Patent Document

RELATED APPLICATIONS 
     This application is a divisional of and claims priority benefits to U.S. patent application Ser. No. 11/703,721, filed Feb. 8, 2007 now U.S. Pat. No. 7,775,685, which is a divisional of application Ser. No. 10/446,532, filed May 27, 2003 now U.S. Pat. No. 7,264,378, the entire contents of which are both hereby incorporated by reference herein. 
    
    
     BACKGROUND 
     Example embodiments in general relate to packaging semiconductor devices which include light emitting diodes. 
     Light emitting diodes (LEDS) are often packaged within leadframe packages. A leadframe package typically includes a molded or cast plastic body that encapsulates an LED, a lens portion, and thin metal leads connected to the LED and extending outside the 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. A portion of the leads extends out from the package body for connection to circuits external to the leadframe package. 
     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 has 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 thereby limiting the amount of light that can be generated by the LED. 
     To increase the capacity of an LED package to dissipate heat, in one LED package design, a heat sink slug is introduced into the package. The heat sink slug draws heat from the LED chip. Hence, it increases the capacity of the LED package to dissipate heat. However, this design introduces empty spaces within the package that is be filled with an encapsulant to protect the LED chip. Furthermore, due to significant differences in CTE (coefficient of thermal expansion) between various components inside the LED package, bubbles tend to form inside the encapsulant or the encapsulant tends to delaminate from various portions within the package. This adversely affects the light output and reliability of the product. In addition, this design includes a pair of flimsy leads which are typically soldered by a hot-iron. This manufacturing process is incompatible with convenient surface mounting technology (SMT) that is popular in the art of electronic board assembly. 
     In another LED package design, the leads of the leadframe package have differing thicknesses extended (in various shapes and configurations) beyond the immediate edge of the LED package body. A thicker lead is utilized as a heat-spreader and the LED chip is mounted on it. This arrangement allows heat generated by the LED chip to dissipate through the thicker lead which is often connected to an external heat sink. This design is inherently unreliable due to significant difference in coefficient of thermal expansion (CTE) between the plastic body and the leadframe material. When subjected to temperature cycles, its rigid plastic body that adheres to the metal leads experiences high degree of thermal stresses in many directions. This potentially leads to various undesirable results such as cracking of the plastic body, separation of the plastic body from the LED chip, breaking of the bond wires, delaminating of the plastic body at the interfaces where it bonds to various parts, or resulting in a combination of these outcomes. In addition, the extended leads increase the package size and its footprint. For this reason, it is difficult to populate these LED packages in a dense cluster on a printed circuit board (PCB) to generate brighter light. 
     Another disadvantage of conventional leadframe design is that the thick lead cannot be made or stamped into a fine circuit for flip-chip mounting of a LED—which is commonly used by some manufacturers for cost-effective manufacturing and device performance. 
     SUMMARY 
     An example embodiment of the present invention is directed to a semiconductor die package including a substrate having conductive traces on a top surface thereof, and a light emitting diode (LED) mounted to the top surface of the substrate via a mounting pad. The mounting pad is electrically connected to the conductive traces on the substrate top surface. The package includes a reflector plate mechanically coupled to the substrate and substantially surrounding the mounting pad and LED, the reflector plate defining a reflection surface, and a lens substantially covering the mounting pad and LED. 
     Another example embodiment is directed to a semiconductor die package having a substrate, a light emitting diode (LED) mounted on the substrate via a mounting pad so that the LED is electrically connected to a top surface of the substrate, and a reflector plate coupled to the substrate and substantially surrounding the mounting pad and LED. The reflector plate has an opening there through. A lens is placed in the opening to substantially cover the mounting pad, LED and opening. The package includes an encapsulant covering the LED within at least part of the opening. The lens adheres to the encapsulant so that the lens floats on the encapsulant within the opening. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a perspective view of a semiconductor die package according to one embodiment of the present invention; 
         FIG. 1B  is an exploded perspective view of the semiconductor package of  FIG. 1A ; 
         FIG. 2A  is a top view of a portion of the semiconductor package of  FIG. 1A ; 
         FIG. 2B  is a side view of a portion of the semiconductor package of  FIG. 1A ; 
         FIG. 2C  is a front view of a portion of the semiconductor package of  FIG. 1A ; 
         FIG. 2D  is a bottom view of a portion of the semiconductor package of  FIG. 1A ; 
         FIG. 3  is a cut-away side view of portions of the semiconductor package of  FIG. 1A ; 
         FIG. 4  is a side view of the semiconductor package of  FIG. 1A  with additional elements; 
         FIG. 5  an exploded perspective view of a semiconductor die package according to another embodiment of the present invention; 
         FIG. 6A  is a top view of a portion of the semiconductor package of  FIG. 5 ; 
         FIG. 6B  is a side view of a portion of the semiconductor package of  FIG. 5 ; 
         FIG. 6C  is a front view of a portion of the semiconductor package of  FIG. 5 ; and 
         FIG. 6D  is a bottom view of a portion of the semiconductor package of  FIG. 5 . 
     
    
    
     DETAILED DESCRIPTION 
     Example embodiments will now be described with reference to the  FIGS. 1 through 6D . 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 in the example embodiments are described with reference to a layer or structure being formed on a substrate or other layer or structure. As will be appreciated by those of skill in the art, references to a layer being formed “on” another layer or substrate contemplates that additional layers may intervene. References to a layer being formed on another layer or substrate without an intervening layer are described herein as being formed “directly on” the layer or substrate. 
     Furthermore, relative terms such as beneath may be used herein to describe one layer or regions relationship to another layer or region as illustrated in the Figures. It will be understood that these terms 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, layers or regions described as “beneath” other layers or regions would now be oriented “above” these other layers or regions. The term “beneath” is intended to encompass both above and beneath in this situation. Like numbers refer to like elements throughout. 
     As shown in the figures for the purposes of illustration, example embodiments of the present invention are exemplified by a light emitting die package including a bottom heat sink (substrate) having traces for connecting to a light emitting diode at a mounting pad and atop heat sink (reflector plate) substantially surrounding the mounting pad. A lens covers the mounting pad. In effect, an example die package comprises a two part heat sink with the bottom heat sink utilized (in addition to its utility for drawing and dissipating heat) as the substrate on which the LED is mounted and connected, and with the top heat sink utilized (in addition to its utility for drawing and dissipating heat) as a reflector plate to direct light produced by the LED. 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 thereby produce more light. 
     Further, the body of the die package itself may act as the heat sink removing heat from the LED and dissipating it. For this reason, the example LED die package may not require separate heat sink slugs or leads that extend away from the package. Accordingly, the LED die package may be more compact, more reliable, and less costly to manufacture than die packages of the prior art. 
       FIG. 1A  is a perspective view of a semiconductor die package  10  according to one embodiment of the present invention and  FIG. 1B  is an exploded perspective view of the semiconductor package of  FIG. 1A . Referring to  FIGS. 1A and 1B , the light emitting die package  10  of the present invention includes a bottom heat sink  20 , a top heat sink  40 , and a lens  50 . 
     The bottom heat sink  20  is illustrated in more detail in  FIGS. 2A through 2D .  FIGS. 2A ,  2 B,  2 C, and  2 D provide, respectively, a top view, a side view, a front view, and a bottom view of the bottom heat sink  20  of  FIG. 1A . Further,  FIG. 2C  also shows an LED assembly  60  in addition to the front view of the bottom heat sink  20 . The LED assembly  60  is also illustrated in  FIG. 1B . Referring to  FIGS. 1A through 2D , the bottom heat sink  20  provides support for electrical traces  22  and  24 ; for solder pads  26 ,  32 , and  34 ; and for the LED assembly  60 . For this reason, the bottom heat sink  20  is also referred to as a substrate  20 . In the Figures, to avoid clutter, only representative solder pads  26 ,  32 , and  34  are indicated with reference numbers. The traces  22  and  24  and the solder pads  32 ,  34 , and  36  can be fabricated using conductive material. Further, additional traces and connections can be fabricated on the top, side, or bottom of the substrate  20 , or layered within the substrate  20 . The traces  22  and  24 , the solder pads  32 ,  34 , and  36 , and any other connections can be interconnected to each other in any combination using known methods, for example via holes. 
     The substrate  20  is made of material having high thermal conductivity but is electrically insulating, for example, aluminum nitride (AIN) or alumina (Al.sub.2O.sub.3). Dimensions of the substrate  20  can vary widely depending on application and processes used to manufacture the die package  10 . For example, in the illustrated embodiment, the substrate  20  may have dimensions ranging from fractions of millimeters (mm) to tens of millimeters. Although the present invention is not limited to particular dimensions, one specific embodiment of the die package  10  of the present invention is illustrated in Figures with the dimensions denoted therein. All dimensions shown in the Figures are in millimeters (for lengths, widths, heights, and radii) and degrees (for angles) except as otherwise designated in the Figures, in the Specification herein, or both. 
     The substrate  20  has a top surface  21 , the top surface  21  including the electrical traces  22  and  24 . The traces  22  and  24  provide electrical connections from the solder pads (for example top solder pads  26 ) to a mounting pad  28 . The top solder pads  26  are portions of the traces  22  and  24  generally proximal to sides of the substrate  20 . The top solder pads  26  are electrically connected to side solder pads  32 . The mounting pad  28  is a portion of the top surface (including portions of the trace  22 , the trace  24 , or both) where the LED assembly  60  is mounted. Typically the mounting pad  28  is generally located proximal to center of the top surface  21 . In alternative embodiments of the present invention, the LED assembly  60  can be replaced by other semiconductor circuits or chips. 
     The traces  22  and  24  provide electrical routes to allow the LED assembly  60  to electrically connect to the solder pads  26 ,  32 , or  34 . Accordingly, some of the traces are referred to as first traces  22  while other traces are referred to as second traces  24 . In the illustrated embodiment, the mounting pad  28  includes portions of both the first traces  22  and the second traces  24 . In the illustrated example, the LED assembly  60  is placed on the first trace  22  portion of the mounting pad  28  thereby making contact with the first trace  22 . In the illustrated embodiment, a top of the LED assembly  60  and the second traces  24  are connected to each other via a bond wire  62 . Depending on the construction and orientation of LED assembly  60 , first traces  22  may provide anode (positive) connections and second traces  24  may comprise cathode (negative) connections for the LED assembly  60  (or vice versa). 
     The LED assembly  60  can include additional elements. For example, in  FIGS. 1B and 2C , the LED assembly  60  is illustrated including an LED bond wire  62 , an LED subassembly  64 , and a light emitting diode (LED)  66 . Such an LED subassembly  64  is known in the art and is illustrated for the purposes of discussing the invention and is not meant to be a limitation of the present invention. In the Figures, the LED assembly  60  is shown die-attached to the substrate  20 . In alternative embodiments, the mounting pad  28  can be configured to allow flip-chip attachment of the LED assembly  60 . Additionally, multiple LED assemblies can be mounted on the mounting pad  28 . In alternative embodiments, the LED assembly  60  can be mounted over multiple traces. This is especially true if flip-chip technology is used. 
     The topology of the traces  22  and  24  can vary widely from the topology illustrated in the Figures while still remaining within the scope of the example embodiments of the present invention. In the Figures, three separate cathode (negative) traces  24  are shown to illustrate that three LED assemblies can be placed on the mounting pad  28 , each connected to a different cathode (negative) trace; thus, the three LED assemblies may be separately electrically controllable. The traces  22  and  24  are made of conductive material such as gold, silver, tin, or other metals. The traces  22  and  24  can have dimensions as illustrated in the Figures and are of a thickness on the order of microns or tens of microns, depending on application. In an example, the traces  22  and  24  can be 15 microns thick.  FIGS. 1A and 2A  illustrate an orientation marking  27 . Such markings can be used to identify the proper orientation of the die package  10  even after assembling the die package  10 . The traces  22  and  24 , as illustrated, can extend from the mounting pad  28  to sides of the substrate  20 . 
     Continuing to refer to  FIGS. 1A through 2D , the substrate  20  defines semi-cylindrical spaces  23  and quarter-cylindrical spaces  25  proximal to its sides. In the Figures, to avoid clutter, only representative spaces  23  and  25  are indicated with reference numbers. The semi-cylindrical spaces  23  and the quarter-cylindrical spaces  25  provide spaces for solder to flow-through and solidify-in when the die package  10  is attached to a printed circuit board (PCB) or another apparatus (not shown) to which the die package  10  is a component thereof. Moreover, the semi-cylindrical spaces  23  and the quarter-cylindrical spaces  25  provide convenient delineation and break points during the manufacturing process. 
     The substrate  20  can be manufactured as one individual section of a strip or a plate having a plurality of adjacent sections, each section being a substrate  20 . Alternatively, the substrate  20  can be manufactured as one individual section of an array of sections, the array having multiple rows and columns of adjacent sections. In this configuration, the semi-cylindrical spaces  23  and quarter-cylindrical spaces  25  can be utilized as tooling holes for the strip, the plate, or the array during the manufacturing process. 
     Furthermore, the semi-cylindrical spaces  23  and the quarter-cylindrical spaces  25 , combined with scribed grooves or other etchings between the sections, assist in separating each individual substrate from the strip, the plate, or the wafer. The separation can be accomplished by introducing physical stress to the perforation (semi through holes at a close pitch) or scribe lines made by laser, or premolded, or etched lines (crossing the semi-cylindrical spaces  23  and the quarter-cylindrical spaces  25 ) by bending the strip, the plate, or the wafer. These features simplify the manufacturing process and thus reduce costs by eliminating the need for special carrier fixtures to handle individual unit of the substrate  20  during the manufacturing process. Furthermore, the semi-cylindrical spaces  23  and the quarter-cylindrical spaces  25  serve as via holes connecting the top solder pads  26 , the side solder pads  32 , and the bottom solder pads  34 . 
     The substrate  20  has a bottom surface  29  including a thermal contact pad  36 . The thermal contact pad  36  can be fabricated using a material having a high thermally and electrically conductive properties such as gold, silver, tin, or another material including but not limited to precious metals. 
       FIG. 3  illustrates a cut-away side view of portions of the semiconductor package of  FIGS. 1A and 1B . In particular, the  FIG. 3  illustrates a cut-away side view of the top heat sink  40  and the lens  50 . Referring to  FIGS. 1A ,  1 B, and  3 , the top heat sink  40  is made from a material having high thermal conductivity such as aluminum, copper, ceramics, plastics, composites, or a combination of these materials. A high temperature, mechanically tough, dielectric material can be used to overcoat the traces  22  and  24  (with the exception of the central die-attach area) to seal the traces  22  and  24  and provide protection from physical and environmental harm such as scratches and oxidation. The overcoating process can be a part of the substrate manufacturing process. The overcoat, when used, may insulate the substrate  20  from the top heat sink  40 . The overcoat may then be covered with a high temperature adhesive such as thermal interface material manufactured by THERMOSET that bonds the substrate  20  to the top heat sink  40 . 
     The top heat sink  40  may include a reflective surface  42  substantially surrounding the LED assembly  60  mounted on the mounting pad  28  (of  FIGS. 2A and 2C ). When the top heat sink  40  is used to dissipate heat generated by the LED in the die package  10 , it can be “top-mounted” directly onto an external heat sink by an adhesive or solder joint to dissipate heat efficiently. In another embodiment, if heat has to be dissipated by either a compressible or non-compressible medium such as air or cooling fluid, the top heat sink  40  may be equipped with cooling fins or any feature that will enhance heat transfer between the top heat sink  40  and the cooling medium. In both of these embodiments, the electrical terminals and the bottom heat sink  20  of the die package  10  can still be connected to its application printed circuit board (PCB) using, for example, the normal surface-mount-technology (SMT) method. 
     The reflective surface  42  reflects portions of light from the LED assembly  60  as illustrated by sample light rays  63 . Other portions of the light are not reflected by the reflective surface  42  as illustrated by sample light ray  61 . Illustrative light rays  61  and  63  are not meant to represent light traces often use in the optical arts. For efficient reflection of the light, the top heat sink  40  is preferably made from material that can be polished, coined, molded, or any combination of these. Alternatively, to achieve high reflectivity, the optical reflective surface  42  or the entire heat sink  40  can be plated or deposited with high reflective material such as silver, aluminum, or any substance that serves the purpose. For this reason, the top heat sink  40  is also referred to as a reflector plate  40 . The reflector plate  40  is made of material having high thermal conductivity if and when required by the thermal performance of the package  10 . In the illustrated embodiment, the reflective surface  42  is illustrated as a flat surface at an angle, for example 45 degrees, relative to the reflective plate&#39;s horizontal plane. The example embodiments are not limited to the illustrated embodiment. For example, the reflective surface  42  can be at a different angle relative to the reflective plate&#39;s horizontal plane. Alternatively, the reflective plate can have a parabolic, toroid or any other shape that helps to meet the desired spectral luminous performance of the package. 
     The reflective plate  40  includes a ledge  44  for supporting and coupling with the lens  50 . The LED assembly  60  is encapsulated within the die package  10  (of  FIGS. 1A and 1B ) using encapsulation material  46  such as, for example only, soft and elastic silicones or polymers. The encapsulation material  46  can be a high temperature polymer with high light transmissivity and refractive index that matches or closely matches refractive index of the lens  50 , for example. The encapsulant  46  is not affected by most wavelengths that alter its light transmissivity or clarity. 
     The lens  50  is made from material having high light transmissivity such as, for example only, glass, quartz, high temperature and transparent plastic, or a combination of these materials. The lens  50  is placed on top of and adheres to the encapsulation material  46 . The lens  50  is not rigidly bonded to the reflector  40 . This “floating lens” design enables the encapsulant  46  to expand and contract under high and low temperature conditions without difficulty. For instance, when the die package  10  is operating or being subjected to a high temperature environment, the encapsulant  46  experiences greater volumetric expansion than the cavity space that contains it. By allowing the lens  50  to float up somewhat freely on top of the encapsulant  46 , no encapsulant will be squeezed out of its cavity space. Likewise, when the die package  10  is subjected to a cold temperature, the encapsulant  46  will contract more than the other components that make up the cavity space for the encapsulant  46 ; the lens will float freely on top of the encapsulant  46  as the latter shrinks and its level drops. Hence, the reliability of the die package  10  is maintained over relatively large temperature ranges as the thermal stresses induced on the encapsulant  46  is reduced by the floating lens design. 
     In some embodiments, the lens  50  defines a recess  52  (See  FIG. 3 ) having a curved, hemispherical, or other geometry, which can be filled with optical materials intended to influence or change the nature of the light emitted by the LED chip(s) before it leaves the die package  10 . Examples of one type of optical materials include luminescence converting phosphors, dyes, fluorescent polymers or other materials which absorb some of the light emitted by the chip(s) and re-emit light of different wavelengths. Examples of another type of optical materials include light diffusants such as calcium carbonate, scattering particles (such as Titanium oxides) or voids which disperse or scatter light. Any one or a combination of the above materials can be applied on the lens  50  to obtain certain spectral luminous performance. 
       FIG. 4  illustrates the die package  10  coupled to an external heat sink  70 . Referring to  FIG. 4 , the thermal contact pad  36  can be attached to the external heat sink  70  using epoxy, solder, or any other thermally conductive adhesive, electrically conductive adhesive, or thermally and electrically conductive adhesive  74 . The external heat sink  70  can be a printed circuit board (PCB) or other structure that draws heat from the die package  10 . The external heat sink can include circuit elements (not shown) or heat dissipation fins  72  in various configurations. 
     An example embodiment having an alternate configuration is shown in  FIGS. 5 through 6D . Portions of this second embodiment are similar to corresponding portions of the first embodiment illustrated in  FIGS. 1A through 4 . For convenience, portions of the second embodiment as illustrated in  FIGS. 5 through 6D  that are similar to portions of the first embodiment are assigned the same reference numerals, analogous but changed portions are assigned the same reference numerals accompanied by letter “a,” and different portions are assigned different reference numerals. 
       FIG. 5  is an exploded perspective view of an LED die package  10   a  in accordance with other embodiments of the present invention. Referring to  FIG. 5 , the light emitting die package  10   a  of the present invention includes a bottom heat sink (substrate)  20   a , atop heat sink (reflector plate)  40   a , and a lens  50 . 
       FIGS. 6A ,  6 B,  6 C, and  6 D, provide, respectively, a top view, a side view, a front view, and a bottom view of the substrate  20   a  of  FIG. 5 . Referring to  FIGS. 5 through 6D , the substrate  20   a  includes one first trace  22   a  and four second traces  24   a . Traces  22   a  and  24   a  are configured differently than traces  22  and  24  of  FIG. 2A . The substrate  20   a  includes flanges  31  that define latch spaces  33  for reception of legs  35  of the reflector plate  40   a , thereby mechanically engaging the reflector plate  40   a  with the substrate  20   a.    
     The example embodiments of the present invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the exemplary embodiments of the present invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.

Technology Category: 5