Semiconductor device having radiator structure and method of producing the same

Abstract: A semiconductor device (1B) is provided with a lead frame (2) having an opening (11) at a central part thereof and a plurality of leads which are respectively made up of an inner lead (2a) and an outer lead (2b), a radiator block (3) provided under the opening (11) of the lead frame (2) and having a top surface part and a bottom part, where a gap is formed between the top surface part of the radiator block (3) and the lead frame (2), a semiconductor chip (5) mounted on the top surface part of the radiator block (3), a plurality of wires (6) bonding the semiconductor chip (5) and the leads, and a package (7) which encapsulates at least a part of the radiator block (3), the semiconductor chip (5), the wires (6) and the inner leads (2a) of the leads so that the outer leads (2b) extend outwardly of the package (7). The package (7) fills the gap between the top surface part of the radiator block (3) and the lead frame (2), and the bottom part of the radiator block (3) projects from the package (7) by a predetermined length to conduct heat generated from the semiconductor chip (5) outside the package (7).

Description: BACKGROUND OF THE INVENTION The present invention generally relates to semiconductor devices and methods of producing the same, and more particularly to a semiconductor device having a radiator structure and to a method of producing such a semiconductor device. Recently, due to the improved integration density of semiconductor devices, the amount of heat generated from the semiconductor device has become large. Hence, there are semiconductor devices having a radiator structure for releasing the heat generated from the semiconductor device. It is desirable that the semiconductor device having such a radiator structure can be produced with ease and with a high dimension accuracy. FIG.1 shows an example of a conventional semiconductor device having the radiator structure. In a semiconductor device 1A shown in FIG.1, a radiator block 3 having a flange 3a is provided at a central opening part of inner leads 2a of a lead frame 2. The radiator block 3 has an inverted trapezoidal cross section. Ends of the inner leads 2a and the flange 3a of the radiator block 3 are fixed by calking at a calked part 4. A semiconductor chip 5 is mounted on the radiator block 3 by soldering, for example, and this semiconductor chip 5 is bonded to the inner leads 2a by wires 6. In other words, a die-bonding is made using one surface of the radiator block 3 as a stage. The semiconductor chip 5, the inner leads 2a and the like are encapsulated by a resin which is molded to form a package 7, and outer leads 2b are thereafter bent in an approximate S-shape or a gull-wing shape to suit the semiconductor device 1A for surface mounting. The radiator block 3 is mounted on the inner leads 2a at the calked part 4. Hence, the thickness of the lead frame 2 must be 300 to 400 µm at the minimum in order to enable the calking. In other words, because the radiator block 3 is fixed to the lead frame 2, it is necessary to make the calking which limits the thickness of the lead frame 2. Furthermore, in order to enable the calking, the radiator block 3 must be made of a metal. On the other hand, when injecting the resin into a metal die so as to mold the resin and form the package 7, the accuracy of the position of the exposed surface of the radiator block 3 becomes poor depending on the injection pressure. That is, it is difficult to control the position of the exposed surface of the radiator block 3. In addition, resin burr or flash occurs if the position of the exposed surface of the radiator block 3 changes. The resin burr or flash may deteriorate the thermal conduction efficiency of the radiator block 3 when the semiconductor device 1A is mounted on a circuit substrate (not shown) or the like. Furthermore, if the resin burr or flash exists, the adhesion of the radiator block 3 to the circuit substrate via an adhesive agent becomes poor when the semiconductor device 1A is mounted on the circuit substrate, because the adhesive agent used would naturally have good adhesion to the metal forming the radiator block 3 and not to the resin forming the resin burr or flash. Moreover, when producing the semiconductor device 1A, the relatively heavy radiator block 3 which is made of a metal must hang from the lead frame 2 when molding the resin to form the package 7. However, the lead frame 2 is thin as described above to enable the calking, and it is difficult to stably support the radiator block 3 in the correct position during the molding process, and unwanted bends may be formed in the lead frame 2. By way of illustration of the prior art, US-A-3902148 discloses a package of the flip-chip type in which a tapered heat sink extends above the top surface of the package. Separate inner leads are used to attach the leads on the TAB tape to the chip, and two encapsulations are employed. SUMMARY OF THE INVENTION Accordingly, it is a general object of the present invention to provide a novel and useful semiconductor device and method of producing the same, in which the problems described above are eliminated or at least reduced. According to one aspect of the present invention there is provided a method of producing a semiconductor device, including the following steps: (a) fitting a bottom part of a radiator block within a tapered hole formed at the bottom of a recess of a jig; (b) positioning on the jig a lead frame which has a central opening, so that the opening is located above the top surface of the radiator block; (c) mounting a semiconductor chip on the top surface of the radiator block and bonding the semiconductor chip to the lead frame by a plurality of wires; (d) positioning at least the radiator block on a lower metal die; (e) positioning an upper metal die on top of the lower metal die so as to form a cavity between them; and (f) injecting a resin into the cavity so as to mold the resin into apackage which encapsulates at least a part of the radiator block, the semiconductor chip, the wires and the inner parts of the leads of the lead frame, so that the outer parts of the leads of the lead frame extend outwardly of the package, while the bottom part of the radiator block projects from the package by a predetermined length in order to conduct heat generated from the semiconductor chip outside the package. In accordance with an alternative aspect of the invention there is provided a semiconductor device comprising a lead frame having an opening at a central part thereof and a plurality of leads each made up of an inner part and an outer part; a radiator block provided under the opening of the lead frame, the radiator block having a top surface part and a bottom part, a gap being formed between the top surface part of the radiator block and the lead frame; a semiconductor chip mounted on the top surface part of the radiator block; a plurality of wires bonding the semiconductor chip and the leads; and a package which encapsulates at least a part of the radiator block, the semiconductor chip, the wires and the inner parts of the leads so that the outer parts of the leads extend outwardly of the package, the package filling the gap between the top surface part of the radiator block and the lead frame, and the bottom part of the radiator block projecting from the package by a predetermined length for conducting heat generated from the semiconductor chip outside the package; characterized in that the radiator block has a tapered shape, the size of the block decreasing towards the bottom surface thereof, and in that the inner parts of the leads have positioning holes adapted to locate the inner parts of the leads during the production of the semiconductor device. With the device of the present invention it is possible to obtain a satisfactory thermal conductivity by selecting an arbitrary material for the radiator block without being restricted in the selection, and the thickness of the lead frame does not need to be limited for the calking because no calking is require to produce this semiconductor device. Moreover, the mounting of the semiconductor device on a circuit substrate or the like is facilitated because the projecting length of the radiator block can be controlled with a high accuracy. According to the method of producing the semiconductor device according to the present invention, it is unnecessary to carry out a calking, and the thickness of the lead frame will not be limited for the purpose of carrying out the calking. In addition, because the jig is used to accurately position the radiator block when molding the resin to form the package, it is possible to suppress the generation of resin burr or flash during the molding process, and furthermore, it is possible to accurately control the projecting length of the radiator block by the thickness of the jig. Other objects and further features of the present invention will be apparent from the following detailed description when read in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS FIG.1 is a side view in cross section showing an example of a conventional semiconductor device; FIG.2 is a side view in cross section showing an embodiment of a semiconductor device according to the present invention; FIG.3 shows side views in cross section for explaining an embodiment of a method of producing the semiconductor device according to the present invention; FIG.4 is a diagram for explaining a palette which is used in the embodiment of the method; FIG.5 are side views showing modifications of a radiator block; and FIG.6 is a side view in cross section showing the embodiment of the semiconductor device mounted on a circuit substrate. DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG.2 shows an embodiment of a semiconductor device according to the present invention. In FIG.2, those parts which are the same as those corresponding parts in FIG.1 are designated by the same reference numerals, and a description thereof will be omitted. In a semiconductor device 1B shown in FIG.2, an opening 11 is formed at the central part of the lead frame 2 on the inner side of the inner leads 2a. The radiator block 3 has an inverted trapezoidal cross section and is positioned below the opening 11 independently of the lead frame 2. This radiator block 3 is made of a material having a satisfactory thermal conductivity compared to the resin which forms the package 7, including metals such as aluminum and non-metals such as ceramics. The semiconductor chip 5 is mounted on the radiator block 3 by solder, silver paste or the like. The semiconductor chip 5 and the inner leads 2a are bonded by the wires 6. The semiconductor chip 5, the inner leads 2a and the like are encapsulated by a resin which is molded to form the package 7. In this case, a tip end 3b of the radiator block 3 projects from the bottom surface of the package 7 by a predetermined length. A tapered part 12 is formed on the radiator block 3 toward the tip end 3b. After the resin is molded to form the package 7, the outer leads 2b are bent in the S-shape or gull-wing shape to suit surface mounting of the semiconductor device 1B on a circuit substrate (not shown) or the like. Of course, the outer leads 2b may have other shapes, and may not be bent, depending on the kind of mounting used. In this embodiment, a vertical length L1 of the outer lead 2b from the bottom surface of the package 7 is selected to 0.05 to 0.2 mm, for example. In addition, a projecting length L2 of the radiator block 3 from the bottom surface of the package 7 is selected so that L2 < L1 to suit surface mounting of the semiconductor device 1B. The surface mounting of the semiconductor device 1B on the circuit substrate will be described later. Next, a description will be given of an embodiment of a method of producing the semiconductor device according to the present invention, by referring to FIG.3. In this embodiment of the method, it is assumed for the sake of convenience that the embodiment of the semiconductor device 1B shown in FIG.2 is produced. This embodiment of the method uses a palette (jig) 21 shown in FIG.3 (A). This palette 21 has a recess 21a, a tapered hole 22 provided at the bottom of the palette 21, and positioning pins 23 provided on the palette 21 to position the lead frame 2. The palette 21 is made of a metal which has a coefficient of thermal expansion close to or approximately the same as that of the lead frame 2. For example, copper, copper-tin alloy or the like is used for the palette 21. As shown in FIG.3 (B), the tip end 3b of the radiator block 3 is fit into the hole 22 of the palette 21 and positioned. In other words, the tapered part 12 of the radiator block 3 and the tapered shape of the wall defining the hole 22 correspond, and the positioning of the radiator block 3 can be made with a high accuracy. The inner leads 2a of the lead frame 2 are respectively provided with a positioning hole, and the positioning pins 23 fit into the positioning holes of the inner leads 2a when the lead frame 2 is placed on the palette 21 as shown in FIG.3 (C). Hence, the lead frame 2 is positioned on the palette 21 by the positioning pins 23. Thereafter, the semiconductor chip 5 is die-bonded on the radiator block 3 by solder, or silver paste for example, as shown in FIG.3 (D). Further, the semiconductor chip 5 and the inner leads 2a are bonded by the wires 6. The structure shown in FIG.3 (D) is positioned within a cavity 24b1 of a lower metal die 24b shown in FIG.3 (E). A passage 25 is provided at the bottom of the cavity 24b1. One end of this passage 25 communicates to a part formed by the palette 21 and the radiator block 3, while the other end of this passage 25 is coupled to a vacuum source (not shown). Thus, the palette 21 and the radiator block 3 are positioned under vacuum suction via the passage 25. Then, an upper metal die 24a is positioned on top of the lower metal die 24b as shown in FIG.3 (E), and a resin is injected into a cavity 26 of the upper metal die 24a via a gate (not shown) so as to mold the resin and form the package 7. After the molding process, the vacuum suction is released, and the upper and lower metal dies 24a and 24b and the palette 21 are removed. As a result, the package 7 which has the tip end 3b of the radiator block 3 projecting by a predetermined length is obtained. The outer leads 2b are bent thereafter if necessary. Because the lead frame 2 and the radiator block 3 of the semiconductor device 1B are independent of each other, it is unnecessary to carry out a calking process and there is no need to limit the thickness of the lead frame 2 therefor. For this reason, the radiator block 3 can be made of a material other than metals. In addition, since the radiator block 3 is positioned by the palette 21 under the vacuum suction, it is possible to accurately control the length of the tip end 3b of the radiator block 3 projecting from the package 7 by the thickness of the palette 21. Furthermore, because the palette 21 and the radiator block 3 are positioned by the vacuum suction, it is possible to prevent the generation of resin burr or flash during the molding process. FIG.4 shows in more detail the palette 21 which is used in this embodiment of the method. FIG.4 (A) shows a side view of the palette 21, and FIG.4 (B) shows a plan view of palette 21. This palette 21 shown in FIG.4 integrally has three recesses 21a which are connected. However, for mass production purposes, it is of course possible to provide more than three recesses 21a. FIG.5 shows modifications of the radiator block 3. The radiator block 3 shown in FIG.5 (A) has slits 31 formed on the tip end part thereof. In this case, the radiator block 3 has a fin shape, and the thermal conduction efficiency can be improved because the effect is basically the same as if the fins were directly mounted on the semiconductor chip 5. The radiator block 3 shown in FIG.5 (B) has a threaded part 32 formed on the tip end part thereof. In this case, an external radiator member can be screwed onto the radiator block 3 by a screw to improve the thermal conduction efficiency. The radiator block 3 shown in FIG.5 (C) has a mounting part 33 formed on top thereof. This mounting part 33 is made of copper, for example, and the semiconductor chip 5 is mounted on the mounting part 33. A part 34 of the radiator block 3 other than the mounting part 33 is made of a metal such as aluminum which is different from the metal forming the mounting part 33. The mounting part 33 and the part 34 are connected by thermocompression bonding. In this case, it is possible to mounting the semiconductor chip 5 on the mounting part 33 not only by silver paste but also by solder. In addition, because the radiator block 3 is made mostly of aluminum, it is possible to reduce the weight of the semiconductor device 1B, and further, the surface of the aluminum can be subjected to an insulation process by an anodic oxidation. Alternatively, the mounting part 33 may be made of aluminum and the part 34 may be made of copper in FIG.5 (C). In this case, the radiator block 3 can be soldered directly when mounting the semiconductor device 1B on the circuit substrate. The radiator block 3 shown in FIG.5 (D) is made of a ceramic. In this case, the semiconductor chip 5 may be mounted on the radiator block 3 by silver paste. In addition, if a metal layer 35 is provided on top of the radiator block 3, it is possible to mount the semiconductor chip 5 on the metal layer 35 by solder. In the case of the radiator block 3 shown in FIG.5 (D), the matching with the semiconductor chip 5 is particularly satisfactory because the coefficient of thermal expansion of the ceramic radiator block 3 becomes approximately the same as that of the semiconductor chip 5. Moreover, the ceramic radiator block 3 can be produced at a low cost, and the relatively rough surface of the ceramic provides good adherence to the circuit substrate when mounting the semiconductor device 1B onto the circuit substrate. Next, a description will be given of the mounting of the semiconductor device 1B described above onto a circuit substrate, by referring to FIG.6. In FIG.6, those parts which are the same as those corresponding parts in FIG.2 are designated by the same reference numerals, and a description thereof will be omitted. In FIG.6, each outer lead 2b is connected to a contact 101 of a circuit substrate 100 by a solder 110. On the other hand, the tip end 3b of the radiator block 3 is adhered on a thermal conduction part 102 by an adhesive agent 111 having a satisfactory thermal conduction. For example, the thermal conduction part 102 is made of a metal identical to that used for the radiator block 3. Therefore, the heat generated from the semiconductor chip 5 is transferred mainly through the radiator block 3 to the thermal conduction part 102 via the adhesive agent 111, and is finally released to a ground layer 104, for example. Because the projecting length of the tip end 3b of the radiator block 3 can be controlled with a high accuracy in the present invention, it is possible to improve the mounting of the semiconductor device 1B on the circuit substrate 100. Further, the present invention is not limited to these embodiments, but various variations and modifications may be made without departing from the scope of the present invention as defined by the appended claims.

Claims: A method of producing a semiconductor device, including the following steps: (a) fitting a bottom part of a radiator block (3) within a tapered hole (22) formed at the bottom of a recess (21a) of a jig (21); (b) positioning on the jig (21) a lead frame (2) which has a central opening, so that the opening (11) is located above the top surface of the radiator block (3); (c) mounting a semiconductor chip (5) on the top surface of the radiator block (3) and bonding the semiconductor chip (5) to the lead frame (2) by a plurality of wires (6); (d) positioning at least the radiator block (3) on a lower metal die (24b); (e) positioning an upper metal die (24a) on top of the lower metal die (24b) so as to form a cavity (26) between them; and (f) injecting a resin into the cavity (26) so as to mold the resin into package (7) which encapsulates at least a part of the radiator block (3), the semiconductor chip (5), the wires (6) and the inner parts (2a) of the leads of the lead frame (2), so that the outer parts (2b) of the leads of the lead frame (2) extend outwardly of the package (7), while the bottom part of the radiator block (3) projects from the package (7) by a predetermined length in order to conduct heat generated from the semiconductor chip (5) outside the package. The method as claimed in claim 1, in which the radiator block (3) has approximately an inverted trapezoidal shape in cross section. The method as claimed in claim 1 or 2, in which the radiator block (3) is made of a material having a thermal conductivity greater than that of the resin which forms the package (7). The method as claimed in claim 3, in which the radiator block (3) is made of a metallic or a non-metallic material, including ceramics. The method as claimed in any of claims 1 to 4, further including a step (g) of forming the outer leads (2b) to an approximate S-shape so that each of the outer leads (2b) projects by a length L1 from the bottom surface of the package (7) to the end of the outer lead, measured in the direction in which the radiator block (3) projects, and in which the jig (21) is shaped so that the bottom part of the radiator block (3) projects by a length L2 from the bottom surface of the package (7), and L1 > L2. The method as claimed in any preceding claim, in which the radiator block (3) includes a plurality of slits (31) formed at the bottom part thereof. The method as claimed in any of claims 1 to 5, in which the radiator block (3) includes engaging means (32) provided at the bottom part thereof for engaging an external radiator member. The method as claimed in any of claims 1 to 5, in which the radiator block (3) has its top surface part (33) and bottom part (34) made of different metals. The method as claimed in any of claims 1 to 5, in which the top surface of the radiator block (3) is made of a metal layer (35) and the bottom part is made of a ceramic material. The method as claimed in any preceding claim, in which step (b) further comprises positioning the lead frame (2) on the jig (21) by fitting positioning pins (23) of the jig (21) into corresponding holes of the lead frame (2). The method as claimed in any any preceding claim, in which step (d) further comprises positioning the radiator block (3) by placing the bottom part of the radiator block (3) at a passage (25) of the lower metal die (24b) so that the radiator block (3) and the jig (21) are positioned under vacuum suction via the passage (25). A semiconductor device comprising a lead frame (2) having an opening (11) at a central part thereof and a plurality of leads each made up of an inner part (2a) and an outer part (2b); a radiator block (3) provided under the opening (11) of the lead frame (2), the radiator block having a top surface part and a bottom part, a gap being formed between the top surface part of the radiator block and the lead frame; a semiconductor chip (5) mounted on the top surface part of the radiator block; a plurality of wires (6) bonding the semiconductor chip and the leads; and a package (7) which encapsulates at least a part of the radiator block, the semiconductor chip, the wires and the inner parts of the leads so that the outer parts of the leads extend outwardly of the package, the package filling the gap between the top surface part of the radiator block and the lead frame, and the bottom part of the radiator block projecting from the package by a predetermined length for conducting heat generated from the semiconductor chip outside the package; characterized in that the radiator block has a tapered shape, the size of the block decreasing towards the bottom surface thereof, and in that the inner parts of the leads (2a) have positioning holes adapted to locate the inner parts of the leads during the production of the semiconductor device. The semiconductor device as claimed in claim 12, in which the radiator block (3) has approximately an inverted trapezoidal shape in cross section. The semiconductor device as claimed in either of claims 12 and 13, in which the package (7) is made of a resin, and the radiator block (3) is made of a material having a thermal conductivity greater than that of the resin. The semiconductor device as claimed in claim 14, in which the radiator block (3) is made of a metallic or non-metallic material, including ceramics. The semiconductor device as claimed in any of claims 12 to 15, in which the outer parts of the leads (2b) have an approximate S-shape, the package (7) has a bottom surface from which the bottom part of the radiator block (3) projects, each outer part of the lead (2b) having a length L1 from the bottom surface of the package to the end of the lead, in the direction in which the radiator block projects, while the bottom part of the radiator block projects a length L2 from the bottom surface of the package, and L1 > L2. The semiconductor device as claimed in any of claims 12 to 16, in which the radiator block (3) includes a plurality of slits (31) formed at the bottom part thereof. The semiconductor device as claimed in any of claims 12 to 16, in which the radiator block (3) includes engaging means (32) provided at the bottom part thereof for engaging an external radiator member. The semiconductor device as claimed in any of claims 12 to 16, in which the top surface part (33) of the radiator block (3) and the bottom part (34) of the radiator block are made of different metals. The semiconductor device as claimed in any of claims 12 to 16, in which the top surface part of the radiator block (3) is made of a metal layer (35), and the bottom part of the radiator block is made of a ceramic material.

Inventors: SAITO KOUJI, SONO MICHIO, TAKENAKA MASASHI, YOSHIMOTO MASANORI, SAITO, KOUJI, SONO, MICHIO, TAKENAKA, MASASHI, YOSHIMOTO, MASANORI, Saito, Kouji, c/o Fujitsu Limited, Sono, Michio, c/o Fujitsu Limited, TAKENAKA, MASASHI, C/O FUJITSU LIMITED, Yoshimoto, Masanori, c/o Fujitsu Limited
Applicants: FUJITSU LTD, FUJITSU LIMITED
Application year: 1992
Main classification: H01L23
IPC codes: H05K1, H05K3, H01L23, H01L33, H01L21