Abstract:
A volatile soldering aid for solder bonding surfaces. A thermally decomposable solid that is suspended in a carrier or dissolved in a solvent is incorporated in a solder assembly having two surfaces separated by a solder preform. The solvent or carrier is subsequently evaporated, and the assembly is heated to decompose the solid and produce a reducing gas. The assembly is then further heated to melt the solder preform. A vacuum may be introduced to remove the gas prior to melting of the solder preform. The solder preform in the assembly may be a monolithic preform or it may be a powder. The solder preform may be provided as a thin film deposited on one or both of the surfaces to be joined. Upon heating, the volatile soldering aid is converted to vapor without forming a liquid phase at the melting point of the solder.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application is a division of application Ser. No. 11/625,345. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    This invention relates to the bonding of electronic components using a solder. In particular, the invention relates to die attach of semiconductors using gold/tin solder. 
         [0004]    2. Description of Related Art 
         [0005]    Electrical components are frequently bonded using solders. Solders may be used as a monolithic preform, or as a powder combined with a flux and applied as a paste. A neutral or reducing gas may also be provided to inhibit oxidation. A solder alloy is distinguished from a braze alloy in that it has a melting point below 427° C. 
         [0006]    Fluxes are effective in removing oxides; however, they generally do so by dissolving the oxides in a liquid phase that is present at the melting point of the solder with which they are used. Upon cooling to room temperature, a solid residue is formed, and depending upon the nature of the flux, removal may or may not be required. Removal is typically recommended for fluxes containing halides such as ammonium chloride or zinc chloride. 
         [0007]    Conventional gas atmospheres (e.g., nitrogen/hydrogen) may be useful for excluding oxidizing agents and avoiding residues, but they are of limited efficacy in reducing or removing native oxides on the surface of solder preforms and powders. For example, when a die attach is performed under a conventional gas blanket, a mechanical “scrubbing” of the die may be required to displace oxides and improve the wetting of the surfaces being bonded. 
         [0008]    Solder pastes containing fluxes may be used to provide enhanced solder flow characteristics, but they generally have a significant volume of residue that must be removed after the attach is complete. The residue may also impede solder flow and contribute to voids when large surface areas are being bonded, particularly if the bonding time is short and the viscosity of the residue at the bonding temperature is high. 
         [0009]    Thus, there is a need for a system and method for soldering that provides an improved capacity for reduction of native oxides, while minimizing the impact of residues on solder flow. 
       BRIEF SUMMARY OF THE INVENTION 
       [0010]    Accordingly, a system for solder bonding surfaces with a locally generated reducing gas mixture is described herein. A volatile soldering aid including a thermally decomposable solid is incorporated in an assembly that includes a solder preform and the surfaces to be bonded. The solid is decomposed at a temperature below the melting point of the solder to provide a reducing gas atmosphere prior to melting of the solder. 
         [0011]    In an embodiment of the present invention, a solution containing a thermally decomposable solid is dissolved in a solvent and applied to two surfaces separated by a solder preform. The separation between the surfaces is small enough to allow capillary forces to draw the solution into the gap on either side of the preform and the adjacent surface. The solvent is subsequently evaporated, and the assembly is heated to decompose the solid and produce a reducing gas. The assembly is then further heated to melt the solder preform. A vacuum may be introduced to remove the gas prior to melting of the solder preform. 
         [0012]    In another embodiment, a powder of a thermally decomposable solid is suspended in a hydrophobic liquid that has a boiling point below or near the melting point of the solder to provide a paste that may be applied to an assembly for soldering. The solder preform in the assembly may be a monolithic preform or it may be a powder that is also suspended in the hydrophobic liquid. In a further embodiment, the solder preform may be provided as a thin film deposited on one or both of the surfaces to be joined. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]      FIG. 1A  shows a solder assembly with a monolithic preform and decomposable solid/solvent solution in accordance with an embodiment of the present invention. 
           [0014]      FIG. 1B  shows the solder assembly of  FIG. 1A  after evaporation of the solvent and prior to decomposable solid decomposition. 
           [0015]      FIG. 1C  shows the solder assembly of  FIG. 1B  after decomposable solid decomposition and solder flow. 
           [0016]      FIG. 2A  shows a solder assembly with a powder preform and decomposable solid/carrier suspension in accordance with an embodiment of the present invention. 
           [0017]      FIG. 2B  shows the assembly of  FIG. 2A  after evaporation of the carrier. 
           [0018]      FIG. 3  shows a solder assembly with a surface coating preform and decomposable solid/carrier suspension in accordance with an embodiment of the present invention. 
           [0019]      FIG. 4  shows a diagram for a soldering system in accordance with an embodiment of the present invention. 
           [0020]      FIG. 5  shows a flow diagram for a soldering process in accordance with an embodiment of the present invention. 
           [0021]      FIG. 6  shows a diagram for thermal and atmospheric profiles for a soldering process in accordance with an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0022]      FIG. 1A  shows an embodiment of a solder assembly  100  with a monolithic preform  115  and a volatile soldering aid  120  disposed between a semiconductor die  105  and a substrate  110 . Soldering aid  120  is a decomposable solid  125  (shown in  FIG. 2 ) dissolved in a liquid solvent. The semiconductor die has a bottom surface that is  106  that may be coated with gold or a gold alloy. The substrate  110  has a top surface  111  that may be coated with gold or a gold alloy. Surfaces  106  and  111  may also be coated with layer of a pure metal that is subsequently alloyed during the formation of a bond. In other embodiments, passive electronic components or mechanical structures may be substituted for the semiconductor die  105  and/or substrate  110 . 
         [0023]    For purposes of this disclosure, a “volatile soldering aid” is defined as a solution of, or a suspension of, a thermally decomposable solid in a liquid. The thermally decomposable solid is entirely converted to a vapor state when heated to the melting point of the solder with which it is being used. Conversion to a vapor phase is not dependent upon chemical reaction with other species (e.g., atmospheric oxygen). The soldering aid as a whole is converted entirely to vapor at the melting point of the solder. The liquid component is converted to vapor through evaporation or decomposition, and the solid component is converted to vapor through decomposition. 
         [0024]    Volatile soldering aid  120  may be an ionic solid dissolved in a polar solvent. For example, ammonium chloride may be dissolved in methanol. In general, decomposable solid  125  is a compound that is thermally decomposable into a gas mixture that is capable of removing oxides associated with surfaces  106  and  101 , and the monolithic preform  115 . The monolithic preform  115  may be a gold/tin eutectic alloy with a melting point of about 283° C. The monolithic preform  115  may also be a gold/tin alloy with a composition that is different from the eutectic composition. 
         [0025]    It should be noted that although ammonium chloride is commonly used as a component in soldering fluxes, it is typically combined with other materials that prevent it from being completely convertible to a vapor state. In the present invention, the thermally decomposable solid does not contribute to the formation of a liquid phase that is used to dissolve oxides. 
         [0026]      FIG. 1B  shows an embodiment of a dry solder assembly  101  that is obtained from the solder assembly  100  of  FIG. 1A  after evaporation of the solvent and prior to decomposition of the decomposable solid  125 . The use of the volatile soldering aid  120  allows for introduction of a dissolved decomposable solid into small gaps after assembly of parts for soldering. The amount of decomposable solid  125  that is deposited may be controlled by the adjusting the concentration of the decomposable solid in the decomposable volatile soldering aid  120 , and by controlling the amount of the volatile soldering aid  120  that is applied. 
         [0027]    Heating of the dry solder assembly  101  may be done to produce in situ decomposition of the decomposable solid  125 , thus producing a volume of reactive gas where it is most desired. The decomposition of ammonium chloride into ammonia and hydrogen chloride produces an expanding volume of gas that sweeps the gap between surfaces  106  and  111 . 
         [0028]    Decomposition of the decomposable solid  125  may be carried out at a pressure other than atmospheric pressure (e.g., vacuum) in order to modify the decomposition behavior over temperature. A vacuum may be introduced after solid decomposition in order to remove residual gas. The removal of residual gas allows the surface tension of the liquid solder to collapse potential voids to a very small size prior to solidification. 
         [0029]      FIG. 1C  shows an embodiment of a finished solder  102  assembly obtained from the solder assembly  101  of  FIG. 1B  after solid decomposition and solder flow. The solder joint  130  (e.g., gold/tin) provides a complete fill of the gap between the semiconductor die  105  and a substrate  110 . A solvent wash may be performed after die attach to remove solid reduction reaction products, if present, and/or initial impurities that may have been present in the decomposable solid. 
         [0030]    When the semiconductor die  105  and substrate  110  have gold metalized surfaces, a volatile soldering aid  120  consisting of methanol and ammonium chloride may be used with an 80/20 gold eutectic preform to achieve full wetting and a specular finish on the exposed surface of the cooled solder joint  130 , without mechanical agitation of the semiconductor die  105 . 
         [0031]      FIG. 2A  shows a solder assembly  200  with a powder preform  215 , and a soldering aid  220  that includes a decomposable solid  225  suspended as a particulate in a volatile carrier  222 . In preparing the assembly  200 , a measured amount of the powder preform  215  and soldering aid  220  may be deposited on the surface of the substrate  210  prior to placing the semiconductor die  205 . Alternatively, a monolithic preform or a surface coating preform may be used in conjunction with a suspension of the decomposable solid  225  in the carrier  220 . 
         [0032]    For hygroscopic solids (e.g., ammonium chloride), the use of a hydrophobic carrier reduces the absorption of moisture by the solid. For components that are sensitive to corrosion in an electrolyte solution, the use of a nonpolar liquid allows an ionic solid to be used in a liquid without forming an electrolyte solution. Thus, a material (e.g., ionic compound) that may normally be corrosive in the presence of moisture may be used as the decomposable solid  225 . Organic compounds may be selected on the basis of viscosity and vapor pressure in order to provide an optimum combination of handling and evaporation behavior as the carrier  222 . 
         [0033]    The carrier  222  may include a mixture of different compounds with different vapor pressures. For example, a low vapor pressure liquid with a boiling point of less than 100° C. may be used to provide dilution and low viscosity, and a high vapor pressure liquid with a boiling point greater than 100° C. may be used to maintain coverage of the decomposable solid  225  so that water absorption is avoided. Carrier  222  may include aromatic, aliphatic, or alicyclic hydrocarbon compounds. 
         [0034]      FIG. 2B  shows an assembly  201  produced by evaporation of the carrier  220  shown in  FIG. 2A . The gap between the semiconductor die  205  and the substrate  210  contains an intimate mixture of the decomposable solid  225  and the solder preform  215 . Alternatively, a monolithic preform or a surface coating preform may be used. A surface coating may be an alloy, or a layered composite of two metals. For example, a first layer of tin may be overlaid with a second layer of gold. 
         [0035]      FIG. 3  shows a solder assembly  300  with a surface coating preform  315 , and a decomposable solid  225  suspended in a carrier  320 . The surface coating preform  315  is deposited on the surface of the substrate  310 . however, the surface coating preform  315  may be deposited on the semiconductor die  305 . The surface coating preform may be deposited as an alloy, or it may be deposited as distinct layers (e.g., gold over tin). Sputtering and electrodeposition may be used to deposit the surface coating preform  315 . 
         [0036]    A surface coating preform is particularly useful for flip-chip bonding of the semiconductor die  305 . For example, the electrical contact pads of transistors are frequently closely spaced and thus vulnerable to bridging by excess solder. The use of a surface coating preform allows a small amount of solder to be precisely placed. When using a minimum amount of solder, it is important to avoid oxidation losses. Since the application of pressure and/or movement is not required during solder flow, soft columnar structures may be used at bonding sites on the semiconductor die  305  and substrate  310 . A columnar structure may be used to provide a localized thermal capacitance for pulsed power applications, and may also be used to provide a buffer between a semiconductor die  305  and a substrate  310  that have different thermal expansion coefficients. 
         [0037]      FIG. 4  depicts an embodiment of a soldering system  400  that may be used to provide a controlled atmosphere for soldering. A chamber  405  contains a stage  410  for supporting a solder assembly. The stage  410  may or may not be used as a heat source for soldering. A radiant heat source  415  may be used, particularly for heating under vacuum. A radiant heat source may be used in combination with a heated stage  410 . 
         [0038]    A gas source  420  may be used to provide a neutral atmosphere such as dry nitrogen. Depending upon the nature of the soldering process, the gas source may simply provide filtered air. The gas source  420  may be adapted to provide more than one gas composition, and may be used to pressurize the chamber  405  to a pressure greater than atmospheric pressure. A positive pressure may be used to purge the chamber  405 , or to improve heat transfer across gaps in a solder assembly. A vacuum pump  425  may be used to exhaust the chamber  405  and provide a working pressure that is below atmospheric pressure. 
         [0039]    Although a gas mixture (e.g., ammonia/hydrogen chloride) could be provided through the gas source  420  as an alternative to in situ decomposition of a solid, the local decomposition of a solid reduces the overall volume of gas required and provides a greater effective concentration of active species at the working surfaces. In order to achieve the same effective concentration, pre-evacuation and backfill at an overpressure would be required with a gas source. Another advantage of a solution or solid/liquid dispersion is that small components may be held in place so that gas flows or static charges will not easily displace them. 
         [0040]    A controller  430  may be used to control the gas source  320 , vacuum pump  425 , radiant heater  415 , and stage  410 , if heated. The controller provides temperature and pressure profiles and controls the composition of the atmosphere within the chamber  405 . 
         [0041]      FIG. 5  shows a flow diagram  500  for an embodiment of a soldering process. In Step  505 , a solder assembly is prepared. In general, a solder assembly includes two or more components to be soldered, with a decomposable solid and a solder preform disposed between the components. A volatile solvent or carrier may be used to dissolve or suspend the decomposable solid. The solder preform may be a powder, an individual piece of solder, or a coating on one or more of the components in the solder assembly. 
         [0042]    In step  510 , the solder assembly is enclosed. This may be done by placing the solder assembly in a chamber that provides for atmospheric and/or temperature control. Atmospheric control may include control of atmospheric composition and/or pressure. Temperature control may be provided by a heated stage that supports the solder assembly, or by radiant heating. 
         [0043]    In step  515 , an atmospheric profile is applied. The atmospheric profile may include segments for purging, pressurizing, and evacuating. Inert or reducing gases may be used for purging and pressurizing. Although satisfactory results have been obtained in air, it is generally desirable to have a vacuum, or an inert or reducing atmosphere in place during solid decomposition and solder flow. 
         [0044]    In step  520 , a thermal profile is applied. Although the thermal profile may be initiated prior to the application of the atmospheric profile, it is generally preferable to create an inert or reducing atmosphere prior to heating. Heat is applied to remove solvents and/or carriers. It is desirable to limit the heating rate so that dislocation of parts due to rapid vapor evolution during the evaporation and decomposition phases is avoided. A fixed temperature dwell below the decomposition temperature of the solid may be used to complete removal of the solvents and/or carriers. Subsequently, the assembly is heated to the solder flow temperature at a rate that allows for the complete decomposition of the solid prior to solder flow. 
         [0045]      FIG. 6  shows a diagram  600  for embodiments of a thermal profile  605  and an atmospheric profile  610  that may be used in a soldering process with a volatile soldering aid. Several steps are shown for each profile, with various ramp segments and dwell segments that may or may not be present in other embodiments. 
         [0046]    Thermal profile  605  is initiated at room temperature (RT) with a dwell time of t 01  that allows for a vacuum evacuation and partial backfill represented by pressure segments t 11  and t 12  of the atmospheric profile  610 . Beginning at atmospheric pressure (P atm ) air is evacuated during segment t 11 , and an inert or reducing gas atmosphere (e.g., N 2  or N 2 /H 2 ) is introduced in segment t 12 . 
         [0047]    During ramp segment t 02 , heat is applied to the solder assembly to evaporate the liquid component of the soldering aid. Thermal ramp segment t 02  begins at room temperature and ends at a temperature T d  at which decomposition of the decomposable solid component of the soldering aid is achieved. During thermal ramp segment t 02 , the pressure ramp segment t 13  shows a return to atmospheric pressure accompanying the evaporation. In general, pressure will be determined by the net mass flow into or out of the chamber, vapor evolution within the chamber, and temperature. Feedback-controlled valves or relief valves may be used to control pressure. 
         [0048]    A thermal dwell segment t 03  occurs at T d  to allow for decomposition of the decomposable solid to a vapor. Due to the solid decomposition during the thermal dwell segment t 03 , pressure rises above P atm  during pressure segment t 14 . Subsequently, the temperature is increased to the melting point of the solder (T m ) during thermal ramp segment t 04 , while the pressure is reduced to a value below Patm as shown in pressure segment t 15 . 
         [0049]    A thermal dwell segment t 05  at T m  allows for melting of the solder, while the pressure dwell segment t 16  provides a low pressure to reduce trapped gas that would prevent collapse of voids in the molten solder. An initial cooling ramp t 06  provides for solidification of the solder and pressure segment t 17  provides a return to room temperature. the chamber may be purged at P atm  to assist in cooling during thermal ramp segment t 07 . 
         [0050]    While the invention has been described in detail with reference to preferred embodiments thereof, it will be apparent to one skilled in the art that various changes can be made, and equivalents employed, without departing from the scope of the invention. For example, embodiments of the invention may include all of the steps shown in  FIG. 5 , or may omit one or more of the disclosed steps (e.g., application of an atmospheric profile). Various embodiments of preforms and soldering aids have been disclosed. Within the scope of the invention, combinations of the aforementioned disclosed components other than those combinations explicitly disclosed may be used in a system for solder bonding with a volatile soldering aid.