Abstract:
A lead frame has a trace embedded in an encapsulant and a plurality of stubs (i) embedded in the encapsulant and (ii) connected to and extending from the trace at different locations along the length of the trace. The stubs inhibit the formation of cracks that may otherwise form along the trace due to thermal or mechanical bending of the lead frame, especially cracks that tend to occur along the four linear edge traces located at the periphery of some conventional embedded lead frames.

Description:
BACKGROUND 
       [0001]    The present invention relates generally to semiconductor packaging, and, more particularly, to lead frames used in assembling semiconductor devices. 
         [0002]      FIG. 1A  shows a top view of one implementation of a conventional embedded lead frame  102 , and  FIG. 1B  shows a top view of a portion of one implementation of a conventional embedded lead frame array  100  having multiple instances of the embedded lead frame  102 . The lead frame array  100  is fabricated using known molded interconnection system (MIS) techniques that enable the lead frame array  100  to be relatively thin (e.g., 0.112 mm) compared to embedded lead frame arrays fabricated using other techniques. 
         [0003]    Referring to  FIG. 1A , the lead frame  102  has a pattern of metal structures embedded in an encapsulant  104  (i.e., a molding compound). In general, a lead frame is a collection of metal leads and possibly other elements (e.g., die flags and power bars) that is used in semiconductor packaging for assembling a single packaged semiconductor device. Prior to assembly, a lead frame may have support structures (e.g., a rectangular metal frame) that keep the elements of the lead frame in place. During the assembly process, the support structures may be removed. As used herein, the term “lead frame” may be used to refer to the collection of elements before assembly or after assembly, regardless of the presence or absence of those support structures. 
         [0004]    In this particular embodiment, the lead frame  102  includes (i) a die flag  106  (also known as a die pad or die paddle), (ii) a plurality of leads  112 , (iii) four corner pads  108 , and (iv) four linear edge traces  110 . The linear edge traces  110  connect different pairs of adjacent ones of the corner pads  108 . 
         [0005]    Each lead  112  has (i) an external pad area  114  that allows the assembled device to be connected to other devices or a printed circuit board, (ii) a wire-bond pad  118  where a bond wire is attached for connecting the lead  112  to an IC die (not shown) subsequently mounted on the die flag  106 , and (iii) a lead trace  116  connecting the external pad area  114  to the wire-bond pad  118 . 
         [0006]    The external pad areas  114  are exposed or formed entirely through the encapsulant  104  so that, for example, a solder ball may be disposed on the bottom of the bond area  114  exposed on the bottom surface of the lead frame  102 . The wire-bond pads  118 , the lead traces  116 , and the linear edge traces  110  are formed part of the way through the encapsulant  104  such that the encapsulant  104  directly under the wire-bond pads  118 , the lead traces  116 , and the linear edge traces  110  is thinner than in areas of the lead frame  102  where no metal structures are present. 
         [0007]    During device assembly, one or more IC dies (not shown) are adhesively mounted on the die flag  106 . Wire bonding is performed, where metal bond wires (not shown) are strung between and bonded to bond pads on each IC die and corresponding wire-bond pads  118  of the lead frame  102 . 
         [0008]    Following wire bonding, the upper surface of the lead frame  102 , the IC die(s), and the bond wires are encapsulated in molding compound. The molding compound is subsequently cured. After encapsulation, solder balls (not shown) may be deposited on the exposed external pad areas  114 . The solder balls, together with the leads  112 , provide electrical connections between electronic components internal to the IC die and electronic components external to the packaged device. External components might include power sources and input/output connections on a printed circuit board (PCB) on which the packaged semiconductor device is mounted. 
         [0009]    Referring to  FIG. 1B , multiple packaged semiconductor devices (not shown) are typically assembled concurrently on an embedded lead frame array such as the embedded lead frame array  100 . The lead frame array  100  includes (i) an array of instances (e.g.,  102   a - d ) of the embedded lead frame  102  upon which the multiple packaged semiconductor devices are assembled and (ii) a peripheral region  120  around the perimeter of the lead frame array  100 . 
         [0010]    In  FIG. 1B , one complete lead frame  102   a , portions of three adjacent lead frames  102   b - d , and a portion of the peripheral region  120  on the top and left-hand sides of the lead frame array  100  are shown. Although not shown, the peripheral region  120  borders all four sides of the lead frame array  100 , and the lead frame array  100  may comprise additional rows and columns of instances of the lead frame  102 . 
         [0011]    The peripheral region  120  includes a plurality of cylindrical metal structures  122  (a.k.a. current stealers) embedded in the encapsulant  104 . The current stealers  122  help to uniformly distribute electrical current across the surface of the lead frame array  100  during electroplating. 
         [0012]    To separate the semiconductor packaged devices assembled on the lead frames  102   a - d  from one another and from the peripheral region  120 , singulation is performed whereby cuts are made along dashed lines  126 . Note that dashed lines  126  do not represent physical markings, but are merely provided to show where the cuts are made. Cutting along the dashed lines  126  leaves a border  128  of encapsulant  104  around the perimeter of each packaged semiconductor device as shown in  FIG. 1A . 
         [0013]    The MIS substrate or lead frame  102  is susceptible to handling and thermally induced warpage cracks along the trace-to-mold interfaces. As these lead frames  102  are ultra thin (0.112 mm total) and the mold compound  104  is thin directly below the traces, there are weaknesses in the structure. This situation is compounded when the lead frames  102  do not have a solder mask coating, which may be left off due to the increased cost involved in including such a coating. Thus, it would be beneficial to have a stronger lead frame or MIS substrate such that it is less susceptible to cracking and warpage. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]    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. 
           [0015]      FIG. 1A  shows a top view of one implementation of a conventional embedded lead frame; 
           [0016]      FIG. 1B  shows a top view of a portion of one implementation of a conventional embedded lead frame array having multiple instances of the embedded lead frame of  FIG. 1A ; 
           [0017]      FIG. 2A  shows a top view of an embedded lead frame according to one embodiment of the present invention; 
           [0018]      FIG. 2B  shows a top view of a portion of an embedded lead frame array according to one embodiment of the present invention having multiple instances of the embedded lead frame of  FIG. 2A ; 
           [0019]      FIG. 3  is a cross-sectional side view of a packaged semiconductor device according to one embodiment of the present invention; and 
           [0020]      FIG. 4  is an enlarged plan view of a portion of a lead frame according to an alternative embodiment having lead traces with metal stubs. 
       
    
    
     DETAILED DESCRIPTION 
       [0021]    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 present invention. 
         [0022]    When the embedded lead frame array  100  of  FIG. 1A  is subjected to temperature changes, the lead frame array  100  can warp, sometimes causing the lead frame array  100  to crack. Further, the lead frame array  100  can crack during normal handling of the lead frame array  100 . Typically, cracks form at the interface between (i) an elongated trace, such as one of the linear edge traces  110  of lead frame  102 , and (ii) the encapsulant  104 . This may be due, at least in part, to the reduced thickness of the encapsulant  104  under the elongated trace. Such cracking may be even more likely to occur when the upper surface of the lead frame array  100  is not coated with soldermask to cut costs. 
         [0023]    When a crack forms, the crack tends to propagate entirely through the thickness of the lead frame array  100  and along the length of the elongated trace. For example, a crack could propagate through the lead frame  102  of  FIG. 1A  along the dashed line  124  defined by the bottom edge of the lead frame  102 . In this case, the lower border  128  of the lead frame  102  (i.e., below dashed line  124  in the view shown in  FIG. 1A ) becomes separated from the adjacent linear edge trace  110 , thereby exposing the adjacent linear edge trace  110  of the lead frame  102  to the ambient environment. 
         [0024]    In the following description, it will be understood that certain embodiments of the present invention are directed to lead frames comprising metal features for preventing cracks such as those discussed above and articles of manufacture comprising such lead frames. Although one particular type of lead frame is described in the embodiment below, it will be understood that embodiments of the present invention are not so limited. According to alternative embodiments of the present invention, these metal features can be implemented in other suitable types of lead frames. 
         [0025]    In one embodiment of the present invention, an article of manufacture comprises a lead frame, wherein the lead frame comprises a trace embedded in an encapsulant, and a plurality of stubs (i) embedded in the encapsulant and (ii) connected to and extending from the trace at different locations along the length of the trace. 
         [0026]      FIG. 2A  shows a top view of an embedded lead frame  202  according to one embodiment of the present invention, and  FIG. 2B  shows a top view of an embedded lead frame array  200  according to one embodiment of the present invention that comprises multiple instances of the lead frame  202 . The lead frame  202  comprises a pattern of metal structures embedded in an encapsulant  204  such as a molding compound, where the metal pattern comprises (i) a die flag  206 , (ii) a plurality of leads  212 , (iii) four corner pads  208 , and (iv) four linear edge traces  210 , which are similar to the analogous components in  FIG. 1A . 
         [0027]    In addition, the lead frame  202  comprises a plurality of outer metal stubs  230  and inner metal stubs  232 . Each outer metal stub  230  extends from a linear edge trace  210  of a lead frame  202  to an edge of the encapsulant border  228  of the lead frame  202 . Each inner metal stub  232  extends from a linear edge trace  210  of a lead frame  202  away from the peripheral edge of the lead frame  202  and terminates before reaching any other metal structures. In this embodiment, each inner stub  232  is positioned between a different pair of adjacent external pads  214  along the perimeter of lead frame  202 ; however, other spacings are possible. For example, the inner stubs  232  could be spaced between every other external pad  214 . 
         [0028]    The metal stubs  230  and  232  strengthen the lead frame  202  such that the lead frame  202  is less susceptible to cracking than the lead frame  102  of  FIG. 1A . In particular, the metal stubs  230  and  232  inhibit bending along the sides of the linear edge traces  210  due to handling and/or temperature-induced warping, where such bending could ultimately lead to cracks. 
         [0029]    Referring now to  FIG. 2B , similar to the embedded lead frame array  100  of  FIG. 1B , the embedded lead frame array  200  comprises an array of instances (e.g.,  202   a - d ) of the lead frame  202  and peripheral region  220  around a perimeter of the lead frame array  200 . The peripheral region  220  comprises a plurality of cylindrical metal structures  222 , analogous to the cylindrical metal structures  122  in  FIG. 1B , which are embedded in the encapsulant  204 . 
         [0030]    In this embodiment, each outer metal stub  230  of each lead frame  202   a - d  is interconnected with either (i) an outer metal stub  230  of an adjacent lead frame or (ii) a cylindrical metal structure  222  of an adjacent portion of the peripheral region  220 . The outer stubs  230  are spaced by a distance that is equal to the distance between every other cylindrical metal structure  222 ; however, other spacings are possible. 
         [0031]    Overall, interconnecting the lead frames  202   a - d  to one another and to the peripheral region  220  using the outer metal stubs  230  provides a lead frame array structure that is more resistant to bending. Further, each metal stub  230  and  232  provides a stop that may prevent a crack from propagating along the length of a linear edge trace  210 . Metal stubs  230  and  232  can be incorporated into lead frame array designs with little, if any, cost, and do not require special routing of the leads  212  to avoid the stubs  230  and  232 . 
         [0032]    Packaged semiconductor devices may be assembled on the embedded lead frames  202   a - d  in a manner similar to that discussed above in relation to the embedded lead frame  102  of  FIG. 1A . Note, however, that the cuts made along the dashed lines  226  during singulation separate each outer metal stub  230  of each lead frame  202   a - d  from either (i) the corresponding metal stub  230  of an adjacent lead frame or (ii) the corresponding cylindrical metal structure  222  of an adjacent peripheral region  220 . As a result, the outer metal stubs  230  of each lead frame  202   a - d  terminate at the outer edge of the border  228  without directly connecting to any other metal structures in the lead frame. 
         [0033]      FIG. 3  shows a cross-sectional view of a packaged semiconductor device  300  assembled on the lead frame  202  of FIG.  2 A according to one embodiment of the present invention. As shown, an IC die  306  is adhesively mounted on the die flag  206  using an adhesive  308  such as a die-attach tape or epoxy, and the IC die  306  is electrically connected to wire-bond pads  218  with bond wires  304 . Note that the lead traces  216  shown in  FIG. 2  interconnecting the wire-bond pads  218  and the external pads  214  extend into or out of the cross-sectional view of  FIG. 3  and are therefore not visible in the view of  FIG. 3 . The upper surface of the lead frame  202 , the IC die  306 , and the bond wires  304  are encapsulated in a molding compound  302 . 
         [0034]    Although one embodiment of the present invention was described as implementing metal stubs  230  and  232  along linear edge traces  210  on the perimeter of lead frame  202 , embodiments of the present invention are not so limited. In general, metal stubs may be implemented on any suitable trace of a lead frame. 
         [0035]    For example,  FIG. 4  shows a view analogous to the detail view of  FIG. 2A  of a lead frame  402  according to an alternative embodiment having lead traces  416  with metal stubs  434 . As shown, the stubs  434  extend along the length of the lead traces  416  between external pads  414  and the wire-bond pads  418 . The length of the stubs is limited to prevent the stubs from electrically coupling with adjacent metal structures such as adjacent lead traces  416 . 
         [0036]    Further, some embodiments of the present invention might not have linear edge traces. In such embodiments, the metal stubs may be implemented on suitable traces within the interior of a lead frame. 
         [0037]    In general, the particular configuration of the lead frame  202  shown in  FIG. 2A  is merely exemplary to illustrate the use of metal stubs along a metal trace. Embodiments of the present invention are not limited to the particular lead frame configuration shown in  FIG. 2A . 
         [0038]    According to alternative embodiments of the present invention, metal stubs such as stubs  230  and  232  may be implemented on types of lead frames other than that shown in  FIG. 2A , including (without limitation) pin grid array lead frames, chip-on-lead (COL) lead frames, quad-flat no-leads (QFN) lead frames, and other lead frames that manufactured in layers using additive manufacturing steps, subtractive manufacturing steps, or a combination of additive and subtractive manufacturing steps. 
         [0039]    Further, according to alternative embodiments of the present invention, the particular features of the lead frame  202  may vary. For example, the size and shape of the lead frame  202 , the number and arrangement of the leads  212 , and the size and shape of the die flag  206  may vary. 
         [0040]    Although  FIG. 2A  shows an embodiment in which the metal stubs  230  and  232  extend substantially perpendicularly from the linear edge traces  210 , embodiments of the present invention are not so limited. According to alternative embodiments of the present invention, metal stubs may be implemented to extend from traces at angles other than 90 degrees. 
         [0041]    Further, although  FIG. 2A  shows an embodiment in which metal stubs extend from both sides of a metal trace, embodiments of the present invention are not so limited. According to alternative embodiments, the metal stubs could extend from only one side of a metal trace. 
         [0042]    Yet further, although  FIG. 2B  shows the outer metal stubs  230  of each lead frame  202   a - d  interconnecting with the outer metal stubs  230  of adjacent lead frames, embodiments of the present invention are not so limited. According to alternative embodiments of the present invention, the metal stubs of adjacent lead frames could be staggered such that they do not interconnect. 
         [0043]    Even further, although  FIG. 2B  shows an embodiment of the present invention in which some of the outer metal stubs  230  extend to corresponding cylindrical metal structures  222  in the peripheral region  220 , embodiments of the present invention are not so limited. According to some alternative embodiments of the present invention, the metal stubs may extend to metal structures in the peripheral region that are not cylindrical. For example, the right-most column of cylindrical metal structures could be replaced with a metal trace. According to other alternative embodiments of the present invention, the metal stubs may extend to the peripheral region without connecting to any metal structure in the peripheral region. 
         [0044]    It will be further understood that various changes in the details, materials, and arrangements of the parts which have been described and illustrated in order to explain the nature of this invention may be made by those skilled in the art without departing from the scope of the invention as expressed in the following claims. For example, more than one IC die may be mounted onto the die flag  206 . As another example, an IC die may be electrically connected to the leads of a lead frame of the present invention using electrical interconnections other than bond wires, such as flip-chip bumps. As yet another example, lead frames of the present invention may be formed using photolithography or other techniques. 
         [0045]    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. 
         [0046]    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.” 
         [0047]    Terms of orientation such as “lower,” “upper,” “horizontal,” “vertical,” “above,” “below,” “up,” “down,” “top,” “bottom,” “right,” and “left” well as derivatives thereof (e.g., “horizontally,” “vertically,” etc.) should be construed to refer to the orientation as shown in the drawing under discussion. These terms of orientation are for convenience of description and do not require that the apparatus be constructed or operated in a particular orientation. 
         [0048]    Unless explicitly stated otherwise, each numerical value and range should be interpreted as being approximate as if the word “about” or “approximately” preceded the value of the value or range. 
         [0049]    Also for purposes of this description, the terms “couple,” “coupling,” “coupled,” “connect,” “connecting,” or “connected” refer to any manner known in the art or later developed in which energy is allowed to be transferred between two or more elements, and the interposition of one or more additional elements is contemplated, although not required. Conversely, the terms “directly coupled,” “directly connected,” etc., imply the absence of such additional elements. 
         [0050]    In this specification including any claims, the term “each” may be used to refer to one or more specified characteristics of a plurality of previously recited elements or steps. When used with the open-ended term “comprising,” the recitation of the term “each” does not exclude additional, unrecited elements or steps. Thus, it will be understood that an apparatus may have additional, unrecited elements and a method may have additional, unrecited steps, where the additional, unrecited elements or steps do not have the one or more specified characteristics. 
         [0051]    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.