Patent Publication Number: US-6710432-B1

Title: Integrated circuit package with low inductance ground path and improved thermal capability

Description:
FIELD OF THE INVENTION 
     The present invention relates to integrated circuit (IC) packages and methods for fabricating them. More specifically, the present invention comprises an IC package with a very low inductance ground path to a printed circuit board ground plane, wherein the same structure that provides the low inductance ground path also provides a significantly improved thermal conduction path. 
     BACKGROUND OF THE INVENTION 
     A known integrated circuit (IC) package, with an exposed die attach pad (DAP), is shown in FIG.  1 . IC package  10  consists of IC die  12  mounted on die attach pad  16  by means of a attach epoxy  14 . IC die  12  is coupled to the input/output pins of metal lead frame  18  by means of a plurality of bond wires  20 . A molded IC body  22  is then formed around IC die  12  an lead frame  18  in a known manner to complete the package. As shown in FIG. 1, lower surface  15  of die attach pad  16  remains exposed and co-planar with the leads of lead frame  18  after IC package  10  is completed. 
     Although IC package  10  is adequate for many purposes, as the operating frequencies of packaged IC dies have increased, several shortcomings of this type of package have emerged. In ICs that contain a large number of fast input/output (I/O) cells, many of the I/O cells can switch states at the same time, or nearly at the same time. This event is referred to as the “simultaneously switching output” (SSO) condition. 
     SSO conditions can cause very high ground bounce to occur on a (shared) on-die ground line which connects multiple I/O cells to the same (shared) on-die ground pad, which is then connected to a (shared) IC package ground pin. Ideally, the voltage on a shared on-die ground line should remain at zero volts when the I/O cells connected to this line switch from the logic high state to the logic low state. However, the bond wires and the lead frame that couple the on-die ground line to an external PCB ground plane present a collective I/O pin inductance. Rising/falling I/O cell currents, which must flow through this inductance, can cause the voltage on the on-die ground line to temporarily rise and/or fall above/below zero volts. These momentary voltage changes are often referred to as “ground bounce.” In the worst case, high ground bounce can cause an IC to malfunction. For example, in ICs which contain analog circuitry, high ground bounce can cause the analog circuitry to generate degraded and/or erroneous waveforms. Furthermore, high ground bounce can also cause digital I/O cells, which are trying to output a logic low level, to temporarily output a logic high level. This temporary “glitch” condition can easily cause system malfunction. 
     Ground bounce depends upon a number of factors, including the total inductance from an on-die ground pad to the PCB ground plane. This total inductance includes all wire inductance plus the inductance of the package lead frame. Ground bounce can be minimized by minimizing the total inductance from an on-die ground pad to the PCB ground plane. If the total inductance could be reduced to zero, then ground bounce would also be reduced to zero. 
     As shown in FIGS. 2 a  and  2   b , another known type of IC package, a plastic ball grid array (PBGA) offers a very direct connection to the ground plane of a PCB. PBGA IC package  30  has an IC die  32  mounted on a substrate  38  using a silver epoxy or other suitable attachment means. Substrate  38  is coupled to IC die  32  by means of bond wires  31 . Those portions of substrate  38  that have been connected to IC die  32  serve as the I/O pins of completed IC package  30 . The ground pads  40  on IC die  32  are coupled to the portion of substrate  38  that directly underlies IC die  32  by means of bond wires  36 . Solder balls  39  are formed on the lower, exposed surfaces of substrate  38 . When placed on a printed circuit board (PCB) and then heated in one of several known ways, solder balls  39  provide electrical connections with substrate  38 . Solder balls  39  which directly underlie IC die  32  provide a direct pathway from the ground pads  40  of IC die  32  through substrate  38  and solder balls  39  into the PCB ground plane. This ground path offers very low inductance to the PCB ground plane and greatly reduces ground bounce. 
     The two different packages shown in FIGS. 1,  2   a  and  2   b  each solve different problems. The exposed die attach pad of the package shown in FIG. 1 offers significant heat sinking, which becomes increasingly important as IC operating speeds continue to increase. Indeed, the heat sinking capabilities of this type of package have been increased in some known packages by extending the die attach pad so that it protrudes outside of the package body. However, according to prior art, these protruding die attach pads are as yet restricted to protruding no further than the top surface of the PCB upon which they are mounted. 
     The package illustrated in FIGS. 2 a  and  2   b  provides a very low inductance ground pathway, which greatly reduces ground bounce while simultaneously reducing the number of leads required for ground connections. 
     An IC package which combines the advantages of the described prior art packages, without increasing the package manufacturing cost, would be a useful advance on known IC packages. 
     SUMMARY OF THE INVENTION 
     In each of its embodiments, the present invention comprises an IC package with a very low inductance ground path to a PCB ground plane, wherein the same structure that provides the low inductance ground path also provides a significantly improved thermal conduction path. A comparatively large metal die attach pad (metal slug), with protrudes outside of the IC package, is electrically connected to IC ground pads through its upper surface, and electrically connected to a PCB ground plane through its lower, exposed surface. This connection provides a very low inductance ground path from the IC die to the PCB ground plane, while also providing a highly efficient heat sink for the IC die. In several embodiments, the metal slug is made sufficiently thick so that it extends well outside of the IC package. In these embodiments, the metal slug is inserted into pre-defined and pre-cut holes in the PCB. When attaching the IC package to the PCB, these embodiments offer the additional advantage of self-alignment of the IC package pins with their corresponding PCB traces. To further improve the present invention&#39;s thermal conduction, in those embodiments where the metal slug extends through the PCB, a heat sink can also be attached to the exposed metal slug after insertion into the PCB. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention, together with its various advantages, may be best understood by referring to the following description and accompanying drawings in which: 
     FIG. 1 is a cross-sectional view of a first known integrated circuit (IC) package (Prior Art); 
     FIGS. 2 a  and  2   b  are, respectively, a top view and a cross-sectional view of a second known IC package (Prior Art); 
     FIGS. 3 a ,  3   b  and  3   c  are, respectively, a first cross-sectional view, a second cross-sectional view, and a bottom view of the first embodiment of the present invention; 
     FIG. 4 is a cross-sectional view of a second embodiment of the present invention; 
     FIG. 5 is another cross-sectional view of the second embodiment of the present invention; 
     FIG. 6 is a cross-sectional view of yet another embodiment of the present invention; and 
     FIGS. 7 a  and  7   b  are cross-sectional views of other embodiments of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A first embodiment of the present invention is shown in FIGS. 3 a ,  3   b  and  3   c . In this embodiment, a Plastic Quad Flat Package (PQFP) IC  100  is shown. Referring to FIG. 3 a , metal lead frame  110 , which is supported by spacers  107 , provides the PQFP I/O pins. As shown in FIG. 3 a , spacers  107  rest on a metal slug  104 , which acts as a die attach pad. So that various embodiments of the present invention can be more clearly shown, metal lead frame  110  is not shown in the other figures illustrating this embodiment, and other embodiments, of the present invention. It should be understood that each embodiment of the present invention incorporates a lead frame for IC I/O signals. IC die  101  is electrically coupled to lead frame  110  by means of bond wires  105 . 
     Referring to FIG. 3 b , IC die  101  is mounted on the top surface of metal slug  104 . IC die  101  is electrically connected to metal slug  104  using an electrically conductive medium such as silver epoxy or a solder preform. Ground pads  102  on IC die  101  are also electrically connected to the top surface of metal slug  104  by bond wires  105 . Bond wires  105  are very short, with very low inductance, and are usually referred to as “downbonds”. IC package body  112  is molded around the lead frame (not shown in FIG. 3 b ), IC die  101  and metal slug  104 . As shown in FIG. 3 a , at the end of the fabrication process, the lower surface of metal slug  104  is exposed and co-planar with the exposed leads of the PQFP. After PQFP  100  is complete, it is mounted on a PCB  106  whose top surface comprises a ground plane  108 . An electrically conductive medium such as silver epoxy, PCB solder reflow, or a solder preform is used to electrically connect the lower surface of metal slug  104  with PCB ground plane  108 . 
     FIG. 3 c  illustrates the bottom surface of a completed PQFP  100 , including a plurality of package pins (signal leads)  114 , on each side of the package. As shown in FIG. 3 c , the lower surface of metal slug  104  is exposed. 
     As shown in FIGS. 3 a ,  3   b  and  3   c , the X, Y dimensions of metal slug  104  are much greater than its thickness. This results in metal slug  104  having an effective inductance which is very nearly equal to zero. Furthermore, as shown in FIG. 3 b , downbond wires  105  also have very low inductance because these wires arc very short. In many applications, the very low inductance of these downbond wires can be reduced even further (made very close to zero) by connecting two or more downbond wires in parallel, to the same ground pad. (In this case, the ground pad must be made wider, but not taller). 
     Referring to FIG. 3 b , the total inductance from any ground pad  102  to the PCB ground plane  108  is equal to the equivalent inductance of the downbond wire(s)  105  connected to ground pad  102 , plus the inductance of metal slug  104 . Since this total inductance is extremely low, it will considerably reduce the ground bounce on shared on-die ground pads and the ground lines connected to them. 
     Since all on-die ground pads  102  can be connected to PCB ground plane  108  through metal slug  104 , no package pins or lead frame “fingers”  110  are needed in order to form these connections. Furthermore, since lead frame fingers  110  are usually the highest inductance component in the ground path, eliminating these fingers from the ground path significantly reduces the overall ground path inductance. 
     For those dies which contain very sensitive “small signal” analog circuitry, it is sometimes desirable to provide one or more “isolated” ground pads which are not connected to metal slug  104 . Referring to FIG. 3 a , these isolated ground pads can be connected in the same manner as normal I/O signal pins, by utilizing bondwire  105  and lead frame  110 . Of course, since these isolated ground pads are connected to lead frame fingers, they will have high inductance in their ground path. This is usually not be a problem for “small signal” analog circuitry, however, because the ground line currents associated with this type of circuitry are often fairly low. 
     Referring to FIGS. 3 a - 3   c , it can be seen that metal slug  104  provides a very efficient thermal heat sink for IC die  101 . IC die  101  is directly attached to metal slug  104 , which in turn is electrically and thermally coupled to the large PCB ground plane. These elements combine to form a significant thermal mass, which can absorb a great deal of the heat generated by IC die  101 . Thus, in addition to significantly reducing ground path inductance, metal slug  104  also acts as a heatsink at no additional packaging cost. 
     As previously described, the first embodiment of the present invention assumes that the PCB ground plane is located on the top surface of the PCB. In those cases where the PCB ground plane is located within the PCB laminate, an array of small plated-thru vias can be used to couple the internal PCB ground plane to a PCB ground “pad” located on the top surface of the PCB. Because the inductance of a plated-thru via is relatively low, and because there are many of these vias in parallel, their total equivalent inductance will be extremely low. 
     FIG. 4 illustrates a second embodiment of the present invention. Referring to FIG. 4, components which are similar or identical to those used in the first embodiment are numbered identically to those used in the first embodiment. (This same convention also applies to the description of all subsequent embodiments of the present invention, as described below). 
     As shown in FIG. 4, metal slug  104  is significantly thicker than the corresponding metal slug utilized in the first embodiment of the invention (as shown in FIG. 3 b ). Referring to FIG. 4, metal slug  104  has been made sufficiently thick so that it now extends be low the bottom surface of IC package  100  and through a cutout  115  in PCB  106 . By examining FIGS. 3 a  and  4 , it can be seen that the bottom surface of metal slug  104  extends below the plane formed by the bottom surfaces of the I/O lead frame fingers  100 . 
     As shown in FIG. 4, cutout  115 , of appropriate dimensions, has been made in PCB  106  during its manufacture. Referring to FIG. 4, cutout  115  has been made in PCB  106  so that metal slug  104  can be inserted into cutout  115 . In this embodiment, wave soldering or another similar process would create solder filet  116 , which physically and electrically attaches metal slug  104  to PCB ground plane  108 . 
     As previously described in the first embodiment of the invention, the second embodiment of the invention, as shown in FIG. 4, maintains a very low inductance path to the PCB ground plane  108 . Furthermore, the second embodiment of the invention offers several additional advantages. For example, since cutout  115  in PCB  106  closely matches the shape of metal slug  104 , metal slug  104  will fit very snugly into cutout  115 . This snug fit allows the pins on IC package  100  to be automatically aligned with their corresponding PCB solder “pads”. Furthermore, because the bottom surface of metal slug  104  is exposed to the air, IC package  100  can be effectively cooled by airflow convection. Comparing IC package  100  (shown in FIG. 4) with prior art IC package  10  (shown in FIG.  1 ), it can be seen that the slug in IC package  10  cannot benefit from airflow convection cooling because its bottom surface is attached to the top surface of the PCB—i.e. the bottom surface of IC package  10  is not exposed to the air. 
     The second embodiment of the invention, shown in FIG. 4, assumes that PCB ground plane  108  is located on the bottom surface of PCB  106 . However, as shown in FIG. 5, in those applications where the PCB ground plane is located within the PCB laminate, an array of small plated-thru vias  117  can be utilized in order to connect internal PCB ground plane  108  to a PCB ground “pad”  118 , located on the bottom surface of PCB  106 . Since the inductance of via  117  is low, and because there are many vias in parallel, these vias collectively present a very low inductance path to ground. Finally, as shown in FIG. 6, the second embodiment of the invention can be modified by attaching a heat sink  119  to metal slug  104 . Heat sink  119  further increases the amount of heat that can be dissipated in this embodiment of the invention. 
     The embodiments shown in FIGS. 4 and 5 can be fabricated by first preparing PCB  106  with appropriate cutouts  115 . In the next step, IC package  100 , with its extended metal slug, would be inserted into the prepared cutouts in PCB  106 . In the final step, metal slug  104  can be attached to PCB ground ( 108  in FIG. 4,  118  in FIG. 5) by using wave soldering techniques, or by using an electrically conductive adhesive (such as silver epoxy), or by using solder reflow techniques. Referring to FIG. 6, when a heat sink must be attached to the metal slug, it can be affixed using any one of several known attachment methods, including thermally conductive adhesive, mechanical attachment using screws, or reflow solder techniques. 
     FIGS. 7 a  and  7   b  show two other embodiments of the present invention. In each of these embodiments, metal slug  104  has been shaped to have shoulder regions  121 . These shoulder regions rest on the top surface of PCB  106 , allowing metal slug  104  to be electrically and mechanically connected to a top surface PCB ground “pad”  118 . This connection can be made using solder reflow techniques, or by means of an electrically conductive adhesive such as silver epoxy. In the embodiments shown in FIGS. 7 a  and  7   b , the bottom surface of slug  104  can be coincident with the bottom surface of PCB  106 . 
     Since the embodiment shown in FIG. 7 a  has a buried PCB ground plane  108 , this ground plane is electrically connected to ground “pad”  118 , on the top surface of PCB  106 , by utilizing an array of vias  117 , connected between PCB ground plane  108  and ground “pad”  118 . Of course, in those cases where ground plane  108  is located on the top surface of PCB  106 , vias  117  will not be required. 
     As shown in FIG. 7 b , in order to maximize thermal heat sinking capacity, a heat sink  119  can be attached to metal slug  104 , whose bottom surface extends through opening  115  in PCB  106 . Referring to FIG. 7 b , heat sink  119  can be attached to slug  104  using thermal grease and screws, or by using thermally conductive bonding agents. 
     In summary, all embodiments of the present invention provide a very low inductance ground path from the on-die ground pads to the PCB ground plane, through the use of short downbond wires to a metal slug, which acts as a very low inductance die attach pad (DAP) for the IC package. This very low inductance ground path can minimize electrical ground bounce to the point where it is no longer a significant problem. Furthermore, all embodiments of the present invention completely eliminate the need for dedicated ground pins on the IC package, significantly increasing the number of I/O pins available. In addition, by extending the thickness of the metal slug, and by providing cutouts in the PCB, all IC package pins can be automatically self-aligned to their corresponding PCB “pads”. Finally, the thickness and size of metal slug  104  provide significant cooling capacity for the IC die. In those cases where maximum cooling capacity is needed, an external heat sink can be affixed to the package&#39;s exposed metal slug, which extends through the bottom surface of the PCB. 
     Although the present invention has been described in some detail for clarity of understanding, certain changes and modifications may be practiced within the scope of the appended claims. Therefore, the described embodiments should be taken as non restrictive illustrations only, and the invention should not be limited to the details presented herein, but should be defined by the following claims and the full scope of their equivalents.