Patent Publication Number: US-2007122646-A1

Title: Solder composition and soldering structure

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
      This application claims the priority benefit of Taiwan application serial no. 94141666, filed on Nov. 28, 2005. All disclosure of the Taiwan application is incorporated herein by reference.  
     BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates to a solder composition. More particularly, the present invention relates to a solder composition and soldering structure applied to aluminum.  
      2. Description of the Related Art  
      Solder is a metallic substance mainly used for joining two identical metallic materials or two different metallic materials structurally together at a relative low temperature. Because soldering techniques can provide an electronic package with a high conductivity, a high heat-dissipating capacity and a high bonding reliability, soldering material has been widely adopted for assembling various electronic components and packaging semiconductor devices.  
      Most conventional soldering material has tin (Sn) as its main component and other metallic elements as secondary components to form a binary or multi-nary alloy, for example, tin-lead (Sn—Pb) alloy, tin-silver (Sn—Ag) alloy, tin-indium (Sn—In) alloy and tin-sliver-copper (Sn—Ag—Cu) alloy. The foregoing soldering materials are now used in vast quantities for joining together some ‘solderable’ metallic objects. At present, the heat dissipating fins or heat dissipating substrates are often fabricated using aluminum in many types of heat dissipating modules. In advanced semiconductor production processes, aluminum wires are frequently used as a means for transmitting signals or connecting power source in active devices or passive devices. However, aluminum is hardly bonded with the aforementioned tin-based conventional soldering material. Thus, before using the soldering material, a metallic pad layer must be coated on the ‘non-solderable’ metal first. In general, the metallic pad layer includes an adhesive layer such as a titanium layer or a chromium layer and a ‘solderable’ metallic layer such as a copper layer or a nickel layer. Nevertheless, the processing complexity and the production cost will be increased.  
      In addition, flux is often applied to remove the oxide material on the surface of the metallic pad layer and increase wettability before performing a conventional reflow process. Yet, the application of the flux not only increases uncertainties and complexities in the production process, but also the structural strength of the bond may be deteriorated due to residual flux and voids generated thereby.  
     SUMMARY OF THE INVENTION  
      Accordingly, at least one objective of the present invention is to provide a solder composition suitable for reacting with aluminum to form stronger bonds.  
      Another objective of the present invention is to provide a soldering structure capable of increasing structural strength.  
      To achieve these and other advantages and in accordance with the purpose of the invention, as embodied and broadly described herein, the invention provides a solder composition that mainly comprises 0.01˜20 wt % of zinc and 0.01˜20 wt % of chromium. The remaining ingredients include at least tin and unavoidable impurity.  
      The present invention also provides a soldering structure that includes an aluminum substrate and a solder composition disposed thereon. The solder composition mainly comprises 0.01˜20 wt % of zinc and 0.01˜20 wt % of chromium. The remaining ingredients include at least tin and unavoidable impurity.  
      The present invention also provides an alternative soldering structure that includes an aluminum substrate, a chromium layer and a solder composition. The chromium layer is disposed on the aluminum substrate and the solder composition is disposed on the chromium layer. The solder composition mainly comprises 0.01˜20 wt % of zinc. The remaining ingredients include at least some and unavoidable impurity.  
      The present invention also provides another soldering structure that includes an aluminum substrate, a zinc layer and a solder composition. The zinc layer is disposed on the aluminum substrate and the solder composition is disposed on the zinc layer. The solder composition mainly comprises 0.01˜20 wt % of chromium. The remaining ingredients include at least tin and unavoidable impurity.  
      In one embodiment of the present invention, the solder composition further includes bismuth (Bi), indium (In) or a mixture thereof.  
      In one embodiment of the present invention, the solder composition further includes the elements of the IVB group, the elements of the VB group or a mixture thereof having an amount of 0.01˜10 wt. %. Furthermore, the IVB group includes at least titanium (Ti), zirconium (Zr), hafnium (Hf) or a combination thereof. In addition, the VB group includes at least vanadium (V), niobium (Nb), tantalum (Ta) or a combination thereof.  
      In one embodiment of the present invention, the solder composition further includes the elements of the IIIB group, or a mixture thereof having an amount of 0.01˜10 wt %. The IIIB group includes at least the lanthanide series, the actinide series, or a combination thereof. The IIIB group includes at least samarium (Sm), neodymium (Nd), lutetium (Lu) or a combination thereof. The lanthanide series includes at least cerium (Ce), praseodymium (Pr), neodymium (Nd), gadolinium (Gd), ytterbium (Yb) or a combination thereof.  
      In one embodiment of the present invention, the solder composition further includes silver (Ag), copper (Cu) or a mixture thereof having an amount 0.01˜10 wt %.  
      In one embodiment of the present invention, the solder composition further includes 0.01˜10 wt % of antimony (Sb).  
      In one embodiment of the present invention, the solder composition further includes nickel (Ni), cobalt (Co), manganese (Mn) or a mixture thereof having an amount 0.01˜10 wt %.  
      In one embodiment of the present invention, the solder composition further includes 0.01˜10 wt % of gallium (Ga).  
      In one embodiment of the present invention, the soldering structure further includes an oxidation-resistant layer disposed over the aluminum substrate and located between the solder composition and the aluminum substrate. In addition, the material of the oxidation-resistant layer can be gold (Au) or platinum (Pt).  
      In one embodiment of the present invention, the soldering structure further includes an oxidation-resistant layer disposed over the chromium layer and located between the solder composition and the chromium layer. In addition, the material of the oxidation-resistant layer can be gold or platinum.  
      In one embodiment of the present invention, the soldering structure further includes an oxidation-resistant layer disposed over the zinc layer and located between the solder composition and the zinc layer. In addition, the material of the oxidation-resistant layer can be gold or platinum.  
      Accordingly, the solder in present invention includes zinc, chromium and a base material of tin. Because chromium has a greater affinity with oxygen, the chromium can easily bond with the oxygen atoms in the oxygen layer on glass, metal or semiconductor substrate. In addition, zinc has a high solubility in aluminum. Hence, an alloy of zinc-aluminum (Zn—Al) solid solution can easily form at their interface. Therefore, the solder composition can increase its wettability in most materials and lower the surface energy between the solder composition in the melted state and the substrate material.  
      It is to be understood that both the foregoing general description and the following detailed description are exemplary, and are intended to provide further explanation of the invention as claimed. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention. In the drawings,  
       FIG. 1  is a soldering structure according to one embodiment of the present invention.  
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
      Reference will now be made in detail to the present preferred embodiments of the invention, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers are used in the drawings and the description to refer to the same or like parts.  
       FIG. 1  is a soldering structure according to one embodiment of the present invention. As shown in  FIG. 1 , the soldering structure  100  in the present embodiment includes an aluminum substrate  110  and a solder composition  120 . The solder composition  120  is disposed on the aluminum substrate  100 . Furthermore, the solder composition  120  is suitable for reacting with the aluminum substrate  110 . In addition, the aluminum substrate  110  can be an aluminum (Al) pad or an aluminum (Al) wire. The solder composition  120  mainly includes 0.01˜20 wt % of zinc (Zn), 0.01˜20 wt % of chromium (Cr). The remaining percentage of the material in the solder composition  120  includes at least tin and unavoidable impurity.  
      If the properties of the solder composition  120  need to be changed, the solder composition  120  in present embodiment may include other ingredients. For example, the solder composition  120  may include the bismuth (Bi), indium (In) or a mixture thereof. In another embodiment, the solder composition  120  may further include the elements of the IVB group, the elements of the VB group or a mixture thereof having an amount of 0.01˜10 wt. %. The IVB group includes at least titanium, zirconium, hafnium or a combination thereof. The VB group includes at least vanadium, niobium, tantalum, or a combination thereof.  
      In another embodiment, the solder composition  120  further includes the elements of the IIIB group or a mixture thereof having an amount of 0.01˜10 wt %. The IIIB group includes lanthanide series, the actinide series or a combination thereof. For example, the IIIB group includes at least samarium, neodymium, lutetium or a combination thereof. The lanthanide group of elements includes at least cerium, praseodymium, neodymium, gadolinium, ytterbium or a combination thereof.  
      In another embodiment, the solder composition  120  further includes silver, copper or a mixture thereof having an amount 0.01˜10 wt %. More specifically, the silver in the solder composition  120  can lower the surface tension and the soldering temperature of the solder composition  120  in the melted state and increase the bonding strength of the final bond. Furthermore, the copper in the solder composition  120  can increase the wettability of the solder composition  120  so that the soldering strength of the final bond will be increased.  
      In another embodiment, the solder composition  120  further includes 0.01˜10 wt % of antimony. Furthermore, the solder composition  120  includes nickel, cobalt, manganese or a mixture thereof having an amount 0.01˜10 wt %. In addition to change the bonding temperature of the solder composition  120 , the aforementioned antimony, nickel, cobalt and manganese can also increase the wettablity of the solder composition  120  and the bonding strength of the solder joint.  
      In another embodiment, the solder composition  120  further includes 0.01˜10 wt % of gallium. More specifically, the gallium can assist the removal of the oxide layer on the bonding material (for example, the aluminum substrate  110 ) and lower the bonding temperature.  
      The bonding temperature of the solder composition  120  in the present invention is roughly between 100° C.˜550° C. In addition, the solder composition  120  in the present invention can be directly applied to the surface of the bonding substrate just like glue without the application of any flux.  
      More specifically, when the solder composition  120  has been heat to melt, the affinity with oxygen of the chromium (Cr) in the solder composition  120  will cause the chromium to congregate on the surface of the solder material. The chromium will react with the oxide material and the oxide layer on the bonding material. Then, the zinc inside the solder composition  120  will react with the aluminum substrate  110  to form an alloy of zinc-aluminum (Zn—Al) solid solution at their interface. The method of melting the solid composition  120  includes plate heating, hot air heating, ultrasonic heating, resistor heating, electromagnetic heating and so on. The bonding strength provided by the solder composition  120  in the present invention is described in detail below.  
                                   TABLE 1                                   Example 1   Example 2   Example 3   Example 4                                                                Prior Technique   48.5   kgf   32.3 kgf   33.3 kgf   53   kfg       Embodiment   54   kgf   53.5 kgf   40.4 kgf   43.4   kgf                  
 
      Table 1 is a tabulation of the results of tests carried out using the experimental equipment 1220WS provided by the DELTA ELECTRONICS. The straining speed is 2 mm/min, the loading range of the measurement is 100kg, the length of travel is 10 mm, the bonding area is 64 mm 2 , and the experimental condition is copper-aluminum bonding. In solder material used in the prior technique experiment is supplied by the S-Bond company and has a Model No. 220-50. The composition of the solder material in the prior technique experiment includes 10 wt % of chromium (Cr), 5 wt % of zinc (Zn) and roughly 85 wt % of tin (Sn). According to the aforementioned experimental results, the embodiment has an average bonding strength better than the prior technique. It should be noted that although only a single solder composition and its experimental data are disclosed in the present embodiment, this solder composition should not be used to limit the scope of the present invention. Similar experimental testing on the other solder composition disclosed in the present embodiment can also be carried out by technicians familiar with such experimental technique to find out the increase in average bonding strength of the solder composition relative to the prior technique. Thus, to simplify the explanations, a detailed description of the experimental results of the other solder compositions is omitted here.  
      The solder composition  120  in the present invention utilizes chromium (Cr) as the main activate component. Due to the affinity of chromium with oxygen, the chromium can easily combine with the oxygen atom on the surface of aluminum to form a chromium-oxygen (Cr—O) bond. Meanwhile, due to the high solubility of zinc in aluminum, zinc can react with aluminum to form a solid solution alloy at their interface. In addition, the solder composition  120  can easily bond with non-solderable metal such as aluminum without the application flux. Furthermore, the solder composition in the present invention has a better coating capability and a rapid oxidation of the soldering surface will be improved.  
      It should be noted that the zinc and chromium in the aforementioned soldering structure  100  does not have to come from the solder composition  120 . For example, when a layer of zinc (not shown) is coated over the aluminum substrate  110 , zinc can be excluded from the solder composition  120 . In addition, a layer of oxidation resistant material (not shown), such as gold (Au) or platinum (Pt) can also be formed over the zinc layer. Similarly, when a layer of chromium (not shown) is coated on the aluminum substrate  110 , chromium can be excluded from the solder composition  120 , and a layer of oxidation resistant material (not shown) can be formed over the chromium layer. Furthermore, a layer of oxidation resistant material (not shown) can also be directly formed over the aluminum substrate  110 . The solder composition  120  is disposed on the oxidation-resistant layer thereafter.  
      It will be apparent to those skilled in the art that various modifications and variations can be made to the structure of the present invention without departing from the scope or spirit of the invention. In view of the foregoing, it is intended that the present invention cover modifications and variations of this invention provided they fall within the scope of the following claims and their equivalents.