Patent Publication Number: US-2011065240-A1

Title: Lead frame and method of forming same

Description:
BACKGROUND OF THE INVENTION 
     This invention relates generally to an apparatus and method of a lead frame for a semiconductor device, and more particularly to a supported stamped lead frame for use in a semiconductor device such as for example an area mounting type semiconductor device. 
     Recent developments in semiconductor device technology have been made to achieve ever smaller size and scale of semiconductor devices and reach ever higher device performances. In view of the smaller scale and size of components achievable, the component densities in semiconductor dies and packages have increased. Consequently, the advancements in semiconductor device technology has outpaced the advances made in semiconductor packaging technology. Although the scale of the individual semiconductor components has reduced, there still remains a scarcity of space for semiconductor components within a semiconductor device package. 
     However, one aspect of semiconductor devices that has remained by comparison for the most part relatively unchanged in the recent developments is lead frame design. Typically, conventional lead frame design does not lend itself to the other advancements made in the smaller scale and size of components. For example, typically in the formation of lead frames in area mounting type semiconductor devices in design and package assembly such as quad flat pack no-lead (QFN), small-outline no leads (SON) leadless integrated circuit (IC) packages have connections made to the devices via contacts on the bottom side of the component to the surface of the connecting substrate of the printed circuit board (PCB). Due to the brittle or pliable nature of lead frame material, conventional lead frames may only be designed having lead sections with a minimum thinness or narrowness as such thin or narrow lead frame sections are susceptible to bending or displacement during the processing environment of the semiconductor device. Conventional lead frame design does not typically easily allow a designer to select any desired configurations for lead positions beyond the limited number of conventional lead designs. For example, in configurations where half etch lead frame is implemented, it is difficult to achieve desired lead configuration under typical lead frame design as it is not possible to reach half etch with the lead frame with a conventional lead frame. Conventional techniques and designs in the placement of leads of a lead frame are not suitable for the new requirements in the industry. 
     Thus, there is a need to extend conventional lead frame technology to allow many desired lead position configurations while maximizing space in semiconductor package assemblies. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In order that embodiments of the invention may be fully and more clearly understood by way of non-limitative examples, the following description is taken in conjunction with the accompanying drawings in which like reference numerals designate similar or corresponding elements, regions and portions, and in which: 
         FIG. 1  pictorially shows lead frame manufacturing in accordance with an embodiment of the invention; 
         FIG. 2A-2D  shows cross-sectional view of a punch and a die for forming a lead frame in different steps of the process in accordance with an embodiment of the invention; 
         FIG. 3  is a flow chart in accordance with a method of an embodiment of the invention; 
         FIG. 4A-4C  shows lead frame after stamping process ( FIG. 4A ), after wire bond ( FIG. 4B ) and after molding ( FIG. 4C ) in accordance with an embodiment of the inventions; 
         FIG. 5A-5B  show areas of the lead frame after wire band in greater detail of areas A ( FIG. 5A ) and B (of  FIG. 5B ) as indicated in  FIG. 3B  in accordance with an embodiment of the invention; and 
         FIG. 6  shows the lead frame after molding of  FIG. 4C  in greater detail in accordance with an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     An apparatus and method for a supported, stamped lead frame for use in a semiconductor device. Such devices include quad flat pack no-lead (QFN)/power quad flat pack no-lead (PQFN) integrated circuit (IC) devices and the like. By adding a support or tape during the stamping of the lead frame process, the supported/taped stamping lead frame undergoes assembly processing including die bond wire bonding, molding and saw singluation without metal or other supporting structure in the saw street during processing. As there is no required metal in saw street, the saw blade life also increases. 
     An aspect of the invention provides a method of fabricating a lead frame, the method comprising stamping a lead frame material into a desired lead frame configuration; and supporting the stamped lead frame material with a support material. 
     In an embodiment the support material is adhered together with the stamped lead frame material. The support material may be adhered to the stamped lead frame material. 
     An aspect of the invention provides a method of processing a semiconductor device having a lead frame, the method comprises stamping a lead frame material into a desired lead frame configuration; supporting the stamped lead frame material with a support material; die bonding a semiconductor die on the lead frame; wire bonding with wires the semiconductor die with the lead frame; molding the lead frame, semiconductor die, and wires to form the semiconductor package with the support material; removing the support material; and singulating the units of the semiconductor device forming unit semiconductor devices. 
     An aspect of the invention provides an apparatus for fabricating a lead frame, the apparatus comprises a punch having a surface for stamping a lead frame; a die for a desired lead frame configuration, the die having edges for defining a desired lead frame configuration; and a lead frame material receiving area for receiving the lead frame material between the surface of the punch and the edges of the die; and a support material receiving area for receiving a support material for supporting the desired lead frame configuration stamped by punch. 
     An aspect of the invention provides a lead frame comprising a lead frame material stamped into a desired lead frame configuration; and a supporting material to support the lead frame. 
     Referring to  FIG. 1 , an apparatus  10  is shown for stamping a lead frame from lead frame material  20 . The lead frame material may be formed from a master coil  12 , master coil units  14 , and/or single coil units  16 . Such lead frame materials may be copper, and the like for example copper or copper material types such as copper CDA 194 ES, and other alloys such as alloy 42 and the like. The apparatus  10  further comprises a stamping device  18  for stamping the lead frame material into the desired shape and configuration. The apparatus also holds a tape material  22  and is arranged such that when the lead frame is stamped by the stamping device the stamped lead frame is fixed directly onto the surface of the support or tape material. The support may be for example acrylic tape, silicon tape and the like. 
       FIG. 2A-2D  show cross-sectional view of the interaction between the punch and the die apparatus in the process of forming a lead frame during different steps of the process in accordance with an embodiment of the invention. In  FIG. 2A  shows a cross-sectional view of the lead frame stamping and support or taping device at an idle start  50 . The device comprises a punch  52  and stripper  54  that are arranged above the lead frame material  56 . The lead frame material  56  is positioned below the punch  52  and above a die  58 . The edges  57  of the die form the shape and form of the desired lead frame. The internal sides  59  of the die are flared or tapered away from the edges  57  to ensure that the lead frame avoids contact with the side of the die during processing. The support or tape  62  is positioned below the die  58  under the lead frame material  56  and the punch  52 . 
     In  FIG. 2B  the lead frame stamping and taping device is shown as the punch starts the stroke process  60  and the punch drops or moves downward along the stripper  55 . 
     In  FIG. 2C  the punch  52  punches  70  the lead frame material  56  with the surface  53  of the punch and the edges of the die  58  to form the desired lead frame  64 . The punch and continues to stroke downward past the edges  57  of the die and past the sides  59  of the die. 
     In  FIG. 2D  the punched lead frame  64  is placed on the tape  62  and the lead and flag  66  of the lead frame is formed. The lead frame material  56  that does not form part of the punched lead frame  64  is scrap  56 . The punched lead frame  64  is fixed to the support  62 . 
     The support  62  supports the punched lead frame  64 . The support may take different forms, for example, the support may be tape, a substrate, a carrier and the like. Other types of supports may be for example acrylic tape, silicon tape and the like. The lead frame is punched off from the coil and attached to the support. Where the support is a tape, the tape has an adhesive that fixes the punched lead frame  64  to the tape. 
     The lead frame in accordance with embodiments of the invention may be designed having thinner or narrower leads than previously achievable. Also, with embodiments of the invention, lead frames may be fabricated for area mounting type semiconductor devices. For example, the lead frames may be arranged in a package assembly such as quad flat pack no-lead (QFN), small-outline no leads (SON) leadless integrated circuit (IC) packages and have connections made to the devices via contacts on the bottom side of the component to the surface of the connecting substrate of the printed circuit board (PCB). As the lead frame is supported by support or tape  62 , the lead frame material is supported to withstand movement or displacement during the processing and package assembly of the semiconductor device package. With embodiments of the invention, the lead frame may be designed to any desired configurations for lead positions having a minimum thickness of lead frame material. Accordingly, the lead frame in accordance with embodiments of the invention may be configured, for example, in half etch lead frame. 
     The support provides support to the punched lead frame to ensure that the lead frame maintains the desired shape of the punched lead frame during semiconductor device processing. By providing the additional support to the punched lead frame during processing the design of the lead frame may be made more precise and intricate. The lead frame may be arranged thinner than was achievable by conventional lead frames using the same material since the material is supported. 
       FIG. 3  shows a method  100  in accordance with an embodiment of the invention. The method shows that the lead frame is stamped or punched  102  and then placed  104  on a carrier or support such as tape. It will be appreciated that the lead frame can be fixed to the support prior to stamping. The substrate that forms the dies for the semiconductor devices is mounted and sawed  106  for forming the dies. The dies are bonded  108  and wire bonded  110  in place with the lead frame. The semiconductor components such as the lead frame, dies, and wires are then molded  112  for the packaging process. The carrier such as tape is removed  114  from the semiconductor package assembly. Saw singulation  116  is performed on the semiconductor package assembly to form individual units  118 . 
     By adding the support or taping process in the stamping lead frame procedure, the lead and flag for individual units may be formed and then the lead frame individual unit is attached directly to the support such as the tape. When the supported stamped lead frame reaches an assembly factory, and goes through the die bond wire bond processing, molding and saw singulation, saw blade life is maximized because there is no metal in saw street. Additionally, as there is no metal in the saw street, only the molding compound is sawed during saw singulation which prevents high temperatures from being reached during saw singulation. By maintaining a lower temperature during saw singulation, saw life is extended as high temperature during saw singulations induces saw blades to wear out. In conventional designs the metal is in the saw street to provide strong physical connection and stability of the individual units during the assembly process. In accordance with embodiments of the invention the additional metal in the saw street is not required as the support provides stability during the assembly process. Advantageously, as there is no metal in the saw street in embodiments of the invention, the material and manufacturing cost of the lead frame is minimized and lower than conventional designs. Additionally, embodiments of the invention provide flexible lead layout and design, and increased capability of multiple rows of leads. 
     Embodiments of the invention are particularly advantageous in area mounting type semiconductor device configurations such as, for example, quad flat pack no-lead (QFN), small-outline no leads (SON) leadless packages have connections made to the devices via contacts on the bottom side of the component to the surface of the connecting substrate of the printed circuit board (PCB). In these type of devices, half etching lead frames are used. With embodiments of the invention, the lead and flag between each unit are connected by saw street for reasons discussed above for preservation of saw blade life, and the lead frame saw street was made to reach half etch. Additionally, the etching lead frame has a relatively higher cost than the stamping lead frame that is in accordance with embodiments of the invention. By adding the taping process in lead frame stamping allows for individual units to be made by punching or stamping and placed directly on the support such as tape. In this process, the support eliminates any requirement or need for the saw street metal, and also reduces the saw blade wearing out during saw singulation process. 
     Accordingly, in view of these advantages, the packaging cost has been minimized. The stamping lead frame has 60-70% cost down than etching lead frame. Packaging cost during saw singulation process can be reduced. In the process of embodiments, by removing the taping process, the saw singluation machine is able to use a single cut mode. 
     It will be appreciated that embodiments may be implemented in other devices such as all quad flat pack no-lead (QFN)/power quad flat pack no-lead (PQFN) products. 
     Add taping process in stamping lead frame procedure. The stamping process make the lead and flag for individual unit, then individual unit was attached on the tape. When the taped stamping lead frame reach assembly factory, and go through die bond wire bond, molding and saw singluation. Because there is no metal in saw street, the saw blade life will increase. 
     Currently, Multiple Array lead frames for Quad Flat No-lead (QFN) packages use a half-etched lead frame. The lead and flag between each unit are connected by a saw street. For preventing high temperatures during saw singulation, which will cause the saw blade to wear quickly, the lead frame saw street was half-etched. The normal, stamped type lead frame could not reach half etch. The etched lead frame has a higher cost than the stamped lead frame. Add the taping process in lead frame stamping which purpose is making the individual unit directly put on the tape to eliminate the saw street metal, to reduce the saw blade wearing out during the saw singulation process, the packaging cost was reduced thereafter. 
     By adding the taping process in lead frame stamping process, no metal saw street lead frame is required as with current conventional lead frame design. Thus the saw blade life and machine in saw singulation is increased in particular with QFN type ICs. Also, the taped stamping lead frame contributes to the overall semiconductor device lower packaging cost, and lower cost of lead frame as less metal is required for use in the lead frame. Lead count can also be increase as multiple rows of leads may be easily designed and applied, which also contributes to minimizing packaging costs. Advantageously, the package is a “green” package that limits any environmental impact in that no Pb plating process is required. 
     While embodiments of the invention have been described and illustrated, it will be understood by those skilled in the technology concerned that many variations or modifications in details of design or construction may be made without departing from the present invention.