Specialized metallurgical processes – compositions for use therei – Processes – Spheroidizing or rounding of existing solid metal particles
Reexamination Certificate
1999-11-24
2001-09-18
Wyszomierski, George (Department: 1742)
Specialized metallurgical processes, compositions for use therei
Processes
Spheroidizing or rounding of existing solid metal particles
C075S363000, C164S046000
Reexamination Certificate
active
06290746
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention pertains to a method of producing a metal ball of Cu, etc., that is effective as a bump for semiconductor packages by heating and melting and then cooling a small metal piece, and a semiconductor package that uses the same ball. The present invention pertains to a method with which good relebiity from the jig during cooling after heating and melting and dimensional accuracy of the metal ball are guaranteed and productivity s markedly improved by, for instance, placing a metal piece on the jig with a layer of fine powder having low wettability with the metal piece in between.
2. Description of the Prior Art
Minute metal balls of Cu, Al, Ni, etc., are used as bumps for BGA (ball grid array)-type semiconductor packages, ornamental materials, wiring electrodes, relay contacts, etc. The method whereby a metal piece cut from a metal wire with dimensions similar to the intended diameter is heated and melted on a flat jig and then made into a sphere under surface tension (Japanese Patent Application No. 58-113586 (Japanese Patent Laid-Open No. 60-5804)) is a method of producing the same.
The method whereby metal pieces are individually shook into and arranged in a plurality of holes formed in a jig and these metal pieces are heated with the jig at a temperature that is the melting point or higher in a non-oxidizing atmosphere and melted is disclosed as a method of increasing sphericity further (WO 95/24113).
Moreover, the method whereby metal pieces are buried in powder that is mainly graphite powder and heated and melted to produce metal balls is presented as a simple method for efficient production (Japanese Patent Laid-Open No. 10-8113).
Methods that use the above-mentioned jig are excellent in terms of handling performance because large quantities of metal pieces can be handled at the same time. However, not only is dimensional accuracy inferior when the surface of the metal ball that comes into contact with the jig is flat, but releasability of the metal ball after melting also becomes poor with degradation of the jig, life of the jig is curtailed, and there is a reduction in production efficiency. This tendency is particularly obvious with a minute diameter of a diameter of 0.4 mm or less.
The surface state is poor because metal pieces are buried in powder by the above-mentioned production methods that use fine powder. Moreover, there are cases where dimensional accuracy is an error of ±0.1 m in terms of the desired dimensions.
SUMMARY OF THE INVENTION
The purpose of the present invention is to improve releasability from the jig of the method wherein minute metal balls are made by heating and melting and then cooling metal pieces of specific dimensions. The purpose of the present invention is to present a method of producing a metal ball with very good dimensional accuracy and sphericity, even though diameter is minute, and a semiconductor package that uses the same ball.
The inventors performed various studies of methods of using fine powder for the purpose of improving releasability from the jig after melting and cooling and improving sphericity. As a result, they discovered that releasability from the jig after melting and cooling is extremely good and long-term use of the jig becomes possible by the method wherein a metal piece is placed on a jig with a layer of fine powder having low wettability with this metal piece material in between, or placing the metal piece on the jig or shaking and arranging individual metal pieces in holes in the same after a layer of this fine powder is made to adhere to the surface of the metal piece. Moreover, the inventors completed the present invention upon discovering that the abovementioned method markedly improves dimensional accuracy and ephericty of the metal ball because the layer of fine powder on the jig or the surface of the metal piece does not prevent spheroidizing under surface tension when the metal piece is melted and there is no deterioration of surface properties of the metal ball that is obtained.
That is, the method of producing a metal ball of the present invention comprises the steps of placing a metal piece of Cu, Al, Ni, etc., at the fixed position on a heat treatment jig with a layer of fine powder, which has an average particle deter of 2 &mgr;m to 10 &mgr;m and which consists of BN, AlN, or C having low wettability with the metal piece material, at a thickness of 5 &mgr;m to 50 &mgr;m in between, or with a layer of this fine powder made to adhere to the surface of the metal piece, heating and melting the metal piece, and then cooling the metal piece so that it solidifies into a spherical shape.
The production method of the present invention provides extremely good releasability between the metal ball and jig after melting and cooling and makes long-term use of the jig possible by using means wherein a metal piece is placed on the jig with a layer of fine powder having low wettability with this metal piece in between, or wherein a layer of this fine powder is made to adhere to the metal piece surface. Moreover, the present invention makes possible marked improvement of dimensional accuracy and sphericity of the metal ball that is obtained because the layer of fine powder on the jig or at the metal piece surface does not prevent spheroidizng of the metal piece under surface tension and there is no deterioration of the surface properties of the same when the metal piece is melted.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
Either metals or alloys can be used as the material from which the metal ball is made in the present invention.
Cu, Al, Ni and their alloys are ideal for use as bumps of semiconductor packages, ornamental materials, wiring electrodes, relay contacts, etc.
The metal piece is made by determining the dimensions of a piece in accordance with the intended volume of the Cu ball and cutting a piece from a Cu wire Cu wire. In order to improve productivity, the relative L/D of the wire material outer diameter and cut length L with which spheroidizing is most efficient can be set in accordance with the jig that is used, the hole shape of the jig, etc.
A powder material having low wettability with the metal piece material that is used can be selected as needed in accordance with this material for the fine powder of the present invention. For instance, BN, AlN, and C are desirable as the fine powder with low wettability with Cu, Al, Ni or their alloys, etc. The average diameter of the abovementioned fine powder is preferably within a range of 2 &mgr;m to 10 &mgr;m. The reason for this is that if it is less than 2 &mgr;m, there will be a reduction in fluidity of the powder and a layer cannot be formed on the jig or the surface of the metal ball and if it exceeds 10 &mgr;m, surface properties of the metal ball will change for the worse.
The fine powder is used in the form of a thin film layer with respect to the metal piece in any case. In the concrete, a layer of the fine powder of 5 &mgr;m to 50 &mgr;m is spread on the jig or Wide the plurality of indentations in the jig and the metal piece is then placed on top, or a layer of the fine powder of 5 &mgr;m to 50 &mgr;m is made to adhere to the surface of the metal piece.
The reason for sing of layer of fine powder with a thickness of 5 &mgr;m to 50 &mgr;m in the present invention is that if the thickness of the layer of fine powder is less than 5 &mgr;m, there will be scatter in the effects of the powder in terms of improving releasability from the jig. Moreover, if it exceeds 50 &mgr;m, it will be difficult to make the fine powder adhere to the surface of a wire material obtained by conventional methods and at the same time, there will be an extreme drop in thermal conductivity and therefore, the cooling temperature will be delayed and the state of the surface of the metal ball will change for the worse.
The following various methods can be used as a method of using a fine powder.
The method wherein after the fine powder has been spread on the jig to form the above-mentioned laye
Dykema Gossett PLLC
Sumitomo Special Metals Co. Ltd.
Wyszomierski George
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