Abstract:
An improved leadframe for packages of integrated power devices which, by virtue of its configuration, allows to press the dissipator on the bottom of the shell during the molding of the plastic case, without the dissipator having exposed portions of its inner face (which is in contact with the chip). In order to achieve this, the leadframe according to the invention comprises a monolithic body which defines a perimetric frame, the leads and the dissipator. The dissipator extends in a depressed plane with respect to the frame and is connected to the frame and to the leads in at least three step-like points which are mutually spaced and non-aligned. During the molding of the plastic case, a pressure is exerted on the frame and is transmitted to the dissipator by the three step-like points, so that the dissipator is effectively pressed flat against the bottom of the mold without using pushers which pass through the plastic case.

Description:
This application is a continuation of application Ser. No.  08 / 228 , 649 , filed Apr.  18 ,  1994 , now abandoned, which is a reissue of Ser. No.  07 / 656 , 386 .  
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to an improved leadframe for packages of integrated power devices. In particular, the present invention is related to medium-power integrated circuits. 
     As is known, in the manufacture of integrated power devices, the chip of semiconductor material which integrates the electronic components is welded to a dissipator which is connected to a leadframe. Generally, the dissipator, which is thicker than the leadframe and is arranged on a different plane, is manufactured separately and is connected mechanically to the leadframe so that its face which is to be welded to the chip is directed toward the leads. Then, after the provision of the interconnections between the chip and the leads or terminals, a plastic case of epoxy resin, with a generally parallelepipedal shape, is molded on one face of the dissipator and completely embeds the chip. In particular, the plastic case only partially covers the face of the dissipator which is in contact with the chip (the inner face), whereas the opposite face of the dissipator must remain as free as possible from resin traces, in order to ensure the required heat dissipation, particularly when the dissipator is fixed to a further external dissipation structure. This entails on one hand the need to prevent as much as possible the resin of the plastic case from reaching the outer face of the dissipator during molding, and on the other hand the need to clean said face from any residues (deflashing). 
     In order to reduce the residues on the exposed or outer side of the dissipator, according to the known solutions, the dissipator is pressed against a bottom of the mold by means of pushers provided in the mold of the plastic case, so as to prevent the penetration of the resin, which is injected in the fluid state and under pressure, between the outer face of the dissipator and the bottom of said mold. However, this solution is disadvantageous. In order to press the dissipator flat so that it adheres as much as possible to the bottom of the mold without lifting in any point, three pushers which act on the “inner” face of the dissipator are in fact used: specifically, one of the pushers acts on a portion of the dissipator which is thus external to the perimeter of the plastic case, and the other two pass through the resin, so that the plastic case has notches or recesses which are not filled with resin and reach the inner face of the dissipator, exposing part thereof. 
     A finished package manufactured in the described manner can be seen for example in FIG. 1, wherein the reference numeral  1  indicates the package as a whole,  2  indicates the dissipator,  3  indicates the molded epoxy-resin plastic case, and  4  indicates the leads or terminals, which are already bent. The reference numeral  5  indicates the portion of the dissipator which is not covered by the plastic of the case  3 , since a first pusher acts thereon and laterally delimits the perimeter of the plastic case, whereas the numerals  6  indicate cavities in the resin which are caused by the presence of the pushers in the mold. Said cavities expose further portions of the inner face of the dissipator, only one of which, indicated by  7 , is visible in the figure. The reference numeral  8  indicates the hole for the passage of the screw for connection to an external structure or plate for increasing heat dissipation. 
     However, the exposure of even only part of the inner face of the dissipator is unwanted, since said face is not protected against external influences during the further manufacturing steps (e.g. nickel-plating for the coating of the leads), and in particular it facilitates the penetration of humidity inside the package, reducing the life and therefore the reliability of the device. 
     Due to the presence of the pushers, the mold furthermore has a relatively complicated and therefore expensive configuration. 
     Nonetheless, part of the resin still succeeds to infiltrate below the dissipator, so that it must be removed. 
     SUMMARY OF THE INVENTION 
     Given this situation, the aim of the present invention is to provide an improved leadframe for packages of integrated power devices which can solve the disadvantages of the prior art. 
     Within the scope of this aim, a particular object of the present invention is to provide an improved leadframe which has such a configuration that no pushers which pass through the resin and cause interruptions therein are required during the molding of the plastic case. 
     In particular, an object of the present invention is to provide a leadframe by virtue of which, after the molding of the plastic case, the inner face of the dissipator which is in contact with the chip is completely covered by the resin, so that it is protected against external contaminating agents, avoiding thus the deposit of resin residues on the outer side of the dissipator. 
     An important object of the present invention is to provide a leadframe which allows to obtain a significant simplification in the mold for the plastic case, with a reduction in manufacturing costs. 
     Not least object of the present invention is to provide a leadframe which does not entail modifications in the other steps of the manufacture of the integrated device. 
     This aim, these objects and others which will become apparent hereinafter are achieved by an improved leadframe for packages of integrated power devices. 
     The present furthermore relates to an intermediate structure for obtaining a non-planar leadframe, to a power package which comprises the non-planar leadframe according to the invention, and to a process for the manufacture of the non-planar leadframe and the molding of the plastic case. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The characteristics and advantages of the invention will become apparent from the description of a preferred embodiment, illustrated only by way of non-limitative example in the accompanying drawings, wherein: 
     FIG. 1 is a perspective view of a package for integrated power devices which uses a leadframe with dissipator of a known type; 
     FIG. 2 is a perspective view of the leadframe according to the invention; 
     FIG. 3 is a plan view of the intermediate structure for obtaining the leadframe according to the invention; 
     FIG. 4 is a bottom plan view of the leadframe according to the invention after the molding of the plastic case; and 
     FIG. 5 is a partially sectional lateral elevation view of the structure of FIG. 4, taken along the line V—V. 
     FIG. 1 is not described hereinafter; reference to the above description is made for said figure. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     With reference to FIG. 2, the leadframe according to the invention is generally indicated by the reference numeral  10  and is part of a strip which comprises for example ten of said leadframes, all identical and arranged side by side. In detail, the leadframe comprises a frame  11  which, according to the invention, extends along a substantially rectangular closed line which defines the outer perimeter of the leadframe. The sides of the frame indicated by  11 a are in common with the adjacent leadframes (not illustrated in the figure) of the same strip. The frame  11  supports the leads  12  (eleven in the illustrated example), some of which are connected to the side  11 b of the frame at their tip, whereas all the leads are connected to an intermediate portion of the sides  11 a of the frame by means of segments  13  which define an interconnection line. Similarly to the prior art, the lead interconnection segments  13  are intended to be removed, together with the frame  11 , after the execution of the plastic case. 
     The leadframe according to the invention furthermore supports a depressed portion  15 , which defines the dissipator, which is manufactured monolithically with the rest of the leadframe and is connected thereto at three points by means of connections  16 a- 16 c and fingers  17 a,  17 b. In particular, the dissipator is substantially shaped like a T, the stem  15 a whereof extends in the space not occupied by the leads (see in particular FIG. 3, which illustrates the leadframe prior to the deformation and lowering of the dissipator, but which also substantially corresponds to the plan view of the deformed leadframe, apart from a slight downward shift of the dissipator toward the side  11 b of the frame due to the deformation of the connections  16 a,  16 b and downward shift of the dissipator toward the side  11 b of the frame due to the deformation of the connections  16 a,  16 b and  16 c so as to form steps ) and is connected to the central lead by means of the step-like connection  16 c. The transverse portion or side of the T, which is indicated by  15 b, is instead connected to the side  11 c of the frame by means of the connections  16 a,  16 b, which extend from the corners of the longitudinal ends of the side  15 b which face the side  11 c; said connections  16 a,  16 b are connected to said side  11 c of the frame by means of fingers  17 a,  17 b. In particular, as is clearly visible in FIG. 3, the fingers  17 a,  17 b (which preferably have a tapered portion  18  suitable for facilitating the subsequent separation of the frame from the package) extend from the side  11 c in a direction which is substantially perpendicular thereto toward the side  11 b, then form a 180° curve and continue with the connections or sides  16 a,  16 b, which are then orientated toward the side  11 c and connect to the dissipator  15 . In practice, the fingers  17 a,  17 b and the connections  16 a,  16 b form an S which is intended, as described in greater detail hereinafter, to be embedded inside the plastic case, so as to minimize the probability of the entry of humidity toward the inner face of the dissipator. 
     The leadframe according to the invention, illustrated in FIG. 2, is obtained, as mentioned, starting from an intermediate planar monolithic structure (monoframe) which is illustrated in FIG.  3 . Said structure, which is obtained by means of conventional methods starting from a strip of appropriate material, typically copper, is deformed mechanically (e.g. by drawing) so as to bend the connections  16 a,  16 b and  16 c and lower the dissipator with respect to the plane of the leads. The lowering of the dissipator is appropriately chosen so that the difference in level between said dissipator and the plane of the frame and of the leads is slightly larger than the depth of the shell against the bottom of which the dissipator is pressed. In practice, preferably, once the dissipator has been placed against the bottom of the related shell, the frame, and in particular the connections  16 a- 16 c, protrude vertically beyond the edge of the shell by 1-2 hundredths of a millimeter, so that after the closure of the mold the pressure exerted on the frame and on the outer part of the leads is transmitted to the dissipator by means of the connections  16 a- 16 c, pressing the dissipator on three points against the bottom of the shell. By virtue of the action on said three points, the resin cannot practically penetrate between the bottom of the mold and the dissipator, except for unavoidable and very small residues, which can be easily removed by means of the conventional deflashing method. The fact should be stressed that since the pressure is exerted directly on the frame, (which is external to the mold and is not affected by the plastic case, and is transmitted through the fingers  17 a,  17 b and the internal portion of the lead which is connected to the connection  16 c, as well as through said connections  16 a- 16 c), the plastic case has no openings or interruptions which expose even only part of the inner face of the dissipator. 
     FIGS. 4 and 5 illustrate the package immediately after the molding of the plastic case, so as to clearly illustrate the position of the dissipator with respect thereto. In particular, FIG. 4 illustrates the outer face of the dissipator  15  (which is opposite to the one visible in FIG. 2, onto which the chip is welded), part of the connections  16 a- 16 c, which are partially deformed due to the pressure which acts during the closure of the mold, and the plastic case  20 . As can be seen from the comparison of FIGS. 4 and 5, the dissipator has no parts which protrude beyond the lateral perimeter of the plastic case, since said plastic case completely covers the internal face of the dissipator and, on its rear side, which is shown in FIG. 4, it extends to the side of the dissipator and flush with its outer face. 
     Furthermore, as can be seen in particular in the right sectional part of FIG. 5, only the ends of the fingers  17 a,  17 b, which are to be removed (when the frame  11  and the segments  13  are separated from the case  20  and from the leads  12 ), protrude from the plastic case. Consequently, when the package is finished, only two small rectangles, related to the cross-section of the fingers  17 a,  17 b at the tapered portions  18 , flush with the wall of the plastic case  20 , will be visible on the side of the case visible in FIG.  5 . However, these points do not constitute a possible entry path for humidity toward the inner face of the dissipator, due to the S-shaped configuration of the sunk part of the fingers  17 a,  17 b and of the connections  16 a,  16 b, which defines a convoluted path inside the case. 
     Finally, it should be noted that the plastic case  20  has two lateral recesses  21 , so that the package can be fixed to an external dissipation structure. However, said recesses are not in contact with any part of the dissipator, so that they do not constitute access paths for humidity. 
     As can be seen from the above description, the invention brilliantly achieves the intended aim. An improved leadframe for packages of integrated power circuits has in fact been provided which does not require the use of pushers which pass through the resin of the plastic case to press the dissipator flat against the bottom of the mold. Consequently, there is no exposure of any part of the inner face of the dissipator, thus minimizing the access paths of humidity and at the same time avoiding as much as possible the penetration of resin below the dissipator. Any residues of resin which might nonetheless reach the outer surface of the dissipator are in any case small enough to be eliminated without great expenditure. 
     Furthermore, by virtue of the elimination of pressers acting inside the mold, said mold has a simpler configuration, so that its manufacture is facilitated and therefore more economical, to the advantage of the overall manufacturing costs of the finished integrated device. 
     The fact is stressed that despite the smaller surface of the dissipator according to the invention with respect to the known solutions, heat resistance is unchanged, and only heat capacity is reduced, so that the solution according to the invention is less advantageous from a thermal point of view only for integrated devices with pulsed-type operation. 
     The leadframe bending step does not entail high costs or expensive machinery, and the molding of the case is indeed simplified, as already mentioned, without requiring modifications in the other steps of the manufacturing process, so that the finished device has manufacturing costs which are compatible with the applications although it more reliable. 
     The invention thus conceived is susceptible to numerous modifications and variations, all of which are within the scope of the invention concept. In particular, the fact is stressed that although the illustrated solution, with three connecting points between the dissipator and the frame, is the most advantageous as regards on one hand the possibility of effectively compressing the dissipator flat on the bottom of the mold and on the other hand the simplicity of the structure, other connecting points, for example halfway along the side  15 b which faces the frame  11 c, may also be provided if this is necessary or advantageous. 
     All the details may furthermore be replaced with other technically equivalent ones.