A mold for manufacturing plastics integrated circuits incorporating a heat sink

Abstract: A mold (10) for the manufacture of integrated circuit packages (1) of plastics, incorporating a heat sink (3,8), comprises a first half-shell (11) having a rest bottom (12) for the heat sink (8), which is associated with a metal frame (7) having a central portion carrying the integrated circuit and being held by filaments (4A), and a second half-shell (13) arranged to overlie the first (11) and having at least one elevated portion (16) which will compress said filaments (4A) on the half-shells (11,13) being laid over each other.

Description: This invention relates to a mold for manufacturing plastics integrated circuit packages, as defined in the preamble of claim 1. It is a well-known fact that, in the specific technical field to which this invention is related, many plastics containers or "packages" have a metal mass conglomerated therewith to promote dissipation of heat developed by the integrated circuit during its operation. This is the case, for instance, with packages for most power circuits. To produce such packages, the prior art has proposed several methods, of which the most widely employed one comprises the following steps. From thin sheet metal, a thin frame is formed as by some blanking process which has a central rest portion whence multiple flat filament conductors branch off, among which are so-called supporting braces. Secured on the central portion is a semiconductor integrated circuit chip having its base in direct contact with said portion. Selected metalized regions of the integrated circuit are connected electrically to said filament conductors by means of thin wires. A metal mass serving as a heat sink is soldered to the central portion of the frame, on the remote side from the integrated circuit. Lastly the frame, complete with the integrated circuit and the heat sink, is conglomerated to a body of synthetic resin formed by injecting a heat setting plastics material, such as an epoxy resin, under a high pressure into a purposely provided mold. From EP-A-0484297 falling under Article 54(3)EPC, published on 06.05.92 and having a priority date of 31.10.90, is known a mold for manufacturing a plastics package incorporating a chip bonded to a base plate and a metal lead frame fastened to the base plate. Leads of the lead frame are in contact with the base plate after welding wires to connect chip and leads. To separate the leads from the base plate, a half mold is provided with ridges which are pressed against braces of the lead frame when the mold is closed. To prevent the conductive filaments of the frame from contacting one another and being shorted out during the injection step, it matters that the resin be a highly flowable one. However, this has a drawback in that plastics flash often develops over the heat sink due to imperfect mating of the heat sink with the mold. Known are methods directed to improve the adhesion of the heat sink to the mold, e.g. through the provision of some pre-load arrangements for biasing the heat sink toward the mold. From JP-A-57191011, Patent Abstracts of Japan vol. 007, no. 041 (M-194) 18 February 1983 is known a mold for manufacturing a plastics package incorporating a chip bonded to a heat sink and a metal lead frame. A half mold is provided with a projection which is pressed against the heat sink when the mold is closed, to prevent resin flash during resin injection. From JP-A-02306639, Patent Abstracts of Japan vol. 015, no. 098 (E-104) 8 March 1991, is known a mold for manufacturing a plastics package incorporating a chip bonded to a heat-sink plate and a lead frame fastened to the heat-sink plate. To press the heat-sink plate against the bottom of the lower half-mold cavity, two pins are provided extending through holes in the upper half mold to the heat-sink plate. Such methods cannot be used, however, with integrated circuit chips of comparatively large size which be associated with extremely thin frames also performing heat sink functions. Flash is customarily removed, anyhow, by subjecting a package to a further step of abrasive processing of the exposed surface of the heat sink. It may be appreciated, however, that this is a cost-intensive and time-consuming procedure penalizing the end product. The underlying technical problem of this invention is to provide a mold which has such constructional and operational features as to yield substantially flash-free packages over the exposed surface of the heat sink, thereby overcoming the drawbacks with which the prior art is beset. This technical problem is solved by a mold as defined in the characterizing part of claim 1 A preferred embodiment is defined in claim 2. The features and advantages of a mold according to the invention will become more clearly apparent from the following detailed description of an embodiment thereof, given by way of example and not of limitation with reference to the accompanying drawing. In the drawing, Figure 1 is an enlarged scale, cross-sectional view taken through a portion of the inventive mold which is closed around a plastics package for integrated circuits, and Figures 2 and 3 are plan and sectional views, respectively, of a part of a package at an intermediate stage of its processing. With reference to the drawing, generally shown at 1 is a container or "package" formed from a plastics material to enclose and protect a semiconductor-type integrated circuit. The package 1 is made by first cutting off from thin sheet metal, as by a blanking or etching process, a thin lead frame 7 which has a supporting central portion 3 and multiple flat filament conductors branching off the central portion 3. These filaments 4, with the exception of some of them referred to as the braces and being shown at 4a in Figure 2 and joined to the portion 3 to support it, have an inward end separated from the central portion 3 but located close to it. Laid onto the portion 3 is a silicon semiconductor chip 5 in which the integrated circuit is formed. The chip base is secured on the central portion 3, and selected metalized regions of the chip 5 are connected electrically to the conductors 4 by thin wires (not shown). The frame 7 also has a depressed part which contains the central portion 3 and serves as a heat sink 8. The frame 7, complete with the chip 5, is then placed into a mold 10, and a resin, such as a heat setting polymer, is injected in a liquid state into the mold. The frame 7 is so inserted into the mold 10 as to have its reverse surface 9, i.e. the surface not carrying the chip 5, in direct contact with the rest bottom 12 of the first half shell 11. The mold 10 consists of a pair of half-shells 11, 13 mating together to overlie each other and enclosing an inner space 14 whereinto the epoxy resin is injected. The half-shells 11 and 13 have corresponding surfaces, facing each other in mating relationship, and defining a closure plane for the mold 10. According to the invention, the second half-shell 13 includes elevated portions 16 which stand proud of the surface 15. These portions rise at least 0.05 mm above said surface 15 (extending downwards in the Fig. 1 view). The first shell 11 has recessed regions opposite the elevated portions 16 and the braces 4a can be located in a space left between the elevated portions 16 and recessed regions when the mold is closed. With the two half-shells 11, 13 of the mold 10 overlying each other and the mold closed, the elevated portions 16 of half-shell 13 press the braces 4a against the bottom of the mold 10 and into the recessed regions, thereby establishing a pressure between the central portion 3 and the bottom 12 of the mold. Upon true sealing adhesion of the two half-shells of the mold 10 being achieved, the injection of liquid state resin is started. After the resin has cured, the package 1 is shaken out of the mold using conventional shake-out means. Further to cutting off those parts of the frame which join together the filaments on the exterior of the formed plastics body, the package is ready for installation on any printed circuit. It will be apparent from the foregoing description that the mold of this invention has a major advantage in that it allows plastics packages to be manufactured which are completely free of flash over the exposed surface of the heat sink. This contributes, of course, to a reduction in the production burden and the product cost.

Claims: A mold (10) for manufacturing plastics integrated circuit packages which incorporate a heat sink (8) which is associated with a metal frame (7) having a depressed central portion (3) carrying an integrated circuit chip (5) and being held by braces (4a), comprising a first half-shell (11) with a rest bottom (12) for the heat sink (8) , a second half-shell (13) arranged to overlie the first half-shell (11) such that a surface thereof mates with a corresponding surface of the first half-shell (11) and encloses an inner space (14) when the mold (10) is closed, characterized in that outside of and adjacent to said inner space (14) said first half-shell (11) has recessed regions and said second half-shell (13) has elevated portions (16) opposite the recessed regions and effective to press said braces (4a) there-between when the mold (10) is closed with the metal frame (7) inside, so as to press the depressed central portion (3) tightly against the rest bottom (12). A mold according to Claim 1, characterized in that said elevated portions (16) rise by at least 0.05 mm above the mating surface plane of the half-shells (11,13).

Inventors: PEREGO CAMILLO, PEREGO, CAMILLO
Applicants: SGS THOMSON MICROELECTRONICS, SGS-THOMSON MICROELECTRONICS S.R.L.
Application year: 1991
Main classification: H01L21
IPC codes: B29C33, B29K105, B29C45, H01L21, B29L31