Abstract:
A semiconductor assembly includes a first subassembly comprising a heat sink and a first patterned polymer layer disposed on a surface of the heat sink to define an exposed portion of the first surface. The exposed portion of the first surface extends radially inward along the heat sink surface from the first layer. The subassembly also includes a second patterned polymer layer disposed on a radially outer portion of the first patterned polymer layer. The first and second layers define a cell for accommodating a power semiconductor die. Solder material is disposed on the exposed portion of the heat sink surface and in the cell. A power semiconductor die is located within the cell on a radially inward portion of the first layer and thermally coupled to the heat sink by the solder material.

Description:
STATEMENT OF RELATED APPLICATION 
       [0001]    This application is related to U.S. patent application Ser. No. ______ (attorney docket no. GS 217), filed on even date herewith and entitled “Subassembly That Includes A Power Semiconductor Die And A Heat Sink Having An Exposed Surface Portion Thereof”, which is incorporated herein by reference in its entirety. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to mounting assemblies and packages for semiconductor devices used in electronic equipment, and more particularly to mounting assemblies and packages power semiconductor devices. 
       BACKGROUND OF THE INVENTION 
       [0003]    The electronics industry has been progressing with the miniaturization of electronic devices. This trend influences semiconductor packaging technology, which enables the connection between bare IC chips and other components, and enables the connection between bare IC chips and other components. Typically, a semiconductor package has a footprint much larger than that of the chip. To adapt to the miniaturization trend, the size difference between the package and the chip has been reduced, producing a new package type called a Chip scale package (CSP). A chip scale package is loosely defined as a package that takes no more than about 20% additional area (length and width) than the bare silicon die. The solder balls of chip scale packages are smaller than ball grid array (BGA) that had arranged according to international standard of Joint Electron Device Engineering Council (JEDEC). When it comes to personal and portable electronic devices, smaller is better, and various products need different chip scale package types, a wide array of which are currently available. 
         [0004]    Certain semiconductor devices are designed to handle relatively high voltages in a compact space. For example, semiconductor devices that are exposed to RMS voltages greater than 100 VAC, such as 265 VAC or 415 VAC, are often mounted in electronic power supplies and the like. These devices may dissipate relatively large amounts of power, and are accordingly often mounted to heat sinks or like devices as well as being electrically connected to electronic equipment of various types. 
         [0005]    Many such semiconductor devices for power applications are commonly available in the JEDEC standard TO-220 and DO-218 packages (www.jedec.org). An illustrative TO-220 package  110  is shown in  FIG. 1 . The TO-220 package  110  includes a pressure clamp  140 , retainer  130 , heat sink  120 , a spacer  150  interposed between the package  110  and the heat sink  120 , and a semiconductor die (not visible in  FIG. 1 ) with leads  114  exiting the package  110  on one side. High-voltage semiconductor devices may also be available in various other packages similar to the TO-220 package. 
         [0006]    The continued emphasis on faster, smaller, lighter, and lower cost electronics systems is making component, board and system packaging more complex each year. The increase in complexity is due to wider use of finer pitch and thinner array surface mount packages, which are the key to miniaturization of electronics products. Most of the components on a typical systems motherboard for desk top computer systems remain at 1.27 and 1.00 mm pitch surface mount components with increasing use of finer pitch (0.80, 0.65, 0.50 &amp; 0.40 mm) array styled packages. Portable systems are moving to the finer pitches at a faster rate. The component pitch and overall profile height plays a critical role in the complexity of manufacturing process. The use of finer pitch, low profile components demands assembly equipment and processes that operate with tighter specification limits. The assembly processes that demand a higher precision include: pick-and-place, solder paste-printing applications, reflow, inspection, and rework. The use of finer pitch low profile components increases the complexity, which could negatively effect yield and rework making assemblies more difficult and costly. 
         [0007]    One aspect of the packaging process that can reduce yield is the accuracy with which the semiconductor die can be mounted to the heat sink or slug. The accuracy of this process relies primarily on the pick and place machine that is employed. In addition, another packaging aspect of the packaging process that can also reduce yield is the accuracy with which the solder thickness can be controlled. 
       SUMMARY OF THE INVENTION 
       [0008]    In accordance with the present invention, a semiconductor assembly is provided. The assembly includes a first subassembly comprising a heat sink and a first patterned polymer layer disposed on a surface of the heat sink to define an exposed portion of the first surface. The exposed portion of the first surface extends radially inward along the heat sink surface from the first layer. The subassembly also includes a second patterned polymer layer disposed on a radially outer portion of the first patterned polymer layer. The first and second layers define a cell for accommodating a power semiconductor die. Solder material is disposed on the exposed portion of the heat sink surface and in the cell. A power semiconductor die is located within the cell on a radially inward portion of the first layer and thermally coupled to the heat sink by the solder material. 
         [0009]    In accordance with one aspect of the invention, the semiconductor assembly may also include a semiconductor package in which the first subassembly, solder and die are located. 
         [0010]    In accordance with another aspect of the invention, the semiconductor package may be is a chip scale package. 
         [0011]    In accordance with another aspect of the invention, at least one of the first and second patterned polymer layers may include polyimide. 
         [0012]    In accordance with another aspect of the invention, the power semiconductor die may have a footprint with a given shape and the first patterned polymer layer conforms to the given shape. 
         [0013]    In accordance with another aspect of the invention, the semiconductor assembly may also include a second subassembly. The second subassembly may include a second heat sink and a third first patterned polymer layer disposed on a surface of the heat sink to define an exposed portion of the surface. The exposed portion of the surface extends radially inward along the second heat sink surface from the third layer. The second subassembly also includes a fourth patterned polymer layer disposed on a radially outer portion of the third patterned polymer layer The third and fourth layers define a cell for accommodating a power semiconductor die. A second solder material is disposed on the exposed portion of the second heat sink surface. The he power semiconductor die is further located within the cell on a radially inward portion of the third layer and thermally coupled to the second heat sink by the second solder material. 
         [0014]    In accordance with another aspect of the invention, a semiconductor assembly is provided that includes a heat sink and a first patterned polymer layer disposed on a surface of the heat sink to define an exposed portion of the first surface. The exposed portion of the first surface extends radially inward along the heat sink surface from the first layer. Solder material is disposed on the exposed portion of the heat sink surface and a power semiconductor die is located on the first patterned layer and thermally coupled to the heat sink by the solder material. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]      FIG. 1  shows an illustrative package for a power semiconductor die. 
           [0016]      FIGS. 2(   a ) and  2 ( b ) show cross-sectional and top views, respectively, of a first heat sink that is to be mounted to a semiconductor die and a first patterned polymer layer formed on the heat sink. 
           [0017]      FIGS. 3(   a ) and  3 ( b ) show cross-sectional and top views, respectively, of the patterned polymer layers formed on the first heat sink. 
           [0018]      FIGS. 4(   a ) and  4 ( b ) show cross-sectional and top views, respectively, of a solder material located on the surface of the first heat sink. 
           [0019]      FIGS. 5(   a ) and  5 ( b ) show cross-sectional and top views, respectively, of a power semiconductor die positioned on the first heat sink and contacting one of the patterned polymer layers. 
           [0020]      FIGS. 6(   a ) and  6 ( b ) show cross-sectional and top views, respectively, of solder material applied to the exposed surface of the semiconductor die. 
           [0021]      FIGS. 7(   a ) and  7 ( b ) show cross-sectional and top views, respectively, of the final semiconductor assembly that includes the semiconductor die mounted to two heat sinks. 
           [0022]      FIGS. 8(   a ) and  8 ( b ) show cross-sectional and top views, respectively, of a first heat sink that is to be mounted to a semiconductor die and a first patterned polymer layer formed on the heat sink when only the x-y position of the die is to be constrained by the polymer. 
           [0023]      FIGS. 9(   a ) and  9 ( b ) show cross-sectional and top views, respectively, of a solder material located on the surface of the first heat sink depicted in  FIGS. 8(   a ) and  8 ( b ). 
           [0024]      FIGS. 10(   a ) and  10 ( b ) show cross-sectional and top views, respectively, of a power semiconductor die positioned on the first heat sink depicted in  FIGS. 9(   a ) and  9 ( b ) 
           [0025]      FIGS. 11(   a ) and  11 ( b ) show cross-sectional and top views, respectively, of solder material applied to the exposed surface of the semiconductor die depicted in  FIGS. 10(   a ) and  10 ( b ) 
           [0026]      FIGS. 12(   a ) and  12 ( b ) show cross-sectional and top views, respectively, of the final semiconductor assembly that includes the semiconductor die mounted to the two heat sinks referred to in connection with  FIGS. 8-11 . 
           [0027]      FIGS. 13(   a ) and  13 ( b ) show cross-sectional and top views, respectively, of a first heat sink that is to be mounted to a semiconductor die and a first patterned polymer layer formed on the heat sink when only the solder thickness is to be controlled by the polymer. 
           [0028]      FIGS. 14(   a ) and  14 ( b ) show cross-sectional and top views, respectively, of a solder material located on the surface of the first heat sink depicted in  FIGS. 13(   a ) and  13 ( b ). 
           [0029]      FIGS. 15(   a ) and  15 ( b ) show cross-sectional and top views, respectively, of a power semiconductor die positioned on the first heat sink depicted in  FIGS. 14(   a ) and  14 ( b ) 
           [0030]      FIGS. 16(   a ) and  16 ( b ) show cross-sectional and top views, respectively, of solder material applied to the exposed surface of the semiconductor die depicted in  FIGS. 15(   a ) and  15 ( b ) 
           [0031]      FIGS. 17(   a ) and  17 ( b ) show cross-sectional and top views, respectively, of the final semiconductor assembly that includes the semiconductor die mounted to the two heat sinks referred to in connection with  FIGS. 13-16 . 
       
    
    
     DETAILED DESCRIPTION 
       [0032]    The present invention provides a mounting system for a semiconductor device that overcomes the aforementioned limitations of prior-art mounting systems. The mounting system is particularly suitable for use with discrete power semiconductor devices such as those employed for power linear and switching applications. Examples of such devices include, without limitation, resistors, rectifiers, transistors and the like. The mounting system discussed herein may be used in connection with surface mount technology packages such as chip scale packages, for example. Examples of standardized packages that may be suitable include, without limitation, JEDEC TO-220 and DO-218 packages. In the detailed description that follows, like element numerals are used to identify like elements appearing in one or more of the figures. 
         [0033]      FIGS. 2(   a ) and  2 ( b ) show cross-sectional and top views, respectively, of a first heat sink  210  that is to be mounted to a semiconductor die. The heat sink  210  may be formed from any suitable thermally conductive material such a, but not limited to, Cu, Al and alloys thereof. In accordance with the present invention, a curable polymer is applied to an upper surface of the first heat sink  210  and patterned using well-known stenciling and screening techniques to form a first patterned polymer layer  212 . Suitable polymers include, without limitation, polymide, silicon rubber, and fluoroelastomer. The first patterned polymer layer  212  defines sidewalls of a cell  211  in which the solder can be placed. Next, in  FIGS. 3(   a ) and  3 ( b ), a second patterned polymer layer  214  is formed over the first polymer layer  212 , again using well-known stenciling and screening techniques. The second patterned polymer layer  214  defines a border within which the die is to be situated. Exposed portions  213  of the first patterned layer  212  (i.e., those portions not covered by the second patterned layer  214 ) define surfaces on which the die ultimately can be mounted. As shown in  FIGS. 4(   a ) and  4 ( b ), after formation of the first and second patterned polymer layers, solder  216  is dispensed in a conventional manner using a syringe, for example, onto the heat sink  210  into the cell  211  that is defined by the first patterned layer  212 . In  FIGS. 5(   a ) and  5 ( b ) a pick and place assembly machine or robot is used to position the semiconductor die  218  onto the exposed portion  213  of the first patterned layer  212 . The border of the second patterned layer  214  facilitates accurate placement and alignment of the die on the heat sink  210 . 
         [0034]    The process depicted in  FIGS. 2-5  may be repeated for a second heat sink that is to contact the side of the die  218  opposing the first heat sink  210 . In this case a second heat sink  220  first undergoes the process steps depicted in  FIGS. 2-4  to form first and second patterned layers  212  and  214  on a second heat sink  220 . Next, as shown in  FIGS. 6(   a ) and  6 ( b ), solder  222  is dispensed onto the exposed surface of the die  218 . The second heat sink subassembly (i.e., heat sink  220  with patterned layers  212  and  214  located thereon) is then positioned over the die  218  so that the die  218  contacts the exposed surface portion of the second patterned layer  212  of the second heat sink subassembly.  FIGS. 7(   a ) and  7 ( b ) show cross-sectional and top views, respectively, of the final semiconductor assembly that includes the semiconductor die mounted to two heat sinks. 
         [0035]    A number of advantages arise from the use of the mounting process depicted in  FIGS. 2-7 . For example, the use of a second patterned layer (e.g., second patterned layer  214 ) to constrain the position of the die on the heat sink limits rotational and out-of plane misalignments of the die. In this way the second patterned layer actively cooperates with the pick and place assembly machine to assist in the placement process and, as a result, the pick and place assembly machine is not solely responsible for placement of the die. In addition, the use of a first patterned layer (e.g., first patterned layer  212 ) that directly contacts the heat sink allows precise control of the overall solder thickness and thickness uniformity. For instance, in some cases the solder thickness in the final package can be maintained within a tolerance of about 0.25 mil to 3 mil. In addition, because the polymer that forms the first and second patterned layers is generally relatively soft and pliable, the level of stress exerted upon the die can be reduced. 
         [0036]    To illustrate the advantages of the present invention, three samples were manufactured in accordance with the technique discussed above. The solder thickness of the samples were selected to be 55 microns, 65 microns and 75 microns, respectively. The 55 micron sample was found to vary in thickness between about 52.8 microns and 54.6 microns. The 65 micron sample was found to vary in thickness between about 64.5 microns and 69.2 microns. The 75 micron sample was found to vary in thickness between about 74.4 microns and 79.2 microns. 
         [0037]    The size and shape of the cells  211  defined by the first and second patterned layers is not limited to those depicted in  FIGS. 2-7 . Rather, the size and shape of the cells can be selected as desired for different die geometries or footprints (e.g., square, hexagonal, round). The cell configuration may also be selected to comply with other factors such as flux overflow, the prevention of shorts and the like. Moreover, the sidewalls of the patterned layers  212  and  214  are not limited to the four linear segments of polymer for each of the two patterned layers that are depicted in  FIGS. 2-7 . Rather, any suitable configuration and number of polymer segments may be employed. For example, a square, rectangular or circular cell can be defined by a single continuous segment of polymer that has a shape defining a square, rectangle or circle, respectively. Alternatively, multiple continuous or non-continuous polymer segments may be employed in any number that is desired. 
         [0038]    In the embodiments of the invention presented above one patterned polymer layer (e.g., patterned layer  214 ) is employed to constrain or control the x-y position of the die on the surface of the heat sink  210  and a second patterned polymer layer (patterned layer  212 ) is used to control the thickness of the solder in the z-direction. In other embodiments of the invention only one polymer layer is employed to control either the x-y position of the die or the thickness of the solder in the z-direction. 
         [0039]      FIGS. 8-10  show an embodiment of the invention in which only a single polymer layer is employed to constrain or control the x-y position of the die on the surface of the heat sink. As shown in  FIGS. 8(   a ) and  8 ( b ), which once again show cross-sectional and top views, respectively, of the heat sink  210 , a curable polymer is applied to an upper surface of the first heat sink  210  and patterned using well-known stenciling and screening techniques to form an orienting patterned polymer layer  214  that is used to constrain or control the x-y position of the die. The orienting layer  214  defines sidewalls of a cell  211  in which the solder can be placed. Next, in  FIGS. 9(   a ) and  9 ( b ), solder  216  is dispensed in a conventional manner using a syringe, for example, onto the heat sink  210  into the cell  211  that is defined by the orienting patterned layer  214 . In  FIGS. 10(   a ) and  10 ( b ), a pick and place assembly machine or robot is used to position the semiconductor die  218  into the cell  211  so that it contacts the solder  216 . The border of the orienting patterned layer  214  facilitates accurate placement and alignment of the die  218  on the heat sink  210 . 
         [0040]    The process depicted in  FIGS. 8-10  may be repeated for a second heat sink that is to contact the side of the die  218  opposing the first heat sink  210 . In this case a second heat sink  220  first undergoes the process steps depicted in  FIGS. 8-9  to form the orienting patterned layer  214  on a second heat sink  220 . Next, as shown in  FIGS. 11(   a ) and  11 ( b ), solder  222  is dispensed onto the exposed surface of the die  218 . The second heat sink subassembly (i.e., heat sink  220  with orienting patterned layer  214  located thereon) is then positioned over the die  218  so that the die  218  is located within the cell defined by the orienting patterned layer  214  of the second heat sink subassembly. The die  218  contacts the solder  222  of the second heat sink assembly to form the complete semiconductor assembly depicted in  FIG. 12 . 
         [0041]      FIGS. 13-15  show an embodiment of the invention in which only a single polymer layer is employed to control the overall thickness and thickness uniformity of the solder in the z-direction. As shown in  FIGS. 13(   a ) and  13 ( b ), which once again show cross-sectional and top views, respectively, of the heat sink  210 , a curable polymer is applied to an upper surface of the first heat sink  210  and patterned using well-known stenciling and screening techniques to form a thickness-controlling patterned polymer layer  212  that is used to control the thickness of the solder in the z direction. Next, in  FIGS. 14(   a ) and  14 ( b ), solder  216  is dispensed in a conventional manner using a syringe, for example, onto the heat sink  210  into the cell  211  that is defined by the thickness-controlling patterned layer  212 . In  FIGS. 15(   a ) and  15 ( b ) a pick and place assembly machine or robot is used to position the semiconductor die  218  onto the thickness-controlling layer  212 . 
         [0042]    The process depicted in  FIGS. 13-15  may be repeated for a second heat sink that is to contact the side of the die  218  opposing the first heat sink  210 . In this case a second heat sink  220  first undergoes the process steps depicted in  FIGS. 13-14  to form the thickness-controlling patterned layer  212  on a second heat sink  220 . Next, as shown in  FIGS. 16(   a ) and  16 ( b ), solder  222  is dispensed onto the exposed surface of the die  218 . The second heat sink subassembly (i.e., heat sink  220  with thickness-controlling patterned layer  212  located thereon) is then positioned over the die  218  so that the die  218  is located on the thickness-controlling patterned layer  212  of the second heat sink subassembly. The die  218  contacts the solder  222  of the second heat sink assembly to form the complete assembly depicted in  FIGS. 17(   a ) and  17 ( b ).