Abstract:
Improved LED die mount. By mounting an LED die on a pedestal which is smaller than the LED die, formation of fillets of die mount material which would block some of the light from the LED die are reduced or eliminated.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to the field of Light Emitting Diodes (LEDs). More particularly, it pertains to a method of attaching the LED die to improve light output.  
         [0003]     2. Art Background  
         [0004]     In use, Light Emitting Diode (LED) dies are packaged by mounting them to a substrate such as a printed circuit board (PCB) or a leadframe. The LED die is mounted to the substrate using a material such as an adhesive, often epoxy-based conductive adhesives, solder, solder paste, or eutectic alloys. When the die is mounted, the mounting material naturally forms a fillet up the side walls of the LED die. Being opaque, the fillet blocks a portion of the light generated by the LED.  
       SUMMARY OF THE INVENTION  
       [0005]     By mounting the LED die on a pedestal smaller than the die, fillets formed by die attach material along the sides of the die are reduced or eliminated. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0006]     The present invention is described with respect to particular exemplary embodiments thereof and reference is made to the drawings in which:  
         [0007]      FIG. 1  shows an LED die mount,  
         [0008]      FIG. 2  shows an LED die mount according to the present invention,  
         [0009]      FIG. 3  shows an additional LED die mount according to the present invention, and  
         [0010]      FIG. 4  shows an additional embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0011]      FIG. 1  shows a LED die mounted according to the prior art. Detail A shows die mount material  100  on substrate  110 . While the substrate shown in  FIG. 1  is flat, it may be curved, such as a curved mount used for single LEDs. The die mount material may be an adhesive such as a conductive epoxy, or may be solder, solder paste, or a eutectic alloy as known to the art.  
         [0012]     Detail B shows collet  130  supporting LED die  140 . Collet  130  may be operated automatically or by hand. Detail C shows the collet and die moving to contact the substrate and die mount material.  
         [0013]     Detail D shows LED die  140  affixed to substrate  110 , with fillets  150  of the die mount material flowing along the sides of LED  140 . The opaque nature of fillets  150  blocks light ray  160 . This is a particular problem if the active PN junction of die  140  is near substrate  110  and subject to being blocked by fillets  150 .  
         [0014]     According to one embodiment of the present invention as shown in  FIG. 2 , Detail A shows die mount material  200  placed on pedestal  220  on substrate  210 . Pedestal  220  is smaller in size than the LED die to be attached. Pedestal  220  may be formed through the stamping process in the case where substrate  210  is a leadframe. Pedestal  220  may be formed through a plating process in the case where substrate  210  is a printed circuit board (PCB). Pedestal  220  may also be a separate component attached to substrate  210 . The height of pedestal  220  is determined mainly by the characteristics of die mount material  200 . The shape of pedestal  220  does not have to match that of die  240 . For example, both die  240  and pedestal  220  may be rectangular in shape. In other instances, die  240  may be square or rectangular, and pedestal  220  may be circular or oval.  
         [0015]     Detail B shows collet  230  supporting LED die  240 . Detail C shows the collet and die moving to contact the die mount material on the pedestal and substrate.  
         [0016]     Detail D shows LED die  240  affixed to pedestal  220  and substrate  210 . Since pedestal  220  is smaller than die  240 , fillets  250  of mount material do not encroach on the sides of LED die  240 . Thus, light ray  260  is not blocked.  
         [0017]     It is also possible to first affix the pedestal to the die, and then affix the combined pedestal and pedestal to the substrate.  
         [0018]      FIG. 3  shows an embodiment of the present invention in which the substrate is curved, as in a leadframe. In this embodiment, leadframe  310  is formed with raised pedestal  320  to which LED die  340  is attached. As pedestal  320  is smaller than LED die  340 , fillets  350  of die attach material do not obscure the sides of LED die  340 , this allowing light ray  360  to pass. Also shown in  FIG. 3 , the sides of pedestal  320  do not have to be normal to the substrate, or parallel to each other.  
         [0019]      FIG. 4  shows an additional embodiment of the invention in which pedestal  420  is formed as part of die  440 . The pedestal feature may be formed by mechanical means such as sawing, or by chemical means such as dry or wet etching.  
         [0020]     The foregoing detailed description of the present invention is provided for the purpose of illustration and is not intended to be exhaustive or to limit the invention to the precise embodiments disclosed. Accordingly the scope of the present invention is defined by the appended claims.