Sn-Cu alloy plating bath

Details Sn-Cu alloy plating bath Sn-Cu alloy plating bath

2000-10-05

2002-10-01

Nguyen, Nam (Department: 1741)

Electrolysis: processes, compositions used therein, and methods

Electrolytic coating

Depositing predominantly alloy coating

C205S253000, C205S254000

Reexamination Certificate

active

06458264

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an acidic Sn—Cu alloy plating bath. More particularly, the present invention relates to an acidic Sn—Cu alloy plating bath which can prevent the Cu substitution-deposition problem which is a serious problem in large scale industrial production.
2. Description of Prior Art
Conventionally, Sn—Cu alloy plating has been applied to bronze plating for decorating purposes. In recent years, however, the Sn—Cu alloy plating is attracting attention as a plating method which can be used in place of solder plating (Sn—Pb alloy plating).
As Sn—Cu alloy plating baths, a copper cyanide-alkaline stannate bath (Japanese Patent Application Laid-open No. 96936/1977), a pyrophosphoric acid bath (Japanese Patent Applications Laid-open No. 72196/1981 and No. 272394/1986), and a copper cyanide-copper pyrophosphate bath (Japanese Patent Application Laid-open No. 60091/1982) are known. In addition, an Sn—Cu alloy plating bath using an inorganic acid in an amount sufficient to maintain 2.0 or less pH has been disclosed as an acidic bath (Japanese Patent Application Laid-open No. 177987/1982).
Of these baths, the acidic bath appears to be more advantageous than other baths in industrial application due to the excellent current efficiency. However, actually the bath has a serious problem which renders the method difficult to be applied. Specifically, a problem with the acidic Sn—Cu alloy plating bath, which is not mentioned in the Japanese Patent Application Laid-open No. 177987/1982, is that copper deposits due to substitution when the plated material is made of a metal with an electric potential lower than copper such as iron and nickel. This is caused due to the presence of free Cu
+2
ion in the acidic Sn—Cu alloy plating bath and unduly impairs plating adhesion. Copper deposits also on the anode when Sn is used as the material of the anode, rendering the anode difficult to be dissolved and interfering with the plating bath stability. Moreover, SnO
2
and the like cause turbidity if Sn
2+
and Cu
2+
are present at the same time.
Use of a complexing agent may be one way of stabilizing Cu
2+
to overcome this type of problems. A complexing agent, however, impairs waste water processability, thus creating another problem of making processing of waste water difficult.
The subject to be solved in the present invention is therefore to provide an acidic Sn—Cu alloy plating bath which can prevent the Cu substitution-deposition problem and turbidity due to production of SnO
2
without using a complexing agent.
The inventors of the present invention have conducted extensive studies to solve the above-described problem and have found that the Cu substitution-deposition problem and turbidity of the plating solution can be prevented without using a complexing agent by adding a thiourea compound to an acidic solution comprising Sn, Cu, and an acid such as analkane sulfonic acid, alkanol sulfonic acid, sulfuric acid, or the like in an amount sufficient to solve Sn and Cu.
SUMMARY OF THE INVENTION
Specifically, an object of the present invention is to provide an acidic Sn—Cu alloy plating bath composition comprising: (a) Sn
2+
ions and Cu ions, (b) at least one acid selected from the group consisting of alkane sulfonic acids, alkanol sulfonic acids, and sulfuric acid, and (c) a thiourea compound.
Another object of the present invention is to provide an acidic Sn—Cu alloy plating bath composition comprising a nonionic surfactant in addition to the above-described components (a) to (c).
Still another object of the present invention is to provide an Sn—Cu alloy plating method comprising subjecting a material to be plated to cathodic electrolysis in either of the above-mentioned acidic Sn—Cu alloy plating bath compositions.
Other objects, features and advantages of the invention will hereinafter become more readily apparent from the following description.
DETAILED DESCRIPTION OF THE INVENTION AND PREFERRED EMBODIMENT
The acidic Sn—Cu alloy plating bath composition of the present invention (hereinafter called “Sn—Cu plating bath”) comprises (a) Sn
2+
ions and Cu
2+
ions, (b) at least one acid selected from the group consisting of alkane sulfonic acids, alkanol sulfonic acids, and sulfuric acid, and (c) a thiourea compound.
Of these components, the Sn
2+
ions and Cu
2+
ions which are the component (a) are made available at the initial time of preparation by providing oxides of these ions, such as tin (II) oxide and copper (II) oxide, and adding an anion of the component (b) to the oxides, or by dissolving the anion salt of the component (b) of Sn
2+
ion or Cu
2+
ion, for example, tin methane sulfonate, copper methane sulfonate, tin ethane sulfonate, copper ethane sulfonate, tin isopropanol sulfonate, copper isopropanol sulfonate, tin sulfate, or copper sulfate. During plating, the Sn
2+
ions and Cu
2+
ions are made available by the anode or the anion salt of Sn
2+
ion or Cu
2+
ion.
On the other hand, the acid selected from the group consisting of alkane sulfonic acids, alkanol sulfonic acids, and sulfuric acid, which is the component (b), is supplied as a free acid or as a salt of Sn
2+
ion or Cu
2+
ion which is the component (a).
As the thiourea compound which is the component (c), thiourea, diethyl thiourea, phenyl thiourea, allyl thiourea, acetyl thiourea, diphenyl thiourea, benzoyl thiourea, and the like can be given.
The respective amount of Sn
2+
ions and Cu
2+
ions contained in the Sn—Cu plating bath of the present invention as component (a) is preferably 0.5-20 wt % and 0.01-2 wt %, and particularly preferably 1-5 wt % and 0.02-0.2 wt %. The ratio of Sn
2+
ions and Cu
2+
ions may be varied according to the composition of the target alloy plating to be deposited. For instance, when an eutectic alloy plating consisting of about 99.3% of Sn and about 0.7% of Cu is desired, the ratio of Sn
2+
ions and Cu
2+
ions may be about 50:1 to 100:1.
The amount of alkane sulfonic acids, alkanol sulfonic acids, and sulfuric acid contained in the Sn—Cu plating bath as component (b) may be about 5-300 g/l, and preferably about 50-150 g/l.
The amount of a thiourea compound contained in the Sn—Cu plating bath as component (c) may be about 0.1-20 g/l, and preferably about 1-10 g/l. Because the thiourea compound has an action of interfering with deposition of Cu, the amount to be added is preferably increased when the Sn—Cu plating bath contains a large amount of Cu
2+
ions.
In addition to the above-described essential components, a nonionic surfactant may be added to the Sn—Cu plating bath as component (d). As specific examples of the component (d), surfactants containing any one of the compounds shown by the following formulas (1) to (4) as a major component can be given.
[Chemical Formula 1]
wherein R
1
represents a hydrogen atom or a residue obtained by excluding a hydrogen atom from the hydroxyl group of an aliphatic alcohol having 8-22 carbon atoms, phenol substituted with an alkyl group having 1-25 carbon atoms, &bgr;-naphtol substituted with an alkyl group having 1-25 carbon atoms, alkoxylated phosphoric acid having 1-25 carbon atoms, sorbitan estrified with a fatty acid having 8-22 carbon atoms, or styrenated phenol in which the hydrogen atom may be substituted by an alkyl group having 1-4 carbon atoms or a phenyl group, R
2
represents an alkyl group having 8-18 carbon atoms, R
3
and R
4
individually represent a hydrogen atom or an alkyl group having 1-5 carbon atoms, A represents —CH
2
CH
2
O—, B represents —CH
2
CH(CH
3
)O—, m
1
and n
1
are individually an integer from 0 to 30, m
2
, n
2
, m
3
, and n
3
are individually an integer from 0 to 40, and m
4
and n
4
are individually an integer from 0 to 20, provided that m
1
and n
1
, m
2
and n
2
, m
3
and n
3
, or m
4
and n
4
are not simultaneously zero, m
1
to m
4
and n
1
to n
4
individually indicate the total number of substituents

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