Abstract:
A solder bonding method comprises the step of solder bonding a first electrode  30  to a second electrode  16  having a solder bump  18  of mainly Sn formed on the upper surface thereof. The first electrode  30  and/or the second electrode  16  includes metal layers  14, 26  formed of an alloy layer containing Ni and P, an alloy layer containing Ni and B, or an alloy layer containing N, W and P. The metal layer of the alloy layer containing impurities, such as P, etc. can prevent the Ni of the metal layer from combining with the Sn in the solder bump. Accordingly, good bonded states can be obtained.

Description:
This application is a Divisional of prior application Ser. No. 09/453,278 filed Dec. 3, 1999 now U.S. Pat. No. 6,457,233. 
    
    
     CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based upon and claims priority of Japanese Patent Application No. Hei 11-14554, filed, the contents being incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     The present invention relates to a solder bonding method, and an electronic device and a process for fabricating the electronic device, more specifically to a solder bonding method using a solder material containing Sn as a main component, and an electronic device and a process for fabricating the electronic device. 
     Recently, in view of the high-speed operation of semiconductor devices, techniques for short wiring lengths have been required. What is noted is flip chip bonding in which specifically solder bumps formed on a semiconductor chip are mounted on a circuit substrate with electrodes formed on, and are melted by heating for bonding. 
     The solder bonding method by the conventional flip chip bonding will be explained with reference to FIG.  4 . 
     First, an electrical wiring  111  is formed of an Al film on a semiconductor substrate  110  with a prescribed device. Next, an electrode  116  is formed of a Ti film  112 , an Ni film  113  and an Au film  114  on an electrical wiring  111 , and a solder bump  118  is formed on the electrode  116 . 
     On the other hand, an electrode  130  is formed of a Cr film  122 , a Cu film  124 , an Ni film  126  and an Au film  128  on an alumina substrate  120  with a prescribed circuit. Thus, the circuit substrate  132  with the electrode  130  formed on is formed 
     Then, the solder bump  118  on the semiconductor substrate  110  is aligned with the electrode  130  on the circuit substrate  120 , and is heated for the flip chip bonding. Such flip chip bonding makes the connection by means of lead wires unnecessary. The wiring length can be short. 
     Conventionally, Pb—Sn (Pb: lead, Sn: tin)-based solder materials have been widely used in the flip chip bonding. However, the Pb contained in Pb—Sn-based solder materials have isotopes, and the isotopes are intermediate products or terminal products of the decay series of U (uranium) and Th (thorium). Uranium (U) and thorium (Th) decay by the emission of He (helium), the solder materials emit α-rays. The α-rays affect the operations of semiconductor devices, often causing the so-called soft errors. In a case that Pb flows into soil, the Pb is solved by acid rain, often affecting environments. From the ecological viewpoint, solder materials containing Pb as a non-main component are required. 
     As a solder material which replaces the Pb—Sn-based solder materials, solder materials containing Sn as a main component is noted. 
     However, in a case that a solder material containing Sn as a main component is used, because the Ni and Cu in the electrodes  116 ,  130  are reactive to the Sn in the solder hump  118 , heat applied by the flip chip bonding produces metal compounds, etc., such as Ni—Sn, Cu—Sn, etc. When the Ni reacts to the Sn, and the Ni film  113  is lost, it is difficult that the bonding between the solder bump  118 , and the electrodes  116 ,  130  can be satisfactory because the Ti film  112 , for example, and the solder bump  118  are incompatible with each other. In reliability test, such as a heat-cycle test, etc., the bonding was defective, and conduction, etc. are unsatisfactory. The reliability is poor. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a solder bonding method, and an electronic device and a process for fabricating the electronic device, which make the bonding satisfactory even by the use of a solder material containing Sn as a main component. 
     The above-described object is achieved by a solder bonding method comprising the step of solder bonding a first electrode to a second electrode having a solder bump of mainly Sn on an upper surface thereof, the first electrode and/or the second electrode including a metal layer of an alloy layer containing Ni and P, an alloy layer containing Ni and B, or an alloy layer containing Ni, W and P. The metal layer of an alloy layer containing impurities, such as P, etc. can prevent the Ni of the metal layer from combining with the Sn in the solder bump. Accordingly, good bonded states can be obtained. 
     The above-described object is achieved by a solder bonding method comprising the step of solder bonding a first electrode to a second electrode having a solder bump of mainly Sn formed on an upper surface thereof, the first electrode and/or the second electrode including a metal layer of mainly Ni, and the solder bonding step being followed by the step of heat treating the alloy layer. The heat treatment can crystallize the metal layer, whereby the Ni of the metal layer can be prevented from combining with the Sn in the solder bump. 
     The above-described object is achieved by an electronic device comprising a first substrate including a first electrode, a second substrate including a second electrode having a solder bump of mainly Sn formed on an upper surface thereof, the first electrode and the second electrode being solder bonded to each other, the first electrode and/or the second electrode including a metal layer of an alloy layer containing Ni and P, an alloy layer containing Ni and B, or an alloy layer containing Ni, W and P. The metal layer of an alloy layer containing impurities, such as P, etc. can prevent the Ni of the metal layer from combining with the Sn in the solder bump. Accordingly, good bonded states can be obtained. Electronic devices having good bonded states can be provided. 
     The above-described object is achieved by an electronic device fabrication process comprising the step of solder bonding a first electrode formed on a first substrate to a second electrode which is formed on a second substrate and has a solder bump of mainly Sn formed on an upper surface thereof, the first electrode and/or the second electrode including a metal layer of an alloy layer containing Ni and P, an alloy layer containing Ni and B, or an alloy layer containing Ni, W and P. The metal layer of an alloy layer containing impurities, such as P, etc. can prevent the Ni of the metal layer from combining with the Sn in the solder bump. Accordingly, a process for fabricating electronic devices having good bonded states can be provided. 
     The above-described object is achieved by an electronic device fabrication process comprising the step of solder bonding a first electrode formed on a first substrate to a second electrode which is formed on a second substrate and has a solder bump of mainly Sn formed on an upper surface thereof, the first electrode and/or the second electrode including a metal layer of mainly Ni, and the step of heat treating the metal layer being followed by the solder bonding step. The heat treatment can crystallize the metal layer, whereby the Ni of the metal layer can be prevented from combining with the Sn in the solder bump 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a sectional view showing the solder bonding method according to a first embodiment of the present invention. 
     FIG. 2 is a sectional view showing the solder bonding method according to a second embodiment of the present invention. 
     FIG. 3 is a sectional view showing the solder bonding method according to a second embodiment of the present invention. 
     FIG. 4 is a sectional view showing the conventional solder bonding method. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     [A First Embodiment] 
     The solder bonding method according to a first embodiment of the present invention will be explained with reference to FIG.  1 . FIG. 1 is a sectional view for explaining the solder bonding method according to the first embodiment of the present invention. 
     First, a semiconductor substrate  10  of a silicon substrate with a prescribed semiconductor device formed on is prepared. Then, a 100 nm-Ti film  12  is formed on the semiconductor substrate  10  by sputtering. The Ti film  12  is patterned into a plane shape of an electrode  16 . The plane shape of the electrode  16  has, e.g., a 70-100 μm-diameter, and a pitch of the electrode  16  with respect to an adjacent one (not shown) is, e.g., 150-210 μm. 
     Then, a plated film  14  containing Ni and P (phosphorus) is formed on the Ti film  12  by electroless plating. A thickness of the plated film  14  is, e.g., 6 μm, and a phosphorus content of the plated film  14  may be, e.g., 5-20 wt %. The plated film  14  contains P because the Ni of the plated film  14  is prevented from combining with the Sn in the solder bump. A phosphorus content of the plated film  14  is not essentially 5-20 wt %. It is preferable to set a phosphorus content suitably to obtain a required bonded state. 
     A film thickness of the plated film  14  is suitably set so that a satisfactory bonded state can be achieved even when the Ni of the plated film  14  is combined with the Sn in the solder bump  18  by heat applied upon the flip chip bonding to resultantly thin the plated film  14 . Thus, the electrode  16  of the Ti film  12  and the plated film  14  is formed. 
     Next, a heat treatment is performed at 400-600° C. for about 0.5-2 hours to crystallize the plated film  14 . The heat treatment is performed for the following reason. That is, the plated film of the Ni film formed simply by electroless plating has amorphous state, and has weak metal combining force and a number of pin holes. Accordingly, when the plated film formed simply by electroless plating is subjected to heat by the flip chip bonding or others, the Ni of the plated film tends to combine with the Sn in the solder bump. A diffusion velocity of the Ni of the plated film  14  formed by electroless plating is higher by 2-3 times a diffusion velocity of an Ni metal plate or a plated film formed by electrolytic plating. As a result, Ni—Sn-based metal compounds are grown by the flip chip bonding in the plated film formed by electroless plating, and the plated film is lost. In the present embodiment, the plated film  14  formed by electroless plating is crystallized by the heat treatment, so that the Ni of the plated film  14  can be prevented from combining with the Sn in the solder bump  18  to thereby produce Ni—Sn-based compounds. Furthermore, as described above, the plated film  14  contains P. Whereby the Ni of the plated film  14  can be furthermore prevented from combining with the Sn of the solder bump  18 . In the present embodiment, the plated film can be formed by electroless plating, which permits the plated film to be formed by simpler process in comparison with electrolytic plating. 
     Next, a solder bump  18  is formed of a solder material containing Sn as a main component on the electrode  16 . As a method for forming the solder bump  18 , dimple plating or others, for example, may be used. It is preferable that a Pb concentration of the solder material of the solder bump  18  is, e.g., below 1 ppm. Preferably, an α-ray amount to be emitted from the solder material of the solder bump  18  is, e.g., below 0.01 cph/cm 2  for the prevention of soft errors. Thus, a semiconductor device  19  with the solder bump  18  formed on the electrode  16  of the semiconductor substrate  10  is fabricated. 
     On the other hand, a Cr film  22  and a Cu film  24  are formed on an aluminum substrate  20  by sputtering. Then, the Cr film  22  and the Cu film  24  are patterned into a plane shape of an electrode  30 . A diameter of the plane shape of the electrode  30  is, e.g., 70-100 μm, and a pitch of the electrode  30  with respect to an adjacent one (not shown) is, e.g., 150-210 μm. 
     Next, a 6 μm-thickness plated film  26  is formed on the Cu film  24  by electroless plating. The plated film  26  may be formed in the same way as the plated film  14 . Then, a 50 nm-thickness Au film  28  is formed by flash plating. The Au film  28 , which is highly reactive to Sn, can contribute to improving a wettability. Thus, the electrode  30  is formed of the Cr film  22 , the Cu film  24 , the plated film  26  and the Au film  28 . Thus, the circuit substrate  32  with the electrode  30  formed on is formed. 
     Next, the semiconductor device  19  and the circuit substrate  32  are aligned with each other to be subjected to the flip chip bonding in a nitrogen atmosphere in a reflow furnace. Thus, the semiconductor device  19  is mounted on the circuit substrate  32 , and an electronic device is fabricated. 
     (Results of Reliability Evaluation Test) 
     Results of a reliability evaluation test made on electronic devices fabricated by using the above-described solder bonding method will be explained with reference to Tables 1-1 to 2-2. Tables 1-1 to 2-2 show results of the reliability evaluation test made on the electronic devices fabricated by using the solder bonding method according to the present embodiment. 
     A diameter of the solder bump  18  was 70-100 μm. A pitch of the solder bump with respect to an adjacent one (not shown) was 150-210 μm. Film thicknesses of the plated films  14 ,  26  were both 6 μm. 
     In the reliability evaluation test, a resistance value was measured immediately after the flip chip bonding, and a heat cycle test (−55° C.-125° C.) was repeated to measure a resistance at every 50 cycle. In Tables 1-1 to 2-2, a residual film thickness means a residual film thickness of the plated film of an Ni-based alloy formed on the circuit substrate  32 . In Controls 1 to 4, films containing no impurity, such as P, etc., were formed by electroless plating, and the heat treatment was not performed. 
     
       
         
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                 TABLE 1-1 
               
               
                   
                   
               
               
                   
                  P Content in 
                 Heat Treatment 
                 Heat Treatment 
               
               
                   
                 Plated Film 
                 Temperature 
                 Time 
               
               
                   
                 (wt %) 
                 (° C.) 
                 (Hours) 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                  Example 1 
                   5 
                 400 
                 1 
               
               
                   
                 Example 2 
                  5 
                 400 
                 1 
               
               
                   
                 Example 3 
                  5 
                 400 
                 1 
               
               
                   
                 Example 4 
                  5 
                 400 
                 1 
               
               
                   
                 Example 5 
                 15 
                 400 
                 1 
               
               
                   
                 Example 6 
                 15 
                 400 
                 1 
               
               
                   
                 Example 7 
                 15 
                 400 
                 1 
               
               
                   
                 Example 8 
                 15 
                 400 
                 1 
               
               
                   
                 Control 1 
                 — 
                 None 
                 None 
               
               
                   
                 Control 2 
                 — 
                 None 
                 None 
               
               
                   
                 Control 3 
                 — 
                 None 
                 None 
               
               
                   
                 Control 4 
                 — 
                 None 
                 None 
               
               
                   
                   
               
             
          
         
       
     
     
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                 TABLE 1-2 
               
               
                   
                   
               
               
                   
                  Content of 
                   
                   
                 Residual 
               
               
                   
                 Elements other 
                   
                   
                 Film 
               
               
                   
                 than Sn in Solder 
                 Heat Cycle 
                 Bonded 
                 Thickness 
               
               
                   
                 Bump (wt %) 
                 (Cycle) 
                 State 
                 (μm) 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                  Example 1 
                  Ag: 3.5, Zn: 5 
                 Above 300 
                 Good 
                 3 
               
               
                 Example 2 
                 Sb: 5 
                 Above 300 
                 Good 
                 3 
               
               
                 Example 3 
                 Ag: 3.5 
                 Above 300 
                 Good 
                 3 
               
               
                 Example 4 
                 Ag: 3.5, In: 5 
                 Above 300 
                 Good 
                 3 
               
               
                 Example 5 
                 Ag: 3.5, Zn: 5 
                 Above 300 
                 Good 
                 4 
               
               
                 Example 6 
                 Sb: 5 
                 Above 300 
                 Good 
                 4 
               
               
                 Example 7 
                 Ag: 3.5 
                 Above 300 
                 Good 
                 4 
               
               
                 Example 8 
                 Ag: 3.5, In: 5 
                 Above 300 
                 Good 
                 4 
               
               
                 Control 1 
                 Ag: 3.5, Zn: 5 
                 200 
                 Fair 
                 0-2 
               
               
                 Control 2 
                 Sb: 5 
                 200 
                 Fair 
                 0-2 
               
               
                 Control 3 
                 Ag: 3.5 
                 150 
                 Fair 
                 0-2 
               
               
                 Control 4 
                 Ag: 3.5, In: 5 
                 150 
                 Fair 
                 0 
               
               
                   
               
             
          
         
       
     
     
       
         
               
               
               
               
             
               
               
               
               
             
           
               
                   
                 TABLE 2-1 
               
               
                   
                   
               
               
                   
                  P Content in 
                 Heat Treatment 
                 Heat Treatment 
               
               
                   
                 Plated Film 
                 Temperature 
                 Time 
               
               
                   
                 (wt %) 
                 (° C.) 
                 (Hours) 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                  Example 9 
                   5 
                 600 
                 1 
               
               
                 Example 10 
                  5 
                 600 
                 1 
               
               
                 Example 11 
                  5 
                 600 
                 1 
               
               
                 Example 12 
                  5 
                 600 
                 1 
               
               
                 Example 13 
                 15 
                 600 
                 1 
               
               
                 Example 14 
                 15 
                 600 
                 1 
               
               
                 Example 15 
                 15 
                 600 
                 1 
               
               
                 Example 16 
                 15 
                 600 
                 1 
               
               
                   
               
             
          
         
       
     
     
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                 TABLE 2-2 
               
               
                   
                   
               
               
                   
                  Content of 
                   
                   
                 Residual 
               
               
                   
                 Elements other 
                   
                   
                 Film 
               
               
                   
                 than Sn in Solder 
                 Heat Cycle 
                 Bonded 
                 Thickness 
               
               
                   
                 Bump (wt %) 
                 (Cycle) 
                 State 
                 (μm) 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                  Example 9 
                  Ag: 3.5, Zn: 5 
                 Above 300 
                 Good 
                 4 
               
               
                 Example 10 
                 Sb: 5 
                 Above 300 
                 Good 
                 4 
               
               
                 Example 11 
                 Ag: 3.5 
                 Above 300 
                 Good 
                 4 
               
               
                 Example 12 
                 Ag: 3.5, In: 5 
                 Above 300 
                 Good 
                 4 
               
               
                 Example 13 
                 Ag: 3.5, Zn: 5 
                 Above 300 
                 Good 
                 4 
               
               
                 Example 14 
                 Sb: 5 
                 Above 300 
                 Good 
                 4 
               
               
                 Example 15 
                 Ag: 3.5 
                 Above 300 
                 Good 
                 4 
               
               
                 Example 16 
                 Ag: 3.5, In: 5 
                 Above 300 
                 Good 
                 4 
               
               
                   
               
             
          
         
       
     
     As shown by Controls 1 to 4 in Tables 1-1 and 1-2, the plated film containing no impurity, such as P or others, was formed by electroless plating and was not subjected to the heat treatment, a residual film thickness of the plated film was as thin as about 0-2 μm. Good bonded state could not be maintained. “Fair” indicating a bonded state means “bonded, but the bonded state is not good”. 
     In contrast to this, as shown in Examples 1 to 16 in Tables 1-1 to 2-2, the plated films  14 ,  26  contained P and were subjected to the heat treatment. Residual film thicknesses of the plated films  14 ,  26  was above 3 μm both with a 5 wt % P content and a 15 wt % P content. Good bonded states were maintained. 
     As described above, according to the present embodiment, the plated films of Ni films formed by electroless plating are crystallized by the heat treatment, whereby the Ni of the plated films can be prevented from combining with the Sn in the solder bump. The bonded state can be good. In the present embodiment, the plated films are formed by the electroless plating, which makes the step of forming the plated films simple. 
     In the present embodiment, the plated film is containing Ni as a main component contain P. Whereby the Ni of the plated films can be prevented from combining with the Ni in the solder bump. The bonded state can be good. 
     [A Second Embodiment] 
     The solder bonding method according tot a second embodiment of the present invention will be explained with reference to FIG.  2 . FIG. 2 is a sectional view explaining the solder bonding method according to the present embodiment. The same members of the present embodiment as those of the solder bonding method according to the first embodiment of FIG. 1 are represented by the same reference numbers not to repeat or to simplify their explanation. 
     The solder bonding method according to the present embodiment is characterized mainly in that a plated film  14   a  containing Ni and B (boron) is formed on a Ti film  12 , by electroless plating, and a plated film  26   a  containing Ni and B on a Cu film  24  by electroless plating. 
     The plated films  14   a ,  26   a  are subjected to the heat treatment as in the first embodiment. Boron contents of the plated films  14   a ,  26   a  may be, e.g., 5-20 wt %. The plated films  14   a ,  26   a  contain B, whereby the Ni of the plated films can be prevented from combining with the Sn in a solder pump, as can be prevented by the P in the plated films in the first embodiment. Accordingly, the present embodiment can provide electronic devices having the bonds in good states. 
     (Results of Reliability Evaluation Test) 
     Then, results of a reliability evaluation test made on electronic devices fabricated by using the above-described solder bonding method. Tables 3-1 to 4-2 show the results of the reliability evaluation test made on the electronic devices fabricated by using the solder bonding method according to the present embodiment. 
     As in the first embodiment, a diameter of the solder bump  18  was 70-100 μm. A pitch of the solder bump  18  with respect to an adjacent one was 150-210 μm. Film thicknesses of the plated films  14   a ,  26   a  were 6 μm as in the first embodiment. The reliability evaluation test was the same as in the first embodiment. 
     
       
         
               
               
               
               
             
               
               
               
               
             
           
               
                   
                 TABLE 3-1 
               
               
                   
                   
               
               
                   
                  B Content in 
                 Heat Treatment 
                 Heat Treatment 
               
               
                   
                 Plated Film 
                 Temperature 
                 Time 
               
               
                   
                 (wt %) 
                 (° C.) 
                 (Hours) 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                  Example 17 
                   1 
                 400 
                 1 
               
               
                 Example 18 
                  1 
                 400 
                 1 
               
               
                 Example 19 
                  1 
                 400 
                 1 
               
               
                 Example 20 
                  1 
                 400 
                 1 
               
               
                 Example 21 
                 10 
                 400 
                 1 
               
               
                 Example 22 
                 10 
                 400 
                 1 
               
               
                 Example 23 
                 10 
                 400 
                 1 
               
               
                 Example 24 
                 10 
                 400 
                 1 
               
               
                   
               
             
          
         
       
     
     
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                 TABLE 3-2 
               
               
                   
                   
               
               
                   
                  Content of 
                   
                   
                 Residual 
               
               
                   
                 Elements other 
                   
                   
                 Film 
               
               
                   
                 than Sn in Solder 
                 Heat Cycle 
                 Bonded 
                 Thickness 
               
               
                   
                 Bump (wt %) 
                 (Cycle) 
                 State 
                 (μm) 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                  Example 17 
                  Ag: 3.5, Zn: 5 
                 Above 300 
                 Good 
                 4 
               
               
                 Example 18 
                 Sb: 5 
                 Above 300 
                 Good 
                 4 
               
               
                 Example 19 
                 Ag: 3.5 
                 Above 300 
                 Good 
                 4 
               
               
                 Example 20 
                 Ag: 3.5, In: 5 
                 Above 300 
                 Good 
                 4 
               
               
                 Example 21 
                 Ag: 3.5, Zn: 5 
                 Above 300 
                 Good 
                 3 
               
               
                 Example 22 
                 Sb: 5 
                 Above 300 
                 Good 
                 3 
               
               
                 Example 23 
                 Ag: 3.5 
                 Above 300 
                 Good 
                 3 
               
               
                 Example 24 
                 Ag: 3.5, In: 5 
                 Above 300 
                 Good 
                 3 
               
               
                   
               
             
          
         
       
     
     
       
         
               
               
               
               
             
               
               
               
               
             
           
               
                   
                 TABLE 4-1 
               
               
                   
                   
               
               
                   
                  B Content in 
                 Heat Treatment 
                 Heat Treatment 
               
               
                   
                 Plated Film 
                 Temperature 
                 Time 
               
               
                   
                 (wt %) 
                 (° C.) 
                 (Hours) 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                  Example 25 
                   1 
                 600 
                 1 
               
               
                 Example 26 
                  1 
                 600 
                 1 
               
               
                 Example 27 
                  1 
                 600 
                 1 
               
               
                 Example 28 
                  1 
                 600 
                 1 
               
               
                 Example 29 
                 10 
                 600 
                 1 
               
               
                 Example 30 
                 10 
                 600 
                 1 
               
               
                 Example 31 
                 10 
                 600 
                 1 
               
               
                 Example 32 
                 10 
                 600 
                 1 
               
               
                   
               
             
          
         
       
     
     
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                 TABLE 4-2 
               
               
                   
                   
               
               
                   
                  Content of 
                   
                   
                 Residual 
               
               
                   
                 Elements other 
                   
                   
                 Film 
               
               
                   
                 than Sn in Solder 
                 Heat Cycle 
                 Bonded 
                 Thickness 
               
               
                   
                 Bump (wt %) 
                 (Cycle) 
                 State 
                 (μm) 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                  Example 25 
                  Ag: 3.5, Zn: 5 
                 Above 300 
                 Good 
                 4 
               
               
                 Example 26 
                 Sb: 5 
                 Above 300 
                 Good 
                 4 
               
               
                 Example 27 
                 Ag: 3.5 
                 Above 300 
                 Good 
                 4 
               
               
                 Example 28 
                 Ag: 3.5, In: 5 
                 Above 300 
                 Good 
                 4 
               
               
                 Example 29 
                 Ag: 3.5, Zn: 5 
                 Above 300 
                 Good 
                 4 
               
               
                 Example 30 
                 Sb: 5 
                 Above 300 
                 Good 
                 4 
               
               
                 Example 31 
                 Ag: 3.5 
                 Above 300 
                 Good 
                 4 
               
               
                 Example 32 
                 Ag: 3.5, In: 5 
                 Above 300 
                 Good 
                 4 
               
               
                   
               
             
          
         
       
     
     As shown by Examples 17 to 32 in Tables 3-1 to 4-2, a residual film thickness of the plated film  26   a  was above 3 μm both with a 1 wt % B content and a 10 wt % B content. The bonded states were good. 
     As described above, according to the present embodiment, the plated film of Ni film formed by electroless plating are subjected to the heat treatment, and the B contained in such plated films can prevent the Ni of the plated films from combining with the Sn in the solder bump. Electronic devices having the bonds in good states can be provided. 
     [A Third Embodiment] 
     The solder bonding method according to a third embodiment of the present invention will be explained with reference to FIG.  3 . FIG. 3 is a sectional view explaining the solder bonding method according to the present embodiment. The same members of the present embodiment as those of the solder bonding method according to the first or the second embodiment shown in FIG. 1 or  2  are represented by the same reference numbers not to repeat or to simplify their explanation. 
     The solder bonding method according to the present embodiment is characterized mainly in that a plated film  14   b  containing Ni, W (tungsten) and P is formed on a Ti film  12  by electroless plating, and a plated film  26   b  containing Ni and B is formed on a Cu film  24  by electroless plating. 
     The plated films  14   b ,  26   b  are subjected to the heat treatment as in the first embodiment. Tungsten (W) contents the plated films  14   b ,  26   b  may be, e.g., 5-15 wt %. Phosphorus (P) contents of the plated films  14   b ,  26   b  may be, e.g., 5-10 wt %. The W and P contained in the plated films  14   b ,  26   b  can prevent the Ni of the plated films  14   b ,  26   b  from combining with the Sn in the solder bump. Accordingly, the present embodiment can provide electronic devices having the bonds in good states. 
     (Results of Reliability Evaluation Test) 
     Next, results of a reliability evaluation test made on electronic devices fabricated by using the above-described solder bonding method will be explained with reference to tables 5-1 to 6-2. Tables 5-1 to 6-2 show the results of the reliability evaluation test made on the electronic devices fabricated by using the solder bonding method according to the present embodiment. 
     As in the first embodiment, a diameter of the solder bump was 70-100 μm, and a pitch of the solder bump with respect to an adjacent one was 150-200 μm. Film thicknesses of the plated films  14   b ,  26   b  were 6 μm as in the first embodiment. The reliability evaluation test was the same as in the first embodiment. A P content was 5 wt %. 
     
       
         
               
               
               
               
             
               
               
               
               
             
           
               
                   
                 TABLE 5-1 
               
               
                   
                   
               
               
                   
                  W Content in 
                 Heat Treatment 
                 Heat Treatment 
               
               
                   
                 Plated Film 
                 Temperature 
                 Time 
               
               
                   
                 (wt %) 
                 (° C.) 
                 (Hours) 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                  Example 33 
                   5 
                 400 
                 1 
               
               
                 Example 34 
                  5 
                 400 
                 1 
               
               
                 Example 35 
                  5 
                 400 
                 1 
               
               
                 Example 36 
                  5 
                 400 
                 1 
               
               
                 Example 37 
                 10 
                 400 
                 1 
               
               
                 Example 38 
                 10 
                 400 
                 1 
               
               
                 Example 39 
                 10 
                 400 
                 1 
               
               
                 Example 40 
                 10 
                 400 
                 1 
               
               
                   
               
             
          
         
       
     
     
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                 TABLE 5-2 
               
               
                   
                   
               
               
                   
                  Content of 
                   
                   
                 Residual 
               
               
                   
                 Elements other 
                   
                   
                 Film 
               
               
                   
                 than Sn in Solder 
                 Heat Cycle 
                 Bonded 
                 Thickness 
               
               
                   
                 Bump (wt %) 
                 (Cycle) 
                 State 
                 (μm) 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                  Example 33 
                  Ag: 3.5, Zn: 5 
                 Above 300 
                 Good 
                 4 
               
               
                 Example 34 
                 Sb: 5 
                 Above 300 
                 Good 
                 4 
               
               
                 Example 35 
                 Ag: 3.5 
                 Above 300 
                 Good 
                 4 
               
               
                 Example 36 
                 Ag: 3.5, In: 5 
                 Above 300 
                 Good 
                 4 
               
               
                 Example 37 
                 Ag: 3.5, Zn: 5 
                 Above 300 
                 Good 
                 3 
               
               
                 Example 38 
                 Sb: 5 
                 Above 300 
                 Good 
                 3 
               
               
                 Example 39 
                 Ag:3.5 
                 Above 300 
                 Good 
                 3 
               
               
                 Example 40 
                 Ag:3.5, In:5 
                 Above 300 
                 Good 
                 3 
               
               
                   
               
             
          
         
       
     
     
       
         
               
               
               
               
             
               
               
               
               
             
           
               
                 TABLE 6-1 
               
               
                   
               
               
                   
                 W Content in 
                 Heat Treatment 
                 Heat Treatment 
               
               
                   
                 Plated Film 
                 Temperature 
                 Time 
               
               
                   
                 (wt %) 
                 (° C.) 
                 (Hours) 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 Example 41 
                 5 
                 600 
                 1 
               
               
                 Example 42 
                 5 
                 600 
                 1 
               
               
                 Example 43 
                 5 
                 600 
                 1 
               
               
                 Example 44 
                 5 
                 600 
                 1 
               
               
                 Example 45 
                 10 
                 600 
                 1 
               
               
                 Example 46 
                 10 
                 600 
                 1 
               
               
                 Example 47 
                 10 
                 600 
                 1 
               
               
                 Example 48 
                 10 
                 600 
                 1 
               
               
                   
               
             
          
         
       
     
     
       
         
               
               
               
               
               
             
           
               
                 TABLE 6-2 
               
               
                   
               
               
                   
                 Content of Elements 
                   
                   
                 Residual 
               
               
                   
                 other than Sn in 
                   
                   
                 Film 
               
               
                   
                 Solder Bump 
                 Heat Cycle 
                 Bonded 
                 Thickness 
               
               
                   
                 (wt %) 
                 (Cycle) 
                 State 
                 (μm) 
               
               
                   
               
             
             
               
                 Example 41 
                 Ag: 3.5, Zn: 5 
                 Above 300 
                 Good 
                 4 
               
               
                 Example 42 
                 Sb: 5 
                 Above 300 
                 Good 
                 4 
               
               
                 Example 43 
                 Ag: 3.5 
                 Above 300 
                 Good 
                 4 
               
               
                 Example 44 
                 Ag: 3.5, In: 5 
                 Above 300 
                 Good 
                 4 
               
               
                 Example 45 
                 Ag: 3.5, Zn: 5 
                 Above 300 
                 Good 
                 4 
               
               
                 Example 46 
                 Sb: 5 
                 Above 300 
                 Good 
                 4 
               
               
                 Example 47 
                 Ag: 3.5 
                 Above 300 
                 Good 
                 4 
               
               
                 Example 48 
                 Ag: 3.5, In: 5 
                 Above 300 
                 Good 
                 4 
               
               
                   
               
             
          
         
       
     
     As shown by Examples 33 to 48 in Tables 5-1 to 6-2, a residual film thickness of the plated films was above 3 μm both with a 5 wt % W content and a 10 wt % W content. The bonded states were good. 
     As described above, according to the present embodiment, the plated films of Ni films formed by electroless plating were subjected to the heat treatment, and the plated films contain W and P, whereby the Ni of the plated films can be prevented from combining with the Sn in the solder bump. Accordingly, electronic devices having the bonded states in good states can be fabricated. 
     [Modified Embodiments] 
     The present invention is not limited to the above-described embodiments and can cover other various modifications. 
     For example, a thickness of the plated films is not limited to the above-described thickness and may be suitably set so as to obtain required bonded states. 
     Contents of impurities, such as P, B, W, etc. contained in the plated films formed of Ni as a main component are not limited to those of the above-described embodiments and may be suitably set so as to prevent to a required extent, the Ni of the plated films from combining with the Sn in the solder bump. 
     In the above-described embodiments, the plated films contain impurities, such as P, etc., but impurities contained in the plated films are not limited to P, etc. The plated films may contain impurities other than P, etc. as long as the impurities can prevent the Ni of the plated films from combining with the Sn in the solder bump. 
     In the above-described embodiments, a heat treatment temperature was 400-600° C., and a heat treatment time was 0.5-2 hours. However, they are not limited to them and may be suitably set so that the plated films can have a required crystal state. 
     In the above-described embodiments, the solder bump was formed by dimple plating. However, the solder bump is not essentially formed by dimple plating and may be formed by, e.g., using a solder paste, vapor-depositing a solder alloy, or transfer. 
     The above-described embodiments exemplify cases that the circuit substrate and the semiconductor device are bonded to each other. The semiconductor device may be any semiconductor device, e.g., LSI or others. The above-described solder bonding method is applicable to fabrication of any electronic device, e.g., fabrication of multi-chip module, etc. 
     In the above-described embodiments, alumina substrates are used but are not essential. Any substrate, e.g., resin substrates, such as BT resin substrates, etc., glass epoxy substrates, AlN substrates, etc., may be used. 
     In the above-described embodiments, the plated films contain impurities, such as P, etc. However, the plated films may contain no impurity, such as P, etc. as long as the plated films are heat-treated, whereby the Ni of the plated films can be prevented, to a required extent, from combining with the Sn in the solder bump. 
     In the above-described embodiment, the plated films are heat-treated, but may not be heat-treated as long as the plated films contain impurities, such as P, etc., whereby the Ni of the plated films can be prevented, to a required extent, form combining with the Sn in the solder bump. 
     In the above-described embodiments, the plated films contain impurities, such as P, etc., and are heat-treated. However, the plated films may contain no impurities, such as P, etc. and may not be heat-treated as long as the plated films have sufficient thicknesses, whereby the bonded state can be good. 
     In the above-described embodiments, the plated films are formed by electroless plating but may not be essentially formed by electroless plating. The plated films may be formed by another film forming technique, such as electrolytic plating or others.