Abstract:
Light emitting die package is disclosed. The die package includes a leadframe, a bottom heatsink, a top heatsink, a reflector and a lens. The top and bottom heatsinks are thermally coupled but electrically insulated from the leadframe. The leadframe includes a plurality of leads and defines a mounting pad for mounting LEDS. The top heatsink defines an opening over the mounting pad. The reflector is coupled to the top heatsink at the opening. The lens is placed over the opening defining an enclosed cavity over the mounting pad. At least one light emitting device (LED) is mounted on the mounting pad within the cavity. Encapsulant optically couples the LED to its surrounding surfaces to maximize its optical performance. When energized, the LED generates light and heat. The light is reflected by the reflector and operated on by the lens. The heat is dissipated by the top and the bottom heatsinks.

Description:
PRIORITY  
       [0001]    This application claims the benefit of the filing date of U.S. Provisional Patent Application No. 60/431,523 filed Dec. 6, 2002 entitled “Leadframe based LED or semiconductor package with improved heat spreading” under 35 USC section  119 , section  120 , or both. 
     
    
     
       BACKGROUND  
         [0002]    The present invention relates to the field of packaging semiconductor devices, and more particularly to packaging light emitting diodes.  
           [0003]    Light emitting devices (LEDS) such as light emitting diodes are often packaged within leadframe packages. A leadframe package typically includes an LED connected to thin metal leads where the LED and most of the leads are completely encapsulated within a plastic body. A part of the plastic body defines a lens. A portion of the leads connected to the LED extends outside the plastic body. The metal leads of the leadframe package serve as the conduit to supply the LED with electrical power and, at the same time, may act to draw heat away from the LED. Heat is generated by the LED when power is applied to the LED to produce light. The portion of the leads that extend out from the package body connects to circuits external to the leadframe package.  
           [0004]    Some of the heat generated by the LED is dissipated by the plastic package body; however, most of the heat is drawn away from the LED via the metal components of the package. The metal leads are typically very thin and have a small cross section. For this reason, capacity of the metal leads to remove heat from the LED is limited. This limits the amount of power that can be sent to the LED. This, in turn, limits the amount of light that can be generated by the LED.  
           [0005]    To increase the capacity of an LED package to dissipate heat, various designs are used in the industry; however, each of these designs results in LED packages with limited heat dissipation capacities while increasing the complexity and the costs of manufacturing the LED packages. For example, in one approach, LED packages are designed to include the LED within a cavity of a heatsink slug. Then, the heatsink slug is surrounded by a plastic body except for its bottom surface. For example, some LUXEON™ LED packages by Lumileds Lighting, LLC embodies such a design. Here, the heatsink slug increases the capacity of the LED package to dissipate heat; however, LED packages of this design are relatively difficult and costly to manufacture. Further, the heat dissipation is limited because of its limited exposed surface (the bottom surface only).  
           [0006]    In another LED package design, the leads of the leadframe are extended (in various shapes and configurations) beyond the immediate edge of the LED package body. This increases the surface area of the portions of the leads exposed to the surrounding air. The increased exposed surface area of the extended leads increases the capacity of the LED package to dissipate heat; however, the extended leads increase the size of the LED package requiring relatively large area on a circuit board. Circuit board area is a scarce and costly factor in many applications.  
           [0007]    Another undesirable aspect of the current leadframe package designs relates to problems associated with thermal expansion of the package. When heat is generated, the LED package experiences thermal expansion. Each of the parts of the LED package has a different coefficient of thermal expansion (CTE). For example, the CTE of the LED, the CTE of the package body, the CTE of the leads, and the CTE of lens are different from each other. For this reason, when heated, each of these parts experience different degrees of thermal expansion resulting in mechanical stresses between the parts of the package thereby adversely affecting its reliability.  
           [0008]    Consequently, there remains a need for an improved LED package that overcomes or alleviates one or more of the shortcomings of the prior art packages.  
         SUMMARY  
         [0009]    The need is met by the present invention. In a first embodiment of the present invention, a light emitting die package includes a leadframe, a bottom heatsink and a top heatsink. The leadframe has a top side and a bottom side and includes a plurality of leads. A portion of the leadframe defines a mounting pad. The bottom heatsink is coupled to the bottom side of the leadframe. The top heatsink is coupled to the top side of the leadframe. The top heatsink defines an opening which generally surrounds the mounting pad.  
           [0010]    In a second embodiment of the present invention, a light emitting die package includes a leadframe, a bottom heatsink and a top heatsink. The leadframe has a top side and a bottom side and includes a plurality of leads. The bottom heatsink is thermally coupled to the bottom side of the leadframe under the mounting pad. The bottom heatsink is electrically insulated from the leadframe. The top heatsink is thermally coupled to the top side of the leadframe and defines an opening which generally surrounds the mounting pad. The top heatsink is electrically insulated from the leadframe and is coupled to a reflector that also surround the mounting pad. At least one light emitting device (LED) is mounted on the mounting pad, the LED adapted to generate light when energized. A lens is coupled to the top heatsink over the opening. The lens is adapted to operate on the light generated by the LED. The lens covers the opening thereby defining an enclosed cavity.  
           [0011]    In a third embodiment of the present invention, a method of manufacturing a light emitting die package is disclosed. First, a leadframe die is fabricated. The leadframe die includes a plurality of leads and die frame, each lead and the die frame having a top side and a bottom side. Then, a top heatsink is coupled over the leadframe die, the top heatsink defining an opening. Next, a bottom heatsink is coupled under the leadframe die. Finally, the leadframe die is stamped to cut-away the die frame.  
           [0012]    Other aspects and advantages of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]    [0013]FIGS. 1A and 1B are perspective views of a light emitting die package according to one embodiment of the present invention;  
         [0014]    [0014]FIG. 1C is a cutaway side view of the light emitting die package of FIG. 1A cut along line A-A;  
         [0015]    [0015]FIG. 2 is an exploded perspective view of the semiconductor package of FIG. 1A;  
         [0016]    [0016]FIG. 3 is a perspective view of a light emitting die package of FIG. 1A during its manufacturing process. 
     
    
     DETAILED DESCRIPTION  
       [0017]    The present invention will now be described with reference to the FIGS. 1 through 3, which illustrate various embodiments of the present invention. As illustrated in the Figures, the sizes of layers or regions are exaggerated for illustrative purposes and, thus, are provided to illustrate the general structures of the present invention. Furthermore, various aspects of the present invention are described with reference to a structure or a portion being formed on other structures, portions, or both. As will be appreciated by those of skill in the art, references to a structure being formed “on” or “above” another structure or portion contemplates that additional structure, portion, or both may intervene. References to a structure or a portion being formed “on” another structure or portion without an intervening structure or portion are described herein as being formed “directly on” the structure or portion. Furthermore, relative terms such as “on” or “above” are used herein to describe one structure&#39;s or portion&#39;s relationship to another structure or portion as illustrated in the Figures. It will be understood that relative terms such as “on” or “above” are intended to encompass different orientations of the device in addition to the orientation depicted in the Figures. For example, if the device in the Figures is turned over, structure or portion described as “above” other structures or portions would now be oriented “below” the other structures or portions. Likewise, if the device in the Figures is rotated along an axis, structure or portion described as “above” other structures or portions would now be oriented “next to” or “left of” the other structures or portions. Like numbers refer to like elements throughout.  
         [0018]    As shown in the figures for the purposes of illustration, embodiments of the present invention are exemplified by a light emitting die package including a leadframe with leads, a bottom heatsink, and a top heatsink with an opening. A light emitting device (LED) such as a light emitting diode is mounted on the leadframe within the opening. A lens covers the opening. In effect, the light emitting die package according to one embodiments of the present invention comprises a two part heat sink sandwiching a leadframe. Because both the bottom and the top heat sinks draw heat away from the LED, more power can be delivered to the LED, and the LED can produce more light. Furthermore, for the same reason, the light emitting die package of the present invention may not require a separate heat sink slugs or leads that extend away from the package. Accordingly, the LED die package of the present invention may be more compact, more reliable, and less costly to manufacture than the die packages of the prior art.  
         [0019]    [0019]FIGS. 1A and 1B are perspective views of a light emitting die package  10  according to one embodiment of the present invention. FIG. 1C is a cutaway side view of the light emitting die package  10  of FIG. 1A cut along line A-A. FIG. 2 is an exploded perspective view of the semiconductor package  10  of FIGS. 1A and 1B. Referring to FIGS. 1A through 2, the semiconductor package  10  includes a leadframe  20 , a bottom heatsink  30 , and a top heatsink  40 .  
         [0020]    The leadframe  20  includes a plurality of leads. In the figures, for illustrative purposes only, leads  22   a ,  22   b ,  22   c ,  22   d , and  22   e  are shown. For convenience, the leads  22   a ,  22   b ,  22   c ,  22   d , and  22   e  are collectively referred to as leads  22  in this document. The leads  22   a ,  22   b ,  22   c ,  22   d , and  22   e  are electrically isolated from each other. To avoid clutter, not all instances of the leads  22  are illustrated with a reference numeral in the Figures. The leadframe  20  includes a top side  24  and a bottom side  26 . Further, a portion  28  of the leadframe  20  defines a mounting pad  28 . The mounting pad  28  is a portion of the leadframe  20  (including a portion of the first lead  22   a ) where an LED assembly  50  is mounted. Typically the mounting pad  28  is generally located proximal to center of the top side  24  of the leadframe  20 . In alternative embodiments of the present invention, the LED assembly  50  can be replaced by other semiconductor circuits or chips. The leadframe  20  is made of electrically conductive material and is generally thin. In one embodiment the leadframe  20  has thickness in order of thousandths or hundredths of inches, and for example, ranges from 0.005 inches to 0.010 inches.  
         [0021]    The bottom heatsink  30  is coupled to the bottom side  26  of the leadframe  20  at least under the mounting pad  28 .  
         [0022]    The bottom heatsink  30  is made of thermally conductive material and is thermally coupled to the bottom side  26  of the leadframe  20  but is electrically separated from the leadframe  20 . The bottom heatsink  30  has a top surface  32  thermally coupled to but is electrically separated from the leadframe  20 , the electrical separation can be accomplished by using a dielectric layer between the leadframe  20  and the bottom heatsink  30 , for example, adhesive filled with ceramic particles. The bottom heatsink  30  has a bottom surface  34  defining a bottom plane for the light emitting die package  10 . The bottom surface  34  of the bottom heatsink  30  can include a metalized bottom as illustrated in FIG. 1B. As illustrated, the leads  22  are bent toward the bottom plane, terminating proximal to the bottom plane.  
         [0023]    The top heatsink  40  is coupled to the top side  24  of the leadframe  20 . The top heatsink  40  defines an opening  42 , the opening  42  generally surrounding the mounting pad  28 . The top heatsink  40  is made of thermally conductive material and is thermally coupled to the top side  24  of the leadframe  20  but is electrically separated from the leadframe  20 , the electrical separation can be accomplished by using a dielectric layer between the leadframe  20  and the bottom heatsink  40 . The bottom heatsink  30  and the top heatsink  40  have generally similar lateral dimensions, or extents, and substantially overlap each other sandwiching the leadframe  20  between them. The top heatsink  40  and the bottom heatsink  30  are made with thermally conductive material such as, for example only, copper, aluminum, or ceramics material.  
         [0024]    The light emitting die package  10  includes the LED assembly  50  including at least one light emitting device (LED) mounted on the mounting pad. In FIG. 2, the LED assembly  50  is illustrated as having four light emitting diodes. The LEDS are adapted to generate light when energized.  
         [0025]    The light emitting die package  10  includes a reflector  60  coupled to the top heatsink  40 , the reflector  60  surrounding the mounting pad  28 . In an alternative embodiment, the reflector  60  is not a separate component but is integrated with and is a portion of the top heatsink  40 . The reflector  60  is adapted to reflect light from the LED assembly  50  toward a lens  70 .  
         [0026]    The light emitting die package  10  includes the lens  70  coupled to the top heatsink  40 , the lens  70  coupled generally over the opening  42 , the mounting pad  28 , and over the reflector  60 . When the lens  70  is placed over the opening  42 , an enclosed cavity  44  is defined by the leadframe  20 , the opening  42  of the top heatsink  40 , and the lens  70 . The lens  70  operates on the light generated by the LED assembly  50  by, for example, reflecting, directing, focusing, and alter wavelength. For example, a bottom surface  72  of the lens  70  can be coated with calcium carbonate to diffuse the light. Alternately, the bottom surface  72  of the lens  70  can be coated with phosphors to alter wavelengths of light from the LED assembly  50 .  
         [0027]    The enclosed cavity  44  is filled by clear encapsulant such as Silicone. The encapsulant affixes the LED assembly  50  to the mounting pad  28 . The enclosed cavity  44  need not be completely filled with the encapsulant. In fact, partially filling the cavity  44  with encapsulant while leaving gaps within the cavity  44  allows the encapsulant to expand (when heat is generated by the LED assembly  50 ) without separating the lens  70  from the top heatsink  40 .  
         [0028]    The method of manufacturing the light emitting die package  10  of FIGS. 1A through 2 can be discussed using FIG. 3. FIG. 3 illustrates the light emitting die package  10  of FIG. 1A before it is manufactured. To manufacture the light emitting die package  10  of Figure, a leadframe die  80  is fabricated. For illustratively purposes, in FIG. 2, the leadframe die  80  is fabricated for manufacturing of two light emitting die packages. In fact, a leadframe die can be fabricated to manufacture multiple light emitting die packages simultaneously. The leadframe die  80  includes a plurality of leads, for example, the leads  22   a ,  22   b ,  22   c ,  22   d , and  22   e  (collectively, “leads  22 ”) and a die frame  82  surrounding the leads  22 . The leadframe die has a top side  24  (that is the same side as the top side  24  of leadframe  20  of FIG. 2) and a bottom side  26  (that is the same side as the bottom side  26  of leadframe  20  of FIG. 2). The leadframe die  80  is fabricated by stamping a sheet of die material such as metal. The thickness of the die material may vary greatly depending on the desired application, for example, the thickness may be in the order of fractions of millimeters (mm), for example, ranging from 0.13 mm to 0.25 mm. Alternately, the leadframe die  80  can be fabricated using etching processes.  
         [0029]    Referring to FIGS. 2 and 3, the top heatsink  40  is coupled to the leadframe die  80 . As already described, the top heatsink  40  defined the opening  42 . The bottom heatsink  30  is coupled to the bottom side of the leadframe die  80 . The bottom heatsink  30  has a top surface  32  thermally coupled to but is electrically separated from the leadframe die  80 . As illustrated in FIG. 1B, the bottom heatsink  30  has a metalized bottom surface  34  defining a bottom plane for the light emitting die package  30 . Dielectric but thermally conductive adhesive layer  38  of FIG. 3 may be used to separate the bottom heatsink  30  from the leadframe  20 .  
         [0030]    The top heatsink  40  and the bottom heatsink  30  have similar lateral extents and substantially overlap each other. For example, the later extents  33  and  35  of the bottom heatsink  30  may vary widely depending on the implementation, for example only, the lateral extents  33  and  35  may be in the order of mm or centimeters (cm), and may range from, for example, seven mm to 20 mm. The bottom heatsink  30  and the top heatsink  40  may have thicknesses in the order of mm or cm, and may range from, for example, 1.5 mm to three mm. These measurements may vary greatly depending on various desired characteristics and applications.  
         [0031]    Referring to FIGS. 2 and 3, the method of manufacturing the light emitting die package  10  is further discussed. The LED assembly  50  including at least one light emitting device (LED) such as a light emitting diode is mounted on at least one lead, such as the lead  22   a , within the opening  42 . Then, the reflector  60  and the lens  70  are attached on the top heatsink  40 , the lens  70  covering the opening  42 . The reflector  60  surrounds the opening  42 . The reflector  60  may be integrated with the top heatsink  40  in which case there is no need for a separate step of coupling the reflector  60  with the top heatsink  40 .  
         [0032]    Finally, the leadframe die  80  is stamped to cut away the die frame  82 . During the stamping, the leads  22  are bent towards the bottom plane as illustrated in FIGS. 1A through 2.  
         [0033]    From the foregoing, it will be apparent that the present invention is novel and offers advantages over the current art. Although specific embodiments of the invention are described and illustrated above, the invention is not to be limited to the specific forms or arrangements of parts so described and illustrated. For example, differing configurations, sizes, or materials may be used to practice the present invention. The invention is limited by the claims that follow.