Patent Publication Number: US-9431328-B1

Title: Power device and preparation method thereof

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This Patent Application is a Divisional Application of a pending application Ser. No. 14/711,969 filed on May 14, 2015. The Disclosure made in the patent application Ser. No. 14/711,969 is hereby incorporated by reference. 
    
    
     FIELD OF PRESENT INVENTION 
     The present invention mainly refers to a power semiconductor device, in particular, to a power semiconductor package device as a power switch and a preparation method thereof. 
     BACKGROUND OF RELATED ART 
     In a conventional power switching system, the voltage is modulated by a power switch to generate a final output voltage with a small ripple, for example, the switch includes a power semiconductor device. In a conventional semiconductor package as shown in  FIG. 1 , a power transistor T 1  is attached on a first die paddle having pins D 1  and a power transistor T 2  is attached on a second die paddle having pins D 2 . The source and the gate of the transistor T 1  are respectively connected to a pin S 1  and a pin G 1  via the bonding wires, and the source and the gate of the transistor T 2  are respectively connected to a pin S 2  and a pin G 2  via the bonding wires. Some power switching system applies a common source for both power transistors, for example, the common source of double-chip T 1  and T 2  in  FIG. 1  can be achieved at the external board level, in which the pin S 1  and the pin S 2  are attached to a common bonding pad of a circuit board or the pin S 1  and the pin S 2  are coupled together via two different conductive paths. The connecting path between two sources of the double-chip forming a common source of the conventional technique still needs to be improved, in particular without an increase in the size of the final package device. In examples of the present disclosure, double-chip common source technique for the source electrodes of two power MOSFETs is achieved by applying a T-shape metal clip. 
     It is within this context that embodiments of the present invention arise. 
    
    
     
       BRIEF DESCRIPTIONS OF THE DRAWINGS 
       The embodiments of this invention are described in more detail with reference to the accompanying drawings. However, the accompanying drawings are for the purpose of descriptions and illustrations only and do not impose limitation to the scope of the present invention: 
         FIG. 1  represents a basic structure of a power package device in prior art. 
         FIG. 2  is a top view of a lead frame used to form a power package device in the present invention. 
         FIG. 3  is a top view of one lead frame unit of the lead frame in  FIG. 2 . 
         FIG. 4  is a top view of a lead frame unit with two semiconductor chips attached thereon. 
         FIG. 5A  to  FIG. 5E  are schematic diagrams of a metal clip and two conductive plates. 
         FIG. 6  is a top view showing the metal clip and two conductive plates mounted on two semiconductor chips attached on the lead frame unit in  FIG. 4 . 
         FIG. 7  is a top view showing a plastic package layer covering a lead frame with the semiconductor chips, metal clip and conductive structures attached thereon. 
         FIG. 8  is a bottom view of a power device with a plastic package body. 
         FIG. 9  is a top view of another power device with the conductive plates of  FIG. 6  being replaced by bonding wires. 
     
    
    
     DESCRIPTIONS OF THE SPECIFIC EMBODIMENTS 
       FIG. 2  is a top view of a portion of a metal lead frame  100 . The lead frame  100  includes a plurality of lead frame units  101 . 
     As shown in  FIG. 3 , each lead frame unit includes a first die paddle  111  and a second die paddle  112 , a first pin  113 , a second pin  114  and a third pin  115  separated from each other. In an embodiment, the first die paddle  111  and the second die paddle  112  can be cuboids or cubes, separated from each other and arranged side by side. The second pin  114  is located on a symmetric line or a cutting line  230  between the die paddle  111  and the die paddle  112 . The first die paddle  111  and the second die paddle  112  are symmetric with respect to the cutting line  230 . The first pin  113  and the third pin  115  are substantially collinear with and located on both sides of the second pin  114 . The pins and the die paddles of the lead frame unit  101  are connected to the lead frame  100  via connecting bands  150 . At least two connecting bands  150  are provided at each two adjacent sides of each of the die paddles  111  and  112  to connect the die paddles to the lead frame. On a common side of the first die paddle  111  and the second die paddle  112 , a connecting band is disposed substantially at a center of each of the die paddle and another connecting band is disposed substantially at each edge where the first die paddle  111  and the second die paddle  112  are separated to stabilize the die paddles during die mounting process. The lead frame units  101  or the lead frame  100  are further subjected to additional processes before a die-attach step. For example, additional processes may include semi-etching or punching/impressing and other similar processes. 
     As shown in  FIG. 3 , the top surface of the die paddles  111  and  112  are etched or punched to form mesa structures  111   a  and  112   a  protruding out of the top surface of the lead frame unit  101 . As an enhancement, upright posts  111   b  and  112   b  are arranged side by side on the common side of the die paddles  111  and  112 , with a portion of each of the upright post  111   b  and  112   b  overlapped with each connecting band  150  connecting the die paddle  111  and die paddle  112  to the lead frame  100 , forming a protruding out top surface by the same semi-etching or punching processes as described above. The upright posts  111   b  and  112   b  strengthen the connecting band  150  thus enhancing the die paddle stability. Furthermore the upright posts  113   a ,  114   a  and  115   a , which are arranged side by side with a portion of each of the upright post  113   a ,  114   a  and  115   a  overlapped with a corresponding connecting band  150  connecting the first pin  113 , the second pin  114  and the third pin  115  to the lead frame  100  respectively, protruding out of the top surfaces of the first pin  113 , the second pin  114  and the third pin  115  respectively, also can be formed via a same semi-etching or punching process. 
     As shown in  FIG. 4 , a first semiconductor chip  121  is attached on the mesa structure  111   a  at the top surface of the first die paddle  111  and a second semiconductor chip  122  is attached on the mesa structure  112   a  at the top surface of the second die paddle  112  via a conductive binding material. The first chip  121  and the second chip  122  can be vertical power MOSFETs, each of which includes a first electrode  121   a  or  122   a , which is the source electrode, and a second electrode  121   b  or  122   b , which is the gate electrode, insulated from each other arranged at the top surface, and the third electrode arranged at the bottom surface of the first chip  121  or the second chip  122  (not shown), which is the drain, attached on the top surface of the mesa structure  111   a  or  112   a  via a soldering paste or a conductive silver paste and other similar conductive binding materials. In addition, the bottom surface of the first chip  121  and the second chip  122  can also be respectively mounted on the top surface of the mesa structure  111   a  and the mesa structure  112   a  by eutectic bonding and other welding methods, so that the third electrode at the bottom surface of the first chip  121  is electrically connected with the first die paddle  111  and the third electrode at the bottom surface of the second chip  122  is electrically connected with the second die paddle  112 . As an option, the upright post  111   b  on the die paddle  111  and the upright post  112   b  on the die paddle  112  are located at the central position of the common side of the die paddles  111  and  112  respectively. When the first chip  121  and the second chip  122  are large in size, the first chip  121  can be attached on the upright post  111   b  and the mesa structure  111   a , and the second chip  122  can be attached on the upright post  112   b  and the mesa structure  112   a . As shown in  FIG. 4 , the first chip  121  and the second chip  122  may be arranged side by side along with the die paddles. 
       FIG. 5A  to  FIG. 5E  are schematic diagrams illustrating a T-shape metal clip  130 , a first conductive plate  141  and a second conductive plate  142  used in the power device of the present invention. The T-shape metal clip  130  includes a first portion  131  and a second portion  132  integrated in one piece, where the second portion  132  is perpendicular to the first portion  131  with one end of the second portion  132  connected to center area of the first portion  131 . The T-shape metal clip  130 , the first conductive plate  141  and the second conductive plate  142  are separated from each other. However, in a preferred embodiment, the T-shape metal clip  130 , the first conductive plate  141  and the second conductive plate  142  can be connected with each other via connecting bands (not shown). In an embodiment, some connecting bands are cut off forming connectors  151  as shown in  FIG. 5A . In another embodiment, the first portion  131  and the second portion  132  of the T-shape metal clip  130  include through holes  133 . The shapes of the through holes  133  may include circles, squares, polygons or cross shapes. The cross shapes, as shown in  FIG. 5A , are used as locked modes that will be described later in the step of coating plastic package. In one example, the T-shape metal clip  130  is connected with the conductive plate  141  and the conductive plate  142 , the first conductive plate  141  and the second conductive plate  142  are located at both sides of the second portion  132  respectively and are symmetric with respect to the second portion  132 , or symmetric with respect to a center line  240  of the second portion  132 . 
       FIG. 5A  shows the top surfaces of the T-shape metal clip  130 , the first conductive plate  141  and the second conductive plate  142 .  FIG. 5B  shows the bottom surfaces of the T-shape metal clip  130 , the first conductive plate  141  and the second conductive plate  142 . Comparing  FIG. 5A  and  FIG. 5B , the positions of the first conductive plate  141  and the second conductive plate  142  are interchanged, and the positions of the end  131   a  and end  131   b  of the first portion  131  of the T-shape metal clip  130  are also interchanged. In  FIG. 5B , a groove is formed at the bottom surface of the first portion  131  in the middle area between the end  131   a  and the end  131   b  using a semi-etching or punching mode, which extends to the second portion  132 .  FIG. 5C  shows the cross-sectional diagram of the first portion  131  along a dotted line AA of  FIG. 5A  and FIG.  5 B.  FIG. 5D  shows the cross-sectional diagram of the first portion  131  and the second portion  132  along a dotted line BB of  FIG. 5A  and  FIG. 5B . One free end of the second portion  132  is firstly bent downward and then extended horizontally to form a down-set part  132   a . The down-set part  132   a  may be made by punch molding. The plane of the down-set part  132   a  is substantially parallel to the plane of the first portion  131  and the plane of the second portion  132 , but the plane of the first portion  131  and the plane of the second portion  132  are slightly higher than the plane of the down-set part  132   a.    
       FIG. 5E  shows a perspective view of the first conductive plate  141 , which is similar to but is in mirror symmetry with respect to the second conductive plate  142 . The first conductive plate  141  is a metal clip including a base  141   b , an extending section  141   d  connected to the base  141   b , a contact portion  141   a  extending from one edge of the extending section  141   d  near the second portion  132  of the T-shape metal clip  130  and away from the base  141   b , and a down-set part  141   c  extending on the outside edge of the base  141   b  away from the extending section  141   d . The down-set part  141   c  is located on a plane lower and parallel with the plane of the base  141   b  and the extending section  141   d . The extending section  141   d  is thinner than the base  141   b , which is formed by a semi-etching or punching process. The bottom surface of the base  141   b  is coplanar with the bottom surface of the contact portion  141   a . The bottom surface of the extending section  141   d  is recessed from the bottom surface of the base  141   b  and the contact portion  141   a . Similarly, as shown in  FIG. 5A-5B , the second conductive plate  142  includes a base  142   b  and an extending section  142   d  connected with each other, a contact portion  142   a  extends from one edge of the extending section  142   d  near the second portion  132  of the T-shape metal clip  130  and away from the base  142   b , and a down-set part  142   c  extending on the outside edge of the base  142   b  away from the extending section  142   d . The down-set part  142   c  is located on a plane below and essentially parallel to the plane of the base  142   b  and the extending section  142   d . The extending section  142   d  is thinner than the base  142   b , which is formed by semi-etching or punching process. The bottom surface of the base  142   b  is coplanar with the bottom surface of the contact portion  142   a . The bottom surface of the extending section  142   d  is recessed from the bottom surface of the base  142   b  and the contact portion  142   a . As such, each of the first conductive plate  141  and the second conductive plate  142  includes different sections integrated in one piece. 
     As shown in  FIG. 6 , a conductive binding material is coated on the first electrode  121   a  and the second electrode  121   b  at the top surface of the first chip  121  and on the first electrode  122   a  and the second electrode  122   b  at the top surface of the second chip  122 . In addition, the conductive binding material is coated on the top surfaces of the first pin  113 , the third pin  115 , the second pin  114 , and the upright post  114   a  if the upright post  114   a  is formed. Then, the T-shape metal clip  130  is attached on the first chip  121 , the second chip  122  and the second pin  114 . The first conductive plate  141  is attached on the first chip  121  and the first pin  113 . The second conductive plate  142  is attached on the second chip  122  and the third pin  115 . Specifically, the first portion  131  of the T-shape metal clip  130  is mounted above the first chip  121  and the second chip  122 . The bottom surface of one end  131   a  of the first portion  131  is aligned with and attached on the first electrode  121   a  of the first chip  121 . The bottom surface of the other end  131   b  of the first portion  131  is aligned with and attached on the first electrode  122   a  of the second chip  122 . The down-set part  132   a  at the free end of the second portion  132  is attached on the top surface of the second pin  114 . The down-set part  132   a  may be aligned with and attached on the top surface of the upright post  114   a . The contact portion  141   a  of the first conductive plate  141  is aligned with and attached on the second electrode  121   b  at the top surface of the first chip  121 . The down-set part  141   c  of the first conductive plate  141  is aligned with and attached on the top surface of the first pin  113 . The contact portion  142   a  of the second conductive plate  142  is aligned with and attached on the second electrode  122   b  at the top surface of the second chip  122  The down-set part  142   c  of the second conductive plate  142  is aligned with and attached on the top surface of the third pin  115 . 
     In an embodiment, if the T-shape metal clip  130 , the first conductive plate  141  and the second conductive plate  142  are connected with each other via the connecting bands, the steps of attaching the T-shape metal clip  130 , the first conductive plate  141  and the second conductive plate  142  on the first and second chips  121  and  122  and the pins  113 - 115  are carried out the same time and the connecting bands are cut off subsequently. Otherwise, the T-shape metal clip  130 , the first conductive plate  141  and the second conductive plate  142  can be attached on the first and second chips  121  and  122  and the pins  113 - 115  separately in an arbitrary sequence. The base  141   b  of the first conductive plate  141  is partially overlapped on the upright post  113   a  of the first pin  113 . The base  142   b  of the second conductive plate  142  is partially overlapped on the upright post  115   a  of the third pin  115 . The symmetrical center line  240  between two long edges of the second portion  132  in  FIG. 5A  can be superimposed with the cutting line  230  between the die paddle  111  and the die paddle  112  in  FIG. 3 . 
     Referring to  FIG. 7 , after the first chip  121  and the second chip  122  are respectively attached on the die paddles  111  and  112  of the lead frame unit  101  in the lead frame  100  and the T-shape metal clip  130 , the first conductive plate  141  and the second conductive plate  142  are subsequently mounted atop of the first chip  121  and the second chip  122 , a plastic package material, such as epoxy resin and the like, is deposited forming a plastic package layer  160 . The plastic package layer  160  covers the first chip  121  and the second chip  122 , the lead frame unit  101 , the T-shape metal clip  130 , the first conductive plate  141  and the second conductive plate  142 . The bottom surfaces of the die paddle  111 , the die paddle  112 , the first pin  113 , the second pin  114  and the third pin  115  in each lead frame unit  101  are exposed (not covered by the plastic package layer  160 ). In addition, the plastic package material also fills in through holes  133  under the molding pressure to firmly lock the T-shape metal clip  130  to the first and second chips  121 ,  122  and the lead frame unit  101 .  FIG. 7  is a top view of the devices after the plastic package layer  160  is formed. Then, the standard package cutting process is carried out to cut through the laminate from the plastic package layer  160  to the lead frame  100  between the adjacent lead frame units  101  along the preset cutting line (dotted line in  FIG. 7 ) forming a plurality of complete package devices. In this step, each lead frame unit  101  is cut off and separated from the lead frame  100  and a plurality of plastic package bodies  161  are formed via cutting the plastic package layer  160 . Each plastic package body  161  correspondingly covers one lead frame unit  101 . In addition, the connecting bands  150  are also cut off, so that the die paddle  111 , the die paddle  112 , the first pin  113 , the second pin  114  and the third pin  115  are separated from the lead frame  100 . 
       FIG. 8  shows the bottom view of a complete package device including a plastic package body  161  coated on the first die paddle  111 , the second die paddle  112 , the first pin  113 , the second pin  114 , the third pin  115 , the first chip  121 , the second chip  122 , the T-shape metal clip  130 , the first conductive plate  141 , the second conductive plate  142 , and the upright posts  111   b ,  112   b ,  113   a ,  114   a  and  115   a . The bottom surfaces of the first pin  113 , the second pin  114 , the third pin  115 , the die paddle  111  and the die paddle  112  are exposed as contact terminals for connecting to external devices. In an optional embodiment, the top surface of the T-shape metal clip  130  can be exposed (not covered by the top surface of the plastic package body  161 ), but the down-set part  132   a  is not exposed (covered by the top surface of the plastic package body  161 ) since the down-set part  132   a  is bent and located in a lower plane. Similarly, the top surfaces of the first conductive plate  141  and the second conductive plate  142  can be exposed (not covered by the top surface of the plastic package body  161 ), but the down-set parts  141   c  and  142   c  is not exposed (covered by the top surface of the plastic package body  161 ). 
     In an optional embodiment shown in  FIG. 9 , the first conductive plate  141  and the second conductive plate  142  of  FIG. 6  are replaced with metal bonding wires  145 . The metal bonding wires  145  connect the second electrode  121   b  at the top surface of the first chip  121  and the second electrode  122   b  at the top surface of the second chip  122  to the first pin  113  and third pin  115  respectively. For example, a first metal bonding wire may connect to the upright post  113   a  and a second metal bonding wire may connect to the upright post  115   a  It can be done before or after attaching the T-shape metal clip  130  on the first and second chips  121  and  122 . The metal bonding wires  145  can also be replaced by metal conduction bands and the likes. As such, double-chip common source for the source electrode  121   a  and the source electrode  122   a  of two power MOSFETs  121   122  respectively via the T-shape metal clip  130  is achieved. 
     Those of ordinary skill in the art may recognize that modifications of the embodiments disclosed herein are possible. For example, the number of lead frame units in a lead frame may vary. Other modifications may occur to those of ordinary kill in this art, and all such modifications are deemed to fall within the purview of the present invention, as defined by the claims.