Electrolysis: processes – compositions used therein – and methods – Electrolytic coating – Depositing predominantly alloy coating
Reexamination Certificate
2000-09-22
2003-08-19
King, Roy (Department: 1742)
Electrolysis: processes, compositions used therein, and methods
Electrolytic coating
Depositing predominantly alloy coating
C205S253000
Reexamination Certificate
active
06607653
ABSTRACT:
TECHNICAL FIELD
The present invention relates to an alloy plating bath containing tin and copper, method for plating with an alloy containing tin and copper using said plating bath and an article provided with a plated coating by said method.
BACKGROUND ART
In recent years, the concern about the effect of lead on the human body and the environment and about the risk of whisker production by pure tin plating has been increasing the demand for the development of a lead-free solder plating bath.
A tin-silver alloy, a tin-bismuth alloy and the like have been studied as the lead-free solder. However, the plating bath of the tin-silver alloy easily decomposes, and the plated coating of the tin-bismuth alloy is prone to cracks. Therefore, these alloys are disadvantageous.
A tin-copper alloy forms a eutectic composition with a copper content of 1.3 mole %. Although the alloy has a relatively high soldering temperature because of its melting point of 227° C., it is unlikely to form cracks, excellent in soldering strength and less expensive than the tin-silver alloy or the like. For these reasons, the tin-copper alloy is a prospective lead-free solder.
Generally, electroplating with the alloy containing tin and copper is conducted while supplying tin(II) ions in the bath using a tin anode. However, since the copper salts contained in the bath have a standard electrode potential higher than the tin of the anode, chemical substitution occurs between the copper and tin. This may lead to the deposit of metallic copper on the anode. If the copper is deposited on the anode, the copper salt concentration in the bath is lowered and the bath composition changes. Therefore, the resulting plated coating of the tin-copper alloy tends to have inconstant composition. Particularly in the plating bath of the tin-copper alloy, the copper salt concentration in the bath is usually lower than the tin(II) salt concentration, and thus the change in the copper salt concentration greatly affect the composition of the coating.
Further, the composition of the plated coating of a tin-copper alloy, tin-copper-silver alloy, tin-copper-bismuth alloy and like alloys containing tin and copper tends to be dependent on cathodic current density. These alloys have the problem that when plating is carried out at a various current density ranging from high density to low density, the composition of the plated coating varies.
For example, in the tin-copper alloy plating, a tin-copper eutectic alloy having a low melting point can be obtained under the condition of a Cu content of 1.3 mole %. When the composition of the coating varies depending on the current density, it is not possible to constantly obtain the tin-copper alloy plated coating having the composition ratio which is suitable for the application.
Further, the plating bath containing tin and copper is likely to become turbid because of its unstability, unlike a tin plating bath, tin-lead alloy plating bath or the like. For example, the plating bath starts to become slightly turbid about one week after preparation, and the entire plating bath becomes turbid 1 month after the preparation.
The bath becomes turbid because divalent tin salt in the bath is oxidized to be tetravalent, thereby producing colloidal particles of tin oxide hydrate. Even the addition of an antioxidant can not completely prevent the bath from becoming turbid. Therefore, the Sn
2+
content in the bath may be considerably lowered, which greatly inhibits obtaining a plated coating of an alloy containing tin and copper which has constant composition.
As the plating bath containing tin and copper, for example, Japanese Examined Patent Publication No. 1996-13185 discloses a tin alloy plating bath comprising (a) Sn
2+
ion, (b) at least one metal ion selected from the group consisting of Ag
+
, Cu
2+
, In
3+
, Tl
+
and Zn
2+
and (c) a nonionic surfactant. Example 3 of this publication discloses a tin-copper alloy plating bath containing tin(II) methanesulfonate, copper methanesulfonate, methanesulfonic acid and ethylene oxide adduct of octyl phenol ethoxylate. Example 4 of the same publication discloses a tin-copper alloy plating bath containing tin(II) methanesulfonate, copper methanesulfonate, methanesulfonic acid and ethylene oxide adduct of laurylamine. According to this publication, the effects of these plating baths are that they can provide a low-melting-point plated coating similar to a tin-lead alloy coating without using lead; they impart good appearance and solderability to the plated coating; they facilitate the bath control; etc.
Meanwhile, Japanese Unexamined Patent Publication No. 1997-143786 discloses a silver alloy plating bath comprising (a) Ag
+
ion; (b) at least one metal ion selected from the group consisting of Sn
2+
, Cu
2+
, In
3+
, Tl
+
, Zn
2+
and Bi
3+
; (c) thiourea, acetyl thiourea, allylthiourea, trimethylthiourea and like thiourea compounds, thiazole compounds, dithiocarbamate compounds, thioglycol, thioglycolic acid, thiodiglycolic acid, &bgr;-thiodiglycol and like sulfur-containing compounds; and (d) a nonion surfactant. Example 5 of the same publication describes a silver-tin-copper alloy plating bath comprising silver methanesulfonate, tin(II) methanesulfonate, copper methanesulfonate, methanesulfonic acid, &bgr;-thiodiglycol, sodium N,N′-diethyldithiocarbamate and ethylene oxide adduct of lauryl ether. According to this publication, the effects of the plating bath are that a fine plated coating and high throwing power can be obtained; the bath control is facilitated; etc.
However, the above plating baths disclosed in Nos. 1997-143786 and 1997-143786 have not completely solved the aforementioned problems such as deposition of copper on the anode by substitution, turbidness of the bath, dependence of the coating composition on the current density, etc.
DISCLOSURE OF INVENTION
An primary object of the present invention is to provide a plating bath containing tin and copper, the bath being capable of preventing deposition of copper on a tin anode by substitution, and having low dependence of plated coating composition on current density, high bath stability and resistance to turbidness.
In view of the aforesaid problems of the plating bath containing tin and copper, the inventors of the present invention conducted extensive research and found out that the above object can be achieved by adding a specific sulfur-containing compound to an alloy plating bath containing tin and copper. The present invention was accomplished based on this finding.
The present invention provides the alloy plating bath containing tin and copper and an article provided with a plated coating using the plating bath mentioned in the following.
1. A tin-copper alloy plating bath comprising:
(A) a soluble tin(II) compound;
(B) a soluble copper compound; and
(C) at least one sulfur-containing compound selected from the group consisting of the following compounds (i)-(v):
(i) a thiourea compound
(ii) a mercaptan compound
(iii) an aliphatic sulfide compound represented by the following formula (1):
Re—Ra—[(X—Rb)
L
—(Y—Rc)
M
—(Z—Rd)
N
]—Rf (1)
(wherein the symbols represent the following:
M is an integer of 1-100, L and N are each 0 or an integer of 1-100; Y represents S or S—S, X and Z are the same or different and each represents O, S or S—S;
Ra represents C
1
-C
12
straight-chain or branched-chain alkylene or 2-hydroxypropylene;
Rb, Rc and Rd are the same or different and each represents methylene, ethylene, propylene, 2-hydroxypropylene, butylene, pentylene or hexylene;
in X—Rb, Y—Rc and Z—Rd, the position of each other is not limited and may be randomly positioned. When the bond X—Rb's, Y—Rc's or Z—Rd's are repeated, the X—Rb's, Y—Rc's or Z—Rd's may be composed of two or more kinds of bonds;
Re and Rf may be the same or different and each represents hydrogen, carboxyl, hydroxyl, alkyl, alkenyl, alkynyl, aralkyl, cycloalkyl, allyl, polycyclic cycloalkyl, aryl, pol
Nawafune Hidemi
Nishikawa Tetsuji
Obata Keigo
Takeuchi Takao
Tsuji Kiyotaka
Armstrong Westerman & Hattori, LLP
Daiwa Fine Chemicals Co., Ltd.
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