Abstract:
A system for assembling electronic chips in a package, including a first lead frame defining chip reception areas; and a second lead frame defining chip coverage areas, the frames including, at least at their periphery, pairs of mutually-cooperating elements for maintaining the frames together.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a U.S. National Stage patent application based on International patent application number PCT/FR2010/052329, filed on Oct. 29, 2010, which application claims the priority benefit of French patent application number 09/58349, filed on Nov. 25, 2009, which applications are hereby incorporated by reference to the maximum extent allowable by law. 
     
    
     BACKGROUND 
       [0002]    1. Technical Field 
         [0003]    The present invention generally relates to electronic circuits and, more specifically, to the assembly of electronic chips in a package. The present invention more specifically relates to so-called clip packages in which, during their assembly, semiconductor chips having contacts on both surfaces are tightened between two conductive plates, each provided with pins of contact transfer on a same plane. 
         [0004]    2. Discussion of the Related Art 
         [0005]    Among the great number of techniques of semiconductor component packaging assembly, the so-called clip assembly is specifically adapted to chips having a small number of contacts. 
         [0006]      FIGS. 1A and 1B  are respective lateral and front views of an electronic device  1  assembled in a so-called clip SMD package (Surface Mount Device). A semiconductor chip  12  forming, for example, a vertical device (for example, a diode or a power thyristor) or an integrated circuit with a small number of contacts (typically from two to four) is assembled on a reception area  142  pertaining to a base  14 , this area having a surface area greater than the surface area of chip  12 . Plate  142  extends outside of the package in at least one coplanar pin  144  of connection to the outside. A cap  16 , called a clip, is placed on the upper surface of chip  12  and is shaped to define at least one pin  164  of connection to the outside of the package in the plane of base  14 . A chip coverage area  162  is connected to pin(s)  164  by a curved portion  166 . In the example of  FIG. 1B , the package is assumed to comprise a single connection pin  144  on the base side and a single connection pin  164  on the cap side. 
         [0007]    Bases  14  and caps  16  are made of a conductive material, most often copper. In the case of a package having one of its elements, base or cap, comprising more than one conductive pin, an insulating region (for example, made of ceramic) is provided on area  142  or  162  to dissociate the contacts. Chip  12  comprises contacts on both its surfaces, which are transferred to areas  142  and  162  of base  14  and of cap  16  by soldering or welding  18  ( FIG. 1B ). Most often, the assembly is encapsulated in an insulating resin  10  (illustrated by dotted lines in  FIG. 1A ). 
         [0008]    Packages such as illustrated in  FIGS. 1A and 1B  are obtained in batches from lead to frames comprising a large number of temporarily interconnected supports and of caps. 
         [0009]      FIG. 2  is a perspective view of an example of a conventional system for assembling chips in packages of devices  1  such as illustrated in  FIGS. 1A and 1B , 
         [0010]    An assembly frame  22  receives a first lead frame  24  comprising supports  14  connected to one another. Then, chips  12  (not shown in  FIG. 2 ) are deposited on plates  142  of supports  14  with an interposed solder layer. Then, a second lead frame  26  comprising caps  16  is placed on the assembly with, here again, an interposed solder layer. The positioning of frames  24  and  26  with respect to each other is performed by slugs  222  protruding from frame  22  and engaged into corresponding openings of frames  24  and  26 . The assembly is held by gravity. 
         [0011]    The assembly is submitted to a thermal processing to weld the chip contacts. 
         [0012]    The upper portion of the assembly may be placed in a mold to embed the different circuits in an epoxy resin. 
         [0013]    Finally, the circuits are individualized by cutting of their pins  144  and  164  ( FIGS. 1A and 1B ) of connection of frames  24  and  26 , to obtain encapsulated electronic devices  1 . 
         [0014]    The use of a handling frame  22  generates a disadvantage linked to the thermal mass of the entire assembly system. This thermal mass increases the manufacturing cost. Further, an inhomogeneity appears in the thermal gradients between the periphery and the center. 
         [0015]    Further, the fact for the assembly to only be held by gravity generates a risk of displacement during manipulations with a thermal processing, or even of deformation of the lead frames in the vertical direction in their central portions which do not rest on the frame. This adversely affects the quality of the assembly. 
       SUMMARY 
       [0016]    An object of an embodiment of the present invention is to overcome all or part of the disadvantages of usual systems of clip assembly of electronic devices. 
         [0017]    Another object of an embodiment of the present invention more specifically aims at avoiding problems due to the thermal mass of the usual support frame. 
         [0018]    An object of an embodiment of the present invention is to improve the evenness of the assembly. 
         [0019]    To achieve all or part of these objects as well as others, the present invention provides a system for assembling electronic chips in a package, comprising a first lead frame defining chip reception areas; and a second lead frame defining chip coverage areas, the frames comprising, at least at their periphery, pairs of mutually-cooperating elements for maintaining the frames together. 
         [0020]    According to an embodiment of the present invention, each pair of elements comprises a pin protruding from one of the frames and an opposite opening in the other frame. 
         [0021]    According to an embodiment of the present invention, the assembly of the lead frames by said elements is of clip type. 
         [0022]    According to an embodiment of the present invention, mutually-cooperating elements are regularly distributed in the lead frames. 
         [0023]    According to an embodiment of the present invention, said elements take part in the alignment of the lead frames with respect to each other. 
         [0024]    According to an embodiment of the present invention, the reception and coverage areas are continued by pins intended to form contact pins. 
         [0025]    The present invention also provide a method for packaging electronic chips by means of a system such as hereabove, comprising the steps of: 
         [0026]    arranging electronic chips on said reception areas, with an interposed solder layer; 
         [0027]    arranging the second connection frame on the first one with an interposed second solder layer; 
         [0028]    submitting the assembly to a thermal processing; and 
         [0029]    scribing the obtained packages. 
         [0030]    According to an embodiment of the present invention, the chips are encapsulated before scribing of the lead frames. 
         [0031]    The present invention also provides a lead frame of a conductive material defining semiconductor chip reception areas of a system such as hereabove, comprising square or rectangular openings intended to receive pins protruding from the second lead frame defining caps covering the chips. 
         [0032]    The present invention also provides a lead frame of a conductive material of a system defining caps for covering semiconductor chips supported by areas for receiving the first lead frame, comprising pins protruding from a surface and intended to cooperate with the first lead frame. 
         [0033]    The foregoing objects, features, and advantages of the present invention will be to discussed in detail in the following non-limiting description of specific embodiments in connection with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0034]      FIGS. 1A and 1B , previously described, are lateral and top views of an example of an electronic device of the type to which the present invention applies; 
           [0035]      FIG. 2 , previously described, is a perspective view of a usual system for assembling chips in a clip package; 
           [0036]      FIG. 3  is a partial perspective view of an embodiment of a lead frame supporting caps; 
           [0037]      FIG. 3A  is a partial cross-section view of the frame of  FIG. 3  along line A-A; 
           [0038]      FIG. 4  is a partial perspective view of an embodiment of a lead frame supporting chip supports; 
           [0039]      FIG. 5  is a partial perspective view of the assembly of the frames of  FIGS. 3 and 4 ; and 
           [0040]      FIG. 6  is a partial lateral cross-section view of the assembly of  FIG. 5 . 
       
    
    
     DETAILED DESCRIPTION 
       [0041]    The same elements have been designated with the same reference numerals in the different drawings. For clarity, only those elements and steps which are useful to the understanding of the present invention have been shown and will be described. In particular, the forming of the semiconductor chips and of their contacts has not been detailed, the present invention being compatible with usual techniques. Further, the soldering and encapsulation operations have not been detailed, the present invention being here again compatible with usual techniques. 
         [0042]    It could be devised to improve the hold on frame  22  ( FIG. 2 ) by placing nuts or similar clamping means on slugs  22 . However, this would solve neither the problem of thermal mass, nor that of a possible deflection in the central portion. 
         [0043]    It is provided to avoid using a frame to support and hold together the lead frames to be assembled with interposed semiconductor chips. For this purpose, elements are provided in each lead frame, each of these elements being intended to cooperate, for example, as a clip, to block the respective positions of the two frames with respect to each other.  FIG. 3  is a perspective view of an embodiment of a frame portion  36  of caps  16  for covering chips in a “clip”-type package assembly. 
         [0044]      FIG. 3A  is a cross-section view shown a portion of frame  36  at the level of line A-A of  FIG. 3 . 
         [0045]      FIG. 4  is a perspective view of an embodiment of a frame portion  34  of supports  14  for receiving chips. For simplification, the chips have not been shown in  FIGS. 3 and 4 . 
         [0046]    Each support  14  comprises, as previously, an area  142  for receiving a chip and one or several pins  144  intended to transfer chip contacts to the outside (two pins in the example of  FIG. 4 ). 
         [0047]    Each cap  16  comprises, as previously, an area  162  for covering the chip and one or several contact transfer pins  164  connected to area  162  by one or several curved connection portions  166  (a single portion in the example of  FIG. 3 ). 
         [0048]    Lead frames  34  and  36  comprise, at their periphery, mutually-cooperating elements for holding the frames together. 
         [0049]    For example, frame  34  comprises openings  42  intended to cooperate with clip-shaped pins  44  made opposite thereto in gate  36 . The clips are for example obtained by stamping, similarly to the way in which the different caps  16  are obtained. In the example of  FIG. 3A , clips  44  have a folded curved shaped to improve the hold. Preferably, and as illustrated in  FIG. 3A , clips of different orientations are provided to properly align frames  34  and  36  with respect to each other. 
         [0050]      FIGS. 3 and 4  further show circular openings  362  and  342 . Such openings are usual and are intended for the step-by-step indexing of machines for placing the chips. 
         [0051]    As a variation, clips may be provided on the side of frames  34  and  36  and openings may be provided opposite thereto in the other frame. However, clips on a single lead frame are sufficient. 
         [0052]    The shape of openings  42  may, instead of being square or rectangular, as shown, rather be oblong, while preferably having linear edges capable of cooperating with pins  44  to clamp the frame together. Further, the tip of pins  44  may be beveled to be easier to introduce into openings  42 . The use of square or rectangular clips prevents the rotation of a frame with respect to the other. 
         [0053]    Frames  34  and  36  are obtained by stamping and scribing and require no machining. 
         [0054]      FIG. 5  is a partial perspective view of assembled lead frames  34  and  36 . 
         [0055]      FIG. 6  is a partial cross-section view of the obtained assembly. 
         [0056]    For simplification, the chips and the corresponding solder areas have not been illustrated in  FIGS. 5 and 6 . 
         [0057]    As a specific embodiment, the interval between the supports and the caps is of a few tenths of a millimeter (for example, between 0.2 and 0.4 millimeter). 
         [0058]    An advantage of the described embodiment is that the mounting assembly may be manipulated until the scribing without requiring any support frame. Accordingly, there is no further thermal mass problem linked to this frame. 
         [0059]    Further, the balance of the assembly system weight is improved. 
         [0060]    Further, by a proper distribution of the clips at the periphery of the frames, or even by the provision of clips regularly distributed across the lead frame surface (for example, in central areas  45 ,  FIG. 4 ), the evenness of the mounting assembly is improved. 
         [0061]    Specific embodiments of the present invention have been described, and different variations and modifications will readily occur to those skilled in the art. In particular, the shapes and dimensions to be given to the clips of assembly of the lead frames may vary according to the application. Further, the practical implementation of the present invention based on the functional indications given hereabove is within the abilities of those skilled in the art, using usual techniques for forming lead frames of semiconductor chips as well as usual encapsulation and welding methods. 
         [0062]    Such alterations, modifications, and improvements are intended to be within the spirit and scope of the invention. Accordingly, the foregoing description is by way of example only and is not intended as limiting. The invention is limited only as defined in the following claims and the equivalents thereto.