the lower surface of the heat dissipation member 41. In the semiconductor device arranged as described above, even when the device is directly mounted on the external mounting member 47 to dissipate heat, since the external mounting member 47 is electrically insulated from the heat dissipation member 41, an insulating member such as a mica plate need not be used. Therefore, the plastic molded semiconductor device can be practically used with good workability, and this structure has been used as a major structure of a power semiconductor device. However, in accordance with a decrease in size of a semiconductor device of this type, various problems are posed. For example, when a semiconductor device sealed by a sealing plastic is mounted on the external mounting member 47, an interval a between an outer lead 441 portion extending from the plastic body 46 of the lead member 44 to the outside and the mounting member 47 has a minimum size determined by the rated dielectric breakdown voltage. In order to decrease the size of the semiconductor device, the thickness of the insulator on the lower surface portion of the heat dissipation member 41 must be decreased as much as possible. However, when the thickness is decreased, the interval a is naturally decreased, and a satisfactory dielectric breakdown voltage cannot be obtained. For this reason, as shown in Fig. 4, the height of the heat dissipation member 41 from the external mounting member 47 is different from that of the lead member 44 to position the lead member 44 at a level higher than the major surface of the heat dissipation member 41. When the heat dissipation member 41 and the lead member 44 are arranged to have a step difference therebetween, not only an arrangement of the member portion is complicated, but a connecting operation of the bonding wire 45 is cumbersome. In addition, since the semiconductor chip 43 on the heat dissipation member 41 is connected to the lead member 44 by the bonding wire 45, a ratio of a size (thickness) above a major surface of the heat dissipation member 41 to a size (thickness) below the major surface of the heat dissipation member 41 is usually "(5 to 10) : 1". Therefore, a thickness b of the semiconductor device is limited, and a decrease in size of the semiconductor device is limited. It is an object of the present invention to provide a plastic molded semiconductor device in which a size of the whole semiconductor device can be sufficiently decreased so that a sufficient dielectric breakdown voltage can be obtained at an outer lead portion in a positional relationship between the device and an external mounting member. It is another object of the present invention to provide a structure of a plastic molded semiconductor device in which an interval between an outer lead portion and an external mounting member can be set to be large without increasing the height of the semiconductor device. It is still another object of the present invention to set a dielectric breakdown voltage at an outer lead portion higher than a voltage corresponding to an interval between the outer lead and a mounting member, to easily improve the reliability of a plastic molded power semiconductor device of this type, and to easily perform a mounting operation for practical assembly. According to the present invention, a plastic molded semiconductor device having the features indicated in claim 1 is provided. Preferred embodiments of the invention are indicated in the dependent claims. In a plastic molded semiconductor device with the above arrangement, the plate made of a plastic is interposed between the outer portions of the leads extending from a sealing plastic body to the outside and the external mounting member to increase an insulation distance between the outer portions and the external mounting member. In this state, since the outer portions are bent in a direction to separate the outer leads from the external mounting member, the insulation distance between the outer leads and the external mounting member can be easily further increased to properly improve the reliability of a power semiconductor device for controlling a power having a high voltage. In addition, the distance between the heat dissipation member having the semiconductor chip and the external mounting member can be easily decreased, and the plastic molded type semiconductor device can be easily designed to have a small size, especially a small height. This invention can be more fully understood from the following detailed description when taken in conjunction with the accompanying drawings, in which: Fig. 1 is a plan view showing a semiconductor chip and a member structure for explaining a plastic molded power semiconductor device according to the first embodiment of the present invention; Fig. 2 is a sectional view showing the semiconductor device taken along a line 2 - 2 in Fig. 1; Figs. 3A and 3B are sectional views for explaining a plastic table integrally formed with a plastic molded body of the semiconductor device in Fig. 2; Figs. 4 and 5 are sectional views for explaining two plastic tables integrally formed with plastic molded bodies according to further embodiments, respectively; and Fig. 6 is a sectional view showing an arrangement for explaining a conventional plastic molded semiconductor device. A power semiconductor device comprises a semiconductor chip 11 constituting a power transistor having, e.g., a collector electrode, an emitter electrode, and a base electrode. As shown in Figs. 1 and 2, the semiconductor chip 11 is provided on a heat dissipation member 12 made of, e.g., copper having good heat and electric conductivity by a Pb-Sn solder 13. In this case, the lower surface of the semiconductor chip 11 serves as the collector electrode of the power transistor, and the heat dissipation member 12 also serves as a collector lead. A lead member 121 is integrally formed with the heat dissipation member 12 in one direction parallel to the surface, and the lead member 121 extends as the collector electrode. With the above heat dissipation member 12, lead members 14 and 15 made of the same material as that of the heat dissipation member 12 are arranged at positions electrically insulated from the dissipation member 12. The lead members 14 and 15 are located at both sides of the lead member 121 formed on the heat dissipation member 12 and are parallel to the lead member 121. The surface level of the heat dissipation member 12 and the surface level of the lead members 14 and 15 are almost equal to each other. Terminals corresponding to the emitter and the base of the semiconductor chip 11 constituting the power transistor are connected to the lead members 14 and 15 by bonding wires 16 and 17 made of an Au or Al thin wire, respectively. In this case, although not shown, when the semiconductor chip 11 must be protected from contamination, silicone encapsulation or the like is applied to the chip 11. As described above, the semiconductor chip 11 is mounted on the heat dissipation member 12, and the main body structure in which the semiconductor chip 11 is connected to the lead members 14 and 15 by the bonding wires 16 and 17 is molded by a sealing plastic body 18 made of an outer-layer plastic material such as epoxy resin. In this case, outer lead portions 122, 141, and 151 respectively extending from the lead members 121, 14, and 15 are parallelly extracted from one side surface of the sealing plastic body 18. A plastic plate 19 is integrally formed with the sealing plastic body 18 in correspondence with a lower surface portion which supports the outer lead portions 122, 141, and 151 so as to project from one side surface of the sealing plastic body from which the outer lead portions 122, 141, and 151 extend. The outer lead portions 122, 141, and 151 extending from the side surface of the sealing plastic body 18 are bent upward at an outlet portion of the sealing plastic body 18 so as to separate the outer lead portions 122, 141, and 151 from the plastic plate 19. That is, in the state wherein the sealing plastic body 18 is formed integrally with the plastic plate 19, as indicated by a broken line in Fig. 2, the outer lead portions 122, 141, and 151 are integrally connected on the plastic plate, as shown in Fig. 3A. Thereafter, the outer lead portions 122, 141, and 151 are bent to have a shape shown in Fig. 2. In this case, the outer lead portions 122, 141, and 151 are bent at the outlet portion away from the sealing plastic body 18 so as to separate the outer leads from the plastic plate 19, and the bending position must be set inside the edge of the plastic table 19. When the outer lead portions 122, 141, and 151 are to be bent away from the plastic plate 19, as shown in Fig. 3A, cracks 20 may be formed in the plastic plate 19 to extend from bonded portions between corners of each lead and the plastic plate 19. In order to prevent a formation of the cracks 20, as shown in Fig. 3B, convex support tables 191 which are higher than other portions are formed on the plastic plate 19 at portions contacting the outer lead portions 122, 141, and 151, respectively. The plastic plate 19 is formed such that each of the outer lead portions 122, 141, and 151 is bonded to the corresponding support table 191. In the above arrangement, when the outer lead portion is bent away from the support table 191, even if cracks are formed, the cracks do not reach the inside of the plastic plate 19. According to the arrangement shown in Fig. 3B, defective products which are produced at a percentage of 0.5 to 5% due to cracks in the arrangement in Fig. 3A can be eliminated. A plastic molded semiconductor device with the above arrangement is mounted and fixed on an external mounting member 21 made of a metal having good heat conductivity by inserting a screw into a mounting hole 22 formed through the sealing plastic body 18 and the dissipation member 12. In this case, although not shown, a heat dissipation fin or the like is provided to the external mounting member 21 as needed to obtain a heat dissipation mechanism of the semiconductor device. In a plastic molded semiconductor device with the above arrangement, plastic molding is performed such that the plastic plate 19 extending from the side surface of the sealing plastic body 18 is provided below the outer lead portions 122, 141, and 151, and the outer lead portions 122, 141, and 151 are bent at their proximal ends in a direction to separate the outer lead portions from the external mounting member 21. Therefore, a sufficiently large interval d between the outer lead portions 122, 141, and 151 and the external mounting member 21 is obtained, as compared with an interval c between the outer lead portions and the external mounting member when the outer lead portions 122, 141, and 151 extend straight from the sealing plastic body 18. In addition, a surface insulation distance including a surface from which the outer lead portions 122, 141, and 151 are bent away is obtained to set a sufficiently high dielectric breakdown voltage at the outer lead portions 122, 141, and 151. In this case, since the surface of the heat dissipation member 12 is set at a level almost equal to the surface level of the lead members 14 and 15, the thickness of the wiring structure including the bonding wires 16 and 17 or the like can be made sufficiently small. The thickness above the heat dissipation member 12 of the sealing plastic body 18 can be made small. Therefore, the plastic molded semiconductor device can be easily formed to be thin, and a compact plastic molded semiconductor device can be formed, thus keeping a sufficiently high dielectric breakdown voltage. Although the semiconductor device has a structure in which a heat dissipation surface of the lower surface portion of the heat dissipation member 12 is electrically insulated by the sealing plastic body 18, the thickness of a plastic layer of the lower surface of the heat dissipation member 12 can be made small, and the thickness of the plastic layer on the surface of the heat dissipation member 12 can be equal to the thickness of the lower surface portion (in the structure shown in Fig. 6, a ratio of (5 to 10) : 1), thereby considerably improving heat dissipation characteristics. For example, although a power transistor had Pc = 20 W and the thickness of heat dissipation plastic layer was set to 400 µm in the structure shown in Fig. 6, the thickness of a plastic layer could be set to 250 µm and a power Pc = 30 W could be obtained according to the structure described in the embodiment. To prevent the formation of cracks, a support table is formed in the structure shown in Fig. 3B. However, formation of cracks can also be prevented by forming, in the plastic plate along both sides of the outer lead portion 151, trenches 201 and 202 as shown in Fig. 4 or trenches 211 and 212 as shown in Fig. 5. In this embodiment, although a power transistor for controlling a high voltage is specifically exemplified, this structure of the present invention can be directly applied to a plastic molded semiconductor device constituting a rectifying device having poor heat dissipation characteristics, an integrated circuit element, or the like. Reference signs in the claims are intended for better understanding and shall not limit the scope.

Claims: A moulded plastic semiconductor device comprising a semiconductor chip (11) incorporating a semiconductor element which generates heat and which is mounted on a metal heat dissipation member (12), lead members (14, 15, 121) electrically connected to terminals of said semiconductor chip (11), said semiconductor chip (11), said heat dissipation member (12) and said lead members (14, 15, 121) being sealed by a sealing plastic body (18), outer portions (141, 151, 122) of said lead members (14, 15, 121) extending from a side surface of said plastic body (18), and wherein said plastic body (18) has a first lower surface mounted and fixed on an external mounting member (21) having a heat dissipation function, characterized in that a plastic plate (19) extends from said side surface of said plastic body (18) so as to lie between said outer portions (141, 151, 122) of said lead members (14, 15, 121) and said mounting member (21) and to contact a lower surface of said outer portions (141, 151, 122), and in that said outer portions (141, 151, 122) are bent, inside the edge of said plastic plate (19), in a direction away from said plastic plate (19) and said mounting member (21). A device according to claim 1, characterized in that said lead members (14, 15, 121) are arranged to have a surface level almost equal to that of said heat dissipation member (12). A device according to claim 1, characterized in that it further comprises a plurality of convex support tables (191) protruding from the surface of said plastic plate (19), each outer lead (141, 151, 122) being bonded to a corresponding support table (191). A device according to claim 1, characterized in that said heat dissipation member (12) is electrically connected to an electrode on the mounting surface of said semiconductor chip (11) and is connected with one of said lead members (121). A device according to any of the preceding claims, characterized in that trenches (201, 202; 211, 212) are formed in said plastic plate (19) along both sides of said outer leads (141, 151, 122).

Inventors: EMOTO TAKAO, EMOTO, TAKAO, EMOTO, TAKAO, C/O INTELLECTUAL PROPERTY DIV.
Applicants: TOSHIBA KK, KABUSHIKI KAISHA TOSHIBA
Application year: 1990
Main classification: H01L23
IPC codes: H01L23