Electrolysis: processes – compositions used therein – and methods – Electrolytic coating – Coating selected area
Reexamination Certificate
2000-05-10
2001-09-11
Dawson, Robert (Department: 1712)
Electrolysis: processes, compositions used therein, and methods
Electrolytic coating
Coating selected area
C205S143000, C205S145000
Reexamination Certificate
active
06287444
ABSTRACT:
TECHNICAL FIELD
The present invention pertains to a method for producing highly efficiently a very small metal ball such as a Cu ball having an outer diameter of not greater than 1 mm, and relates to a method for obtaining a high-precision very small metal ball having an outer diameter of not greater than 1 mm by cutting a metal wire of Cu or the like having a diameter not greater than 0.3 mm into a predetermined length, melting and solidifying the metal wires to form starting metal pieces, and plating the surfaces of the starting metal pieces with a metal such as Cu, or an alloy such as solder by means of cyclic repetition of forward rotation/reverse rotation using a plating tank which is turnable in a horizontal direction.
BACKGROUND ART
Conventionally, the very small metal balls used as bump cores in semiconductor packages of BGA (Ball Grid Array) type have outer diameter of about 0.1 mm~1.0 mm and consist of solder of predetermined composition as the material; additionally, there has recently been proposed in consideration of electrical characteristics and mechanical characteristics a chip carrier of a core material of a metal ball of Kovar (Ni—Co—Fe alloy), Cu, 42Ni—Fe alloy, or the like coated with a soldering material (Japanese Publication of Unexamined Patent Application S62-112355).
As methods for producing the aforementioned very small metal balls there have been proposed a so-called drip-into-liquid method wherein molten metal is dripped into a liquid of predetermined temperature, rendered spherical through the surface tension of the molten metal per se, and solidified as-is (Japanese Publication of Unexamined Patent Application H7-252510); so-called mechanical plastic working methods such as forming with a mold or the like (Japanese Publication of Unexamined Patent Application H4-354808); a vibrating heating method in which a metal particle or metal piece is arranged on a flat board in a non-oxidizing atmosphere, heated and melted while applying vibration thereto, rendered spherical through the surface tension thereof, and solidified as-is (Japanese Patent Publication H2-50961), and the like.
In the aforementioned drip-into-liquid method and mechanical plastic working methods, it is possible to produce very small balls of relatively large diameter, but all have poor workability and are unsuited to mass production. With the drip-into-liquid method in particular there is large deviation in dimensional precision, and to ensure very small balls having the dimensional precision required as bumps for BGA type semiconductor packages, it is necessary to separate metal balls of the desired diameter by classification, so yield is extremely poor and this is a factor contributing to increased cost.
Further, in mechanical plastic working methods, various die are required depending on ball diameter, and in addition to the aforementioned workability this is a factor contributing to increased cost.
In methods of obtaining metal balls by heating and melting in a jig as well, deviation of the diameter of the metal balls occurs as a result of deviation in individual metal wire piece unit weight; for example, in the case of a soft metal such as solder, it is difficult to obtain metal balls having identical unit weight, and therefore deviation in diameter is generated so that, like the drip-into-liquid method, there is the problem of extremely poor yield.
SUMMARY OF THE INVENTION
It is an object of the present invention to solve the aforementioned problems and provide a method for producing a very small metal ball affording highly efficient production of a very small metal ball having exceptional mass production qualities effective as a bump for semiconductor packages of BGA (Ball Grid Array) type as well as high dimensional precision, and particularly for a very small metal ball having an outer diameter of not greater than 1 mm.
As a result of various studies with the object of a method for efficient production of a very small metal ball such as a Cu ball having an outer diameter of not greater than 1 mm, the inventors discovered that by cutting a metal wire of Cu or the like having a diameter not greater than 0.3 mm into a predetermined length, melting and solidifying the metal wires to form starting metal pieces, and plating the surfaces of the starting metal pieces with a metal such as Cu, or an alloy such as solder by means of cyclic repetition of normal rotation/reverse rotation using a plating tank which is turnable in a horizontal direction, there is obtained a high-precision very small metal ball having an outer diameter of not greater than 1 mm, and perfected this invention.
Specifically, this invention is a method for producing a very small metal ball using, for example, a horizontal turning plating tank of a design wherein a plating solution charged into a tank having a cathode arranged at the inner peripheral portion within a tank turnable in a horizontal direction about a vertical shaft and an anode arranged at the center inside the tank is discharged from the turning peripheral portion; the method comprising the steps of cutting a metal wire having a diameter not greater than 0.3 mm into a predetermined length, for example, a ratio LID of metal wire length L and diameter D of 0.7~1.5; melting and solidifying the metal wires to obtain starting metal pieces, charging the starting metal pieces into the aforementioned tank; and conducting electroplating with a desired metal or alloy by inducing electrical contact of the starting metal pieces with the cathode at the inner peripheral portion within the tank under plating bath conditions, for example, of an ion concentration of 1~70 g/l and current density of 0.05~10 A/dm
2
, while making the plating tank to periodically repeat forward rotation in a desired direction at 50~800 rpm followed by reverse rotation to produce very small metal balls having an outer diameter of not greater than 1 mm.
REFERENCES:
patent: 5487824 (1996-01-01), Griego
Kikui Fumiaki
Nishiuchi Takeshi
Yoshimura Kohshi
Dawson Robert
Dykema Gossett PLLC
Feely Michael J
Sumitomo Special Metals Co. Ltd.
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