Process for producing a solder preform having high-melting metal particles dispersed therein
[Problems] A conventional process for producing a solder preform in which a predetermined amount of high-melting metal particles are directly put into molten solder and stirred, requires a long time for dispersing the high-melting metal particles by stirring. Therefore, in the conventional method for producing a solder preform, dissolution of the high-melting metal particles into the molten solder occurred during stirring, and their particle diameters became small. If a semiconductor chip and a substrate are soldered with a solder preform containing metal particles having such decreased diameters, the space between portions being soldered becomes narrow, and a sufficient bonding strength is not obtained. [Means for Solving the Problems] In the present invention, a premixed master alloy having a higher proportion of the high-melting metal particles in solder is first prepared, then the premixed master alloy is put into molten solder to disperse the high-melting metal particles. As a result, the high-melting metal particles can be uniformly dispersed in solder in a short length of time. Accordingly, a solder preform which is obtained by the process for producing a solder preform according to the present invention can maintain a predetermined clearance between portions being soldered, and a sufficient bonding strength is obtained.
1. A process for producing a solder preform having high-melting metal particles dispersed therein comprising:
preparing a premixed master alloy having high-melting metal particles dispersed in a solder such that the proportion of the high-melting metal particles is 5-30 mass percent based on the amount of the solder;
placing the premixed master alloy into a first molten solder in an amount such that the proportion of the high-melting metal particles is 0.1-3 mass percent based on the first molten solder followed by stirring;
casting the first molten solder having the high-melting metal particles mixed therein into a mold to form a billet; and
working the billet to form a solder sheet and then forming a solder preform from the solder sheet.
2. A process as claimed in claim 1 including subjecting the high-melting metal particles to surface treatment prior to preparing the premixed master alloy in order to make the surfaces of the high-melting metal particles easily wettable by the first molten solder.
3. A process as claimed in claim 1 including preparing the premixed master alloy by placing a mixture of a flux and the high-melting metal particles into a second molten solder.
4. A process as claimed in claim 3 wherein preparing the premixed master alloy includes stirring the second molten solder after placing the mixture into the second molten solder to disperse the high-melting metal particles in the second molten solder, and solidifying the second molten solder with the high-melting metal particles dispersed in the second molten solder in an as-stirred state.
5. A process as claimed in claim 3 wherein the first molten solder and the second molten solder have the same composition.
6. A process as claimed in claim 1 including preparing the premixed master alloy by heating a mixture of a flux, the high-melting metal particles, and solder powder to melt the solder powder.
7. A process as claimed in claim 1 wherein forming the billet comprises solidifying the first molten solder in the mold with the high-melting metal particles dispersed in the first molten solder in an as-stirred state.
8. A process as claimed in claim 1 including solidifying the premixed master alloy prior to placing it into the first molten solder.
9. A process as claimed in claim 1 wherein preparing the premixed master alloy comprises dispersing the high-melting metal particles in a second molten solder, and then solidifying the second molten solder at a rate such that there is no substantial decrease in the diameter of the high-melting metal particles.
10. A process as claimed in claim 1 wherein preparing the premixed master alloy comprises dispersing the high-melting metal particles in a second molten solder by stirring, and then solidifying the second molten solder with the high-melting metal particles dispersed in the second molten solder in an as-stirred state.
11. A process for producing a solder preform having high-melting metal particles dispersed therein comprising:
preparing by one of the following methods (i) and (ii) a premixed master alloy having high-melting metal particles dispersed in a solder having a prescribed composition such that the proportion of the high-melting metal particles is 5-30 mass percent based on the amount of the solder;
(i) heating a mixture of a flux, a solder powder having the prescribed composition, and the high-melting metal particles to melt the solder powder; or
(ii) adding a mixture of a flux and the high-melting metal particles to a first molten solder having the prescribed composition;
incorporating the premixed master alloy into a second molten solder in an amount such that the proportion of the high-melting metal particles is 0.1-3 mass percent based on the second molten solder following by stirring;
casting the second molten solder having the high-melting metal particles mixed therein into a mold to form a billet; and
working the billet to form a solder sheet and then forming a solder preform from the solder sheet.