Joint structure, joining method, wiring board and method for producing the same

1. A joint structure, comprising:

a conductive member containing copper as a major component thereof;

an electrode member containing copper as a major component thereof; and

a joint portion through which the conductive member and the electrode member are joined together, the joint portion consisting of a central part integrally formed with both the conductive member and the electrode member and an outer circumference part integrally formed with the central part so as to surround the central part, the outer circumference part being integrally formed with only the central part and the electrode member, wherein

both the central part and the outer circumference part of the joint portion are formed of a copper-tin alloy obtained by melting a brazing material containing tin as a major component thereof and containing substantially no copper, and diffusing copper atoms of the copper contained in the conductive member and the electrode member into the brazing material;

a concentration of copper atoms in the central part is higher than that in the outer circumference part and is 40 to 80 atom %; and

in the case where the concentration of the copper atoms in the central part is defined as S1, and the concentration of the copper atoms in the outer circumference part is defined as S2, a difference between S1 and S2 (S1−S2) is 10 to 30 atom %.

2. The joint structure as claimed in claim 1 , wherein the conductive member is a protruding electrode.

3. The joint structure as claimed in claim 1 , wherein the brazing material further contains silver in addition to the tin.

4. A wiring board comprising the joint structure defined by claim 1 .

5. A method of manufacturing the joint structure defined by claim 1 ,

wherein the joint portion is formed by melting the brazing material so that the brazing material is transformed into a copper-tin alloy by controlling a molten state of the brazing material.

6. The method as claimed in claim 5 , wherein in the molten state, the brazing material is maintained at a temperature higher than a melting point thereof, and a difference between the temperature and the melting point thereof is 25° C. or higher.

7. The method as claimed in claim 5 , wherein in the molten state, the brazing material is maintained at a temperature of a melting point thereof or higher, for 60 seconds or more.

8. A method of manufacturing the joint structure defined by claim 1 , the method comprising:

forming a fillet through which the conductive member and the electrode member are joined together by melting the brazing material so that the brazing material is transformed into the fillet;

forming a film shaped alloy part within the fillet by heating the fillet so that copper atoms of the copper contained in the conductive member and the electrode member are diffused into the fillet to allow the tin to be transformed into a copper-tin alloy, wherein a concentration of copper atoms contained in the copper-tin alloy of the film shaped alloy part is 15 atom % or less; and

forming island shaped alloy parts by further heating the fillet so that copper atoms of the copper contained in the conductive member and the electrode member are further diffused into the fillet to separate the film shaped alloy part into the island shaped alloy parts, wherein a concentration of copper atoms contained in the copper-tin alloy of each of the island shaped alloy parts is 15 atom % or less.

9. The method as claimed in claim 8 , further comprising forming the joint portion by further heating the fillet so that copper atoms of the copper contained in the conductive member and the electrode member are further diffused into the fillet to eliminate the island shaped alloy parts.

10. The method as claimed in claim 8 , wherein in the film shaped alloy part forming step, the fillet is brought into a molten state.

11. The method as claimed in claim 8 , wherein in the island shaped alloy parts forming step, the fillet is brought into a molten state.

12. The method as claimed in claim 9 , wherein in the joint portion forming step, the fillet is brought into a molten state.

13. A method of manufacturing a wiring board comprising joining a first substrate and a second substrate using the joint structure defined by claim 1 , such that said first substrate is provided with said conductive member, and said second substrate is provided with said electrode.