Abstract:
Disclosed are spring structures that provide solderless electrical connections in semiconductor die packages. An exemplary spring structure comprises a first portion adapted to make an electrical connection to a conductive region of a semiconductor die, a second portion adapted to make an electrical connection to a conductive region of a leadframe, and a third portion disposed between the first and second portions. During a molding process, the third portion is compressively strained to impart forces to the first and second portions that maintain these portions in contact with the conductive regions of the die and leadframe. After the molding material sets, the third portion remains in a state of compressive strain, and imparts forces on the first and second portions that maintain the electrical connections. The spring structure may be made of less expensive materials, and does not require cleaning, fluxing, or reflowing, thereby reducing manufacturing cost and time.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
       [0001]    NOT APPLICABLE 
       BACKGROUND OF THE INVENTION 
       [0002]    Small semiconductor die packages are widely used in electronic devices, such as computers, cell phones, televisions, appliances, etc. Such a semiconductor die package typically comprises a semiconductor die having its back surface mounted to a leadframe, a plurality of wire bonds that connect pads at the top surface of the die to respective leads of the leadframe, and a molding material disposed over the wire bonds, die, and leadframe. For power transistor applications, where there may only be one to three large pads on the die&#39;s top surface, there may be several wire bonds used per pad, and/or a soldered-on die clip may be used to connect a die pad to a lead. There continues to be pressure in the electronics industry to reduce the time and cost of manufacturing semiconductor die packages. 
       BRIEF SUMMARY OF THE INVENTION 
       [0003]    As part of making their invention, the inventors have recognized that using multiple wire bonds per pad and solder-on die clips adds significant time and costs in manufacturing semiconductor die packages. Multiple wire bonds per pad use significant wire bonding material, which generally comprises expensive gold material, and the solder-on die clips require cleaning, fluxing, and reflowing steps. The solder-on die clips also require the formation of a solderable metal layer on the die pad, which requires an additional processing step. Each of the above processing steps adds time and cost. Also as part of making their invention, the inventors have discovered that the solder-on die clips and the multiple wire bonds per pad can be replaced by an electrically conductive spring structure (e.g., spring clip) that is compressed against a conductive region of the die and a conductive region of a leadframe during the molding process, and held in a compressed state by the solidified molding material to provide an electrical connection between the die and the leadframe. The spring clip provides an electrically conductive structure that has a first portion abutting an electrically conductive region of the die, a second portion abutting an electrically conductive region of the leadframe, and a third portion located between the structure&#39;s first and second portions, where the third portion is compressively strained to impart forces to the first and second portions that maintain these portions in contact with the conductive regions of the die and leadframe. The spring clip may be made of less expensive materials, and does not require cleaning, fluxing, or reflowing steps, thereby reducing manufacturing cost and time. 
         [0004]    Accordingly, a first general embodiment of the invention is directed to a semiconductor die package comprising a leadframe, a semiconductor die, an electrically conductive structure, and a body of molding material. The leadframe has a first electrically conductive region. The semiconductor die has a first surface, a second surface attached to a portion of the leadframe, and a first electrically conductive region disposed on the die&#39;s first surface. The electrically conductive structure has a first portion abutting the die&#39;s first electrically conductive region, a second portion abutting the leadframe&#39;s first electrically conductive region, and a third portion located between the structure&#39;s first and second portions, with the third portion being compressively strained. The body molding material is disposed over at least a portion of the first electrically conductive structure, over at least a portion of the die&#39;s first surface, and over at least portions of the leadframe&#39;s first and second electrically conductive regions. The body molding material maintains the third portion of the conductive structure in a compressively strained state. 
         [0005]    Another general embodiment of the invention is directed to a method for forming a semiconductor die package, the method comprising attaching a semiconductor die to a conductive region of a leadframe, and assembling an electrically conductive structure with the die and leadframe such that a first portion of the electrically conductive structure at least faces a first electrically conductive region of the die, and a second portion of the electrically conductive structure at least faces a first electrically conductive region of the leadframe. The electrically conductive structure has a third portion located between the structure&#39;s first and second portions, and the third portion may be placed in a state of compressive strain. The method further comprises applying a force to the electrically conductive structure such that the structure&#39;s first portion abuts the die&#39;s first conductive region, the structure&#39;s second portion abuts the leadframe&#39;s first conductive region, and the structure&#39;s third portion is compressively strained. The method further comprises disposing a molding material over at least a portion of the electrically conductive structure, at least a portion of the die, and at least a portion of the leadframe. The molding material may be disposed before, during, or after the initiation of the force. The method further comprises maintaining the application of the force while the molding material undergoes a transition from a liquid state to a solid state. 
         [0006]    Another general embodiment of the invention is directed to a system, such as an electronic device that comprises a semiconductor die package according to the invention. 
         [0007]    These and other embodiments of the invention are described in detail in the Detailed Description with reference to the Figures. In the Figures, like numerals may reference like elements and descriptions of some elements may not be repeated. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]      FIG. 1  is an expanded perspective view of a portion of a first exemplary semiconductor die package according to the present invention, and 
           [0009]      FIGS. 2 and 3  are partially assembled perspective views thereof. 
           [0010]      FIGS. 4 and 5  are side views the first exemplary semiconductor die package according to the present invention before and during an exemplary molding process according to the present invention. 
           [0011]      FIGS. 6 and 7  are top and bottom perspective views of the completed first exemplary semiconductor die package according to the present invention. 
           [0012]      FIGS. 8 and 9  are side views of a portion of a second exemplary semiconductor die package according to the present invention before and during an exemplary molding process according to the present invention. 
           [0013]      FIGS. 10 and 11  are side views of additional embodiments of the spring structure according to the present invention. 
           [0014]      FIG. 12  is a perspective view of a system that comprises a semiconductor die package according to the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0015]      FIG. 1  shows a partial expanded perspective view of a first exemplary semiconductor die package  10  according to the present invention. Semiconductor die package  10  comprises a semiconductor die  5 , a leadframe  20 , at least one electrically conductive wire-type structure  30  (shown in  FIGS. 2-5 ), and at least one electrically conductive spring structure  40  (shown in  FIGS. 3-5 ). Leadframe  20  may comprise a base layer of copper (Cu) that is coated or alloyed with the following order of metal sub-layers: nickel (Ni), palladium (Pd), and gold (Au). Leadframe  20  has a first electrically conductive region  24 , a second electrically conductive region  26 , and a third electrically conductive region  28 . First electrically conductive region  24  comprises a plurality of end caps  25  (also called tabs) at one of its edges. Similarly, second electrically conductive region  26  comprises a plurality of end caps  27  at one of its edges, and third electrically conductive region  28  comprises at least one end cap  29  at one of its edges. In preferred implementations, electrically conductive regions  24 - 28  will be encapsulated by a body of electrically-insulating molding material (described in greater detail below), except that the bottom portions of end caps  25 - 29  will be left exposed by the molding material. These bottom portions of the end caps will serve as electrical connection points for package  10 . In some implementations, a substantial portion of the bottom surface of second conductive region  26 , or entire bottom surface thereof, may be left exposed by the molding material. Still referring to  FIG. 1 , semiconductor die  5  has a first surface  6 , a second surface  7 , a first electrically conductive region S disposed on the die&#39;s first surface  6 , a second electrically conductive region D disposed on the die&#39;s second surface  7 , and a third electrically conductive region G disposed on the die&#39;s first surface  6 . In an exemplary implementation, semiconductor die  5  comprises a vertical power device, preferably a power MOSFET device, having a first conduction terminal (e.g., source) at first conductive regions S, a second conduction terminal (e.g., drain) at second conductive region D, and a modulation terminal (e.g., gate) at third conductive region G. However, semiconductor die  5  may comprise other power devices, such as rectifiers, controlled rectifiers (e.g., SCRs), bipolar transistors, insulated-gate field-effect transistors, etc., and may comprise non-power devices such as digital circuits and analog circuits. 
         [0016]    In an exemplary manufacturing method, the second surface  7  of semiconductor die  5  is attached to a portion of leadframe  20  such that die&#39;s second conductive region D is attached and electrically coupled to portion  23  of the leadframe&#39;s second conductive region  26 . A body  15  of conductive adhesive may be used to attach the components. For power devices, adhesive body  15  preferably comprises solder material, which may be initially disposed on region  23  as a preform or solder paste layer, and thereafter reflowed while in contact with the die&#39;s second conductive region D. The resulting structure is shown in  FIG. 2 , where the reference numbers shown in the figure are the same as previously described above. This attachment and conductive regions D and  26  collectively provide an electrical interconnection between semiconductor die  5  and a system that utilizes package  10 . Next, wire-type conductive structure  30  is assembled onto package  10  such that a first portion  31  of the structure is attached and electrically coupled to the die&#39;s third conductive region G, and a second portion  32  of the structure is attached and electrically coupled to the leadframe&#39;s third conductive region  28 . Wire-type conductive structure  30  may comprise a wire bond, a ribbon bond, a tape-automated bond (“TAB bond”), and the like. Conductive structure  30  and conductive regions G and  28  collectively provide another electrical interconnection between semiconductor die  5  and a system that utilizes package  10 . 
         [0017]    Next in the exemplary method, as shown in  FIG. 3 , electrically conductive spring structure  40  is assembled with semiconductor die  5  and leadframe  20  such that a first portion  41  of the spring structure  40  faces and contacts the die&#39;s first electrically conductive region S, and a second portion of spring structure  40  faces and contacts the leadframe&#39;s first electrically conductive region  24 . Spring structure  40  also has a third portion  43  located between the structure&#39;s first and second portions  41  and  42 . In one implementation, spring structure  40  has a general U-shape (shown upside-down in  FIG. 3 ), with portion  43  having two short sides and a long back side (which is the bottom of the U-shape). Other implementations of spring structure  40  are possible, and are described below. Spring structure  40  preferably comprises a core sheet of resilient elastic material, such as spring steel or a polymer, which is coated with at least one layer of electrically conductive material, such as aluminum (Al), copper (Cu), or gold (Au), with one or more optional barrier layers between it and the core sheet (such as nickel and palladium). The core sheet may be heated (to temporarily lower the sheet&#39;s elastic limit) and bent to shape to the desired shape, or in some cases may be stamped at room temperature to the desired shape with forces that exceed sheet&#39;s elastic limit. The other reference numbers described in  FIG. 3  have been previously described with reference to  FIGS. 1 and 2 . 
         [0018]    Next in the exemplary method, a force F is applied to spring structure  40  such that the structure&#39;s first portion  41  abuts and makes electrical connection with the die&#39;s first conductive region S, such that the structure&#39;s second portion  42  abuts and makes electrical connection with the leadframe&#39;s first conductive region  24 , and such that the structure&#39;s third portion  43  is compressively strained, but preferably not stressed beyond its elastic limit. As is known in the materials science art, a structure is strained when it is distorted from its intrinsic shape by external or internal forces acting on it. In the exemplary spring structure shown in  FIG. 3 , force F is preferably applied to the long back side of portion  43  by a mold plate during a molding step, and the short sides of portion  43  are compressed and placed in a state of compressive strain. Before, during, or after the initiation of force F, a body of molding material, preferably in viscous form (i.e., liquid form), is disposed over spring structure  40 , die  5 , and leadframe  20  and allowed to undergo a transition from a liquid state to a solid state while force F is applied. After the molding material is solidified, force F may be removed. While currently not preferred, it is possible to initially dispose the molding material in a powdered form (i.e., solid particles, which may comprise a thermoplastic material), to thereafter heat the powder to turn it into a liquid form, and to thereafter allow the liquid form to solidify. The solidified molding material maintains the compressive strain state of portion  43 , which in turn keeps first portion  41  in contact with the die&#39;s first electrically conductive region S and second portion  42  in contact with the leadframe&#39;s first electrically conductive region  24 . 
         [0019]    The latter steps of the exemplary method can be implemented using a dual-side, film-assisted molding process, which is illustrated by the side views of  FIGS. 4 and 5 , wherein the reference numbers shown therein have been previously described. Prior to the molding process, assembled instances of leadframe  20  and die  5  are releasably attached to a first carrier film (with leadframe  20  contacting the first carrier film), and instances of spring structure  40  are releasably attached to a second carrier film (with the back side of portion  43  contacting the second carrier film). As used herein, the state of “releasably attached” means that the carrier films may be later removed without damage to the finished package. Each carrier film may comprise a polymer sheet having dimensional stability that is coated with a releasable adhesive on one side. The first carrier film may be attached to a roll of leadframes  20  before or after the dice  5  are assembled with the leadframes, and may be attached so as to not interfere with the indexing apertures of the roll. (Typically, the first carrier film is already part of the roll, and no special step is needed.) Automated pick-and-place equipment may be used to assemble the spring structures  40  on to the second carrier film. Some molding equipment, such as that sold by Boschman Technologies, have pick-and-place capabilities. With such equipment, the second carrier film may be transported across the bottom molding plate without the need for indexing apertures, and each spring structure  40  may be disposed on the second carrier film while in the molding chamber, just prior to the molding operation. If such equipment is not available, the second carrier film may include indexing apertures to assist the pick-and-place equipment and the molding equipment with the alignment of the spring structures  40 . In this latter approach, the second carrier film may be attached to a roll of leadframe carrier rings that have blank areas to receive the spring structures, and the spring structures may be attached to the blank areas by the pick-and-place equipment. 
         [0020]    Both carrier films are then fed into a film-assisted molding machine. If spring structures  40  are already assembled on the second carrier film, then the carrier films are aligned with another so that portions  41  and  42  of spring structure  40  will face conductive regions  24  and S, respectively, when films are transported into the molding chamber. In this case, the first carrier film may be transported along either the top or bottom molding plate, and the second carrier film along the other molding plate. If the spring structures are not already assembled on the second carrier film, then the second carrier film is transported along the bottom molding plate, and a pick-and-place tool takes a spring structure from a stock source, and places it on the second carrier film in a predetermined position with respect to the bottom molding plate (the predetermined position may be inside the molding chamber or outside the molding chamber, such as at an up-stream assembly area next to the molding chamber). The first carrier film is transported along the top molding plate, and aligned to bring conductive regions  24  and S of leadframe  20  and die  5 , respectively, into alignment with the structure&#39;s portions  41  and  42 , respectively. Once in the molding chamber, as shown in  FIG. 4 , two molding plates press the carrier films toward one another to press spring structure  40  against leadframe  20  and die  5 , causing the short sides of portion  43  of the spring structure to move outward and become compressively strained, and causing portions  41  and  42  to abut and make electrical contact with conductive regions  24  and S, respectively, as shown in  FIG. 5 . Before, during, or after the molding plates are pressed together, a body  50  of molding material is injected into the space between the carrier films, and covers at least portions of leadframe  20 , die  5 , and spring structure  40 , and preferably covers all of these components once the plates are at their compressed positions. The molding plates are preferably held in their compressed positions until body  50  solidifies. After body  50  solidifies, the plates are retracted, and the carrier films are moved to position the next instance into the mold. Several instances of package  10  may be processed in this manner. 
         [0021]    If molding material is initially present in the gap between portion  41  (or  42 ) and conductive region  24  (or S), the pressing of the portion against the conductive region closes the gap and ejects the molding material to enable an electrical coupling to be made. To minimize the changes of any remaining molding material degrading the electrical coupling, one or more of the following actions may be taken: (1) the second carrier film may be transported along the bottom molding plate, (2) the molding material may be disposed along one or more sides of spring structure  40 , and (3) portions  41 - 42  and conductive regions  24 , S may be brought into at least light contact before the molding material is dispensed. 
         [0022]    In the above way, an electrical connection may be made between conductive region S of die  5  and conductive region  24  of leadframe  20  within an existing molding operation, and with the addition of simple, fast, and inexpensive pick-and-place operation. The previously-used fluxing, soldering, and cleaning operations are thus eliminated, with a substantially savings is time and cost. 
         [0023]    After processing, the carrier films are peeled away from the package instances, and the instances are trimmed of excess material. The final outline of the package&#39;s side dimensions, after molding and trimming, is shown by the dashed rectangles in  FIGS. 4 and 5 .  FIG. 6  shows a top perspective view of the completed package  10 , and  FIG. 7  shows a bottom perspective view. There it can be seen that end caps  25 ,  27 , and  29  are exposed, that conductive region  26  is exposed (which can enhance thermal conduction and cooling of package  10 ), and that the back side of spring portion  43  is exposed, which can provide an additional electrical connection point. 
         [0024]    In some applications of package  10 , it is preferred that the back side of spring portion  43  is not exposed. This can be achieved by using one or more retractable pins during the mold transfer process, as illustrated by a second exemplary embodiment in  FIGS. 8 and 9 , where the reference numbers shown therein have been previously described. Each retractable pin compresses the spring clip during the molding process, and is then retracted just before the molding material fully sets (e.g., fully cures), preferably at a stage where the material is soft enough to allow the pin to retract, but firm enough to hold the spring portion  43  in a compressive strained state. Each pin will leave a small, characteristic aperture in the molding material, and this aperture typically has uneven side walls (because the pin is retracted when the material is not fully set) and/or will have vertical streak marks caused by burrs on pin. The first carrier film may have an aperture formed in it for each retractable pin. The retractable pins may be coated with a non-stick material. As a result, the back side of portion  43  is covered by molding material, and is not exposed. 
         [0025]    Spring structures according to the present invention may have shapes that are different from the U-shape illustrated above.  FIG. 10  shows a spring structure  40 ′ with a portion  43  that has a shallow V-shape, and  FIG. 11  shows a spring structure  40 ″ that has an oval shape. Each has portions  41  and  42  that provide electrical connections, and a portion  43  that is compressively strained. For each of spring structures  40 ,  40 ′,  40 ″, and variations thereof, each of their portions  41  and  42  exerts a force against the opposing conductive (e.g., regions  24  and S, respectively) that is greater than the portion&#39;s gravitational force (i.e., weight), and that is preferably greater than the gravitational force of the spring structure. While the present invention has been illustrated with one spring structure per semiconductor die package, it may be appreciated that multiple spring structures may be used per package. 
         [0026]      FIG. 12  shows a perspective view of a system  200  that comprises semiconductor package  10  according to the present invention. System  200  comprises an interconnect substrate  201 , a plurality of interconnect pads  202  to which components are attached, a plurality of interconnect traces  203  (only a few of which are shown for the sake of visual clarity), an instance of package  10 , second package  100 , and a plurality of solder bumps  205  that interconnect the packages to the interconnect pads  202 . Package  10  is shown with the aforementioned characteristic aperture. 
         [0027]    The semiconductor die packages described above can be used in electrical assemblies including circuit boards with the packages mounted thereon. They may also be used in systems such as phones, computers, etc. 
         [0028]    Some of the examples described above are directed to “leadless” type packages such as MLP-type packages (microleadframe packages) where the terminal ends of the leads do not extend past the lateral edges of the molding material. Embodiments of the invention may also include leaded packages where the leads extend past the lateral surfaces of the molding material. 
         [0029]    Any recitation of “a”, “an”, and “the” is intended to mean one or more unless specifically indicated to the contrary. 
         [0030]    The terms and expressions which have been employed herein are used as terms of description and not of limitation, and there is no intention in the use of such terms and expressions of excluding equivalents of the features shown and described, it being recognized that various modifications are possible within the scope of the invention claimed. 
         [0031]    Moreover, one or more features of one or more embodiments of the invention may be combined with one or more features of other embodiments of the invention without departing from the scope of the invention. 
         [0032]    While the present invention has been particularly described with respect to the illustrated embodiments, it will be appreciated that various alterations, modifications, adaptations, and equivalent arrangements may be made based on the present disclosure, and are intended to be within the scope of the invention and the appended claims.