Patent Publication Number: US-2015060898-A1

Title: Method for low temperature bonding of electronic components

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is a continuation of U.S. Ser. No. 12/674,804, filed on Nov. 24, 2010 as a United States National Stage under 35 U.S.C. §371 of International Application Serial No. PCT/US2008/074809, having an International Filing Date of Aug. 29, 2008 and is related to, and claims priority from, U.S. Provisional Patent Application Ser. No. 60/969,534, filed on Aug. 31, 2007, which is herein incorporated by reference. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH 
     Not Applicable. 
     BACKGROUND OF THE INVENTION 
     The present invention is related generally to methods for bonding semiconductor devices to substrates, and in particular, to a method for low temperature solder bonding of temperature sensitive devices such as light-emitting diode (LED) assemblies to a substrate containing a heat sink, without exposing the entire semiconductor device or the adjacent substrate to damaging heat. 
     Light-emitting diodes, or LED&#39;s, are semiconductor light emitting components that are increasingly replacing incandescent bulbs in many lighting application. As shown in  FIG. 1 , a prior art typical LED assembly consists of an LED chip (not shown) encased in a protective package  31  (LED package). Two or more connection leads  34  with feet  36  extend from the sides of the package  31 . A thermally coupling slug  32 , commonly formed from copper, is disposed at the base of the LED package  31  and is coated with a wetting layer  33  to facilitate bonding to a substrate which incorporates a heat sink such as a metal core printed circuit board (MCPCB) (not shown in  FIG. 1 ). A lens  35  may be attached to the top of the LED package  31  for directing emitted light. An exemplary LED package  31  may have dimensions of approximately 5 mm width×5 mm length by 2 mm height, without the lens  35 . 
     For economic fabrication the LED package  31 , the lens  35 , and the MCPCB substrate desirably comprise polymer or plastic materials. The MCPCB substrate is typically a printed circuit board that includes a metal core within one or more layers of dielectric material. Copper traces and pads are disposed outside the dielectric material, and a solder mask is disposed surrounding the copper pads and covering the traces.  FIG. 2  shows a typical pad pattern to match the connection leads  34  and slug  32  of the LED package  31  shown in  FIG. 1 . For operation of the LED package  31 , the slug  32  is bonded to pad  55   b  while the two leads  34  are each bonded to an associated pad  55   a.    
     In one conventional bonding process, the various bonds between the LED package  31  and the MCPCB pads  55   a ,  55   b  are formed by a high temperature solder reflow process. However, polymer lenses  35  may be damaged by the solder reflow process due to exposure to the high temperature reached during the solder reflow, and particularly to the high temperatures used with lead-free solder reflow procedures. Similarly, where large heat sinks are needed, plastic packaging and the LED semiconductor electronics may be damaged by reflow heating because the thermal mass of the heat sinks necessitates a lengthy solder reflow cycle at an elevated temperature. If the solder reflow cycle is not long enough, poor bonding may result due to inadequate melting of the solder. In addition, the high temperature of reflow may adversely affect brightness of the LED or the lifetime of the device. 
     Alternatively, the connection leads  34  may be soldered individually using laser or hot bar (thermode) soldering and the center slug  32  may be attached with a thermally conductive adhesive. An electrical connection between the center slug  32  and the pad  55   b  is not needed, but since the center slug  32  serves to convey heat from the LED package  31  to the metal-core PCB, there must be low thermal resistance between the center slug  32  and the MCPCB. Thermally conductive adhesives have reasonable but limited thermal conductivity, and suffer from inconsistent viscosity during application which can lead to varying bond line thicknesses. If the thickness of the bond line between the center slug  32  and the pad  55   b  is too great, the thermal resistance of the bond increases. In extreme situations, the thickness of the bond at the center slug  32  can be so great that the feet  36  of the connection leads  34  fail to contact the pads  55   a , making laser soldering difficult or impossible. Consequently, conventionally fabricated LED assemblies often fall short of their potential thermal, electronic and optical performance potential. 
     The same problems may occur in other electronic components and semiconductor packages with integrated thermal management pads or slugs that must be bonded to an external heat sink, and which are sensitive to high bonding temperatures. Such components and packages include but are not limited to electrolytic capacitors, power amplifiers, and photovoltaic devices. Many components that were designed for lead-tin eutectic solders are damaged at the higher reflow temperatures required for the use of lead-free solders. 
     Accordingly, it would be advantageous to provide a uniform and reliable method for bonding electronic packages, such as semiconductor devices and LEDs which are sensitive to extreme bonding temperatures, to a supporting substrate. It would be further advantageous to provide such a bonding method which does not generate excessive heat outside of the bonding region, and which results in bonds having a uniform thickness and strength, and which are capable of conducting electrical current and thermal energy. 
     BRIEF SUMMARY OF THE INVENTION 
     In one embodiment, the present invention sets forth a method for bonding an LED assembly or other electronic package to a substrate containing a heat-sink, which utilizes a reactive multilayer foil with nanoscale layers disposed between the LED assembly connecting leads, center slug, and the associated contact pads on the supporting substrate. By initiating an exothermic reaction in the reactive foil with nanoscale layers, together with an application of pressure, sufficient heat is generated between the connecting leads, center slug, and the associated contact pads to melt adjacent solder material to obtain good electrically and thermally conductive bonds between the connecting leads, center slug, and contact pads without thermally damaging the LED assembly, heat-sensitive electronic package, or other heat-sensitive components such as polymer lenses. 
     In an alternate embodiment the present invention sets forth an electronics package, such as an LED assembly, bonded to a supporting substrate by one or more bonds which include the remnants of reacted foils of reactive composite material. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         FIG. 1  is a schematic illustration of a prior art light-emitting diode package; 
         FIG. 2  is a schematic top view of the contact pads on a prior art printed circuit board for receiving an LED package of  FIG. 1 ; 
         FIG. 3  is an illustration of prior art reactive multilayer bonding process; 
         FIG. 4  is a schematic illustration of an LED package bonded to a PCB using the methods of the present disclosure; 
         FIG. 5  illustrates a simplified cross-section of a bond formed by the method of the present disclosure, incorporating remnants of a reactive foil; 
         FIGS. 6A and 6B  shows two different prior art LED package lead geometries; 
         FIG. 7  shows a bare LED package bonded to a substrate; and 
         FIG. 8  shows an arrangement for testing the shear strength of a bonded LED package. 
     
    
    
     DETAILED DESCRIPTION 
     The following detailed description illustrates the invention by way of example and not by way of limitation. The description enables one skilled in the art to make and use the present disclosure, and describes several embodiments, adaptations, variations, alternatives, and uses of the present disclosure, including what is presently believed to be the best mode of carrying out the present disclosure. 
     In general, the present disclosure sets forth a method for bonding various contacts between electronic packages, such as LED packages, semiconductor devices, or photovoltaic devices, and their supporting substrates such as metal-core printed circuit boards, using pieces or segments of reactive foil with nanoscale layers to provide sufficient thermal energy to melt and flow connecting bonding materials, such as solder, without damaging adjacent heat-sensitive components in the electronic packages or supporting substrates. Throughout this specification, the numerous descriptions and examples are described in the context of LED packages, but are not limited thereto, will readily be understood to be equally applicable to a wide variety of electronic devices, particularly those which are sensitive to damage by excess heat or thermal energy. Additional types of electronic packages which may be suitably bonded utilizing the methods of the present disclosure include flip chips, chip-on-board, bare dies, chip scale packages, package on package, ball grid arrays, fine-pitch no-lead, leadless chip carriers, quad flat packs, plastic leaded chip carriers, flat leads, small outline packages, DPAKs, and D2PAKs. 
     The reactive foil with nanoscale layers utilized in the bonding methods of the present disclosure are typically formed by magnetron sputtering and consist of thousands of alternating nanoscale layers of materials, such as nickel and aluminum. The layers react exothermically when atomic diffusion between the layers is initiated by an external energy pulse, and release a rapid burst of heat in a self-propagating reaction. If the reactive foil with nanoscale layers are sandwiched between layers of a bonding material, such as a solder or braze alloy, the heat released by the exothermic reaction of the reactive foil with nanoscale layers can be harnessed to melt these layers of bonding material, as is illustrated in  FIG. 3 . The resulting bonding layer comprises a solder or braze layer that includes the reaction products of the reactive foil with nanoscale layers. By controlling the properties of the reactive foil with nanoscale layers, the amount of heat released by the reactive foil with nanoscale layers during the exothermic reaction can be tuned to ensure there is sufficient heat to melt the bonding material layers, but at the same time maintain the bulk of the adjacent components at or close to room temperature. Further details concerning reactive foil with nanoscale layers, bonding with them and their reaction products can be found in U.S. Pat. No. 6,736,942 which is incorporated herein by reference. 
     Turning to  FIG. 3 , an arrangement  9  is shown for performing the general process of reactive multilayer joining of two components  10 A and  10 B. A discrete sheet, piece, or segment of reactive composite material  12 , such as a reactive foil with nanoscale layers, is disposed between two sheets or layers of fusible bonding material  14 A and  14 B, typically solders or brazes which, in turn, are sandwiched between the contact surfaces (not visible) of the components  10 A and  10 B. The sandwiched assembly is then pressed together, as symbolized by vise  16 , and the reactive foil with nanoscale layers is ignited by any suitable means, as is illustrated by a match and flame  18 . The reaction propagates rapidly through the reactive foil with nanoscale layers  12 , melting the fusible bonding material  14 A and  14 B, which subsequently cools, joining the components  10 A and  10 B together at the contact surfaces. The bond areas contemplated herein may range from 0.25 mm 2  up to many square centimeters. 
     Referring to  FIG. 4 , in one embodiment of the invention, an electronic device such as an LED package  31 , having a center slug  32  and at least one connection lead  34  is bonded at one or more contacts to a metal-core printed circuit board  52  using a bonding method employing reactive foil with nanoscale layers. For example, the LED package  31 , having a plurality of contacts defined by a center slug  32  and plurality of connection leads  34  is bonded to corresponding contacts of a printed circuit board (PCB)  52  defined by the pads  55   a  and  55   b  using reactive multilayer joining. 
     As shown in  FIG. 4 , the PCB  52  comprises a metal core  56 , typically aluminum, a dielectric layer  53 , and pads  55   a  and  55   b . Surrounding and partially covering the pads  55   a  and  55   b  is a polymer solder mask  57 . The pads  55   a  and  55   b  may be formed from copper, and may optionally be coated with a surface layer  54  of bonding material, such as tin applied by hot air solder leveling (HASL) to form a tin layer that is typically 4 μm to 8 μm thick. Those of ordinary skill in the art will recognize that the surface layer  54  may be selected from any of a variety of conventional surface treatments, including immersion tin, electroless nickel/immersion gold (ENIG), lead-tin HASL, immersion silver, electroless palladium/gold, electrolytic nickel/gold, reflowed tin/lead, or any other surface treatment known in the art. 
     Advantageously, the LED package connection leads  34  and center slug  32  are similarly coated with a layer of bonding material, such as tin between 4 μm and 30 μm thick. As with the surface layer  54 , the layer of bonding material on the connection leads  34  and center slug  32  of the LED package  31  may be any suitable bonding material such as a solder, solder alloy, or braze, and may be applied via electroplating, dipping, or any common method known in the art. Such solders and brazes include but are not limited to pure metals and alloys of indium, silver, and lead, as well as commercial alloys such as Georo® and Incusil®. 
     Traditionally, pure tin has not been commonly used as a bonding material due to its high melting point, but the method of the present disclosure enables the use of pure tin as a solder or bonding material, providing the advantage of higher ductility when compared to other bonding materials such as lead-free solder alloys. 
     To bond the center slug  32  and the connection leads  34  to pads  55 , one or more pieces of a reactive foil with nanoscale layers  51  are placed between the contacts, i.e., between the connection lead feet  36  and their adjacent pads  55   a , and between the center slug  32  and the adjacent pad  55   b . A load designed to hold the contact surfaces against the reactive foil with nanoscale layers  51  is applied to each connection lead foot  36 , and to the LED package  31  to exert a load on the center slug  32 . Concurrently, an energy pulse is applied to each piece of reactive foil with nanoscale layers  51  which is sufficient to initiate an exothermic reaction therein. The resulting exothermic reaction of each piece of reactive foil with nanoscale layers  51  generates sufficient thermal energy to melt the surrounding bonding material or surface layer  54 , which solidifies upon cooling to bond the connection leads  34  and center slug  32  of the electronic device to their respective pads  55  on the supporting substrate or printed circuit board  52 . The thermal energy generated by the reaction of the reactive foil with nanoscale layers is not sufficient to raise the temperature of the adjacent LED package or supporting substrate (printed circuit board  52 ) appreciably above ambient temperature. 
     Advantageously, the discrete segments of reactive foil with nanoscale layers  51   a  and  51   b  are sized to fit within the polymer solder mask  57  which surrounds each pad  55  on the supporting substrate. Preferably, for the exemplary LED package  31  described above and shown in the figures, the applied load results in a pressure which is between 10 MPa and 100 MPa on each connection lead foot  36 , and between 1 MPa and 9 MPa on the LED package  31  (to apply a load to the center slug  32 ) If the applied pressures are too low, the bonds resulting from the exothermic reaction of the reactive foil with nanoscale layers  51  may be weak. If the applied pressures are too high, the printed circuit board  56 , connection leads  34 , or LED package  31  may be damaged, and molten bonding material may be extruded from the bonding region during the exothermic reaction of the reactive foil with nanoscale layers  51 . 
     The resulting bond which is formed, such as between the adjacent contact surfaces of the connection lead feet  36  and the pad  55   a , as shown in  FIG. 5 , incorporates the remains  51   r  of the reactive foil with nanoscale layers  51 , which typically comprise a nickel aluminide. During the exothermic reaction, the reactive foil with nanoscale layers  51  typically cracks across its thickness to form smaller pieces and the bonding material and surface layer  54 , such as tin or other fusible material, flows through the cracks to fill them, resulting in a distinctive microstructure shown schematically in  FIG. 5 . 
     The supporting substrate  52  or printed circuit board (PCB) may be another variety of printed circuit board, including but not limited to fiberglass, FR4, polybutadiene, or PTFE. The supporting substrate may comprise multiple layers of dielectric and copper pads. Alternatively, the electronic device or LED package  31  may be bonded to a metal heat sink or a flex-circuit material. 
     Those of ordinary skill in the art will recognize that the electronic device or LED package connection leads  34  may be any of a variety of shapes, including but not limited to gull-wing leads, as shown in  FIG. 6   a , a “leadless” configuration as shown in  FIG. 6   b , J-leads, or flat leads. Ball grid arrays for connection of the electronic device or LED package  31  may also be bonded using the methods of the present invention. Preferably, a large fraction of the surface or contact area of each connection lead foot  36  is parallel to the corresponding connection pad  55  on the supporting substrate  52 , as shown in  FIG. 4 , to provide for a large area in contact with the reactive foil with nanoscale layers  51  to produce a strong bond. Advantageously, the area is selected to be greater than 50% of the area of the connection lead foot  36 . 
     During the bonding process, the load to the electronic device  31  and connection leads  34  is preferably applied with a compliant element such as a spring, elastomer or foam, configured to maintain at least 80% of the initial load during and after the initiation of the exothermic reaction in the segments of reactive foil with nanoscale layers  51 . The compliant element advantageously responds to any increase or decrease in thickness of the reactive foil with nanoscale layers  51 , bonding materials, and surface layers  54  while the materials are molten in order to ensure sufficient contact is maintained throughout the duration of the bonding process. 
     In a related embodiment of the present disclosure, only a portion of the bonds between the electronic device or LED package  31  and the supporting substrate  52  are formed using the exothermic reaction of segments of reactive foil with nanoscale layers  51 , and the remaining bonds are formed using any conventional bonding process. For example, the center slug  32  may be bonded by reactive multilayer joining and the connection lead feet  36 , which may be displaced from potentially heat-sensitive components of the electronic device  31  may be bonded by another method known in the art, such as laser soldering, gap welding, or hot-bar (thermode) soldering. In a similar embodiment, the connection lead feet  36  may be bonded by reactive multilayer joining while the center slug  32  is bonded by another method known in the art, such as with thermally conductive epoxy. 
     In a related embodiment, either the contacts such as the pads  55 , the connection leads  34 , or the center slug  32 , are not coated with 4 μm to 30 μm of a tin alloy, but are instead prepared with a wettable surface layer, such as gold, silver, or palladium. For this embodiment, a fusible bonding material such as a tin-based solder alloy is advantageously combined with the pieces of reactive foil with nanoscale layers  51 , by magnetron sputtering, warm cladding, electroplating, or other methods known in the art. Preferably, a layer of fusible bonding material is adhered to one or both major surfaces of a piece of reactive foil with nanoscale layers  51 . Alternatively, the fusible bonding material may be in the form of a free-standing preform. Fusible bonding material may be disposed on one or both sides of the reactive foil with nanoscale layers  51 . The fusible bonding material is placed adjacent the wettable surface layer and the bonding process is carried out as described above by the exothermic reaction of the reactive foil with nanoscale layers  51 . For good adhesion and strength, the total thickness of fusible bonding material in a bond, including any bonding material layer on the surfaces of the connection leads  34 , center slug  32 , and copper pads  55 , should be at least 6 μm. More fusible bonding material can accommodate surface roughness or unevenness and improve reliability. The practical upper limit on fusible bonding material thickness is determined by the amount of heat provided by the exothermic reaction of the reactive foil with nanoscale layers  51  (because too little heat will not melt the fusible bonding material and form the bond), or by the escape of fusible bonding material from the bond area (which increases with increasing fusible bonding material thickness). This escape is undesirable because it may lead to short circuits or damage to the LED package or supporting substrate. Thus, the fusible bonding material thickness should be kept below 100 μm, and preferably below 50 μm. 
     Fusible bonding materials appropriate for combining with reactive foil with nanoscale layers include but are not limited to pure metals and alloys of tin, indium, silver, gold, and lead. In a related embodiment, the fusible bonding material may be a higher melting-point alloy such as Georo® or Incusil® which is advantageous when a ceramic substrate or printed circuit board is used in place of the metal-core PCB (MCPCB) described above, for high-temperature applications. 
     In another related embodiment, the pads  55 , the leads  34 , the center slug  32 , or any combination, may be coated with a tin alloy or other fusible bonding material and a fusible bonding material may also be present on the surfaces of the pieces of reactive foil with nanoscale layers  51  before the bonding process. The addition of tin or solder layers to the reactive foil with nanoscale layers  51  allows good bonding with lower pressure than is necessary in the absence of such layers. 
     In another embodiment, illustrated in  FIG. 7 , the LED device  31  is not encased in a ceramic or polymer package with a copper slug  32 , but instead is in the form of a bare die  71 , comprising silicon or another semiconductor material. The contact areas to be bonded  72  are nickel and gold plated, and the silicon is bonded to a printed circuit board or supporting substrate  73  via the reactive multilayer joining process of the present disclosure using pieces of reactive foil with nanoscale layers  74  including a layer of fusible material  75  on the side facing the LED bare die  71 . If the supporting substrate  73  does not have adequate fusible bonding material in the bonding contact regions, the reactive foil with nanoscale layers  74  may further include a second layer of fusible bonding material  76  on the side facing the supporting substrate  73 . 
     In another embodiment, the LED package  31  contains multiple LED dies  71 . Each LED die  71  may have its own copper slug  32  and connection leads  34  or it may not. 
     The following example describe exemplary results for bonding electronic devices to supporting substrates utilizing the methods of the present disclosure. It will be recognized that these examples are for illustrative purposes, and are not intended to limit the scope of the present disclosure in any way. 
     In the first example, LED packages  31  (without lenses  35 ) and MCPCB&#39;s  52  were bonded by the methods of the present disclosure as shown in  FIG. 4 . The connection leads  34  and center slugs  32  were each electroplated with a 5 μm layer of tin and the copper pads  55  were similarly coated with 5 μm to 7 μm of tin via hot air solder leveling. The connection leads  34  as received were in the gull wing configuration, shown in  FIG. 6   a . To improve contact between the leads  34  and pads  55   a , some leads  34  were conditioned, by methods known in the art, to bring the feet  36  to ±2° of horizontal and co-planar with the base of the LED package  31 , similar to  FIG. 1 . Segments of reactive foil with nanoscale layers  51 , comprising nickel and aluminum layers with a 1 μm thick Incusil® braze alloy bonding material layer on each outer surface, totaling approximately 40 μm in thickness, were utilized to make the bonds between the connection leads  34 , center slug  32 , and pads  55 . The center slugs  32  were 4 mm in diameter and the copper pads  55   b  were 4.3 mm in diameter. The connection lead feet  36  were each approximately 2 mm×2 mm after conditioning, and the associated pads  55   a  were 3 mm×3 mm. For one package, discrete pieces of reactive foil with nanoscale layers  51   a  each having dimensions of 2 mm×2 mm were placed between the pads  55   a  and connection lead feet  36 , while a 4 mm diameter disk of reactive foil with nanoscale layers  51   b  was placed between pad  55   b  and the center slug  32 . Loading pressure was applied to the upper surface of the LED package  31  via a spring plunger and the disk  51   b  was ignited to initiate the exothermic reaction. Loading pressure was then applied to one connection lead  34  via a spring plunger and the piece  51   a  was ignited. Similarly, loading pressure was then applied to the other connection lead  34  via a spring plunger and the other piece  51   a  was ignited. The exothermic reaction of each piece of reactive foil with nanoscale layers piece generated sufficient thermal energy to melt the surrounding bonding materials, which subsequently solidified to form the intended bonds between the respective contacts. 
     Similar bonds were made wherein only the copper slug  32  was bonded to pad  55   b  and the shear strength was tested. Table 1 below shows the effect of pressure and thickness of the reactive foil with nanoscale layers on bond strength. 
     
       
         
           
               
               
               
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                   
                   
                   
                 Average 
                   
               
               
                   
                 Foil 
                   
                 Joining 
                 Shear 
                 Std. Dev. 
               
               
                   
                 Thickness 
                   
                 Pressure 
                 Strength 
                 of Shear 
               
               
                   
                 (μm) 
                 Foil Shape 
                 (MPa) 
                 (MPa) 
                 Strength 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 1 
                 40 
                 4 mm × 4 mm square 
                 1.38 
                 32.78 
                 8.60 
               
               
                 2 
                 30 
                 4 mm × 4 mm square 
                 1.38 
                 13.78 
                 7.96 
               
               
                 3 
                 30 
                 4 mm × 4 mm square 
                 2.76 
                 18.58 
                 8.43 
               
               
                 4 
                 40 
                 3.9 mm diameter disk 
                 1.38 
                 34.30 
                 4.80 
               
               
                   
               
            
           
         
       
     
     Similar center slug bonds were made using MCPCB&#39;s that were coated with a 1 μm layer tin via an immersion tin process on the connection pads  55   b . Table 2, below, shows the effect of pressure on bond strength when only 1 μm of tin is on the connection pad  55   b . Pieces of reactive foil with nanoscale layers with 1 μm Incusil on each side and with 1 μm Incusil plus 4 μm pure tin on each side applied via electroplating were used. Four millimeter diameter disks of reactive foil with nanoscale layers were used for these tests. 
     
       
         
           
               
               
               
               
               
               
             
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                   
                   
                   
                 Average 
                   
               
               
                   
                 Foil 
                 Foil 
                 Joining 
                 Shear 
                 Std. Dev. 
               
               
                   
                 Thickness 
                 Surface 
                 Pressure 
                 Strength 
                 of Shear 
               
               
                   
                 (μm) 
                 Layer 
                 (MPa) 
                 (MPa) 
                 Strength 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 1 
                 40 
                 1 μm Incusil 
                 1.38 
                 3.47 
                 1.37 
               
               
                 2 
                 40 
                 1 μm Incusil 
                 4.14 
                 12.2 
                 4.98 
               
               
                 3 
                 40 
                 1 μm Incusil 
                 7.09 
                 38.3 
                 3.86 
               
               
                 4 
                 40 
                 4 μm Sn       
                 2.13 
                 47.6 
                 1.03 
               
               
                 5 
                 40 
                 4 μm Sn       
                 7.09 
                 49.1 
                 1.79 
               
               
                   
               
            
           
         
       
     
     In a separate set of tests, the connection leads  34  were bonded to pads  55   a  using the methods of the present invention and shear tested. The connection leads  34  were originally in the gull-wing configuration as shown in  FIG. 5   a , but were conditioned as described above to provide increased contact area. The reactive foil with nanoscale layers  51  used was 40 μm thick, with 1 μm Incusil on each side, as described above. The shear test geometry is illustrated in  FIG. 8 , with a load  61  applied to the center of the side of the LED package  31  and balanced by forces  62  on the two connection lead bonds. Table 3, below, shows shear strength for one lead bond under three conditions: first, in the gull-wing shape, second in a conditioned configuration, and third in a conditioned configuration with double the joining pressure. 
     
       
         
           
               
               
               
               
               
               
             
               
                 TABLE 3 
               
               
                   
               
               
                   
                   
                   
                 Average 
                   
                   
               
               
                   
                   
                 Average 
                 Shear 
                 Average 
               
               
                   
                 Joining 
                 Bonded 
                 Fracture 
                 Shear 
                 Std. Dev. 
               
               
                   
                 Pressure 
                 Area 
                 Load 
                 Strength 
                 of Shear 
               
               
                 Leads 
                 (MPa) 
                 in 2 /mm 2   
                 (lbs/kg) 
                 (MPa) 
                 Strength 
               
               
                   
               
             
            
               
                 Gull-wing 
                 5.7 
                  0.00120/0.774 
                 3.33/1.51 
                 9.95 
                 5.44 
               
               
                 Conditioned 
                 3.5 
                 0.00394/2.54 
                 7.35/3.33 
                 6.58 
                 0.96 
               
               
                 Conditioned 
                 7.3 
                 0.00380/2.45 
                 7.57/3.43 
                 6.57 
                 1.31 
               
               
                   
               
            
           
         
       
     
     As can been seen from Table 3, above, conditioning the connection leads  34  to increase contact surface area greatly increased the average shear fracture load, but not the average shear strength, since the bond area was much greater when the leads were conditioned, however, the uncertainty in the shear strength dropped considerably when the leads were conditioned, indicating much improved consistency and repeatability. Doubling the load from the second case to the third case made almost no difference in either shear load or shear strength. 
     It can now be seen that in one aspect the present disclosure sets forth an improved method for bonding an electronic device, such as a light emitting diode (LED) package  31 , to a supporting substrate  52  such as a printed circuit board. The method comprises providing the electronic device or LED package  31  including at least two electrical connection leads  34  and a thermal path (e.g. the copper slug)  32  for transferring heat from the device  31  to the supporting substrate  52 , and further providing the supporting substrate  52  for securing the device or LED package  31  and for extracting heat from it. The supporting substrate  52  preferably includes a thermal sink, connection pads  55   a  for electrically connecting to the package connection leads  34 , and a connection pad  55   b  for thermally coupling the thermal sink to the thermal path. The thermal path of the electronic device or LED package  31  is thermally coupled to the heat sink by the process of disposing between the thermal path and the heat sink, one or more layers of bonding material, such as solder or braze, together with a piece of a reactive foil with nanoscale layers  51 , and by subsequently triggering an exothermic reaction in the reactive foil with nanoscale layers  51  to generate sufficient thermal energy to melt the bonding material, forming a bond between the thermal path and the connection pad associated with the heat sink. Advantageously the package connection leads  34  are also electrically connected to the substrate pads  55   a  by layers of bonding material such as solder or braze melted by the initiation of an exothermic reaction of an interposed piece of reactive foil with nanoscale layers  51 . The thermal and electrical connections are thus achieved without significantly raising the temperature of the electronic package or thermally damaging the LED semiconductor electronics or other heat sensitive materials such as polymers in the electronic package  31  or supporting substrate  52 . 
     In another aspect the present disclosure provides an improved light emitting diode (LED) assembly that comprises an LED package, a supporting substrate, and an interconnecting bonding layer between the LED package and the supporting substrate. The LED package includes an LED component, at least two electrical leads to the LED component, and thermal path (e.g. the copper slug) for coupling heat from the LED to a thermal sink in the supporting substrate. The substrate for supporting the LED package comprises the thermal sink, connection pads for electrically connecting to the package leads, and a pad for thermally connecting the thermal sink to the thermal path. The bonding layers, disposed between each of the electrical leads, thermal path, and associated pads on the supporting substrate each comprise a layer of solder or braze and the reaction products resulting from the exothermic reaction of a piece of reactive foil with nanoscale layers. 
     In summary, in one embodiment the present disclosure sets forth a method for bonding an electronic package having at least one electrically or thermally conductive contact to a supporting substrate having at least one associated electrically or thermally conductive contact disposed in a bonding region, comprising: coating each of said contacts with a layer of bonding material; disposing a reactive foil with nanoscale layers between each of said contacts of said electronic package and said associated contacts of said supporting substrate; applying a pressure to said contacts in the bonding region; and initiating an exothermic reaction in said reactive foil with nanoscale layers, said exothermic reaction resulting in the formation of a bond containing said bonding material and the remains of said reactive foil with nanoscale layers between said electronic package contact and said associated substrate contact. 
     In an alternate embodiment, the present disclosure sets forth a bonded electronic assembly, comprising: an electronic package having at least one electrically or thermally conductive contact lead coated with a first bonding material; a supporting substrate comprising at least one electrically or thermally conductive contact pad coated with a second bonding material, and the remains of a reactive foil with nanoscale layers disposed within at least one bond formed between said contact lead and said contact pad by said first and second bonding materials. 
     In an alternate method, the present disclosure sets forth a method for bonding a temperature-sensitive electronic package having at least one contact to a supporting substrate receiving contact disposed in a bonding region, comprising: disposing a reactive foil with nanoscale layers between said contact of said electronic package and said associated receiving contact of said supporting substrate; applying a controlled pressure to said layer of reactive foil with nanoscale layers through said contacts; and initiating an exothermic reaction in said reactive foil with nanoscale layers, said exothermic reaction resulting in the formation of a bond containing the remains of said reactive foil with nanoscale layers between said electronic package contact and said associated substrate receiving contact. 
     It will be recognized that various aspects of the present disclosure can be embodied in-part in the form of computer-implemented processes and apparatuses for practicing those processes. These may include, but are not limited to, computer controlled application of pressure to the various contact regions during the bonding process. The present disclosure can also be embodied in-part in the form of computer program code containing instructions embodied in tangible media, such as floppy diskettes, CD-ROMs, hard drives, or an other computer readable storage medium, wherein, when the computer program code is loaded into, and executed by, an electronic device such as a computer, micro-processor or logic circuit, the device becomes an apparatus for practicing the present disclosure. 
     The present disclosure can also be embodied in-part in the form of computer program code, for example, whether stored in a storage medium, loaded into and/or executed by a computer, or transmitted over some transmission medium, such as over electrical wiring or cabling, through fiber optics, or via electromagnetic radiation, wherein, when the computer program code is loaded into and executed by a computer, the computer becomes an apparatus for practicing the present disclosure. When implemented in a general-purpose microprocessor, the computer program code segments configure the microprocessor to create specific logic circuits. 
     As various changes could be made in the above constructions without departing from the scope of the disclosure, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.