Abstract:
In one aspect, an embodiment of an IC package includes an IC chip electrically connected to a substrate, a heatspreader disposed over the IC chip, wherein the heatspreader does not directly contact the IC chip, and an encapsulant material encapsulating at least a portion of the IC chip and a portion of the heatspreader such that a top portion of the heatspreader is exposed to the surroundings of the IC package. In another embodiment, the heatspreader comprises at least one castellation to improve adhesion to the encapsulation compound. A method of manufacturing such IC package is also disclosed.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims benefit under 35 U.S.C. §119(e) of U.S. Provisional Application having Ser. No. 61/185,970 filed on Jun. 10, 2009, the entirety of which is enclosed herewith by reference. 
     
    
     FIELD OF INVENTION 
       [0002]    This invention relates generally to integrated circuit (IC) packaging technology and, in particular to IC packages having enhanced heat dissipation and related methods of manufacture. 
       BACKGROUND OF INVENTION 
       [0003]    IC packaging is one of the final stages involved in the fabrication of IC devices. During IC packaging, one or more IC chips are mounted on a package substrate, connected to electrical contacts, and then coated with an encapsulation material comprising an electrical insulator such as epoxy or silicone molding compound. The resulting structure, commonly known as an “IC package,” may then be mounted onto a printed circuit board (PCB) and/or connected to other electrical components. In most IC packages, the IC chip is completely covered by the encapsulation material, while the electrical contacts are at least partially exposed so that they can be connected to other electrical components. 
         [0004]    IC chips generate a great deal of heat during normal operation. As the speed of the IC chips has increased, so too has the amount of heat generated by them. It is desirable to dissipate this heat from an IC package in an efficient manner. 
       SUMMARY OF INVENTION 
       [0005]    The present invention relates to IC packages having enhanced heat dissipation and related methods of manufacturing. More particularly, in one aspect, the invention features an IC package with an IC chip electrically connected to a substrate and a heatspreader disposed over the IC chip, wherein the heatspreader does not directly contact the IC chip. In some embodiments, the heatspreader may have a castellated texture on a bottom side thereof for enhanced adhesion with the encapsulation material and increased heat dissipation. 
         [0006]    In one embodiment, the IC package comprises a leadframe having a first surface and an opposing second surface. A first surface of an IC chip is mounted on the first surface of the leadframe. An encapsulation embeds the first surface of leadframe and a second surface of the IC chip at a first side. A heat sink comprising a base sheet and at least one castellation is disposed on top of the leadframe. The castellation extends from a first surface of the base sheet and they are all embedded in a second side of the encapsulation. A second surface of the base sheet is exposed to the environment. 
         [0007]    In another embodiment, the IC package further comprises a central castellation that extends from the first surface of the base sheet. A center of the central castellation is aligned to a center of the IC chip to minimize the distance between the heat sink and the IC chip to improve heat dissipation performance. 
         [0008]    In another embodiment of the present invention, the IC package further comprises a metal clip on at least one side that clamps the second surface of the base sheet and the surface opposing that. The metal clip further prevents the second leadframe from detaching and also improves heat dissipation performance of the IC package by directly contacting the PCB when the leadframe is soldered to the PCB, such that heat can be transferred from the heat sink to the PCB so it can be spread away even faster. 
         [0009]    According to another aspect of the present invention, a method of manufacturing the IC package as described above is disclosed. The method comprises the steps of etching a leadframe strip to define a leadframe having a first surface and an opposing second surface, mounting a second surface of an IC chip on the first surface of the leadframe, and fixing the leadframe and a heat sink strip in a mold, such that the heat sink strip is disposed in close proximity to the IC chip and a cavity exists therebetween. The cavity is then encapsulated by an encapsulation compound such that the first surface of the leadframe strip and the second surface of the IC chip are embedded in one side of the encapsulation and the heat sink strip is embedded in an opposing side of the encapsulation. The IC package is then singulated from the leadframe strip, the heat sink strip and the encapsulation. 
         [0010]    In one embodiment, the method further comprises a step of etching a first surface of the heat sink strip to define at least one castellation. 
         [0011]    In another embodiment, the method further comprises a step of attaching a metal clip to at least one side of the IC package. 
         [0012]    There are many advantages to the present invention. A heat sink with castellations greatly increases the surface area of contact between the heat sink and the encapsulation. As a result, the adhesion of the heat sink to the encapsulation is increased. 
         [0013]    In this invention, a thin layer of encapsulation separates the heat sink from the IC chip, preventing them to be in direct contact. This reduces the chance that the IC chip will be damaged during the fabrication process, and also prevents short circuiting the IC chip and the heat sink. A surface of the heat sink is further exposed to the external environment of the IC package to improve heat dissipation to the environment. 
         [0014]    If the castellations are etched from the heat sink strip, the etching process produces undercuts at the side surfaces of the castellations. By being concavely curved, the undercuts further increases the amount of surface area of contact between the heat sink and the encapsulation. The undercuts also provide better interlocking between the heat sink and the encapsulation to further decrease the chance of detaching. 
         [0015]    The central castellation is aligned to the IC chip, with a thin layer of encapsulation separating the two. That way, the distance between the two is minimized regardless of the package thickness while still preventing the two to be in direct contact. 
     
    
     
       BRIEF DESCRIPTION OF FIGURES 
         [0016]      FIG. 1  is a top view of a leadframe strip according to an embodiment of the present invention. 
           [0017]      FIG. 2A  is a top view of a leadframe according to an embodiment of the present invention. 
           [0018]      FIG. 2B  is a bottom view of the leadframe in  FIG. 2A . 
           [0019]      FIG. 2C  is a horizontal cross sectional view of the leadframe in  FIG. 2A . 
           [0020]      FIG. 3  is a top view of a heatspreader strip according to an embodiment of the present invention. 
           [0021]      FIG. 4A  is a top view of a heatspreader according to an embodiment of the present invention. 
           [0022]      FIG. 4B  is a bottom view of the heatspreader in  FIG. 4A . 
           [0023]      FIG. 4C  is a horizontal cross sectional view of the heatspreader in  FIG. 4A . 
           [0024]      FIG. 4D  is a side view of a castellation according to an embodiment of the present invention. 
           [0025]      FIG. 5A  is a top view of a heatspreader according to another embodiment of the present invention. 
           [0026]      FIG. 5B  is a bottom view of the heatspreader in  FIG. 5A . 
           [0027]      FIG. 5C  is a horizontal cross sectional view of the heatspreader in  FIG. 5A . 
           [0028]      FIG. 6A  is a top view of a heatspreader according to another embodiment of the present invention. 
           [0029]      FIG. 6B  is a bottom view of the heatspreader in  FIG. 6A . 
           [0030]      FIG. 6C  is a horizontal cross sectional view of the heatspreader in  FIG. 6A . 
           [0031]      FIGS. 7A to 7F  are a process flow chart of a method of fabricating an IC package according to an embodiment of the present invention. 
           [0032]      FIG. 8A  is a bottom view of an IC package with metal clip according to an embodiment of the present invention. 
           [0033]      FIG. 8B  is a horizontal cross sectional view of the IC package in  FIG. 8A . 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0034]    Various embodiments of the present invention will now be described more fully with reference to the accompanying drawings. The invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, the embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. 
         [0035]    Throughout the specification and claims, the terms “IC chip” and “semiconductor die” are used interchangeably. The same applies to the terms “heatspreader” and “heat sink”. 
         [0036]    Referring now to  FIG. 1 , an embodiment of a leadframe strip  100  used in an IC package manufacturing process is shown having a pattern etched on a top surface thereof. In various embodiments, the leadframe strip  100  is made of either a rigid material, such as, for example, BT, FR4, or ceramic, or a flexible material, such as, for example, polymide, or is formed of a metal, such as for example, copper or copper alloy, aluminum or aluminum alloy, or other metal or metal alloy. In the embodiment shown, the leadframe strip  100  comprises four leadframe matrices  100   a - 100   d,  each matrix being subdivided into a plurality of leadframes  102 . As described in more detail below, each leadframe  102  ( 288  shown) of the leadframe strip  100  may be used to form an individual IC package. The leadframe strip  100  also has a plurality of guides  122  disposed around a periphery thereof and/or interposed between the matrices  100   a - d  thereof. As will be explained in more detail below, in various embodiments, the guides  122  are used as reference points when lining up the leadframe strip  100 . While the leadframe strip  100  shown in  FIG. 1  comprises a 4×1 leadframe matrix of 16×18 leadframes, any size, arrangement, and number of matrices and/or leadframes  102  may be utilized.  FIG. 1  therefore, is an exemplary arrangement of one embodiment. 
         [0037]    Referring now to  FIGS. 2A-2C , one of the leadframe  102  is shown. From this view, it can be seen that an IC chip  104  may be mounted to a central region of the leadframe  102  and wire bonded to the leadframe  102  via wire bonds  105 . 
         [0038]    Referring now to  FIG. 3 , an embodiment of a heatspreader strip  108  is shown having a layout similar to the leadframe strip  100  of  FIG. 1 . In the embodiment shown, the heatspreader strip  108  is a strip having four heatspreader matrices  108   a - 108   d,  each matrix being subdivided into a plurality of heatspreaders  106 . As will be described in more detail below, in various embodiments, the arrangement of the plurality of heatspreaders  106  of the heatspreader strip  108  corresponds to the arrangement of the plurality of leadframes  102  of the leadframe strip  100 . In the embodiment shown, a plurality of guides  122  is disposed around the heatspreader strip  108  and/or between the matrices  108   a - 108   d.  In some embodiments, the guides  122  of the heatspreader strip  108  correspond to the guides  122  of the leadframe strip  100  to facilitate lining up of the heatspreader strip  108  and the leadframe strip  100 . 
         [0039]      FIGS. 4A-4C  show an embodiment of a heatspreader  106  of the heatspreader strip  108 . Referring now to  FIG. 4A  in particular, a heatspreader  106  of an embodiment of a heatspreader strip  108  comprises a base sheet  107  having a generally flat or untextured top surface and a bottom surface, with at least one castellations (also called crenellations)  110  extending from the bottom surface of the base sheet  107 . In various embodiments, the castellations  110  are formed by removing portions of the bottom surface of the heatspreader  106 , such as, for example, via an etching process. In various embodiments, the castellations  110  are formed by selectively layering or adding material onto the bottom surface of the base sheet  107 . In the embodiment shown, the castellations  110  are shown as generally cuboidal and uniformly disposed on the bottom surface of the base sheet  107 . In various embodiments, the castellations  110  are of varying sizes and shapes and are disposed on the bottom surface in a variety of patterns. In various embodiments, the heatspreader  106  is a metal sheet, such as a copper sheet, having a thickness of on the order of 10 mils (approximately 250 microns). Recesses are then etched into a surface of the heatspreader  106  such that the castellations  110  would extend outwardly from the recesses In one embodiment, a generally concave undercut  111  is etched into at least one side surface of the castellations  110 , such that the area at the bottom surface of the castellations  110  is larger than the cross-sectional area at the middle of the castellations. 
         [0040]    Having the castellations  110  extending from the base sheet  107  as opposed to the bottom surface being a flat surface greatly improves the adhesion and attachment of the heatspreader  106  to an encapsulation compound which embeds the bottom surface of the heatspreader  106 . In this invention, the contact area between the heatspreader  106  and the encapsulation compound is substantially increased over the traditional flat heat sinks. Also, the castellations  110  extending into the encapsulation compound means that the heatspreader  106  is able to withstand a larger shear force from the side as the castellations  110  interlocks with the encapsulation compound in the horizontal plane, while traditional flat heat sinks can only rely on adhesion force to keep the heat sink from moving sideways. 
         [0041]    A side view of a castellation  110  is shown in  FIG. 4D . The concave undercuts  111  that are etched into the side surfaces of the castellations  110  increase the surface area of the castellations  110  compared to flat side surfaces (shown in dashed lines), thus improving the adhesion of the heatspreader  106  to the encapsulant. The undercuts  111  also improves the physical interlocking performance of the heatspreader  106  to the encapsulant because the encapsulant fills up the undercuts  111  such that an area directly above the bottom surface of the castellation  110  is filled, thus preventing the castellations  110  from moving directly upwards after molding. 
         [0042]    Referring now to  FIGS. 5A-5C , a heatspreader  106  of an embodiment of the heatspreader strip  108  is shown having a central castellation  112  and a plurality of castellations  110  disposed around a periphery of the base sheet  107  where the central castellation  112  is bigger than the castellations  110  at the periphery. As can be seen from the side cross sectional view of  FIG. 5B , in various embodiments, the central castellation  112  extends out farther from the base sheet  107  than the castellations  110 . In the embodiment shown, the central castellation  112  and the castellations  110  disposed thereabout are integrally formed from the heatspreader strip  108 . For example, portions of the heatspreader  106  may be etched away around the central castellation  112  so that the central castellation  112  is thicker than those castellations  110  disposed around the periphery as shown in  FIG. 5C . As will be explained in more detail below, enhanced heat dissipation may be obtained when a central castellation  112  extends outwardly from the base sheet  107 . 
         [0043]    In thin IC packages, heat generated from the IC chip  104  only needs to travel for a short distance to reach the environment through the base sheet  107 . However, as the IC packages get thicker, the distance between the IC chip  104  and the base sheet  107  also increases. If a central castellation  112  is absent, heat will need to travel through a thick layer of encapsulation compound before reaching the base sheet  107 . With the central castellation  112 , the thickness of the encapsulation compound between the heatspreader  106  and the IC chip  104  can be kept to a minimum. As the central castellation  112  is made of a thermally conductive material with thermal properties superior to the encapsulation compound, heat dissipation performance for the IC package is increased. 
         [0044]    Referring now to  FIGS. 6A-6C , a heatspreader  106  of an embodiment of the heatspreader strip  108  is shown having a plurality of castellations  110  disposed around a periphery of a bottom surface of the base sheet  107  and a central castellation  112  where, in some embodiments, the central castellation  112  is larger than the plurality of castellations  110 . As can be seen from the side cross sectional view, the centrally disposed castellation  112  is attached to the base sheet  107 . In various embodiments, the heatspreader  106  is formed of, for example, a metal such as copper and the central castellation  112  is formed of the same metal or is formed of a different metal. In various embodiments, the central castellation  112  is attached to the base sheet  107  via bonding, welding, epoxy, riveted, clipped, or other attachment method. One advantage of attaching one or more of the castellations  110  and/or central castellation  112  is that less material is wasted in the etching away process. 
         [0045]    The main difference between this embodiment and the one shown in  FIGS. 5A-5C  is that the central castellation  112  is attached rather than etched. When the whole heatspreader  106  is formed by etching, the initial thickness of the heatspreader strip  108  will be large since it must be at least as thick as the base sheet  107  and the central castellation  112  combined. Heatspreader strip material is then etched away to form the castellations  110  and the central castellation  112 . In this embodiment, the heatspreader strip  108  can be much thinner, with the thickness of the attached central castellation  112  chosen to keep the heatspreader  106  as close to the IC chip  104  as possible. 
         [0046]    Having an attached central castellation  112  also makes it easy for the heatspreader  106  to adapt to different IC package thicknesses. In the previous embodiment, in order to keep the layer of encapsulation between the IC chip  104  and the central castellation  112  to be thin, heatspreaders  106  etched from heatspreader strips  108  will need to be of different thicknesses. In this embodiment, the same configuration and thickness of base sheet  107  and the castellations  110  can be used for IC package of all thicknesses, as the thickness of the central castellation  112  can be changed independently to the base sheet  107  to adapt to different IC package thicknesses. 
         [0047]    Referring now to  FIG. 7A-7F , the steps of a manufacturing process for forming an IC package having a heatspreader are illustrated. In step S 300  as shown in  FIG. 7A , a pattern is etched on a leadframe  102  of a leadframe strip  100 . In the embodiment shown, the leadframe  102  has been etched all the way through. In various embodiments, the leadframe  102  is partially etched. In step S 302  as shown in  FIG. 7B , an IC chip  104  has been attached to the leadframe  102  and electrically coupled thereto via wire bonds  105 . In step S 304  as shown in  FIG. 7C , a heatspreader  106  of a heatspreader strip  108  aligned over the leadframe strip  100  and positioned such that the central castellation  112  is disposed directly over the IC chip  104 . In various embodiments, the central castellation  112  does not directly contact the IC chip  104 . In some embodiments, there is a gap between a bottom surface of the central castellation  112  and the IC chip  104 . As can be seen, the central castellation  112  is formed such that when the central castellation  112  is disposed directly above the IC chip  104 , the wire bonds  105  coupled to the IC chip  104  do not touch the central castellation  112 . In various embodiments, this is accomplished by increasing the gap between the IC chip  104  and the castellation  112  and/or decreasing the width of the central castellation  112  relative to the width of the IC chip  104 . In various embodiments, the leadframe strip  100  is secured in place by a lower-half mold base and the heatspreader strip  108  is secured in place by an upper-half mold base. The leadframe strip  100  and the heatspreader strip  108  is aligned, for example, by using guides  122  (shown in  FIGS. 1 and 3 ). In some embodiments, the heatspreader strip  108  is held in place in the upper-half mold base by vacuum suction, clipping, screwing, rotatable tabs, or other way of securing the heatspreader strip  108  in place. As can be seen in the figure, when the heatspreader strip  108  is disposed over the leadframe strip  100 , a cavity is formed therebetween. 
         [0048]    At step S 306  as shown in  FIG. 7D , an encapsulation compound  114  is interposed between the heatspreader strip  108  and the leadframe strip  100 . In various embodiments, the encapsulation compound  114  is injected into the cavity between the heatspreader strip  108  and the leadframe strip  100  using a variety of known methods. As can be seen, the encapsulation compound  114  does not cover a top surface of the heatspreader strip  108 . In such embodiments, not covering the heatspreader strip  108  enhances heat dissipation from a top surface thereof. In the embodiment shown, the plurality of castellations  110  on a bottom surface of the heatspreader  108  have undercuts  111  etched into sides thereof. In such embodiments, the undercuts  111  increase adhesion between the heatspreader strip  108  and the encapsulation compound  114  as explained above. At step S 308  as shown in  FIG. 7E , the leadframe strip  100  and the heatspreader strip  108  are singulated at singulation points  116  to separate a single IC package  118  from the rest of the IC packages formed from the leadframe strip  100  (shown in  FIG. 1 ) and the heatspreader strip  108  (shown in  FIG. 3 ). In step S 310  as shown in  FIG. 7F , a single IC package  118  is shown. 
         [0049]    The IC package  118  as shown in  FIG. 7F  is described in more detail below. The leadframe  102  and the IC chip  104  is the same as described in  FIG. 2C . The bottom surface of the leadframe  102  is exposed to the environment at a bottom side of the IC package  118 . Above the leadframe  102  and the IC chip  104  is the encapsulation compound  114 . The bottom side of the encapsulation compound  114  embeds the top surface of the leadframe  102  along with the top and side surfaces of the IC chip  104 . The side surfaces of the encapsulation compound  114  are exposed to the environment at the side surfaces of the IC package  118 . The top side of the encapsulation compound  114  embeds the bottom surface of the base sheet  107  and the bottom and side surfaces of the castellations  110 , along with the bottom and side surfaces of the central castellation  112 . The top surface of the base sheet  107  is exposed to the environment at a top side, and the side surfaces of the base sheet  107  are also exposed to the environment at the side surfaces of the IC package  118 . 
         [0050]    Referring now to  FIGS. 8A and 8B , a bottom view and a side cross sectional view of a single IC package  118  is shown having a heatspreader  108  disposed over an IC chip  104  and having a metal clip  120  disposed on either side thereof. In various embodiments, the metal clip  120  is clamped around a top and bottom surface of the IC package  118 . The top surface of the IC package  118  is also the top surface of the base sheet  107 . As the metal clip  120  cannot make contact to the leadframe  102  or else it will create an electric short-circuit, the leadframe  102  is designed such that the encapsulation compound  114  covers the part of the bottom surface of the leadframe  102  that contacts the metal clip  120 . 
         [0051]    The metal clip  120  decreases the likelihood of separation between the heatspreader  106  and the leadframe  102 . Additionally, the metal clip  120  increases the heat dissipation from the heatspreader  106  by thermally coupling the heatspreader  106  to a surface below the IC package  118  such as, for example, a PCB. In one embodiment, the metal clip  120  contacts the PCB when the leadframe  102  is bonded or soldered to the PCB. In another embodiment, the metal  120  is in close proximity to the PCB. The metal clip  120  also increases the surface area that heat can be dissipated from, so heat dissipation performance is further improved. 
         [0052]    To produce the integrated circuit package as shown in  FIGS. 8A and 8B , the metal clip  120  is attached to the individual IC package  118  after the singulation step S 310 . 
         [0053]    Although various embodiments of the method and system of the present invention have been illustrated in the accompanying Drawings and described in the foregoing Detailed Description, it will be understood that the invention is not limited to the embodiments disclosed, but is capable of numerous rearrangements, modifications and substitutions without departing from the spirit of the invention as set forth herein. 
         [0054]    For example, the top surface of the base sheet  107  is flat or untextured in the embodiments described. However, it is obvious that the top surface can also be partially etched or otherwise plated with additional material to form a patterned surface to further increase the surface area of the heatspreader  106 . In an embodiment, the mold follows the contour of the top surface of the base sheet  107  such that encapsulation compound  114  will not be injected between the upper mold half and the top surface of the base sheet  107 . 
         [0055]    The embodiments above described that the castellations can be formed by either etching and layering. It is clear to a person skilled in the art that both methods can be used in the same heatspreader if they so wish. 
         [0056]    The IC package  118  as shown in  FIG. 7F  uses the embodiment of the heatspreader  106  as shown in  FIGS. 6A-6C . However, it is clear that all embodiments of the heatspreader  106  shown above or with variations described in the paragraphs above can be used in the IC package  118 . 
         [0057]    The side surfaces of the castellations  110  are shown as concave in the embodiment, but it could be convex or having a sharp corner as long as the shape has a larger surface area than flat surface while improves interlocking.