Abstract:
A circuit assembly containing a surface mount (SM) IC package wire bonded to a substrate and configured to conduct heat from the package into a heat sink through a heat-conducting member instead of the substrate. The package contains an IC device with input/output pads on a surface thereof that are connected with leads to conductors on the substrate. The heat sink is located adjacent the package so as not to be separated from the package by the substrate. The heat-conducting member is positioned adjacent the surface of the device opposite its input/output pads, and is bonded to the device and heat sink to provide a heat path between the package and heat sink that does not pass through the substrate.

Description:
BACKGROUND OF THE INVENTION 
   The present invention generally relates to surface mount (SM) integrated circuit (IC) devices. More particularly, this invention relates to thermal management of circuit assemblies containing one or more SM IC packages. 
   IC devices generate heat during their operation, resulting in increased junction temperatures for the devices. Because IC reliability and function are adversely affected by high junction temperatures, various techniques for removing heat from IC devices have been proposed. One approach entails the use of a thermally-conductive substrate or a substrate structurally modified to promote its heat conduction capability. For example, heat-generating semiconductor devices, such as power flip chips and SM packages, are often mounted to alumina substrates that conduct and dissipate heat in the vertical direction beneath the device. 
   Another approach used with SM IC packages is represented in  FIG. 1 , and involves an IC package  114  containing a metal slug  116  that conducts thermal energy from the package  114  to a substrate  112  beneath the package  114 . Heat can then be removed from the substrate  112  by conduction into a separate heat sink  118  (such as the product case) contacting the backside of the substrate  112  opposite the package  114 . As is conventional,  FIG. 1  shows the input/output (I/O) pads  122  of the device  120  on the surface of the device  120  opposite the PCB substrate  112  and wire bonded to signal lines (not shown) on the substrate  112 . The metal (e.g., copper) slug  116  is soldered to the side of the device  120  facing the PCB substrate  112  (i.e., opposite the I/O pads  122 ). In the case where the substrate  112  is a printed circuit board (PCB) with limited thermal conductivity, heat removal from the IC device  120  must often be promoted through the use of through-hole vias  124 , which are shown as thermally contacting the heat sink  118  through a thermally conductive compound  126 . 
   More recently, commonly-assigned U.S. Pat. Nos. 6,180,436, 6,262,489, and 6,873,043 and U.S. Patent Application Publication Nos. 2005/0040518 and 2005/0078456 teach thermal management techniques compatible with flip chips on PCB&#39;s by making use of heat sinks with pedestals that contact the back side of the flip chip, i.e., the surface of the chip opposite the surface on which its microcircuitry is formed. As evident from  FIG. 1 , such an approach cannot be used with SM packages because of the presence of the I/O pads  122  and bond wires on the side of the package opposite the substrate  112 . 
   BRIEF SUMMARY OF THE INVENTION 
   The present invention provides a circuit assembly containing a surface mount (SM) IC package soldered to a substrate and configured to conduct heat from the package into a heat sink through a heat-conducting member and not through the substrate. The invention is particularly adapted for SM IC&#39;s mounted to PCB circuit boards that exhibit low thermal conductivity, and can be used with SM IC packages having backside electrical contacts. 
   The circuit assembly includes a substrate having oppositely-disposed first and second surfaces and conductors on the first surface, and at least one surface mount package on the first surface of the substrate. The package contains an integrated circuit device with input/output pads on a first surface thereof, an oppositely-disposed second surface, and leads connecting the input/output pads to the conductors on the first surface of the substrate. A metal heat sink is located adjacent the package so as not to be separated from the package by the substrate. Finally, a heat-conducting member is positioned adjacent the second surface of the circuit device and bonded to the circuit device and heat sink to provide a heat path between the package and heat sink that does not pass through the substrate. As such, the substrate can be a PCB or other relatively low-conductivity substrate material without limiting the thermal management of the circuit device. 
   Other objects and advantages of this invention will be better appreciated from the following detailed description. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a cross-sectional view depicting a surface mount IC package with a heat conductor (metal slug) located between the package and its substrate for thermal management of the package in accordance with prior art practice. 
       FIG. 2  is a cross-sectional view depicting a surface mount IC package with a heat conductor located between the package and a through-substrate heat sink pedestal for thermal management of the package in accordance with a first embodiment of the present invention. 
       FIG. 3  is a cross-sectional view depicting a surface mount IC package with its I/O pads facing the substrate to which the package is electrically connected with leads, and with a heat conductor contacting the opposite side of the package for thermal management of the package in accordance with a second embodiment of the present invention. 
       FIG. 4  is a cross-sectional view depicting a surface mount IC package with its I/O pads facing the substrate to which the package is electrically connected with leads, and with a heat conductor contacting the opposite side of the package and contacting a case for thermal management of the package in accordance with a third embodiment of the present invention. 
       FIGS. 5 and 6  are cross-sectional and plan views, respectively, depicting a dual-sided PCB assembly with multiple surface mount IC packages with I/O pads facing the substrate to which the packages are electrically connected with leads, and with a heat conductor contacting the opposite side of each package and contacting a case for thermal management of the packages in accordance with a fourth embodiment of the present invention. 
       FIGS. 7 and 8  are cross-sectional views depicting a surface mount IC package with its I/O pads facing the substrate to which the package is electrically connected with leads, and a backside electrical contact equipped with a backside heat conductor for thermal management in accordance with a fifth embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIGS. 2 through 8  represent circuit assemblies  10 ,  110 ,  210 ,  310 , and  410  in accordance with different embodiments of this invention. Each circuit assembly  10 ,  110 ,  210 ,  310 , and  410  is shown as including a SM IC package  14 , in which is contained an IC device  20  of any type suitable for SM packaging. Each device  20  is shown as having input/output (I/O) pads  22  electrically connected with wirebonds and leads  38  to conductors (not shown) on a surface  28  of a substrate  12 , which may be a thin laminate PCB or any other suitable substrate material. The orientation of the device  20  to the substrate  12  can be the conventional with its I/O pads  22  located on a surface  32  of the device  20  facing away from the substrate  12  ( FIG. 2 ), or can be unconventional with the surface  32  carrying the I/O pads  22  facing the substrate  12  ( FIGS. 3-5  and  8 ). In all cases, heat generated by the device  20  is conducted from the package  14  through a heat conductor  16 , which is preferably in the form of what is commonly called a slug. As such, the heat conductor  16  can be a metal plate, such as aluminum, copper, or another material with similar thermally conductivity. The heat conductor  16  contacts and is preferably bonded with solder  36  to the surface  34  of the device  20  opposite the I/O pads  22  to provide a highly thermally conductive path to a heat sink  18  adjacent the package  14 . The heat conductor  16  can be bonded directly or indirectly to the heat sink  18  with a thermally-conductive adhesive or solder, so that the conductive path does not pass through the substrate  12  and avoids the prior practice of through-hole vias (e.g.,  124  of  FIG. 1 ). 
   Referring to  FIG. 2 , the package  14  is shown as being mounted over a through hole  40  formed in the substrate  12 . The heat sink  18  is located adjacent the lower surface  30  of the substrate  12  opposite the package  14 , and includes a pedestal  24  that projects up through the hole  40  and engages the heat conductor  16 . The facing surface of heat conductor  16  is aligned with and preferably bonded to the heat sink pedestal  24  with a thermal contact material  26 . The interface between the heat conductor  16  and pedestal  24  is not required to be electrically conductive. Therefore, while the thermal contact material  26  may be solder such as indium or an indium alloy, a thermal adhesive may also be used. Suitable thermal adhesives contain an adhesive matrix material (e.g., an epoxy or silicone) and a dispersion of metal and/or ceramic particles. 
   Because the package  14  is bonded to the substrate  12  through the leads  38  and bonded to the heat sink  18  through the heat conductor  16 , it may be desirable to also bond the package  14  directly to the substrate  12  to increase the solder joint interconnect life of the solder bonding the leads  38  to the substrate  12 . For this purpose,  FIG. 2  depicts an adhesive  42  deposited to encapsulate the lead solder joints and extend up along the sides of the package  14 . 
   As noted above, the circuit assembly  110  represented in  FIG. 3  differs from the embodiment of  FIG. 2  by reversing the orientation of the package  14 , i.e., the surface  32  carrying the I/O pads  22  of the device  20  faces the substrate  12 . With this orientation, the leads  38  contact the lower surface ( 32  as viewed in  FIG. 3 ) of the device  20 , instead of its upper surface ( 34  as viewed in  FIG. 3 ). An advantage of this orientation is the ability to engage the heat conductor  16  (again located on the surface  34  of the device  20  opposite the I/O pads  22 ) with the heat sink  18  located above the package  14 , instead of requiring a through-hole  40  through which the pedestal  24  of the heat sink  18  projects as done in  FIG. 2 . Another advantage is that a larger surface region of the package  14  can be directly bonded to the substrate  12  with the adhesive  42  to promote the solder joint interconnect life of the package interconnects. To promote thermal contact between the heat conductor  16  and the heat sink  18 ,  FIG. 3  shows a biasing member  44  engaging the lower surface  30  of the substrate  12 , in accordance with commonly-assigned U.S. Pat. No. 6,180,436, the relevant teachings of which are incorporated herein by reference. The biasing member  44  can be formed of an elastomeric material or be in the form of a mechanical spring, and can permit the use of a thermal grease or pad in place of the thermal contact material  26  used in  FIG. 2  to bond the heat conductor  16  to the heat sink pedestal  24 . Flexing of the substrate  12  by the biasing member  44  occurs to some degree to ensure good thermal contact, necessitating a sufficiently thin or otherwise flexible substrate material. 
   The circuit assembly  210  represented in  FIG. 4  primarily differs from the embodiment of  FIG. 3  by forming the heat sink  18  as part of a casing  46  that completely encloses the package  14  on the surface  28  of the substrate  12 . The casing  46  can be bonded to the substrate  12  with solder or a structural adhesive, such as an epoxy or filled epoxy known in the art. Because the heat conductor  16  is not biased into contact with the heat sink  18 , the thermal contact material  26  is preferably a thermally conductive adhesive or solder. 
     FIGS. 5 and 6  depict multiple packages  14  mounted to both surfaces  28  and  30  of the substrate  12 , with the entire substrate and package assembly enclosed within a two-piece casing formed by casing halves  18   a  and  18   b  that each serve as a heat sink for packages  14  located on their respective sides of the substrate  12 . The casing halves  18   a  and  18   b  are assembled and held together with fasteners  48  that determine the force applied by the halves  18   a  and  18   b  on the packages  14 , as evident from  FIG. 5 . As with the embodiment of  FIG. 3 , the ability of the circuit assembly  310  to apply a controlled amount of contact force between the individual pairs of heat conductors  16  and pedestals  24  permits the use of a thermal grease or pad in place of an adhesive as the thermal contact material  26 . 
   Finally,  FIGS. 7 and 8  depict an embodiment in which the device  20  within the package  14  includes a vertical semiconductor device, necessitating a backside electrical contact (not shown) located on the backside surface  34  of the device  20 . Consequently, in addition to the leads  38  electrically connecting the I/O pads  22  to conductors on the substrate surface  28  (as shown in the section of the assembly  410  represented by  FIG. 8 ), the circuit assembly  410  includes backside leads  50  that electrically connect the backside electrical contact to one or more other conductors on the substrate surface  28  (as shown in a different section of the assembly  410  represented by  FIG. 7 ). The heat conductor  16  is shown as forming part of the electrically conductive path from the backside electrical contact to the backside leads  50 , and therefore must be bonded to the device  20  and the backside leads  50  with electrically conductive materials, such as solder  36 . However, to electrically isolate the heat sink  18 , the thermal contact material  26  between the heat conductor  16  and the heat sink  18  is preferably a thermal adhesive. Because the backside leads  50  are attached to the heat conductor  16  and the substrate  12 , a CTE mismatch does not exist between the two and stress on the solder joints within the assembly  410  is minimal. 
   While our invention has been described in terms of a preferred embodiment, it is apparent that other forms could be adopted by one skilled in the art. Accordingly, the scope of our invention is to be limited only by the following claims.