Abstract:
A semiconductor device has a die mounted on a die paddle that is elevated above and thermally connected via tie bars to a heat sink structure. Heat generated by the die flows from the die to the die paddle to the tie bars to the heat sink structure and then to either the external environment or to an external heat sink. By elevating the die/paddle sub-assembly above the heat sink structure, the packaged device is less susceptible to delamination between the die and die attach adhesive and/or the die attach adhesive and the die paddle. An optional heat sink ring can surround the die paddle.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present invention relates generally to packaged semiconductor devices and, more particularly, to techniques for relieving stress in semiconductor sensor devices. 
         [0002]    In conventional packaged semiconductor devices, dies are typically mounted, using epoxy or other suitable adhesive, directly on die paddles, which, when configured for use, may themselves be connected directly to external heat sinks designed to conduct heat away from the heat-generating dies. Such configurations can induce mechanical stresses to the paddles and dies potentially resulting in undesirable de-lamination of the interface between the die and the epoxy and/or undesirable de-lamination of the interface between the epoxy and the paddle. If the weak adhesion between the epoxy and the die/paddle is addressed, the mechanical stresses can shift from the epoxy to the die itself, resulting in mechanical failure (e.g., cracks) in the die. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0003]    Embodiments of the present invention are illustrated by way of example and are not limited by the accompanying figures, in which like references indicate similar elements. Elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the thicknesses of layers and regions may be exaggerated for clarity. 
           [0004]      FIG. 1  shows a cross-sectional side view of a packaged semiconductor device mounted on an external heat sink in accordance with an embodiment of the invention; and 
           [0005]      FIG. 2  illustrates a step in a process for assembling a packaged semiconductor device in accordance with an embodiment of the present invention; 
           [0006]      FIG. 3  illustrates a step in a process for assembling a packaged semiconductor device in accordance with an embodiment of the present invention; and 
           [0007]      FIG. 4  illustrates a step in a process for assembling a packaged semiconductor device in accordance with an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0008]    Detailed illustrative embodiments of the present invention are disclosed herein. However, specific structural and functional details disclosed herein are merely representative for purposes of describing example embodiments of the present invention. Embodiments of the present invention may be embodied in many alternative forms and should not be construed as limited to only the embodiments set forth herein. Further, the terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of example embodiments of the invention. 
         [0009]    As used herein, the singular forms “a,” “an,” and “the,” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It further will be understood that the terms “comprises,” “comprising,” “has,” “having,” “includes,” and/or “including” specify the presence of stated features, steps, or components, but do not preclude the presence or addition of one or more other features, steps, or components. It also should be noted that, in some alternative implementations, the functions/acts noted may occur out of the order noted in the figures. For example, two figures shown in succession may in fact be executed substantially concurrently or may sometimes be executed in the reverse order, depending upon the functionality/acts involved. 
         [0010]    One embodiment of the invention is an article of manufacture comprising a lead frame for a packaged semiconductor device. The lead frame comprises a heat sink structure defining a first plane; a die paddle defining a second plane substantially parallel to the first plane and displaced from the first plane; a plurality of tie bars physically and thermally interconnecting the heat sink structure and the die paddle to form a heat sink/paddle sub-assembly; and a plurality of leads positioned adjacent to the heat sink/paddle sub-assembly. 
         [0011]    Another embodiment of the invention is a method for assembling an article of manufacture and the resulting article. A die is die-bonded onto a die paddle of a lead frame, wherein the lead frame comprises a heat sink structure defining a first plane; the die paddle defining a second plane substantially parallel to the first plane and displaced from the first plane; a plurality of tie bars physically and thermally interconnecting the heat sink structure and the die paddle to form a heat sink/paddle sub-assembly; and a plurality of leads positioned adjacent to the heat sink/paddle sub-assembly. The die is wire bonded to one or more of the leads using one or more corresponding bond wires. Mold compound is applied to encapsulate the die and the one or more bond wires, wherein a portion of the heat sink structure and a portion of each lead are exposed. 
         [0012]      FIG. 1  shows a cross-sectional side view of a packaged semiconductor device  100  mounted on an external heat sink  150  in accordance with an embodiment of the invention. The packaged device  100  has a die  102  die-bonded to a die paddle  104  with a die-attach adhesive  106  and wire bonded to a plurality of leads  108  with bond wires  110 . The die paddle  104  is connected to a heat sink ring  112  by multiple tie bars  114 . The die  102 , die paddle  104 , bond wires  110 , and tie bars  114  are encapsulated and the leads  108  and heat sink ring  112  are embedded all within an electrically insulating mold compound  116  to form the QFP (quad flat pack) packaged semiconductor device  100  having leads that extend beyond the lateral dimensions of the mold compound  116 . 
         [0013]    Those skilled in the art will understand that alternative embodiments of the invention include QFN (quad flat no-lead) packaged semiconductor devices whose leads do not extend beyond the lateral dimensions of the mold compound. Further alternative embodiments include (without limitation) ball grid array (BGA) packages, molded array packages (MAP), and other leaded packages. 
         [0014]    Whether the packaged device is a QFP device or a QFN device or some other type of device, at least some surface of each lead  108  is exposed (i.e., not completely encapsulated within the mold compound  116 ). Similarly, as viewed in  FIG. 1 , the bottom surface of the heat sink ring  112  is also exposed. 
         [0015]    Conventional, the electrically insulating die-attach adhesive  106  may be used to attach the die  102  to the die paddle  104 . Those skilled in the art will understand that suitable alternative means, such as die-attach tape, may be used. The leads  108  may be formed of copper, an alloy of copper, a copper plated iron/nickel alloy, plated aluminum, or the like. Often, copper leads are pre-plated first with a nickel base layer, then a palladium mid layer, and finally with a very thin, gold upper layer. The bond wires  110  are formed from a conductive material such as aluminium, silver, gold, or copper, and may be either coated or uncoated. The mold compound  116  may be a plastic, an epoxy, a silica-filled resin, a ceramic, a halide-free material, the like, or combinations thereof, is known in the art. 
         [0016]    As shown in  FIG. 1 , the heat sink ring  112  lies in a first plane at the bottom of the packaged device  100 , while the die paddle  104  lies in a second plane substantially parallel to and displaced above the first plane, with the tie bars  114  providing down steps between the die paddle  104  and the heat sink ring  112 . Similarly, each lead  108  provides a down step (middle section) from an upper or inner lead portion lying in the second plane to a lower or outer lead portion lying in the first plane. Depending on the particular implementation, the leads  108  may have any suitable shape, such as “gull wing” or J-shaped. 
         [0017]    In the configuration of  FIG. 1 , the packaged device  100  is mounted on an external heat sink  150  using thermally conductive solder  152 . In operation, heat generated within the die  102  is thermally conducted from the die  102  to the die paddle  104  via the adhesive  106 , from the die paddle  104  to the heat sink ring  112  via the tie bars  114 , and from the heat sink ring  112  to the heat sink  150  via the solder  152 . Thus, in this embodiment, the heat sink ring  112  forms a heat sink structure that is part of the thermal path between the die paddle  104  and the external heat sink  150 . 
         [0018]    In a conventional packaged semiconductor device, the die paddle would lie in the first plane such that, when configured for use, the die paddle would be in direct or at least relatively close thermal contact with a heat sink such as heat sink  150 . In the configuration shown in  FIG. 1 , however, the die paddle  104  lies in the second plane, such that the die  102  and the die paddle  104  are displaced from the first plane and therefore from the heat sink  150 . As a result, the mechanical stresses in configurations of conventional packaged devices that result from the close thermal contact with a heat sink are eliminated or at least reduced in the configuration of  FIG. 1 . As a result, the corresponding undesirable de-laminations of the prior art can be avoided. 
         [0019]      FIGS. 2-4  illustrate one possible process for assembling packaged semiconductor device  100  of  FIG. 1 . 
         [0020]      FIGS. 2(A) and 2(B)  show a cross-sectional side view and a top plan view, respectively, of a lead frame  200  comprising the die paddle  104 , the leads  108 , the heat sink ring  112 , and the tie bars  114  of  FIG. 1 . Note that the lead frame  200  is an un-embedded lead frame whose elements are not yet embedded in mold compound. As such, those elements need to be suitably supported prior to subsequent application of mold compound during an encapsulation phase of the assembly process (described below with respect to  FIG. 4 ). 
         [0021]    Those skilled in the art will understand that a two-dimensional array of different instances of the packaged device  100  of  FIG. 1  are assembled using a lead-frame assembly consisting of a two-dimensional array of instances of the lead frame  200  of  FIG. 2 , where, except for the outermost leads of the lead-frame assembly, each lead  108  is part of a lead structure in the lead-frame assembly that includes a corresponding lead for the adjacent instance of the lead frame  200 . At the end of the assembly process, the individual packaged devices  100  are separated during a saw or laser singulation step in which each lead structure is sliced into two leads  108 , one for each of two different instances of the packaged device  100 . The leads may then be appropriately shaped (e.g., gull wing or J) for the particular implementation. 
         [0022]    As shown in  FIG. 2(B) , the lead frame  200  also includes four support bars  202  which provide mechanical support for each sub-assembly consisting of the die paddle  104 , heat sink ring  112 , and tie bars  114  in the two-dimensional lead-frame assembly prior to application of the mold compound  116 . Although not shown in the figures, the support bars  202  also have down steps analogous to those of the leads  108 . 
         [0023]      FIGS. 3(A) and 3(B)  show a cross-sectional side view and a top plan view, respectively, of the lead frame  200  of  FIG. 2  after the die  102  has been (i) die-bonded to the die paddle  104  using the adhesive  106  and (ii) wire bonded to the leads  108  using the bond wires  110 . 
         [0024]      FIGS. 4(A) ,  4 (B), and  4 (C) show a cross-sectional side view, an X-ray top plan view, and a bottom plan view, respectively, of the assembly of  FIG. 3  after the mold compound  116  has been applied to (i) encapsulate the die  102 , die paddle  104 , bond wires  110 , and tie bars  114  and (ii) embed the leads  108  and heat sink ring  112 , leaving exposed the ends of the leads  108  and the bottom of the heat sink ring  112 . 
         [0025]    One way of applying the molding compound  116  is using a mold insert of a conventional injection-molding machine, as is known in the art. The molding material is typically applied as a liquid polymer, which is then heated to form a solid by curing in a UV or ambient atmosphere. The molding material can also be a solid that is heated to form a liquid for application and then cooled to form a solid mold. Subsequently, an oven is used to cure the molding material to complete the cross linking of the polymer. As known in the art, the mold compound  116  may be applied using a dam bar design on the leads, where any excess mold compound is later removed using a suitable de-junking process. In alternative embodiments, other encapsulating processes may be used. 
         [0026]    Note that this implementation involves a single application of mold compound. In an alternative implementation, the lead frame  200  is pre-molded prior to the die-bonding and wire bonding of  FIG. 3 , in which case the corresponding encapsulation step of  FIG. 4  would involve the application of the mold compound  116  on top of the pre-molded lead frame. 
         [0027]      FIGS. 5(A)-5(D)  show respectively a 3D perspective view, a top view, a bottom view, and a partial side view of the leads  502 , the die paddle  504 , and the four pads  506  of a lead frame for a packaged semiconductor device  500  according to an alternative embodiment of the invention. As shown in the figures, the die paddle  504  has two intersecting arms that form an X shape, while the four pads  506  are all triangular in shape. 
         [0028]    As shown in  FIG. 5(D) , the four pads  506  lie in a first plane at the bottom of the packaged device  500 , while the die paddle  504  lies in a second plane substantially parallel to and displaced above the first plane, with multiple tie bars  508  providing down steps between the die paddle  504  and the four pads  506 . Similarly, each lead  502  provides a down step from an upper lead portion to a lower lead portion. In the particular implementation of  FIG. 5 , the leads  502  are “gull wing” shaped. 
         [0029]    In the configuration of  FIG. 5 , the packaged device  500  could be mounted on an external heat sink using thermally conductive solder in a manner analogous to that shown in  FIG. 1 . In operation, heat generated within the die or dies (not shown) mounted on the die paddle  504  would be thermally conducted from the die(s) to the die paddle  504  via the adhesive used to secure the die(s) to the die paddle  504 , from the die paddle  504  to the four pads  506  via the 12 tie bars  508 , and from the four pads  506  to the external heat sink (not shown) via the solder used to secure the packaged device  500  to the external heat sink. Thus, in this embodiment, the four pads  506  form a heat sink structure that is part of the thermal path between the die paddle  504  and the external heat sink. 
         [0030]    As described previously, in a conventional packaged semiconductor device, the die paddle would lie in the first plane such that, when configured for use, the die paddle would be in direct or at least relatively close thermal contact with an external heat sink such as heat sink  150 . In the configuration shown in  FIG. 5 , however, the die paddle  504  lies in the second plane, such that the die(s) and the die paddle  504  are displaced from the first plane and therefore from the external heat sink. As a result, the mechanical stresses in configurations of conventional packaged devices that result from the close thermal contact with a heat sink are eliminated or at least reduced in the configuration of  FIG. 5 . As a result, the corresponding undesirable de-laminations of the prior art can be avoided. 
         [0031]    As used herein, the term “mounted on” as in “a first die mounted on a die paddle” covers situations in which the first die is mounted directly on the die paddle with no other intervening dies or other structures (as in the mounting of the die  102  on the die paddle  104  in  FIG. 3 ) as well as situations in which the first die is directly mounted on another die, which is itself mounted directly on the die paddle. Note that “mounted on” also covers situations in which there are two or more intervening dies between the first die and the die paddle. Depending on the situation, the term “mounted” can imply electrical connection in addition to physical attachment, where the electrical connection may be provided by one or more bond wires, one or more solder bumps, and/or any other suitable technique. 
         [0032]    By now it should be appreciated that there has been provided an improved packaged semiconductor device and a method of forming the improved packaged semiconductor device. Circuit details are not disclosed because knowledge thereof is not required for a complete understanding of the invention. 
         [0033]    Although the invention has been described using relative terms such as “front,” “back,” “top,” “bottom,” “over,” “above,” “under” and the like in the description and in the claims, such terms 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. 
         [0034]    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. Further, 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. 
         [0035]    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. 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. 
         [0036]    It should be understood that the steps of the exemplary methods set forth herein are not necessarily required to be performed in the order described, and the order of the steps of such methods should be understood to be merely exemplary. Likewise, additional steps may be included in such methods, and certain steps may be omitted or combined, in methods consistent with various embodiments of the invention. 
         [0037]    Although the elements in the following method claims, if any, are recited in a particular sequence with corresponding labeling, unless the claim recitations otherwise imply a particular sequence for implementing some or all of those elements, those elements are not necessarily intended to be limited to being implemented in that particular sequence. 
         [0038]    Reference herein to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment can be included in at least one embodiment of the invention. The appearances of the phrase “in one embodiment” in various places in the specification are not necessarily all referring to the same embodiment, nor are separate or alternative embodiments necessarily mutually exclusive of other embodiments. The same applies to the term “implementation.” 
         [0039]    The embodiments covered by the claims in this application are limited to embodiments that (1) are enabled by this specification and (2) correspond to statutory subject matter. Non enabled embodiments and embodiments that correspond to non statutory subject matter are explicitly disclaimed even if they fall within the scope of the claims.