CA1269344A - Tin-lead alloy plating bath - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
A tin-lead alloy plating bath based on a principal plating bath comprising an alkanesulfonic or alkanolsulfonic acid and both bivalent tin and lead salts thereof is charac-terized by the addition of a guanamine compound having the general formula wherein R1 and R2, which may be the same or different, represent each a hydrogen atom, C1-18 straight- or branched-chain alkyl radical, C1-18 straight- or branched-chain alkoxy-lower alkyl radical, or a C3-7 cycloalkyl radical, and R1 and R2 may combine with the adjoining nitrogen atom to form a piperidine, morpholine or piperazine cycle, and A
represents a lower alkylene radical.

Description

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BAC~ÇROUND OF THE INVENTIQN

This invention relates to a sulfonic acid bath for tin-lead alloy plating capable of giving a deposit of stabilized tin-lead alloy composition.
For tin-lead alloy plating the use of commonly employed borofluoride baths has been subject to varied limitations due to the necessity of disposing of the resulting fluorine-containing wastewater. From this viewpoint tin-lead alloy plating baths using organic sulfonic acids of relatively low toxicity havs recently been proposed. For example, Canadian Patent No.
1,222,476 to Obata et al, issued June 2, 1987 and Japanese Patent Application Public Disclos.ure No.
182986/1984 disclosed that light-grayish, uniform, fine-grainsd electroplated coatings of tin-lead alloy could be obtained by adding to an organic sulfonic acid bath a nonionic surface active agent, such as an adduct of styrenated phenol with an alkylene oxide (e.g., polyoxyethylene tristyrylphenyl ether, POETSPE) and an additive, such as certain sulfanilic acid le.g., N-(3-hydroxybutylidene)-p=sulfanilic acid, HBPSA] and/or triazine [e.g., 2,4-diamino-6=(2'-alkylimidazolyl(1'))-ethyl-1,3,5-triazine, DAAIMET].
Tin-lead alloy (generally known as ~older) plating is used extensively in light electric and electronic industries for joining metallic surfaces of componsnts.
For applications wherein occurrence of whisker i6 undesixable, solder deposits containing from a few % to 20 % of lead are applied. For applications wherein resistance to corrosion is r.eguired, solder deposits containing from 70 ~ to 80 % of lead are applied.
Further, in fabricating printed-circuit boards, 60~40 eutectic solder deposits are applied as an etching resist.

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Thus, s$nce deposits having variou~ compositions are required for tin-lead alloy plating according to their applications, it is ideal to always obtain deposits having a constant composition even if the current density changes from a low to high.
For example, the printed-circuit boards with the tin-lead alloy are usually subjected to ~using, a treatment for removing overhangs and enhancing the solderability. The treatment, however, ~ill give uneven, rough treated surfaces if the deposit produced by electroplating on the ~urface regions of the printed-circuit board is dissimilar in composition to that ~ormed in through-hole plating with consequent difference in melting point between the two deposits. Therefore, in plating printed-circuit boards with a tin-lead alloy, it is necessary to assure deposition of a uniform aomposition throughout the surface regions and holes of the boards.
For the tin-lead alloy plating of printed-circuit boards æemibright plating techniques are in wide use because in many cases brightness is not the first consideration and because the techniques permit smooth and even electroplating with good fusibility.
The ~lating baths described in the above-mentioned patent applications produce tin-lead alloy plates with fairly improved throwin~ power and fusing property.
Under low current density conditions, however, they tend to increase the lead contents in the resulting deposits of tin-lead alloy, rendering it impossible to form plated coatings of the desired ~n/Pb ratio. In order to ensure high reliability r~quired of printed-circuit boards, it is imperative that the Sn/Pb ratio in the deposits be stable, the deposits be improved in the fusing property and in stability against heat to be applied in ~ubsequent process steps, and the plating bath be easy to control.

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In view of the foregoing, we have investigated various addition agents. As a result, it has now been found that a certain group of guanamine compounds give tin-lead alloy plated coatings having a constant Sn/Pb ratio, under not only low current density conditions but even high current density conditions, the Sn/Pb ratio being substantially the same as that of the plating bath. It has also been found that these guanamine compounds yield plates possessing good throwing power, fusing property, and heat resistance without the addition of any such nonionic surface active agent or additive as referred to in the cited patent applications.

~RIEF SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide a tin-lead alloy plating bath capable of giving a deposit of a constant Sn/Pb ratio under high as well as low current density conditions.
In accomplishing these objects, there has been provided in accordance with the present invention a tin-lead alloy plating bath consisting essentially of tin and lead salts of an organic sulfonic aoid and a free organic - sulfonic acid, characterized by the addition of at lsast one guanamine compound.
Guanamine compounds which may be employed in the invention have the general formula R1 \ / N = C ~
N - A - C ~ N
R2 / ~ N - C
\ NH2 wherein R1 and R2, which may be the same or different, ~ ~ , ' ' ~9~

represent each a hydrogen atom, C1_18 straight- or branched-chain alkyl radical, C1_18 straight- or branched-chain alkoxy-lower alkyl radical, or a C3_7 cycloalkyl radical, or R1 and R2 may combine to form a carbon cycle or hetero cycle, and A represents a lower alkylene radical.

BRI EF DESCR~TI QN OF THE_DRAWI N(~, FIG. 1 is a graphic repre~entation of the relations between varied current densities for tin-lead alloy plating uslng various guanamine compounds ancl the lead contents in the resulting deposit6, FIG. 2 is a graphic representation of the surface conditions after ~using of the tin-lead alloy plates in the embodiment of the invention;
FIG. 3 is a curve showing the relation between the current density and the lead content in the deposit in another embodiment of the invention; and FIG. 4 is a graphic representation of the surfac~
condition after fusing of the tin-lead alloy plate in the above embodiment of the invention.

DETAILED DESCRIPTION

De~irable guanamine compounds for the purposes of the invention include those of the above-mentioned general formula in which either R1 or R2 represent~ a hydrogen atom and the other represents a C5_14 alkyl (e.g., pentyl, hexyl, heptyl, octyl, nonyl, decyl, or dodecyl), C5_14 alkoxy-ethyl or alkoxy-propyl (e.g., pentyloxy-, hexyloxy-, peptyloxy-, octyloxy-, 2-ethyl-hexyloxy-, or decyloxy-ethyl or -propyl), or cyclohexyl radical, and those in which R1 and R2 combine to form a piperidine, morpholine, or piperazine cycle. A desirable "~ ~
..~

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, .
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lower alkylene radical i8 ethylene or propylene radical.
Example~ of usable guanamine compound~ are mentioned in Table 1 to be given later.
Among particularly desirable ones are those in which either R1 or R2 Of the general formula represents a C1_18 alkoxy-lower alkyl radical, e.g., B-N-(2-ethylhexyloxy-propylamino)propioguanamine.
A guanamine compound in accordance with the invention is added in an amount of 0.01 to 30 g, preferably 0.1 ~o 10 g, per liter of the plating solution.
The principal plating solutlon according to the invention consists basically of at least one of organic sulfonic, alkanesulfonic, and alkanolsulfonic acids and a tin salt and a lead salt of such a sulfonic acid.
The alkane- or alkanolsulfonio acid employed has the general formula wherein R is a C1_12 alkyl radical, or wherein R is a C1_12 alkyl radical and OH may be located in any desired position.
Examples of alkanesul~onic acids are methane-, ethane-, propane-, 2-propane-, butane-, 2-butane-, pentane-, hexane-, decane-, and dodecane6ul~0nic acids~
These alkanesulfonic acids may be used singly or as a mixture of two or more.
Examples of alkanolsulfonic acids are isethionic acid and 2-hydroxyethane-1-, 2-hydroxypropane-1-, 1-hydroxypropane-2-, 3-hydroxypropane-1-, 2-hydroxybutane-1-, 4-hydroxybutane-l-, 2-hydroxypentane-1-, 2-hydroxy-hexane-1-, 2-hydroxydecane-1-, and 2-hydroxydodecane-l-sulfonic acids. These hydroxyl-containing alkanesulfonic acids may be employed alone or in a combination of two or more.

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The total concentration of tin and lead salts is, in terms of the respective metallic elements, in the range of 0.5-200 g/Q, preferably in the range of 10-100 g/Q. The concentration of the free alkanesulfonic or alkanolsulfonic acid present in the plating bath is 30-400 g/Q, preferably 70 - 150 g/Q. In accordance with ths invention, a plated coating having substantially the same Sn/Pb ratio as that of the plating bath can be obtained under a broad range of ourrent densities including low current density conditions.
The tin-lead alloy plating bath of the invention may contain a ~urface active agent, especially a nonionic one, which improves the disper6ibility of the bath and allows the bath to form an `adherent, smooth plated coating. Nonionic surface active agents have proved effective in enhancing the throwing power in electroplating at a low current density.
The nonionic surface active agents that may be effectively utilized in the plating bath of the invention have the general formula (I) R~
RA (CH2 - CH ~ )m - (CH2 - CH - O)nH (I) wherein RA represents a residue of a C8_20 alkanol, C1_ 25 alkylphenol, C1_25 alkyl-B-naphthol, C3 ~2 fatty acid amide, C1 25 alkoxylated phosphoric acid, C8_22 higher-fatty-acid-esterified sorbitan ester, or of a ~tyrenated phenol (in which the hydrogen of the phenol nucleus may be substituted with a C1_4 alkyl or phenyl radical, R' and R" represent each a hydrogen atom or methyl radical wlth the proviso that when R~ is a hydrogen atom R" iG a methyl radical or vice versa, and m and n represent each an integer of 1 to 30.
Such a useful nonionic surface active agent of the , 12~3~4 formula (I) for the plating bath of the invention may be one well known in the art. It may be prepared in the usual manner, for example, by addition condensation of a C8_22 higher alcohol, alkylphenol, alkyl-B-naphthol, C3_ 22 fatty acid amide, alkoxylated phosphoric acid, C8_22 higher-fatty-acid esterified sorbitan or 6tyrenated phenol with ethylene oxide (or propylene oxide) and further with propylene oxide (or ethylene oxide).
Among the higher alcohols that can be addition condensed with ethylene oxide or propylene oxide are octanol, decanol, lauryl alcohol, tetradecanol, hexadecanol, ~tearyl alcohol, eicosanol, cetyl alcohol, oleyl alcohol, and docosanol. Useful alkylphenols are mono-, di-, or trialkyl-substituted phenols, ~.g., p-butylphenol, p-isooctylphenol, p-nonylphenol, p-hexylphenol, 2,4-dibutylphenol, 2,4,6-tributylphenol, p-dodecylphenol, p-laurylphenol, and p-stearylphenol.
Alkyl radicals for alkyl=B-naphthols inclu~e methyl, ethyl, propyl, butyl, hexyl, octyl, decyl, dodecyl, and octadecyl. They may assume any desired position in the naphthalene nucleus. Examples of fatty acid amide~ are the amides of propionic, butyric, caprylic, capric, lauric, myristic, palmitic, staaric, and behenic acids.
Alkoxylated phosphoric acids are represented by the formula Ra \ ,~

Rb / OH

wherein Ra and Rb are C1_25 alk~l radicals, and either of them may be a hydrogen atom. They are obtained by esterifying one or two of the hydroxyl groupæ of phos-phoric acid with an alcohol of a suitable chain length (C1_25). Usable styrenated phenol is a mono-, di-, or , .
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tristyrenated phenol having the formula OH
~ CH ~

wherein Rc is hydrogen, C1_4 alkyl radical, or phenyl radical, and x has a number of 1 to 3. The hydrogen in the phenol nucleus may be substituted with an alkyl or phenyl radical. A suitable example is a mono-, di-, or tristyrenated phenol, mono- or distyrenated cresol, or mono- or distyrenated phenylphenol. It may be a mixture of these phenols. Typical sorbitans esterified with higher fatty acids are mono-, di-, or triesterified 1,4-1,5-, and 3,6-sorbitans, e.g., sorbitan monolaurate, Rorbitan mono-palmitate, sorbitan monostearate, sorbitan oleate, sorbitan dilaurate, sorbitan dipalmitate, sorbitan distearate, sorbitan dioleate, and sorbitan mixed fatty acid esters.
The afore-mentioned nonionic sur~ace active agents may be used singly or in combination.
The concentration of the nonionic surface active agent to be employed is usually in the range of 0.01-50 g/Q~ preferably in the range of 0.03-2~0 g/Q.
To improve the smoothns6s of the plate surface, the plating bath of the invention may contain one of certain smoothing or leveling additives. Such an additive i~
used together with the nonionic surface active agent to achieve a ~ynergetically favorable effect. The leveling additives that have proved particularly effective include those having the formulas (A) and (B) .
' . ~ ` .:

~ 10 -(A) Rc - CH - CH - CH = N - ~ - S03H
Rd Re wherein Rc is hydrogen, C1_4 alkyl radical, or phenyl radical, Rd is hydrogen or hydroxyl group, B is a C1_~
alkylene, phenylene, or benzyl radical, and Re is hydrogen or C1_4 alkyl radical.
(B) ~H2 N ~ C~2 ~ C~2 N~
\~ N y N~2 wherein Rf and Rg are each C1_18 alkyl radical.
Of these leveiling additives, particularly desirable are N-(3-hydroxybutylidene)-p-sulfanylic acid, n-butylidenesulfanilic acid, N-cinnamoylidenesulfanilic acid, 2,4-diamino-6-[2'-methylimidazolyl(l')]ethyl-1,3,5-t r i a z i n e, 2, 4 - d i a m i n o - 6 - [ 2' ~ e t h y l - 4 -methylimidazolyl(l')]ethyl-1,3,5-triazine, 2,4-diamino-6-[2'-undeculimidazolyl(l')]ethyl-1,3,5-triazine, and the like.
The concentration of such a leveling additive ranges from 0.01 to 30 g/Q, preferably from 0.03 to 5 g/Q.
The concentrations of the individual constituents of the plating bath according to the present invention may be optionally chosen depending on whether the plating is performed by the barrel, rack, high-speed continuous, or

2~ through-hole plating technique.
The plating bath of the invention is capable of producing uniform, dense plated coatings at a wide range , ~, ~ ' ' ' , ~2~344 of current densities.
The present invention will now be illustrated by the following examples 6howing typical plating bath compositiona and operating conditions. It should be S noted, however, that the invention i8 not limited thereto but may be variously embodied with free modifications of the bath composition and plating conditions to realize the afore-described objects.
Example 1 In this example various guanamine compounds listed in Table 1 were added to divided portions of a tin-lead alloy plating bath of a fundamental bath composition shown in Table 2, and plating was carried out on copper pieces.

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Table 1 No, Compound Abrid~ed Chemical Formula ~NH2 ~-N-Dodecylamino- C12H2sNH-CH2-cH2-c ~ \ N
1 n-N-DPG N-C
propioguanamine . \NH2 ~ NH2 ",N=C
~-N=Hexylamino- C6H13NH-CH2-cH2 C~ \ N
2 . ~-N-HPG ~ N-C~
proploguanamlne \NH2 ... .. ~
~NH2 A ~N-C~
Piperidine- PPA ~ N-CH2-~H2-C N
propioguanamine ~ NH2 . . .
~NH2 Clyclohexylamino- O -NH-CH2-CH2-C \ N
4 CHAA ~N-C~
propioguanamine \ NH2 . . .
~ NH2 Morpholine- O ~ N CH2 CH2 C N
MMA ~ ~N-C\
propioguanamine NH2 ~-N-(2-Ethylhexyl- C4Hg-CH-CH2-0-c3H6~NH-c2H4 6 oxypropylamino)- C2C60PAA C2H5 / C~
N N
propioguanamine C

NH / \ N /i \ NH
~ ~-N-(Laurvloxy- / NH2 ~N=C~
7 pr~pylamino)- C12CPAAC12H25--C3H6-NH-C2H4-C~ ~N
propioguansmine ~NH2 _ _ _ . . _ . , .

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~able 2 Stannous 2-hydroxypropanesulfonate 12 g/Q
(as a bivalent tin) Lead 2 hydroxypropanesulfonate 8 "
(as a bivalent lead) Free methanesulfonic acid 100 ~' POETSPE 5 "
HBPSA 0.1 "
DAAIMET 0.7 "

The plating was performed by galvanostatic electrolysis with 600 coulombs at a predetermined current density, using a copper wire, 2 mm dia. and 250 mm long, as the cathode and moving it at a rate of 2 m/min. The individual deposits thus obtained were dissolved in 6N
HC1 and their lead contents (in percent by weight) were determined by atomic absorption analysis. The results are shown in FIG. 1.
Example ~
The test specimens tin-lead alloy plated from the baths prepared by adding the stabilizers of Table 1 to the basic bath composition of Table 2 were subjected to infrared fusing at varied temperatures. The sur~ace conditions treated at the different temperatures were evaluated. The plating was carried out under a relatively low current density condition (0.25 A/dm2) to a plate thickness of 10 ~m. A constant heating time of 3 seconds was used for fusing at each predetermined temperature.
~or the fusing tests a near-infrared-ray planar heater, automatic SCR power controller, and temperature controller were used. The plated test piece was sst on a copper sheet for temperature control connected to an iron-constantan thermocouple, and was irradiated with ' , ~
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.

~2~3~4 infrared rays from the direction perpendicular to the piece.
The test results are shown in FIG. 2.
As a criterion of the fusing property, complete fusing at a relatively low temperature is desired. The addition of the 6tabilizer, irrespective of which was employed, mads it possible for the bath to yield a completely ~usible surface as compared with the surface from the nonstabilized bath of the fundamental composition.
Example 3 A plated coating of tin-lead alloy was formed from a bath of the composition comprising stannous 2-hydroxypropanesulfonate 12 g/Q
(as a bivalPnt tin) lead 2-hydroxypropanesulfonate 8 "
(as a bivalent lead) free methanesulfonic acid100 "
C2C60PAA 2 ~

and the relation between the current density and the lead content in the resulting deposit was determined and further the fusing property of the plated coating wa~
evaluated. The xesults are given, respectively, in FIGS.

3 and 4. With the C2C60PAA alone the plate composition and the fusing properties were both stable.
Example 4 The stabilizers of Table 1 were added to divided portions of a fundamental plating bath of the composition shown in Table 3. The alloy compositions of the tin-lead alloy plated coatings thus obtained from the individual plating baths were analyzed in the same way as described in Example 1. The re~ults are given in Table 4.

~9344 Stannous ethanesulfonate 18 g/Q
tas a blvalent tin) Lead ethanesulfonate 2 (as a bivalent lead) Free ethanesulfonicacid 100 "
POETSPE 5 "
~BPSA 0.1 "
DAAIMET 0.07 "

Table 4 Leaa content in de~osit (wt%) Current density Guanamine additive0.25 A/dm~ 5 A/dm2 No additive 25.4 15.0 CHAA 1 g~ 9.5 12.1 C2C60PAA 2 " 11.2 10.5 Example 5 The ~tabilizer6 of Table 1 were added to divided portions of a fundamental plating bath of the composition 6hown in Table 5. The alloy compositions of the tin-lead alloy plated coating6 thus obtained from the individual plating baths were analyzed in the same way as described in Example 1. The~reæults are given in Table 6.

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3~!4 Table 5 Stannous 4 g/Q
2-hydroxypropanesulfonate (as a bivalent tin) Lead 2-hydroxypropanesulfonate 16 (as a bivalent lead) Free 80 "
2-hydroxypropanesulfonic acid POETSPE 3 ~
HBPSA 0.2 "
DAAIMET 1 "

Table 6 Lead cQntent in de~Qsit (wt%) Current density Guanamine additive Q~~ 32 4 A~dm2 No additive 98.2 85.3 B-N-HPG 5 g/ 79.6 81.4 C120PAA 2 " 78.3 80.6 Example 6 The stabilizers of Table 1 were added to divided portions of a fundamental plating bath of the compo~ition shown in Table 7. The alloy compositions of the tin-lead alloy plated coatings thus obtained from the individual plating baths were analyzed in the same way as d~scribed in Example 1. The results are given in Table 8.

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Table 7 Stannous 54 g/Q
2-hydroxyethanesulfonate (as a bivalent tin) Lead 6 "
2-hydroxyethanesulfonate (as a bivalent lead) Free 120 2-hydroxyethanesulfonic acid POETSPE , 5 ~
HBPSA 1 "
DAAIMET 2 "

T~ble 8 Lçad aontent in de~osit (wt%) Current density Guanamine additiv~ 0.5 A/dm2 30 A~dm2 No additive 24.2 12.7 C2C6OPAA 2 g/10.7 11.5 Exampl.e l The stabilizers of Table 1 we,re added to divided portions of a fundamental plating bath of the composition shown in Table 9. The alloy compo6itions of the tin-lead alloy plated coatings thus obtained from the individual plating baths were analyzed in the same way as descxibed in Example 1. The results are given in Table 10.

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Stannous methanesulfonate 3h g/Q
(as a bivalent tin) Lead methanesulfonate 24 (as a bivalent lead) Free methanesulfonic acid 80 POETSPE 10 ~
HBPSA 0.2 "
DAAIMET 0.7 "

~
~ead con~ent in dçpo~it (wt~) Current density Çuanamine additive . 0.25 A~dm2 30 A/dm2 No additive 75.7 43 7 B-N-DPG 6 g/ 36.7 38.0 NMA 6 1' 37.1 42.0 PPA 6 " 41.2 45.2 Example 8 The alloy compositions of tin-lead alloy plated coatin~s, produced from different plating baths prepared by adding the stabilizers of Table 1 to portio~s of the fundamental plating bath shown in Table 11, were determined by the same method as used in ~xample 1.
Table 12 summarizes the results.

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TabLe 1~
Stannous propane 5 ul fonate 24 g/Q
(as a bivalent tin) Lead propane6ulfonate 16 (as a bivalent lead) Free propanesulfonic acid 100 POETSPE 2 "
HBPSA 0.1 DAAI~ET 0.07 "

Table 12 Lead ~on~ent in d~osit (wt%) Current density Guanamine additiye 0.25 A/dm2 30 A/dm2 No additive 70.8 42.1 B-N-DPG 0.5 g/ 39.5 41.3 MMA 0.5 " 42.8 44.1 C2C60PAA 0.5 " 36.5 40.3 - : .

Claims (3)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS.

1. A tin-lead alloy plating bath based on a principal plating bath comprising an alkanesulfonic or alkanolsulfonic acid at a concentration of 30 to 400 g/litre and both bivalent tin and lead salts thereof said tin and lead salts having a concentration, in terms of the respective metallic elements, of 0.5 to 200 g/litre of the bath, characterized by the addition of a guanamine compound having the general formula wherein R1 and R2, which may be the same or different, represent each a hydrogen atom, C1-18 straight- or branched-chain alkyl radical, C1-C18 straight- or branched-chain alkoxy-lower alkyl radical, or a C3-7 cycloalkyl radical, and R1 and R2 may combine with the adjoining nitrogen atom to form a piperidine, morpholine or piperazine cycle, and A represents a lower alkylene radical, said guanamine compound having a concentration of 0.01 to 30 g/litre of the bath.

2. A plating bath acaording to claim 1 characterized in that said alkane- or alkanolsulfonic acid has, respectively, the general formula wherein R is a C1-12 alkyl radical, or wherein R is a C1-12 alkyl radical and OH may be located in any desired position.

3. A plating bath according to claim 1 which further comprises a nonionic surface active agent and/or a leveling additive.