Abstract:
An electronic assembly includes a Light Emitting Diode (LED) and a substrate. The LED has a solderable surface other than the contacts. The substrate has an opening. The solderable surface is mounted substantially over the opening. When the opening is filled with solder, the solderable surface is metallically bonded with the solder in the opening.

Description:
DESCRIPTION OF THE RELATED ART 
       [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). There are various ways of transferring heat from the LED device to an intermediate heat conducting structure, and then to other structures on the circuit board (for example, a metal core or heat sink). Typically, there are multiple thermal interfaces, comprising multiple layers of dissimilar materials, which impede heat flow. 
         [0003]    Sometimes, a LED package includes a heat slug (a mass of metal, typically copper) and heat generated by the LED is dissipated by the heat slug, or transferred through the heat slug. The LED is typically attached to the heat slug using an epoxy or a mixture of metals having a low melting point (eutectic) such as gold-tin or tin-copper. In order to attach the LED to the heat slug, proper metallic configurations are needed at each of the mating surfaces. For example, the LED may have metal layers having gold-tin and nickel, and the copper heat slug may have metal layers comprising nickel and gold. In addition, if the heat slug is soldered to a circuit board (for example, a metal-core printed circuit board), then the circuit board also needs compatible metal plating, for example, copper, nickel, and silver/gold. All these various plating layers with dissimilar materials impede heat flow and add manufacturing cost. 
         [0004]      FIG. 1  illustrates another prior art example of a LED package mounted on a circuit board. In  FIG. 1 , an LED package  100  is mounted on a circuit board  102 . A package typically also includes a reflector and housing, which are not illustrated. A LED  104  has two electrodes  106 . The LED  104  is attached to an intermediate package substrate  108 . Each electrode  106  is over an opening  110  in the intermediate package substrate  108 . Each opening  110  is filled with a heat conducting material (for example, a metal paste containing a polymer for curing). The openings  110  connect to a metal layer  112  in the intermediate package substrate  108 , which in turn connects to contacts  114 . The package  100  is attached (for example, soldered) to the circuit board  102 , which may contain other circuit elements  116 . The filled openings  110 , metal layer  112 , and contacts  111  all provide electrical connection for LED electrodes  106 , as well as heat transfer from LED  104 . Note that heat, generated from the light producing layer of the LED  104 , must flow through various semiconductor layers to the electrodes  106 , through the electrodes  106  (with various plating layers), the heat-conducting-paste filled openings  110 , metal layer  112 , and contacts  114 , and possibly other structures such as a heat sink (not illustrated) on or in the circuit board. 
         [0005]      FIG. 2  illustrates another prior art example of a LED package mounted on a circuit board. In  FIG. 2 , a LED package  200  is mounted on a circuit board  202 . An LED  204  has a first electrode  206  on a bottom surface, and a second electrode  208  on a top surface. LED  204  is attached to an intermediate package substrate  210 . Electrode  206  is connected (for example, soldered) to a contact  212  on the intermediate package substrate  210 , and electrode  208  is wire bonded to a contact  214  on the intermediate package substrate  210 . Electrode  206  also interfaces with a via  216  in the intermediate package substrate  210 . Via  216  may be filled with a heat-conducting material (for example, metal paste or solder). The LED package  200  is connected (for example, soldered) to circuit board  202 , which may contain other circuit elements  218 . The circuit board  202  may also contain other structures (not illustrated) for heat sinking. The heat from the light-producing layer of the LED  204  must then pass through various semiconductor layers to the electrode  206 , through plating layers on the electrode  206 , through the solder in the via  216 , and through a thermal interface between the solder-filled via  216  and the circuit board  202 . 
         [0006]    There is a need for a LED device, mounted on a circuit board or other substrate, with a reduced number of thermal interfaces layers of dissimilar materials), and with improved heat flow, and reduced manufacturing cost. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]      FIG. 1  is a block diagram illustrating an example prior art LED package mounted on a circuit board. 
           [0008]      FIG. 2  is a block diagram illustrating another example prior art LED package mounted on a circuit board. 
           [0009]      FIG. 3  is a block diagram of an example embodiment of an LED device. 
           [0010]      FIG. 4  is a block diagram of the example embodiment of  FIG. 3 , with additional example heat sinking structure. 
           [0011]      FIG. 5A  is a flow chart of an example embodiment of a method for making an electronic assembly. 
           [0012]      FIG. 5B  is a flow chart showing an example additional method step for the method of  FIG. 5A . 
           [0013]      FIG. 5C  is a flow chart showing an example additional method step for the method of  FIG. 5B . 
       
    
    
     DETAILED DESCRIPTION 
       [0014]      FIG. 3  illustrates an LED  300  mounted directly on a substrate or circuit board  302 . The LED  300  has two electrodes  304  and  306  on a top surface. Electrode  304  is wire bonded to a contact  308  on the substrate or circuit board  302 . Electrode  306  is wire bonded to a contact  310  on the substrate or circuit board  302 . The substrate or circuit board  302  has a via  314 . LED  300  has a solderable surface  312 , and the solderable surface is mounted substantially over the via  314 . The substrate or circuit board  302  may contain other circuit elements  316 . 
         [0015]    The electrodes  304  and  306  are illustrated as being on a top surface of the LED and wire bonded, but the location of the electrodes and how they are connected is not important as long as they are not in the primary path for heat flow. The LED  300  may be contained in a package that may include a reflector, encapsulant, and other housing materials (not illustrated), but it is important to note that the resulting package does not contain an intermediate package substrate or a heat plug. In addition, if the LED  300  is contained in a package, the solderable surface  312  is directly exposed to the outside of the package to enable a metallurgical bond, as discussed further below. 
         [0016]    The LED  300 , with or without a package, may be mounted on a variety of substrates  302 , which may or may not include other components. For example, substrate  302  may be a printed circuit board made from glass epoxy, phenolic, or polyimide. Alternatively, the substrate  302  may be an inorganic material such as ceramic or a semiconductor material such as silicon. 
         [0017]    The LED  300  has a solderable surface  312 , separate from the electrodes  304  and  306 , that is intended to be directly metallurgically bonded to a heat conducting material in a substrate or circuit board, without an intermediate package substrate. The solderable surface  312  may comprise, for example, a layer of nickel next to the semiconductor surface, and a layer of gold-tin on the outside surface. The via  314  may be filled with solder at the time other components  310  are soldered. Alternatively, the via  314  may be prefilled with a solder paste which is later melted. For either alternative, during assembly of the substrate or circuit board  302 , a metallurgical bond is formed between the solderable surface of the LED and the solder in the via. After soldering, heat flow from LED  300  flows through the solderable surface  312  and then directly into the solder in the via  314 . The solder filled via then acts as a heat plug, without requiring an intermediate package substrate or a package that includes a heat plug. 
         [0018]    The substrate or circuit board  302  may contain a metal layer or other structures for heat sinking.  FIG. 4  illustrates an optional heat sink  318  on the surface opposite from the LED  300 . Heat sink  318  may be, for example, aluminum. For aluminum, various metal layers may be needed for solderability, and a dielectric layer may be needed for electrical insulation. For example,  FIG. 3  illustrates three layers (no reference numbers) which may be, for example, a layer of solder next to the circuit board, a dielectric layer next to the aluminum surface, and a layer of copper between the dielectric layer and the solder. The resulting structure has only two heat transfer interfaces: (1) from the LED body to the solder-filled via, and (2) from the solder-filled via to the substrate or circuit board (or other heat sink). Furthermore, the thermal interfaces are metallurgical or intermetallic, and the thermal conductivity of solder is superior to many other heat conducting materials such as thermal pastes. In addition, solder is commonly used for circuit board assembly, eliminating the need for a separate process step for a heat conducting material.