Combination of an electronic semiconductor device and a heat sink

Abstract: A combination of an electronic semiconductor device (13') comprising a metal plate (21') and a plastics body (20') which encapsulates the metal plate leaving at least a major surface (21a) thereof exposed, a heat sink (10'), and means of fastening the heat sink to the device is disclosed. To enable securement of the heat sink on the device without any external fastening arrangement having to be used, and without unduly straining the solder spot of the device pins to a printed circuit, the device is provided with undercut regions (22') on opposite sides adjacent to the exposed surface (21a) of the plate for releasable engagement by the fastening means (11').

Description: This invention relates to mounting structures for electronic circuits, and in particular, to a combination of an electronic semiconductor device having a plastics body which includes a metal plate with a major portion of its surface left exposed, and a heat sink attached to the electronic device so as to be in contact with the exposed surface portion of the plate. An electronic semiconductor device of the kind outlined above is a composite structure typically adapted to accommodate an integrated circuit thereon. As is known, an integrated circuit is implemented on a die (chip) of a semiconductor material having a surface area of a few square millimeters, and requires, for its connection to an external circuit such as a printed circuit on a board made of an insulating material, a special holding structure which also enables the circuit to be interconnected electrically. A known structure that fills this demand is basically comprised of a plastics body enclosing the chip, the chip being connected, as by thin wire leads soldered to purposely provided metallized areas of its surface, to corresponding terminal conductors led out of the plastics body. In the instance of power integrated circuits, that is devices designed to operate on comparatively large currents and accordingly liable to become heated to a considerable extent, such structures further include a metal plate whereby the chip secured thereon can transfer the heat developed during its operation to the outside. The manufacture of the last-named structures provides for the plate to be formed from sheet metal, e.g. a copper sheet, by a blanking process; the chip is then attached to the metal plate either by a soldering process using a low melting point solder, such as a lead-tin solder, or by gluing using a suitable adhesive, such as an epoxy-based adhesive; thereafter, a set of metal strips intended to form the device pins or terminal conductors, and which have been blanked off sheet metal but are still held together by interconnecting bridge portions, are mounted on the plate in an electrically isolated fashion therefrom; thin wires, usually golden wires, are then soldered with one end to the metallized areas of the chip using a low melting point solder, and with the other end, to the ends of the metal strips by a process wherein heat and ultrasound are applied simultaneously; subsequently, the whole is placed in a suitable mold whereinto a plastics material, such as a thermosetting epoxy resin, in a liquefied state is poured; a resin curing step yields then a solid plastics body encapsulating the above-described elements but for one face of the metal plate and part of the metal strips, i.e. the device pins, and the interconnecting bridges therebetween; the latter are then removed as by blanking to yield the finished electronic device. In the instance of devices intended for surface mounting, that is for attachment to a printed circuit board by soldering their pin ends to specially arranged metallic areas on the board surface, the pins are bent such that their end portions will all lie in the same plane as one face of the device. This face may alternatively be the exposed surface of the metal plate or the surface of the plastics body opposite to it. A prior art device according to the preamble of claim 1 is disclosed in EP-A-0 484 180. The bend direction of the pins, and hence the face of the device which is to contact the printed circuit board, is selected to suit the device own heat dissipation requirements in a specific circuit application. Where the amount of heat to be so dissipated is relatively small, or where a broad metal surface is available on the board for heat dissipation purposes, the pins are bent to cause the exposed surface of the plate to contact the board, specifically a metallic area to which the plate is soldered simultaneously with the pin soldering operation. With a larger amount of heat to be dissipated, or where it is undesirable or impossible to use a broad surface of the board for heat dissipation, the pins are bent in the opposite direction and a large-size heat sink is attached to the exposed surface of the plate, which will locate here in the remote plane from that of contact with the board. The last-described situation is illustrated by Figure 1 of the drawings that accompany this specification. For securing the heat sink, indicated there at 10, it is common pratice to use a clip 11 affixed to the board 12 and effective to push, with a middle portion 11b thereof, the heat sink 10 centrally against the board, and therefore, against the metal plate 13a of the intervening electronic device, designated 13. In the example shown, the clip 11 is a shaped strip of spring steel having enlarged ends 11a arranged to fit in respective openings 12a of appropriate shape formed in the board 12. This mounting arrangement has, however, some drawbacks. In particular, it requires that two areas of the board be made available for applying the clip, which detracts from the compactness of the printed circuit as a whole and possibly from the quality of the electric contact between the device pins, designated 13b, and the printed circuit, shown at 12b, because the pressure exerted on the heat sink 10 by the clip 11 will be transferred to the solder spots, not shown, of the pins 13b in the purposely provided areas 12c of the printed circuit. It is the object of this invention to provide a combination of an electronic semiconductor device, a heat sink and attachment means, which generally enables the heat sink to be attached to the electronic device without any assistance from external fasteners and without unduly straining the pin-to-printed circuit solder spots. This object is achieved by a combination as generally defined and characterized in the first of the appended claims to this specification. The invention can be more clearly understood by making reference to the following detailed description of an exemplary, and hence in no way limitative, embodiment thereof, to be read in conjunction with the accompanying drawings, in which: Figure 1 is an exploded perspective view, drawn to an enlarged scale, of a conventional combination of an electronic device, heat sink, and attachment means as previously described; Figure 2 is an enlarged perspective view of a conventional electronic device; and Figure 3 shows in an exploded perspective view, drawn to an enlarged scale, a combination according to this invention. A suitable conventional electronic semiconductor device for use in the combination of this invention would be structurally much like that shown in Figure 2. This has a body 20 made of a plastics material which encapsulates a metal plate 21, leaving a major surface, not visible in the Figure, of the plate and two edge areas 22 on the opposite side from this major surface exposed. Two sets of electric contact terminals 23 juts out from opposite sides of the body 20 and are bent twice such that their end portions will lie in the same plane as the exposed surface of the plate. The device shown in Figure 2 is mounted onto a printed circuit board, not shown, by soldering the exposed surface of the plate 21 and the end portions of the contact terminals 23 at specially provided metallic areas of the board itself. A mold and a manufacturing method for a structure of this type are described in Italian Patent Application MI91A003440, filed by this Applicant on December 20, 1991. The edge areas 22 unoccupied by the plastics are provided by specially designing the manufacturing mold to hold the metal plate 21 closely against the bottom of the mold cavity, thereby preventing the resin in a liquefied state from seeping in during its injection into the mold. Shown in Figure 3, where similar or corresponding parts to those shown in Figures 1 and 2 are denoted by the same reference numbers with a prime added, is an electronic semiconductor device 13' which is adapted for installation on a printed circuit board 12' by having the end portions of its pins 13b' soldered to respective metallic areas 12c' of the printed circuit 12b' on the board 12'. As can be seen, this device only differs from that shown in Figure 2 by that the pins 13b' are first bent in the opposite direction from that of the device in Figure 2, so that the exposed surface 21a of the metal plate 21' will now locate on the remote side from the mounting face of the device 13' to the board 12'. The two exposed edge areas 22' of the plate define here two undercut regions which are utilized, in accordance with the invention, to attach a heat sink 10'. This is accomplished, in the illustrated embodiment, using a clip 11', similar to that of the conventional combination shown in Figure 1 but having its ends 11a' bent inwardly so as to engage the undercut regions 22' from the device 13'. The various parts making up the inventive combination are assembled by first securing the device 13' on the board 12', as by soldering its pins 13b' to the metallic areas 12c', arranging the heat sink 10' to bear with a flat surface 10a thereof on the exposed surface 21a of the device metal plate 21', and then applying the clip 11', as by deforming it elastically until it snaps with its end portions lla' over the inner undercut regions 22' of the plate 21'. The heat sink can be disassembled by suitably deforming the clip 11' to release it from the undercut regions 21'. Thus, a structure has been provided which can dissipate a large amount of heat and has the same requirements of printed circuit board area as a device with no external heat sink, and which imposes no particular strain on the solder spots of the device pins. It matters to observe that the combination of this invention does not involve for its implementation the provision of any specific electronic devices, it admitting instead of devices made in the same molds as are employed for manufacturing devices intended for mounting with the metal plate in contact with the board surface on condition, of course, that they have exposed edge areas of the plate on the remote side from the mounting face, and this using the same pin finishing and bending equipment. Understandably, the invention may be also applied just as advantageously to devices other than the one illustrated, provided that they are formed, on opposite sides adjacent to the exposed surface of the plate, with grooves or recesses constituting undercut regions for the securement of the external heat sink fastening means.

Claims: Combination of an electronic semiconductor device (13') comprising a metal plate (21') and a plastics body (20') encapsulating the metal plate to leave at least a major surface (21a) thereof substantially exposed, a heat sink (10'), and fastening means (11') effective to hold a surface (10a) of the heat sink in heat-conduction contact with the exposed surface (21a) of the metal plate, characterized in that the device has undercut regions (22') on opposite sides adjacent to said exposed surface (21a), and that the fastening means (11') releasably engage the undercut regions (22'). Combination according to Claim 1, characterized in that each of the undercut regions (22') comprises an edge area of the metal plate (21') surface opposite from the exposed surface (21a). Combination according to either Claim 1 or 2, characterized in that the fastening means comprise a clip (11') in the form of a shaped strip of spring steel having a middle portion (11b') adapted to engage a central region of the heat sink which is opposite from the surface of heat-conduction contact with the exposed surface (21a) of the plate (21') and ends adapted to engage the undercut regions (22'), thereby pushing the heat sink (10') against the exposed surface (21a) of the plate. Combination according to any of the preceding claims, characterized in that the electronic semiconductor device has a plurality of electric contact terminals (13b') led out from opposite sides of the plastics body adjacent to the exposed surface of the plate, said terminals being soldered to a printed circuit board (12'). Combination according to any of the preceding claims, characterized in that the plurality of electric contact terminals (23) are bent in the opposite direction to ones whose end portions locate substantially in the same plane as the exposed surface of the plate (21) and whereby the exposed surface of the plate (21,21') locates on the opposite side from the device mounting face to the printed circuit board and the undercut regions engaged by the fastening means (14') of the heat sink (10') are formed by the edge areas (22,22') of the plate (21,21').

Inventors: CASATI PAOLO, MARCHISI GIUSEPPE, CASATI, PAOLO, MARCHISI, GIUSEPPE
Applicants: SGS THOMSON MICROELECTRONICS, SGS-THOMSON MICROELECTRONICS S.R.L.
Application year: 1993
Main classification: H01L23
IPC codes: H01L23