Abstract:
A lead frame assembly for an integrated circuit package includes a lead frame, a die, first and second information signal-carrying bond wires, first and second ground bond wires, and first and second ground down bond wires. Each ground bond wire shares a die pad with one of the ground down bond wires.

Description:
BACKGROUND 
     In several types of integrated circuit (IC) packaging technologies, the die is mounted on a portion of the upper surface of a lead frame referred to as the die-attach region, and wire bonds electrically connect pads on the die to areas known as pins on regions of the lead frame surrounding the die-attach region. For example, in a quad-flat no-lead (QFN) package, a lead frame assembly of this type is encapsulated in plastic, and the pins, which are distributed about the four sides of the package, extend to the lower surface of the package to define landing pads that are surface-mountable to a printed circuit board. 
     As illustrated in  FIG. 1 , a lead frame assembly  10  of a conventional QFN package includes a lead frame  12  on which a die  14  is mounted. In an exemplary portion of lead frame assembly  10 , bond wires  16 ,  18 ,  20 ,  22 ,  24  and  26  connect die pads  28 ,  30 ,  32 ,  34 ,  36  and  38  to pins  40 ,  42 ,  44 ,  46 ,  48  and  50 , respectively. Only a portion of the rectangular lead frame assembly  10  is shown in  FIG. 1  for purposes of clarity, and a similar arrangement of bond wires, die pads and pins exists on each of the four sides of lead frame assembly  10 . Lead frame  12  can also include a ground paddle region  52  that is maintained at a ground potential during operation. 
     Signal crosstalk is a common problem in IC packages of the above-described type. Such crosstalk is commonly caused by electromagnetic coupling of signals between bond wires that are in close proximity with each other. To inhibit such crosstalk, it is known to interpose ground bond wires between information signal-carrying bond wires susceptible to crosstalk with each other. For example, as illustrated in  FIG. 1 , bond wire  22  is maintained at ground potential (as indicated by its depiction in solid line as a contrast with broken line) to inhibit crosstalk between the differential pair of bond wires  18  and  20  and the differential pair of bond wires  24  and  26 , which carry information signals (as indicated by their depiction in broken line as a contrast with solid line). Note that information signal-carrying bond wires are commonly paired in instances in which signals are differential rather than single-ended. However, the same principle of interposing a ground bond wire between otherwise adjacent information signal-carrying bond wires applies in an instance (not shown) in which signals are single-ended. Bond wires  22  and  16  are disposed on either side of the differential pair of bond wires  18  and  20  to inhibit crosstalk between the differential pair of bond wires  18  and  20  and other bond wires. 
     A shortcoming of the above-described use of ground bond wires to inhibit crosstalk is that maintaining some of the pins at ground potential limits the number of pins available to carry information signals. Increasing the total number of pins so that more pins are available for ground bond wires increases the outside dimensions of the QFN package. That is, if the total number of pins is increased, each of the four sides must be lengthened to accommodate the additional pins. As it is highly desirable to minimize package size, alternatives to maintaining many pins at ground potential have been developed. 
     As illustrated in  FIG. 2 , a lead frame assembly  54  of a conventional QFN package includes a lead frame  56  on which a die  58  is mounted. In an exemplary portion of lead frame assembly  54 , so-called “down” bond wires  60 ,  62  and  64  connect die pads  66 ,  68  and  70  to the ground paddle region  72  of lead frame  56 . As down bond wires  60 ,  62  and  64  are not connected to pins, all or many of the pins of lead frame  56  (most of which are not shown in  FIG. 2  for purposes of clarity) are available to carry information signals. Such information signals are carried by bond wires  74 ,  76 ,  78  and  80  which connect die pads  82 ,  84 ,  86  and  88  to pins  90 ,  92 ,  94  and  96 . In a manner similar to that described above with regard to  FIG. 1 , down bond wire  62 , for example, inhibits crosstalk between a differential pair of bond wires  74  and  76  and a differential pair of bond wires  78  and  80 . 
     It has been found that ground down bond wires ( FIG. 2 ) do not inhibit crosstalk to as great an extent as ground pin bond wires ( FIG. 1 ). It has been theorized that because ground down bond wires do not extend all the way from the die pads to the pins, some electromagnetic signal coupling (i.e., crosstalk) can occur in the region where the information signal-carrying pins are immediately adjacent one another. Such crosstalk may not occur to as great an extent in an arrangement ( FIG. 1 ) in which a ground pin bond wire and ground pin are fully interposed between the information signal-carrying bond wires and corresponding pins that are otherwise susceptible to crosstalk. However, as described above, it is desirable to minimize the number of ground pins so that the number of signal pins can be maximized without increasing package dimensions. 
     Although the above-described conventional bond wire arrangements help minimize crosstalk, crosstalk can remain problematic in some instances. It would be desirable to provide an improved bond wire arrangement that further inhibits crosstalk without providing an excessive number of ground pins. 
     SUMMARY 
     Exemplary embodiments of the present invention relate to a lead frame assembly for an integrated circuit package that includes a lead frame, a die, first and second information signal-carrying bond wires, first and second ground bond wires, and first and second ground down bond wires. Each ground bond wire shares a die pad with one of the ground down bond wires. 
     The lead frame has a die-attach region, a plurality of pins defining a pin array, and a ground paddle region between the die-attach region and the pin array. The die is mounted on the die-attach region of the lead frame. The die has a plurality of die pads defining a die pad array parallel to the pin array. The first information signal-carrying bond wire has a first end connected to a first signal die pad of the die. The first information signal-carrying bond wire extends across the ground paddle region and has a second end connected to a first signal pin of the lead frame. The second information signal-carrying bond wire has a first end connected to a second signal die pad of the die. The second information-carrying bond wire extends across the ground paddle region and has a second end connected to a second signal pin of the lead frame. The first ground bond wire has a first end connected to a first ground die pad of the die interposed in the die pad array between the first signal die pad and second signal die pad. The first ground bond wire extends across the ground paddle region and has a second end connected to a first ground pin of the lead frame interposed in the pin array between the first signal pin and the second signal pin. A first ground down bond wire has a first end connected to the first ground die pad and a second end connected to the ground paddle region. A second ground bond wire has a first end connected to a second ground die pad of the die interposed in the die pad array between the first signal die pad and second die pad. The second ground bond wire extends across the ground paddle region and has a second end connected to the first ground pin of the lead frame. The second ground down bond wire has a first end connected to the second ground die pad and a second end connected to the ground paddle region. 
     In an exemplary method of operation of the above-described lead frame array in an IC package, a first information signal is communicated via the first information signal-carrying bond wire, and a second information signal is communicated via the second information signal-carrying bond wire, while the first and second ground bond wires and first and second ground down bond wires are maintained at a ground potential. 
     Other systems, methods, features, and advantages will be or become apparent to one with skill in the art upon examination of the following figures and detailed description. It is intended that all such additional systems, methods, features, and advantages be included within this description, be within the scope of the specification, and be protected by the accompanying claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention can be better understood with reference to the following drawings. The components in the drawings are not necessarily to scale, emphasis instead being placed upon clearly illustrating the principles of the present invention. 
         FIG. 1  is a plan view of a portion of a lead frame assembly, in accordance with the prior art. 
         FIG. 2  is a plan view of a portion of another lead frame assembly, in accordance with the prior art. 
         FIG. 3  is a plan view of a portion of a lead frame assembly, in accordance with the present invention. 
         FIG. 4  is a plan view of a portion of another lead frame assembly, in accordance with the present invention. 
         FIG. 5  is a plan view of a portion of still another lead frame assembly, in accordance with the present invention. 
         FIG. 6  is a plan view of a lead frame assembly, in accordance with the present invention. 
         FIG. 7  is a generalized side elevation view of the lead frame assembly of  FIG. 6 . 
         FIG. 8  is a plot comparing crosstalk isolation of lead frame assemblies having the bond wire arrangements shown in  FIGS. 3 ,  4  and  5 . 
     
    
    
     DETAILED DESCRIPTION 
     As illustrated in  FIG. 3 , a lead frame assembly  100  of a QFN package includes a lead frame  102  on which a die  104  is mounted. In an exemplary portion of lead frame assembly  100 , ground down bond wires  106 ,  108 ,  110  and  112  connect die pads  114 ,  116 ,  118  and  120  to the ground paddle region  122  of lead frame  102 . In this exemplary portion of lead frame assembly  100 , information signals are carried by signal bond wires  124 ,  126 ,  128  and  130 , which connect die pads  132 ,  134 ,  136  and  138  to pins  140 ,  142 ,  144  and  146 . Note that a pair of the ground down bond wires  108  and  110  are immediately adjacent one another (i.e., which have no other bond wires interposed between them). Ground down bond wires  108  and  110  are maintained at ground potential. Note that the pair of ground down bond wires  108  and  110  is interposed between a differential pair of signal bond wires  124  and  126  and a differential pair of signal bond wires  128  and  130 . Because ground down bond wires  108  and  110  are immediately adjacent one another, the pair of ground down bond wires  108  and  110  inhibits crosstalk between the differential pair of signal bond wires  124  and  126  and the differential pair of signal bond wires  128  and  130  more effectively than a single ground bond wire would inhibit crosstalk between two such differential pairs of bond wires. 
     As illustrated in  FIG. 4 , a lead frame assembly  148  of a QFN package includes a lead frame  150  on which a die  152  is mounted. In an exemplary portion of lead frame assembly  148 , down bond wires  154  and  156  connect die pads  158  and  160  to the ground paddle region  162  of lead frame  150 . In this exemplary portion of lead frame assembly  148 , information signals are carried by signal bond wires  164 ,  166 ,  168  and  170 , which connect die pads  172 ,  174 ,  176  and  178  to pins  180 ,  182 ,  184  and  186 . A pair of ground bond wires  188  and  190 , which are immediately adjacent one another (i.e., which have no other bond wires interposed between them), connect a corresponding pair of die pads  192  and  194  to a common or shared ground pin  196 . Ground bond wires  188  and  190  are maintained at ground potential. Note that the pair of ground bond wires  188  and  190  is interposed between a differential pair of signal bond wires  164  and  166  and a differential pair of signal bond wires  168  and  170 . Because ground bond wires  188  and  190  are immediately adjacent one another, conventional wisdom would suggest that the pair of ground bond wires  188  and  190  would inhibit crosstalk between the differential pair of signal bond wires  164  and  166  and the differential pair of signal bond wires  168  and  170  at least as effectively as, if not more effectively than, the pair of ground down bond wires  108  and  110  inhibits crosstalk in the bond wire arrangement described above with regard to  FIG. 3 . Surprisingly, in accordance with the present invention, it was found that the bond wire arrangement illustrated in  FIG. 3  inhibits crosstalk more effectively than the bond wire arrangement illustrated in  FIG. 4 . That is, an arrangement that uses ground pins and corresponding ground bond wires instead of only ground down bond wires is less effective at inhibiting crosstalk between adjacent differential pairs of signal bond wires. 
     A comparative analysis between the bond wire arrangements described above with regard to  FIGS. 3 and 4  revealed that the apparent anomaly was rooted in overall loop inductance and inductive coupling between adjacent differential pairs of signal bond wires. More specifically, modeling of the electric field (E-field) distributions of lead frame assembly  100  ( FIG. 3 ) and lead frame assembly  148  ( FIG. 4 ) revealed that the E-field in lead frame assembly  148  spread further in all directions from signal bond wires  164 ,  166 ,  168  and  170  ( FIG. 4 ) than the E-field in lead frame assembly  100  spread from signal bond wires  124 ,  126 ,  128  and  130  ( FIG. 3 ). This difference in E-field spreading is believed to be rooted in loop inductance of the signals and their return paths in each differential pair. Consider the information signals carried on signal bond wires  168  and  166  in lead frame assembly  148  ( FIG. 4 ) and the information signals carried on signal bond wires  126  and  128  in lead frame assembly  100  ( FIG. 3 ). There is a considerably greater loop area formed by the signal and its corresponding return path in the case of bond wires  166  and  168  ( FIG. 4 ) than in the case of bond wires  126  and  128  ( FIG. 3 ). In lead frame assembly  100  ( FIG. 3 ), the presence of ground down bonds  108  and  110  helps provide the shortest path for the return current. Since lowest crosstalk is obtained when the loop inductance formed by the signal and the return path is minimized, lead frame assembly  100  ( FIG. 3 ) provides lower crosstalk than lead frame assembly  148  ( FIG. 4 ). Further analysis led to a focus on regions nearer the die pads than regions nearer the pins. Specifically, greater E-field coupling was observed around the ends of signal bond wires  164 ,  166 ,  168  and  170 , where they connect to die pads  172 ,  174 ,  176  and  178  ( FIG. 4 ) than around the ends of signal bond wires  124 ,  126 ,  128  and  130 , where they connect to die pads  132 ,  134 ,  136  and  138 . Replacing ground down bond wires  108  and  110  in  FIG. 3  with bond wires  188  and  190  in  FIG. 4  increases crosstalk between adjacent differential pairs, thereby defeating the purpose of adding ground pin  196  at lead frame  150  ( FIG. 4 ). The lowering of loop area formed by the signal and return path is believed to explain the observations of greater insertion loss and greater crosstalk in lead frame assembly  148  ( FIG. 4 ) than in lead frame assembly  100  ( FIG. 3 ). 
     As illustrated in  FIG. 5 , a lead frame assembly  200  of a QFN package includes a lead frame  202  on which a die  204  is mounted. In an exemplary region of lead frame assembly  200 , down bond wires  206  and  208  connect die pads  210  and  212  to the ground paddle region  214  of lead frame  202 . In this exemplary region of lead frame assembly  200 , information signals are carried by signal bond wires  216 ,  218 ,  220  and  222 , which connect die pads  224 ,  226 ,  228  and  230  to pins  232 ,  234 ,  236  and  238 . A pair of ground bond wires  240  and  242 , which are immediately adjacent one another (i.e., which have no other bond wires interposed between them), connect a corresponding pair of die pads  244  and  246  to a common or shared ground pin  248 . Ground bond wires  240  and  242  are maintained at ground potential. In addition, a ground down bond wire  250  further connects die pad  244  to ground paddle region  214 . Thus, die pad  244  is connected to both ground bond wire  240  and ground down bond wire  250 . Likewise, a ground down bond wire  252  further connects die pad  246  to ground paddle region  214 . Thus, die pad  246  is connected to both ground bond wire  242  and ground down bond wire  252 . 
     Note that lead frame assembly  200  ( FIG. 5 ) is nearly identical to lead frame assembly  148  ( FIG. 4 ) with the difference being the addition of ground down bond wires  250  and  252 . Thus, in lead frame assembly  200  there are four bond wires interposed between a differential pair of signal bond wires  216  and  218  and a differential pair of signal bond wires  220  and  222 : ground bond wire  240 , ground down bond wire  250 , ground bond wire  242 , and ground down bond wire  252 . Ground down bond wire  250  shares die pad  244  with ground bond wire  240 , and ground down bond wire  252  shares die pad  246  with ground bond wire  242 . This sharing of die pads in lead frame assembly  200  helps to achieve less coupling than the bond wire arrangements in lead frame assemblies  100  ( FIG. 3) and 148  ( FIG. 4 ). This result is achieved because the additional ground down bond wires  250  and  252  (i.e., “additional” to the bond wire arrangement described above with regard to  FIG. 4 ) provides minimal loop area between the signal and return path, thereby reducing the inductive coupling and crosstalk. The additional ground down bond wires  250  and  252  also help to provide isolation between the differential pair of signal bond wires  216  and  218  and the differential pair of signal bond wires  220  and  222  in the region where bond wires  216 - 222  connect to pins  232 - 238  by making use of (a common or shared) pin  248 . Absent a shared pin such as pin  248 , a lead frame assembly (not shown) otherwise having the same bond wire arrangement as lead frame assembly  200  would require two additional die pads to make such a configuration possible, which would require a larger lead frame. 
     It should be understood that, for purposes of clarity, each of  FIGS. 1-5  illustrates only a portion of a lead frame assembly.  FIG. 6  is intended to illustrate that any suitable portion of a lead frame assembly  254  can include a bond wire arrangement of the type described above with regard to, for example,  FIG. 5 . Lead frame assembly  254  includes a lead frame  256 . A die  258  is mounted on a die-attach region of lead frame  256 . Bond wires  260  having an arrangement of the type described above with regard to, for example,  FIG. 5 , can be arrayed along any one or more of the four sides  262 ,  264 ,  266  and  268  of lead frame assembly  254 . Note that one or more portions of lead frame assembly  254  can include a bond wire arrangement of the type described above with regard to  FIG. 5 , while still other portions of lead frame assembly  254  can include one or more other types of bond wire arrangements, such as those described above with regard to  FIGS. 3 and 4 . Bond wires  260 , die  258 , and portions of lead frame assembly  254  can be encapsulated in plastic or similar encapsulation  269  to form a QFN package, as illustrated in  FIG. 7 . However, in other embodiments, lead frame assemblies as described herein can be included in any other suitable type of IC package. 
     As illustrated in  FIG. 8 , simulations involving computer models of lead frame assembly  100  ( FIG. 3 ), lead frame assembly  148  ( FIG. 4 ), and lead frame assembly  200  ( FIG. 5 ) result in plots  270 ,  272  and  274 , respectively. Comparing plot  272  with plot  270  indicates that merely adding ground pins and corresponding ground bond wires as described above with regard to  FIG. 4  does not reduce crosstalk and actually worsens crosstalk. Rather, it is only by adding ground pins and corresponding ground bond wires in combination with ground down bond wires that share bond pads with the ground bond wires in the manner described above with regard to  FIG. 5  that crosstalk is further reduced, as indicated by a comparison of plot  274  with plot  270 . 
     One or more illustrative embodiments of the invention have been described above. However, it is to be understood that the invention is defined by the appended claims and is not limited to the specific embodiments described.