Patent Publication Number: US-8969139-B2

Title: Lead frame array package with flip chip die attach

Description:
This application is a divisional application of a U.S. patent application entitled “Lead Frame Array Package With Flip Chip Die Attach”, having a Ser. No. 13/758,263, having a filing date of Feb. 4, 2013, having common inventors, and having a common assignee, all of which is incorporated by reference in its entirety. 
    
    
     BACKGROUND 
     1. Field 
     This disclosure relates generally to semiconductor device packaging, and more specifically, to providing sufficient input/output contacts on small form-factor packages. 
     2. Related Art 
     Smaller electronic devices and demands for increased functionality of electronic circuits in the same space create the need for near chip scale packages. One type of near chip scale package is the flat no-leads package, such as dual-flat no-leads (DFN) and quad-flat no-leads (QFN) packages. Flat no-leads packages provide a semiconductor device encapsulated in a molding material and coupled to the input/output contacts, or lands, on the perimeter of the device package through a lead frame substrate. 
     Continued demands for smaller package footprints have resulted in decreasing package sizes, in some cases to 2 mm×2 mm or less. At these scales, contact pitch limitations restrict the number of contacts that can be provided along the perimeter of the package. In addition, demand for increased functionality in these small packages makes it desirable for larger semiconductor devices to be incorporated in the package. But traditional flat no-leads packaging techniques require wire bonding of die contacts to the lead frame, which takes up space within the package. Further, this increased functionality often requires additional input/output contacts that cannot be provided along the perimeter of traditional small flat no-leads packages. In addition, the traditional techniques of forming the flat no-leads packages can be resource and time intensive due to repeated mechanical processes, such as performing wire bonding. 
     It is therefore desirable to provide a small, near chip scale package that can provide increased numbers of input/output contacts, while at the same time providing capacity for larger semiconductor devices in the same package footprint. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention may be better understood, and its numerous objects, features, and advantages made apparent to those skilled in the art by referencing the accompanying drawings. 
         FIG. 1  is a simplified block diagram illustrating a perspective view of a conventional dual flat no leads (DFN) package. 
         FIG. 2  is a simplified block diagram illustrating a bottom surface of the DFN package subsequent to singulation. 
         FIG. 3  is a simplified block diagram of a semiconductor device package and that provides additional input/output contacts in accord with embodiments of the present invention. 
         FIG. 4  is a simplified block diagram illustrating one embodiment of a semiconductor device die configured for use with embodiments of the present invention. 
         FIG. 5  is a simplified block diagram illustrating one embodiment of the semiconductor device die subsequent to formation of conductive bumps on the bonding pads of the active surface of the semiconductor device die. 
         FIG. 6  is a simplified block diagram illustrating a portion of an example array lead frame strip usable for embodiments the present invention. 
         FIG. 7  is a simplified block diagram illustrating a perspective view of the semiconductor device die attached to an array lead frame unit subsequent to a flip chip die attach process. 
         FIG. 8  is a simplified block diagram illustrating a perspective view of a semiconductor device structure at a step in processing subsequent to that illustrated in  FIG. 7 . 
         FIG. 9  is a simplified block diagram illustrating a perspective view of the semiconductor device structure after partial saw through and singulation, in accord with embodiments of the present invention. 
         FIG. 10  is a simplified block diagram illustrating a cross section of a semiconductor device package electrically coupled to a printed circuit board (PCB). 
     
    
    
     The use of the same reference symbols in different drawings indicates identical items unless otherwise noted. The figures are not necessarily drawn to scale. 
     DETAILED DESCRIPTION 
     A small form factor near chip scale package is provided that includes input/output contacts not only along the periphery of the package, but also along the package bottom area. Embodiments provide these additional contacts through use of an array lead frame coupled to under die signal contacts through the use of flip chip bonding techniques. The array lead frame contacts are electrically isolated through the use of a partial sawing process performed during package singulation. 
     Certain electronics applications have led to the demand for smaller and smaller semiconductor device packages. For some applications, flat no-leads packages on the order of 2 mm×2 mm or less are being suggested. These small package sizes result in a very limited region in which input/output contacts can be placed. Given pitch restrictions (currently of about 0.4 mm), only five input/output contacts can be placed along opposing sides of a 2 mm×2 mm DFN package. But some applications, while desirous of such small form factors, require more than the limited number of input/output contacts. Embodiments of the present invention provide a remedy to this issue. 
       FIG. 1  is a simplified block diagram illustrating a perspective view of a conventional dual flat no leads (DFN) package  100 . A semiconductor device die  110  is mounted active side up on a die pad (not illustrated) of a lead frame  120 . For purposes of this discussion, active side up refers to the major surface of semiconductor device die  110  having die bonding pads  130  and  135 , for example, thereon. The die bonding pads on the active surface of semiconductor device die  110  (e.g., die bonding pads  130  and  135 ) are electrically coupled to corresponding leads (e.g., package leads  140  and  145 ) using corresponding wire bonds (e.g. wire bonds  150  and  155 ). Semiconductor device die  110 , lead frame  120 , and wire bonds  150  and  155  are encapsulated in a molding material  160 . Upon singulation, the package leads are electrically isolated from one another and provide exposed contacts along sides of DFN package  100 . 
       FIG. 2  is a simplified block diagram illustrating a bottom surface of DFN package  100  subsequent to singulation. Along the bottom surface of DFN package  100  are exposed package leads, including package leads  140  and  145 . A molding material  160  forms the bulk of the bottom surface of DFN package  100 . The package leads on the package bottom surface provide electrical connections between semiconductor device die  110  and electrical contacts formed on a printed circuit board on which the DFN package is mounted for operation. 
     Typically, a flat no-leads package such as DFN package  100  is electrically coupled to the printed circuit board using solder reflow techniques. Limitations of solder reflow techniques and techniques involved in manufacturing of printed circuit boards restrict how close package leads along the perimeter of the DFN package can be placed. A lead pitch of 0.4 mm (i.e., the distance from the top of one lead to the top of the neighboring lead) is the practical limitation of lead spacing on no-leads packages today. Thus, for a 2 mm×2 mm DFN package, five leads can be placed along lead bearing peripheral edges of the package, resulting in a maximum of 10 leads total on the package. For some applications and desired functionality, this is too few input/output contacts. 
       FIG. 3  is a simplified block diagram of a semiconductor device package  300  and that provides additional input/output contacts in accord with embodiments of the present invention. As illustrated, semiconductor device package  300  provides not only input/output contacts along opposite edges of a bottom surface of the package (e.g.  310  and  320 ) but also a set of center input/output contacts  330 . The set of center input/output contacts  330  are provided in a region of the package beneath a location of a semiconductor device die within the encapsulated package. Center input/output contacts  330  increases the number of input/output contacts for semiconductor device package  300  by ½, while continuing to hold the area footprint of the semiconductor device package to the same area as a traditional DFN package (e.g., DFN package  100 ). As will be discussed more fully below, embodiments of the semiconductor device package  300  can be provided using an array lead frame to which a semiconductor device die is electrically coupled through the use of flip chip die attach techniques, rather than wire bonding. 
       FIG. 4  is a simplified block diagram illustrating one embodiment of a semiconductor device die  400  configured for use with embodiments of the present invention. The active surface of semiconductor device die  400  includes three rows of bonding pads. Bonding pads rows  410  and  420  are provided along a perimeter edges of semiconductor device die  400 , while bonding pad row  430  is provided in a center region of semiconductor device die  400 . As will be made clearer below, locations of the bonding pads should correspond with lead locations for the array lead frame incorporated in the semiconductor device package. The composition of, and techniques for, providing the bonding pads on semiconductor device die  400  correspond to those commonly used in the art of semiconductor device die fabrication. Further, the bonding pads are metalized, or otherwise treated, to make the bonding pads more receptive to conductive bumps used for a flip chip assembly process. 
       FIG. 5  is a simplified block diagram illustrating one embodiment of semiconductor device die  400  subsequent to formation of conductive bumps on the bonding pads of the active surface of the semiconductor device die. As for a typical flip chip attachment process, conductive bumps are formed on each bonding pad of the semiconductor device die (e.g., conductive bumps  510 ,  520 , and  530 ). Each bump provides a conductive path from a contact of semiconductor device die  400  to a lead frame to which the chip will be attached (as will be discussed in further detail below), as well as forming the mechanical mounting of the semiconductor device die to the lead frame. Embodiments of the present invention are not limited by the type of bumping process used, and therefore conductive bumps such as solder bumps formed by sputtering or plating, nickel-gold bumps formed by electroless nickel plating, gold stud bumps, conductive adhesive bumps, and the like can all be used. A preferred embodiment uses copper pillar bumps formed by a plating process, in which the copper pillar bumps have a solder cap. Copper pillar bumps are preferred in part because such a technique is well-suited to fine pitch applications, and uses existing bump infrastructures and standard materials and process flows and is therefore more easily and cost-effectively incorporated into the manufacturing process flow. 
       FIG. 6  is a simplified block diagram illustrating a portion of example array lead frame strip  600  usable for embodiments the present invention. Array lead frame strip portion  600  is part of a larger strip of array lead frames onto which many semiconductor device die can be placed. Each array lead frame unit  605  includes a number of leads corresponding to the number of semiconductor device die contacts (e.g., bonding pad rows  410 ,  420 , and  430 ). As illustrated, edge lead sets  610  and  620  correspond to bond pad rows  410  and  420 , respectively, of semiconductor device die  400 . Likewise, center lead set  630  corresponds to center bond pad row  430 . Edge leads  610  are coupled to center leads  630  by corresponding tie bars  615  and edge leads  620  are coupled to center leads  630  by corresponding tie bars  625 . 
     Each lead provides a surface to which a corresponding conductive bump on semiconductor device die can be attached. Each lead also provide a separate surface that will be exposed external to a semiconductor device package subsequent to a molding and singulation process, as will be discussed in greater detail below. As illustrated, each lead also can incorporate a locking feature (e.g., locking feature  640 ) configured to aid in restricting movement of an associated lead within an encapsulant. Lead frames within array lead frame strip  600  are coupled to one another via strip tie bars  650 . Array lead frame strip  600  can include as many array lead frame units  605  as desired for an application. In one embodiment, a strip that is 250 mm×70 mm can provide 2496 2 mm×2 mm array lead frame units. 
       FIG. 7  is a simplified block diagram illustrating a perspective view of semiconductor device die  400  attached to array lead frame unit  605  subsequent to a flip chip die attach process. Semiconductor device die  400  is placed on array lead frame unit  605 , using a typical pick and place technique. As discussed above, bumps on bonding pad row  410  are coupled to edge lead set  610 , bumps on bonding pad row  420  are coupled to edge lead set  620 , and bumps on bonding pad row  630  are coupled to center lead set  630 . A typical flip chip die attach process as known in the art can be used in which solder on the bumps (or forming the bumps) forms the attachment and the conductive path. In this manner, embodiments avoid the use of wire bonding techniques found in prior art flat no-leads packages. Since wire bonding is avoided, the semiconductor device die attached to the lead frame extends over the edge lead sets, and therefore can be larger in area than a semiconductor device die incorporated in a standard flat no-leads package. 
       FIG. 8  is a simplified block diagram illustrating a perspective view of a semiconductor device structure  800  at a step in processing subsequent to that illustrated in  FIG. 7 . A molding material is applied to the semiconductor device die affixed to array lead frame strip  600 , forming an encapsulant  810  that encapsulates the structures within the molding material and forms a panel. The molding material can be any appropriate encapsulant including, for example, silica-filled epoxy molding compounds, plastic encapsulation resins, and other polymeric materials such as silicones, polyimides, phenolics, and polyurethanes. The molding material can be applied by a variety of standard processing techniques used in encapsulation including, for example, printing, pressure molding and spin application. Once the molding material is applied, the panel can be cured by exposing the materials to certain temperatures for a period of time, or by applying curing agents, or both. In a typical encapsulation process, a depth of encapsulant  810  exceeds a maximum height of structures embedded in the molding material. Only a portion of a panel is illustrated I  FIG. 8 . 
       FIG. 9  is a simplified block diagram illustrating a perspective view of semiconductor device structure  800  after partial saw through and singulation, in accord with embodiments of the present invention. Exposed edge leads  610  and  620  are isolated from center leads  630  by removing tie bars  615  and  625 . In one embodiment, the tie bars are removed using a partial saw process that cuts through the tie bars and the encapsulant to a depth sufficient to remove all the tie bar material. The partial sawing can also provide cuts of a width sufficient to expose sides of the center leads while leaving sufficient center lead material to form good contacts with a printed circuit board to which the semiconductor device structure will be attached during use. In an alternative embodiment, the tie bars can be removed using a mask and etching process. One advantage of using a mask and etching process is that the etching can be selective to the mold material, and thus does not remove any molding material during the etching process. Once the tie bar material is removed, by either partial sawing or etching, individual semiconductor device package  910  can be singulated from the panel by performing a through cut sawing, thereby removing strip tie bars  650  and electrically isolating all the package leads. One advantage of using the partial sawing method to remove the tie bars is that such partial sawing can be performed during the same process step as singulation, thereby simplifying the process flow. 
       FIG. 10  is a simplified block diagram illustrating a cross section of semiconductor device package  910  electrically coupled to a printed circuit board (PCB)  1005 . PCB  1005  has electrical contacts  1010 ,  1020 , and  1030  that correspond to edge leads  610  and  620 , and center lead  630 , respectively. The leads of semiconductor device package  910  are placed over the PCB contacts and can be electrically coupled to the PCB contacts using solder reflow techniques known in the art, for example. 
     As discussed above, embodiments of the present invention provide numerous advantages over standard near chip scale packages. Due to the small form factors being demanded, standard near chip scale packages are limited in the number of input/output contacts that can be provided along the perimeter of the package. The discussed embodiments provide at least an increase of 50% the number of leads by providing a row of leads along the center region of the semiconductor device package. This allows for increased input/output capacity of the packaged semiconductor device. In addition, by using flip chip attachment techniques to bond the semiconductor device die to the array lead frame, wire bonding is avoided. Thus, the size of the semiconductor device die can extend closer to the edges of the semiconductor device package, allowing for potentially increased functional capabilities of the semiconductor device. Further, the use of a batch attachment process such as flip chip over wire bonding allows for a simpler, faster, and potentially less expensive process flow for assembling the semiconductor device package. 
     By now it should be appreciated that there has been provided an array lead frame for a semiconductor device package, the array lead frame includes: a first row of a first plurality of edge leads; a second row of a second plurality of edge leads; a third row of a third plurality of center leads; a first set of tie bars coupling a center lead of the third plurality of center leads to one or more edge leads of the first plurality of edge leads; and, a second set of tie bars coupling the center lead of the third plurality of center leads to one or more edge leads of the second plurality of edge leads. Each lead is configured for electrical coupling to a corresponding die bonding pad using a flip chip die attach. 
     In one aspect of the above embodiment, a quantity of the first plurality of edge leads is the same as a quantity of the second plurality of edge leads. In a further aspect, a quantity of center leads is the same as the quantity of the first plurality of edge leads, the first set of tie bars includes one tie bar from the center lead to a corresponding edge lead of the first plurality of edge leads, and the second set of tie bars includes one tie bar from the center lead to a corresponding edge lead of the second plurality of edge leads. 
     Another embodiment provides a semiconductor device package that includes: an array lead frame having a first row of a first plurality of edge leads, a second row of a second plurality of edge leads, a third row of a third plurality of center leads, a first set of tie bars coupling a center lead to one or more edge leads of the first plurality of edge leads, and a second set of tie bars coupling the center lead to one or more edge leads of the second plurality of edge leads; a semiconductor device die; and a mold compound formed over and around the semiconductor device die and encapsulating a portion of each lead of the array lead frame. The semiconductor device die has a first major surface and a plurality of bond pads on the first major surface. Each bond pad is located in a geometric location corresponding to an edge lead or center lead of the array lead frame. Each bond pad is electrically coupled to the corresponding edge lead or center lead of the array lead frame. 
     In one aspect of the above embodiment, the semiconductor device die further includes a conductive bump formed on each bond pad. The electrical coupling of each bond pad to a corresponding edge lead or center lead is performed using the conductive bump for a flip chip die attach. In a further aspect, the conductive bump is one of a copper pillar bump, a solder bump, a stud, and a conductive adhesive bump. 
     In another aspect of the above embodiment, the semiconductor device package further includes the first plurality of edge leads being exposed along a first edge of the semiconductor device package on a first major surface of the semiconductor device package, the second plurality of edge leads exposed along a second edge of the semiconductor device package on the first major surface of the semiconductor device package where the second edge is opposite the first edge of the semiconductor device package, and the third plurality of center leads is exposed in a center region of the first major surface of the semiconductor device package. In another aspect of the above embodiment, the first plurality of edge leads, the second plurality of edge leads, and the third plurality of center leads are electrically isolated by removal of the first set of tie bars and the second set of tie bars subsequent to formation of the mold compound. In a further aspect, removal of the first set of tie bars and second set of tie bars is performed by a partial sawing operation. 
     Another embodiment of the present invention provides for a method of forming a semiconductor device package, where the method includes: providing an array lead frame having a plurality of leads; conductively attaching a semiconductor device die to the array lead frame; and, encapsulating the semiconductor device die and a portion of the array lead frame by forming a mold compound over and around the semiconductor device die and the portion of the array lead frame. The semiconductor device die includes a plurality of bonding pads on a major surface of the semiconductor device die. Each bonding pad has a corresponding lead of the plurality of leads. Conductively attaching comprises a flip chip die attach process. 
     One aspect of the above embodiment further includes partially sawing the semiconductor device package and singulating the semiconductor device package from a panel formed by said encapsulating. The partially sawing is performed subsequent to the encapsulating and removes a plurality of tie bars coupling pairs of the leads of the array lead frame such that the leads are rendered electrically isolated. In a further aspect, subsequent to partially sawing and singulating, the semiconductor device package includes a plurality of leads exposed along two or more perimeter edges of a major surface of the semiconductor device package and a plurality of leads exposed on a center region of the major surface of the semiconductor device package. 
     Another aspect of the above embodiment further includes forming a conductive bump on each bonding pad of the semiconductor device die. In a further aspect, the conductive bump includes one of a copper pillar bumps, a solder bump, a stud, and conductive adhesive. A still further aspect of the above embodiment includes placing the semiconductor device die on the array lead frame such that each conductive bump is in contact with a lead on the array lead frame. 
     Because the apparatus implementing the present invention is, for the most part, composed of electronic components and circuits known to those skilled in the art, circuit details will not be explained in any greater extent than that considered necessary as illustrated above, for the understanding and appreciation of the underlying concepts of the present invention and in order not to obfuscate or distract from the teachings of the present invention. 
     Moreover, the terms “front,” “back,” “top,” “bottom,” “over,” “under” and the like in the description and in the claims, if any, are used for descriptive purposes and not necessarily for describing permanent relative positions. It is understood that the terms so used are interchangeable under appropriate circumstances such that the embodiments of the invention described herein are, for example, capable of operation in other orientations than those illustrated or otherwise described herein. 
     Although the invention is described herein with reference to specific embodiments, various modifications and changes can be made without departing from the scope of the present invention as set forth in the claims below. For example, depending upon advances made in pitch size, additional input/output leads can be placed in any row of a semiconductor device package, and potentially multiple rows of connectors could be provided along the center area of a package. Accordingly, the specification and figures are to be regarded in an illustrative rather than a restrictive sense, and all such modifications are intended to be included within the scope of the present invention. Any benefits, advantages, or solutions to problems that are described herein with regard to specific embodiments are not intended to be construed as a critical, required, or essential feature or element of any or all the claims. 
     The term “coupled,” as used herein, is not intended to be limited to a direct coupling or a mechanical coupling. 
     Furthermore, the terms “a” or “an,” as used herein, are defined as one or more than one. Also, the use of introductory phrases such as “at least one” and “one or more” in the claims should not be construed to imply that the introduction of another claim element by the indefinite articles “a” or “an” limits any particular claim containing such introduced claim element to inventions containing only one such element, even when the same claim includes the introductory phrases “one or more” or “at least one” and indefinite articles such as “a” or “an.” The same holds true for the use of definite articles. 
     Unless stated otherwise, terms such as “first” and “second” are used to arbitrarily distinguish between the elements such terms describe. Thus, these terms are not necessarily intended to indicate temporal or other prioritization of such elements.