Abstract:
An electronic assembly includes a Light Emitting Diode (LED) mounted on a top surface of a heat spreader, at least two electrical contacts co-planar with the heat spreader, and at least one heat slug mounted on the top surface of the heat spreader, where the heat slug is made of high thermal conductive plastic.

Description:
[0001]    Light emitting diodes (LED&#39;s) are semiconductor devices that emit light when they are forward biased and current is flowing. There is an ongoing demand for increasing light intensity, resulting in higher currents, and more heat. Heat is detrimental to the performance of a LED because light output generally drops with increasing temperature. In addition, the life of a LED device may be shortened by high temperatures. Therefore, heat removal is extremely important in systems using LED&#39;s. 
         [0002]    Semiconductor LED devices are typically mounted on a substrate that is part of a package, and the package is attached to a circuit board (for example, by soldering). Sometimes, a LED package includes a heat slug (a mass of metal, typically copper) between the semiconductor die and the printed circuit board, and heat generated by the LED is dissipated by the heat slug, or transferred through the heat slug to heat dissipating structures on the printed circuit board. 
         [0003]    There is an ongoing need for LED devices with improved heat dissipation, reduced manufacturing complexity, and lower cost. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0004]      FIG. 1A  is a cross-section side view illustrating an example embodiment of a LED package. 
           [0005]      FIG. 1B  is a cross-section side view orthogonal to the cross-section of  FIG. 1A . 
           [0006]      FIG. 1C  is a cross-section top view of the LED package of  FIGS. 1A and 1B . 
           [0007]      FIG. 2  is a cross-section side view of an example embodiment of a variation of the example of  FIGS. 1A-1C . 
           [0008]      FIG. 3  is cross-section side view of an example embodiment of a LED package with optional heat dissipating structures formed on the top heat slugs. 
           [0009]      FIG. 4  is a cross-section side view of an example embodiment of a LED package with an optional secondary reflector cup. 
           [0010]      FIG. 5  is a flow-chart of an example method of manufacturing for a LED package. 
       
    
    
     DETAILED DESCRIPTION 
       [0011]      FIG. 1A  illustrates an example embodiment of a LED package  100 . A LED semiconductor die  102  is mounted onto a heat spreader  104 . The semiconductor die  102  is wire bonded to two electrical contacts  106 . The heat spreader  104  and the electrical contacts  106  are stamped from a metal sheet, for example, copper, aluminum, or iron. A non-conductive plastic  108  electrically insulates the electrical contacts  106  from the heat spreader  104 . A reflector  110  may also be molded from the non-conductive plastic  108 . The package may be encapsulated, for example, in epoxy or silicone ( 112 ). The electrical contacts  106  are exposed at the bottom exterior of the package for attaching to a substrate, for example by soldering to a PC board. The heat spreader  104  may also be exposed at the bottom exterior of the package to facilitate heat transfer through the bottom of the package to an attached substrate. 
         [0012]      FIG. 1B  illustrates a cross-section side view orthogonal to the cross-section of  FIG. 1A . In  FIG. 1B , at least one heat slug  114 , made of high thermal conductive plastic, is mounted on top of the heat spreader  104 . In the example of  FIG. 1B , heat flows from the semiconductor die  102  through the heat spreader  104  into the top mounted heat slug(s)  114 , where it is dissipated through the top of and sides of the package  100 . The package may also be mounted onto a substrate (not illustrated), for example a printed circuit board, and heat may also be conducted through the heat spreader  104  to heat dissipating structures (not illustrated) on the substrate. 
         [0013]      FIG. 1C  illustrates a cross-section top view of the package  100  of  FIGS. 1A and 1B . Again, the semiconductor die  102  is mounted onto the top surface of the heat spreader  104 . The semiconductor die is wire bonded to electrical contacts  106 . A non-conductive plastic  108  electrically insulates the electrical contacts  106  from the heat spreader  104 . The non-conductive plastic  108  may also form a reflector  110 . At least one heat slug  114  is mounted on top of the heat spreader  104 . 
         [0014]    The non-conductive plastic  108  may be, for example, polyphthalamide (PPA). The high thermal conductive plastic slug(s)  114  may be, for example, a high heat-resistant resin, such as Liquid Crystal Polymer (LCP), Polyphenylene Sulfide (PPS), PolyEtherEtherKetone (PEEK), or polysulfone, which has been loaded with a thermally conductive additive, for example, graphite fibers, aluminum nitride, or boron nitride. Suitable high thermal conductive plastics are commercially available from, for example, Cool Polymers, Inc., 333 Strawberry Field Rd, Warwick, R.I. 02886 USA. 
         [0015]      FIG. 2  illustrates a variation of the example of  FIGS. 1A-1C . For the LED package in  FIG. 2 , the tops, and at least one side, of the heat slugs  202  are exposed and are not covered by the non-conductive plastic or any encapsulating material. This improves heat dissipation. 
         [0016]      FIG. 3  illustrates another optional enhancement. In  FIG. 3 , a package  300  has heat slugs  302  extending through the top of the package, and the heat slugs have additional surface structure, for example fins, to increase the surface area for improved heat dissipation. The heat slugs  302  may be high thermal conductive plastic and the surface structure may be molded as an integral part of the heat slugs 
         [0017]      FIG. 4  illustrates an optional enhancement to the LED package  100  of  FIGS. 1A-1C . In  FIG. 4 , a supplemental reflector  400  is attached to the top of heat slugs  114  (or heat slugs  202  in  FIG. 2 ). The supplemental reflector may be made of a high thermal conductive material to provide additional heat dissipation from the top of the overall assembly. 
         [0018]      FIG. 5  illustrates an example method for manufacturing an LED package. At step  500 , a heat spreader and electrical contacts are formed (for example, stamped from a sheet of metal). At step  502 , the heat spreader is insulated from the electrical contacts. At step  504 , a reflector is formed (for example, steps  502  and  504  may be combined into one injection-molding step with a non-conductive plastic). At step  506 , at least one heat slug is formed (for example, injection molding) onto the top side of the heat spreader. At step  508 , a semiconductor LED die is attached to the heat spreader. At step  510 , the semiconductor die is electrically connected to the electrical contacts (for example, by wire bonding). At step  512 , the package is encapsulated (for example, by filling with epoxy or silicone).