Electrolysis: processes – compositions used therein – and methods – Electrolytic coating – Depositing predominantly alloy coating
Reexamination Certificate
2000-02-03
2002-12-31
Moore, Margaret G. (Department: 1712)
Electrolysis: processes, compositions used therein, and methods
Electrolytic coating
Depositing predominantly alloy coating
C205S238000
Reexamination Certificate
active
06500327
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an Sn—Bi alloy plating bath, and more particularly to an Sn-Bi alloy plating bath which does not erode a piece to be plated, and has a high stability.
2. Description of the Related Art
In the electronic industrial field, Sn—Pb alloy platings have been widely used for soldering electrodes. In recent years, there have been anxieties about the influence of the Pb contained in the Sn—Pb alloy platings which may be exerted over the environment. Sn alloy platings which to not contain Pb have been demanded. Therefore, greater attention has been paid to Sn—Bi alloy platings, which have a low melting point and excellent soldering properties.
Many of Sn—Bi alloy plating baths have a strong acidity, namely, pH 1.0 or lower, in order to dissolve large amounts of bismuth. Since a large part of the electronic components pieces to be plated are composites containing ceramics, glass, ferrite, and so forth, there has been the problem that the electronic components become eroded by such high strong acidic baths, causing the deterioration of their characteristics.
For the purpose of improving the problem of the eroding properties, Japanese Unexamined Patent Publication No. 6-340994 and Japanese Unexamined Patent Publication No. 7-138782 disclose Sn—Bi alloy plating baths containing various complexing agents and having a pH of 2.0-9.0. Bismuth ions and tin ions are stabilized in the baths by addition of the complexing agents. As a result, plating baths within the range of from weak acidity to neutral are realized. However, these plating baths have problems of stability, and should be improved further to be used industrially.
SUMMARY OF THE INVENTION
The present invention is directed to a Sn—Bi alloy plating bath which is stable enough to use continuously in the electronic industrial field and a method of plating using the Sn—Bi alloy plating bath. The Sn—Bi alloy plating bath has a pH of about 2.0 to 9.0 and comprises Bi
3+
ions, Sn
2+
ions, a complexing agent (I) and a complexing agent (II).
The complexing agent (I) is selected from the group consisting of (a) aliphatic dicarboxylic acids having alkyl groups of 1-3 carbon atoms, (b) aliphatic hydroxymonocarboxylic acids having alkyl groups of 1-3 carbon atoms, (c) aliphatic hydroxypolycarboxylic acids having alkyl groups of 1-4 carbon atoms, (d) monosaccharides, polyhydroxycarboxylic acids produced by partially oxidizing the monosaccharides, and their cyclic ester compounds, and (e) condensed phosphoric acids.
The complexing agent (II) is selected from the group consisting of (s) ethylenediaminetetraacetic acid (EDTA), (t) nitrilotriacetic acid (NTA), and (u) trans-1,2-cyclohexanediaminetetraacetic acid (CyDTA).
The Sn—Bi alloy plating bath has a pH of about 2.0 to 9.0 and comprises Bi
3+
ions, Sn
2+
ions, complexing agent (I) and complexing agent (II).
Preferably, the concentration ratio of complexing agent (II) in mol/l to the Bi
3+
ions in mol/l is at least about 10, the concentration ratio of complexing agent (II) in mol/l to the Sn
2+
ions in mol/l is at least about 1, and the concentration ratio of complexing agent (I) in mol/l to the Sn
2+
ions in mol/l is at least about 0.1.
According to the present invention, electronic components pieces made of ceramics, glass, ferrite or the like, can be plated at a high cathode current density without eroding the electronic components. The plating bath of the present invention has a high bath stability and can be used for a long time without the bath decomposition occurring.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
As a result of the intensive examination by the inventors of the present invention, it has been found that the stability of a bath in the weak acidic range can be remarkably enhanced by adding to the bath a complexing agent (I) selected from the below-described (a) through (e) and a complexing agent (II) selected from the aminocarboxlic acids of the below-described (s) through (u) as complexing agents for the plating bath. The practical application of the Sn—Bi alloy plating bath enables avoidance of eroding electronic component pieces to be plated made of ceramics, glass, ferrite or the like, and it can be used at a relatively high cathode current density and has an excellent bath-stability.
For complexing agent (I), (a) aliphatic dicarboxylic acids having alkyl groups of 1-3 carbon atoms, (b) aliphatic hydroxymonocarboxylic acids having alkyl groups of 1-3 carbon atoms, (c) aliphatic hydroxypolycarboxylic acids having alkyl groups of 1-4 carbon atoms, (d) monosaccharides, polyhydroxycarboxylic acids produced by partially oxidizing the monosaccharides, and their cyclic ester compounds, and (e) condensed phosphoric acids can be employed.
For complexing agent (II), (s) ethylenediaminetetraacetic acid (EDTA), (t) nitrilotriacetic acid (NTA), and (u) trans-1,2-cyclohexanediaminetetraacetic acid (CyDTA) can be used.
Preferable examples of (a)-(e) as the complexing agent (I) are described below. As the aliphatic dicarboxylic acids (a) having alkyl groups of 1-3 carbon atoms, malonic acid, succinic acid or the like; as the aliphatic hydroxymonocarboxylic acids (b) having alkyl groups of 1-3 carbon atoms, glycolic acid, lactic acid or the like; as the aliphatic hydroxypolycarboxylic acids (c) having alkyl groups 1-4 carbon atoms, citric acid, tartaric acid, malic acid or the like; as the monosaccharides, the polyhydroxycarboxylic acids produced by partially oxidizing the monosaccharide, and their cyclic ester compounds (d), gluconic acid, glcoheptic acid, &dgr;-gluconic lactone, or the like; and as the condensed phosphoric acids (e), pyrophosphoric acid, tripolyphosphoric acid or the like, are exemplified.
In the present plating bath, preferably, the concentration ratio of complexing agent (II) (mol/l)/Bi
3+
(mol/l) is at least about 10, the concentration ratio of complexing agent (II) (mol/l)/Sn
2+
(mol/l) is at least about 1, and the concentration ratio of complexing agent (I) (mol/l)/Sn
2+
(mol/l) is at least about 0.1. With the above-described concentration ratios, a plating bath which has a high bath stability and which can be used at a high current density can be realized.
The standard electrode potential (Bi
3+
/Bi E
0
=+0.215 V) based on the standard hydrogen electrode with respect of the oxidation of Bi, is nobler than the standard electrode potential (Sn
4+
/Sn
2+
E
0
=0.154 V) based on the standard hydrogen electrode at which Sn
2+
is oxidized to Sn
4+
. Therefore, in a Sn—Bi alloy plating bath, Bi
3+
is reduced with Sn
2+
so that the decomposition of the bath, such as the deposition of Bi, readily occurs. Accordingly, it is important for the purpose of stabilizing the bath to select the kinds and ratios of complex ions to be in the bath. The order of magnitude of the complex stability constants between Sn and Bi with the complexing agent (I) and the complexing agent (II) used in this invention is
complexing agent (II)—Bi> complexing agent (II)—Sn>>
complexing agent(1)—Bi> complexing agent (I)—Sn.
The ratios of the respective complex ions to be produced in the bath are determined by this relationship between the magnitudes of the complex stability constants, and the concentration ratios of the respective metals to the complexing agents. A complex having a higher complex stability constant is formed precedently, and the formed complex has a higher stability.
In the composition of the plating bath of the present invention, substantially the total amount of Bi
3+
forms a complex with complexing agent (II) precedently. The complexing agent (II) remaining, not coordinated to Bi
3+
, then forms a complex with Sn
2+
. The Sn
2+
remaining and not forming the complex with complexing agent (II) then produces a complex with the complexing agent (I). Accordingly, three kinds of complexes, namely, the complexes of complexing agent (II) with Bi, complexing agen
Hamaji Yukio
Kunishi Tatsuo
Saitoh Junichi
Feely Michael J
Moore Margaret G.
Murata Manufacturing Co. Ltd.
Ostrolenk Faber Gerb & Soffen, LLP
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