Abstract:
A process for manufacturing a semiconductor device envisages the steps of: positioning a frame structure, provided with a supporting plate carrying a die of semiconductor material, within a molding cavity of a mold; and introducing encapsulating material within the molding cavity for the formation of a package, designed to encapsulate the die. The frame structure is further provided with a prolongation element mechanically coupled to the supporting plate inside the molding cavity and coming out of the molding cavity, and the process further envisages the steps of: controlling positioning of the supporting plate within the molding cavity with the aid of the prolongation element; and, during the step of introducing encapsulating material, separating and moving the prolongation element away from the supporting plate.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a process and to a system for manufacturing an encapsulated semiconductor device; in particular, the ensuing treatment will make reference, without this implying any loss of generality, to the production by molding of a power package for a semiconductor device, of the full insulated type. 
         [0003]    2. Discussion of the Related Art 
         [0004]    Power semiconductor devices, for example power MOSFETs, are known that comprise a plastic package designed to encapsulate a die of semiconductor material integrating a corresponding integrated circuit, wherein the plastic package is commonly obtained by molding. 
         [0005]    For example,  FIG. 1   a  and  1   b  show a semiconductor device  1  (in particular a power device) encapsulated in a package  2 , made of plastic material, for example epoxy resin, of the type known as JEDEC TO-220. The semiconductor device  1  comprises a die  3  of semiconductor material and a leadframe  4  set at least partially within the package  2  and designed to support the die  3  within the same package  2 , and to provide the electrical connection towards the outside of the integrated circuit within the die  3 . The leadframe  4  comprises: a metal plate (known in general as “die pad”)  5 , set entirely within the package  2  and having a top surface  5   a , to which the die  3  is coupled (for example, via interposition of adhesive material); and a plurality of leads  6 , for example three, which come out of the package  2 . In a way not illustrated, the die pad  5  is made of a single piece with one of the leads  6  (in particular, with the lead set in a central position), consequently constituting an electrode of the semiconductor device  1 , and the die  3  is connected electrically to the remaining leads  6  by means of bond wires, which extend from a respective contact pad, carried by a top surface of the die  3  not in contact with the die pad  5 , and a respective lead  6 . The package  2  moreover has a through hole  7  at an end portion thereof (opposite to the one from which the leads  6  come out), for coupling, for example, by means of a screw or rivet, of the semiconductor device  1  to a heat sink (not illustrated). In this regard, the die pad  5  transfers the heat generated in use by the circuit integrated in the die  3  towards the aforesaid heat sink. 
         [0006]    Given the need to ensure a good transfer of heat towards the heat sink, and (at least in the case of insulated packages) to insulate the die pad  5  electrically from the outside of the package  2 , during manufacture of the semiconductor device  1 , and in particular of molding of the package  2 , a controlled thickness of the encapsulating material of the package underneath the die pad  5  needs to be guaranteed (in particular of the material in contact with a bottom surface  5   b  of the die pad  5 , opposite to the top surface  5   a  to which the die  3  is coupled). The thermal and electrical performance of the resulting semiconductor device  1  can vary even considerably according to the aforesaid thickness, which is markedly dependent on the technique for manufacturing the package  2 , and in particular on the correct positioning of the leadframe  4  during the molding step. An incorrect alignment of the leadframe  4  with respect to the mold used for the formation of the package  2  can cause a degradation of the thermal performance (in terms of thermal resistance R th ) if thickness of the encapsulating material is greater than an upper specification limit (USL), or the exposure of the bottom surface  5   b  of the die pad  5  if thickness of the encapsulating material is considerably lower than a lower specification limit (LSL). If the aforesaid thickness is smaller than the lower specification limit, an inadequate flow of encapsulating material during molding may also occur, causing the creation of voids at the backside of the package  2 . 
         [0007]    In a known manner, according to the molding technique and the resulting structure of the package, power packages for semiconductor devices are divided into “full molded” and “full insulated.” In both cases, the die  3  is entirely coated with the encapsulating material, but in full molded packages areas of exposed metal may remain (for example, portions of the die pad  5  may be accessible from the outside of the package  2 ), whereas in full insulated packages the total absence of exposed metal needs to be guaranteed. It is evident that, especially in the case of full insulated packages, the presence of an excessively thin layer of encapsulating material on the backside of the device can irreparably jeopardize its performance. 
         [0008]    It is consequently necessary to arrange the leadframe  4  and keep it in a proper and pre-set position within a corresponding mold during the step of molding of the package  2 , in particular during injection of the encapsulating material and its subsequent hardening (polymerization). In the past, a wide range of molding processes have been proposed, designed to address this need. 
         [0009]    For example, one of the proposed techniques envisages the use of fixed pins (so-called “fixed-pin” technique), fixedly coupled to the mold, and such as to come to abut on opposed portions of the top and bottom surfaces of the die pad  5  within the molding cavity, thus keeping the die pad in a desired position upon closing of the mold. Once the molding step is terminated, as illustrated in  FIG. 2 , the package  2  has, however, voids  10 , which leave the die pad  5  exposed, in positions corresponding to the ones occupied by the fixed pins during molding. This technique can consequently be used for the production of full molded packages, but not for the production of full insulated packages. 
         [0010]    In the case where the production of a full insulated package is required, the process described previously can be completed with a final off-line step (i.e., one distinct from and subsequent to the molding step) of filling of the voids  10  left by the fixed pins, with an epoxy compound (commonly known as “potting”), which is subsequently cured.  FIG. 3  shows the resulting semiconductor device  1 , in which reference  11  designates the filling portions that totally close the voids  10 . The resulting process suffers, however, from a series of drawbacks, amongst which: the longer duration of the manufacturing method; the need for additional equipment and the associated additional costs; and the possibility of occurrence of reliability problems due to the fact that the epoxy compound is a material “external” to the encapsulating material that forms the body of the package. 
         [0011]    To overcome these drawbacks, an alternative molding method has been proposed, which envisages the use of retractable ejector pins. In detail, in an initial step,  FIG. 4   a , the leadframe  4  is inserted within a molding cavity  12  of a mold  13 . An input channel (or gate)  14  is in fluid communication with the molding cavity  12  and enables the introduction of encapsulating material. The leadframe  4  is kept in a desired position by means of the use of retractable pins  15 , which are brought into contact with the die pad  5  (as highlighted by the arrows in  FIG. 4   a ) so as to abut on a respective top surface  5   a  or bottom surface  5   b  thereof. For this purpose, guides  16  are provided in the mold  13 , and the retractable pins  15  can slide within the guides  16  by the action of suitable actuators (not illustrated). Subsequently ( FIG. 4   b ), encapsulating material  17  is injected within the mold  13 ; the retractable pins  15  keep the die pad  5  in the proper position so as to guarantee the thickness required by specifications of the encapsulating material  17  on the backside of the package  2 . Next ( FIG. 4   c ), once the molding cavity  12  is entirely filled with the encapsulating material, but before it polymerizes, the retractable pins  15  are retracted and moved away from the die pad  5  (causing them to slide again in the guides  16 , as indicated by the arrows in  FIG. 4   c ). The pressure of injection of the encapsulating material  17  is then increased so that it comes to occupy the empty spaces left by the retractable pins  15 , and subsequently the required compactness of the package is achieved by hardening. 
         [0012]    This method enables formation of a full insulated package with a good accuracy of the thickness of the encapsulating material  17  on the back of the leadframe  4 . However, it has the problem of rapid wear of the retractable pins  15  and of the corresponding guide within the molding cavity  12 , due to the abrasive characteristics of the encapsulating material  17  used for the manufacturing of the package  2  (in general, epoxy resin containing an inorganic part, known as filler, with abrasive properties). This aspect has a negative impact both on the costs of production and on the quality of the devices thus produced. 
       SUMMARY OF THE INVENTION 
       [0013]    One aim of the present invention is consequently to provide a process that enables the aforesaid disadvantages and problems to be overcome and in particular that enables a good precision in the control of the thickness of the encapsulating material to be obtained, as well as having reduced costs. 
         [0014]    According to one aspect of the present invention, a process for manufacturing a semiconductor device is provided, comprising a process for manufacturing a semiconductor device, comprising: positioning a frame structure, provided with a supporting plate carrying a die of semiconductor material, within a molding cavity of a mold; and introducing encapsulating material within said molding cavity for the formation of a package, designed to encapsulate said die, wherein said frame structure is provided with a prolongation element, mechanically coupled to said supporting plate inside said molding cavity and coming out of said molding cavity, and by further comprising: controlling positioning of said supporting plate within said molding cavity with the aid of said prolongation element; and during said step of introducing encapsulating material, separating and moving said prolongation element away from said supporting plate. 
         [0015]    According to another aspect of the present invention, a system for manufacturing a semiconductor device is provided, comprising a system for manufacturing a semiconductor device, comprising: a mold defining a molding cavity, designed to house a frame structure of said semiconductor device provided with a supporting plate carrying a die of semiconductor material; and an introduction unit for introduction of encapsulating material within said molding cavity for the formation of a package designed to encapsulate said die, wherein said frame structure is further provided with a prolongation element, mechanically coupled to said supporting plate inside said molding cavity and coming out of said molding cavity, and by further comprising: a positioning-control unit, configured to control positioning of said supporting plate within said molding cavity with the aid of said prolongation element; and an actuation unit, configured to separate and move said prolongation element away from said supporting plate during introduction of said encapsulating material. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0016]    For a better understanding of the present invention, preferred embodiments thereof are now described, purely by way of non-limiting example and with reference to the attached drawings, wherein: 
           [0017]      FIG. 1   a  is a schematic perspective view of an encapsulated semiconductor device of a known type; 
           [0018]      FIG. 1   b  is a cross-sectional view of the device of  FIG. 1 ; 
           [0019]      FIG. 2  is a cross-sectional view of a semiconductor device with full molded package of a known type; 
           [0020]      FIG. 3  is a cross-sectional view of a semiconductor device with full insulated package of a known type; 
           [0021]      FIGS. 4   a - 4   c  show sections of a further semiconductor device with full insulated package of a known type, in successive steps of a corresponding molding process; 
           [0022]      FIGS. 5   a - 5   d  show sections of an encapsulated semiconductor device in accordance with an embodiment of the present invention, in successive steps of a corresponding process for manufacturing by molding; and 
           [0023]      FIG. 6  is a simplified block diagram of a molding system according to a further embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0024]      FIG. 5   a  relates to an initial step of a molding process for a semiconductor device according to an embodiment of the present invention. In this and in the subsequent figures, parts that are similar to other described previously will be designated with the same reference numbers, and will not be described again in detail. In particular, a leadframe  4  of the semiconductor device  1  is positioned within the molding cavity  12  of a mold  13  of a molding apparatus (of a known type that is not herein described in detail). The leadframe  4  comprises a die pad  5 , arranged totally within the molding cavity  12 , and a plurality of leads  6  (just one of which is illustrated in  FIG. 5   a ), which come out of the molding cavity  12 . Conveniently, the die pad  5  and the leads  6  have one and the same thickness, for example of a few millimeters, and are obtained by shaping and processing of one and the same ribbon of metal material, for example, copper. 
         [0025]    The leadframe  4  further comprises a prolongation element  20  coupled to the die pad  5  inside the molding cavity  12  and coming out on the outside of the same molding cavity  12 . In particular, the prolongation element  20  comprises a connection portion  20   a , arranged within the molding cavity  12 , and a grip portion  20   b , arranged on the outside of the molding cavity  12 . The connection portion  20   a  is mechanically connected to the die pad  5  at an area with facilitated breaking (facilitated breaking area  22 ). In a preferred embodiment, the prolongation element  20  is made of a single piece with the die pad  5  (in particular, starting from the same ribbon of metal material), and the facilitated breaking area  22  is an area of weakening of the same metal ribbon situated between the die pad  5  and the connection portion  20   a . For instance, the facilitated breaking area  22  is obtained by means of removal of material and consequent reduction of the thickness of the aforesaid metal ribbon. 
         [0026]    The grip portion  20   b , which comes out of the molding cavity  12 , enables positioning, in a desired way, the die pad  5  of the leadframe  4  during molding operations. In detail, both a first end of the die pad  5 , connected integrally to the lead  6  coming out of the molding cavity  12 , and a second diametrally opposite end of the same die pad  5 , connected mechanically to the grip portion  20   b , also coming out of the molding cavity  12 , have a position established in a precise way by clamping, between a top half  13   a  and a bottom half  13   b  of the mold  13 , of the leads  6  and grip portion  20   b , respectively. The die pad  5  is hence correctly and stably positioned, in particular with an end portion thereof centered with respect to the molding cavity  12  of the mold  13 , ensuring that a repeatable and accurate thickness of encapsulating material is obtained on the backside of the die pad  5  (this thickness is designated by h in  FIG. 5   a , and is indicated by the arrows). 
         [0027]    In a subsequent step of the molding process ( FIG. 5   b ), the encapsulating material  17 , in this case an epoxy resin (or other electrically non-conductive thermosetting plastic material), is injected under pressure within the molding cavity  12 , through an input channel  14 , set for example at the point where the leads  6  come out of the molding cavity  12 . In this step, the molding cavity  12  is filled entirely with the encapsulating material, which has not, however, yet reached the desired compactness (the thermosetting process is still in progress). 
         [0028]    Next ( FIG. 5   c ), the grip portion  20   b  is pulled mechanically, moving it away from the molding cavity  12  (in the direction indicated by the arrow), for example by means of an appropriate hydraulic actuator (not illustrated). This operation causes breaking of the facilitated breaking area  22 , the separation of the connection portion  20   a  from the die pad  5 , and the formation of an empty space  23 , without encapsulating material, within the molding cavity  12 . In this step, the prolongation element  20  is not removed altogether from the mold  13 , but displaced until it is positioned exactly at the end of the molding cavity  12 , closing it laterally (in other words, the displacement stops substantially when the connection portion  20   a  comes out entirely of the molding cavity  12 ). At this point, the transfer of the encapsulating material  17  continues so as to fill the empty space  23  previously occupied by the connection portion  20   a , and then proceeds until the desired compactness is reached (completion of the polymerization process). During this step, the presence of the prolongation element  20  at the end of the molding cavity  12  prevents undesirable exit of the encapsulating material  17  from the molding cavity  12 . 
         [0029]    Next ( FIG. 5   d ), once the process for molding of the package  2  of the semiconductor device  1  is terminated, the prolongation element  20  is fully removed from the molding cavity  12  and from the molding apparatus. The mold  13  is opened, separating the top half  13   a  from the bottom half  13   b  so as to enable extraction of the semiconductor device  1 , which is now encapsulated and ready for subsequent processing steps. 
         [0030]    As illustrated in  FIG. 6 , a molding system  30  for implementation of the process previously described comprises: the mold  13 , designed to receive the leadframe  4  and the corresponding die  3  for their encapsulation with the encapsulating material  17 ; an actuation unit  32 , designed to co-operate with the leadframe  4  during the molding operations, and in particular to separate and move the prolongation element  20  away from the die pad  5 ; an introduction unit  34 , designed to control introduction under pressure of the encapsulating material  17  within the molding cavity  12  of the mold  13 ; and a position-control unit  36 , designed to co-operate with the prolongation element  20  for control of positioning of the die pad  5  within the molding cavity  12 , during the initial steps of the molding process. 
         [0031]    The process described has a number of advantages. 
         [0032]    In particular, it enables a semiconductor device  1  to be obtained, which comprises a package  2  of a full insulated type, made up of just one plastic encapsulation material, in a simple and economically advantageous way (given that no additional equipment is required), without the drawbacks of the known art (and in particular without the disadvantages associated to the use of retractable ejector pins or the like, and with a better precision with respect to the use of fixed pins). 
         [0033]    The process described ensures an accurate and controlled positioning of the leadframe  4  during molding, and consequently a repeatable and precise thickness of the encapsulating material  17  can be obtained on the backside of the package  2 . Semiconductor devices  1  are thus obtained with pre-established and repeatable mechanical and electrical characteristics, preventing an increase in the defectiveness and a decrease in the performance. 
         [0034]    Finally, it is clear that modifications and variations can be made to what is described and illustrated herein, without thereby departing from the scope of the present invention, as defined in the annexed claims. 
         [0035]    In particular, the described process can be advantageously used with further types of packages, in which control of the thickness of the encapsulating material is required. For instance, it can be used with non-insulated packages, or with any type of package (also for signal applications, and not power applications) requiring a complete encapsulation of the leadframe in the compound of encapsulating material. 
         [0036]    It is clear that different shapes can be envisaged for the package  2 , as likewise there may be envisaged a different number of leads  6  (the same electrical connections between the die  3  and the leads  6  may vary). 
         [0037]    Furthermore, the connection portion  20   a  of the prolongation element  20  may be mechanically coupled to the die pad  5  in a different way through a corresponding area with facilitated breaking, without being made of a single piece with the die pad; the molding method previously described remains in any case substantially unchanged.