High-purity palladium-nickel alloy plating solution and process

The present invention relates to a high-purity palladium-nickel alloy plating, and particularly to a high-purity palladium-nickel alloy plating solution and process suitable as plating for electronic parts, said plating solution and process producing a deposit having a nickel content of 1 to 15%, an article plated with said alloy, and an article plated with said alloy and plated further with gold or a gold alloy.

FIELD OF THE INVENTION 
The present invention relates to a high-purity palladium-nickel alloy 
plating, and particularly to a high-purity palladium-nickel alloy plating 
solution and process suitable for plating electronic parts, said plating 
solution and process producing a deposit having a low nickel content, an 
article plated with said alloy, and an article plated with said alloy and 
further with gold or a gold alloy. 
BACKGROUND OF THE INVENTION 
In general, a plating which contains palladium is known to have excellent 
electrical properties and be economical as compared to precious metal 
platings such as a gold plating. However, a pure palladium plating 
containing no other metal has such an intrinsic property that the deposit 
is liable to occlude hydrogen, leading to high internal stress and easy 
cracking. Therefore, a palladium-nickel alloy plating by co-depositing 
nickel has hitherto been widely used, as disclosed, for instance, in 
Japanese Patent Publication Nos. 46-25604 (1971), 47-33177 (1972) and 
47-33178 (1972). The deposit obtained by the palladium-nickel alloy 
plating is glossy, occludes little hydrogen and is malleable, as 
contrasted to the pure palladium plating; therefore, a thick plating of 
the alloy can be obtained with extremely few cracks and favorable adhesion 
properties. 
However, in the case of the conventional palladium-nickel alloy plating, 
the nickel content of the deposit is high (15% or more), so that the 
deposit is liable to be attacked by chemicals as nitric acid, and 
accordingly, such plating can hardly be utilized in the field where 
high-quality plating is required, for instance, electronic parts. 
The present inventor, as a result of various experiments, has found that 
the abovementioned problems can be overcome if the nickel content of the 
deposit is not higher than 15%. 
The present invention has been attained, based on the abovementioned 
finding, while paying attention to the prior art. Accordingly, an object 
of the present invention is to provide a plating solution and a plating 
process for a high-purity palladium-nickel plating by which a deposit 
comparable to a pure palladium plating in resistance to chemicals such as 
nitric acid can be obtained without sacrificing the intrinsic features of 
palladium-nickel alloy plating, an article plated with the 
palladium-nickel alloy and an article plated with the palladium-nickel 
alloy and further with gold or a gold alloy. 
DISCLOSURE OF THE INVENTION 
To attain the abovementioned object, the high-purity palladium nickel alloy 
plating solution and process, the article plated with the alloy and the 
article plated with the alloy and further with gold or a gold alloy 
according to the present invention are constituted as follows: 
(1) A high-purity palladium-nickel alloy plating solution which is 
characterized in containing at least 5 g/l of palladium added in the form 
of palladous ammine chloride and 0.5 to 5 g/l of nickel. 
(2) A process for plating with a high-purity palladium-nickel alloy which 
is characterized in comprising adjusting a high-purity palladium-nickel 
alloy plating solution containing at least 5 g/l of palladium added in the 
form of palladous ammine chloride and 0.5 to 5 g/l of nickel to a pH 6 to 
8, and carrying out plating so that the nickel content of the deposit will 
be 1 to 15%. 
(3) An article plated with a palladium-nickel alloy having a nickel content 
of 1 to 15%. 
(4) An article plated with a palladium-nickle alloy having a nickel content 
of 1 to 15% and plated further with gold or a gold alloy. 
If at least 5 g/l of palladium is not added as palladous ammine chloride, 
the so-called "burnt deposit" will be generated in the plating, thereby 
spoiling the intrinsic action of the palladium-nickel alloy plating, 
namely, the action of obtaining a glossy, malleable deposit which occludes 
little hydrogen and can have a large thickness with few cracks and 
favorable adhesion properties. Palladium can be used in any large amount 
not less than 5 g/l. 
Like palladium, nickel is used in an amount of not less than 0.5 g/l, in 
order that the nickel content of the deposit will be not less than 1%. In 
order to restrict the nickel content of the deposit to 15% or less, the 
amount of nickel added must be less than 5 g/l. It is essential to 
maintain the pH of the plating solution in a substantially neutral range 
of 6 to 8. If the pH is less than 6, the deposit will be blackish in color 
and brittle. Although a favorable deposit can be obtained at a pH of more 
than 8, as the pH increases, ammonium of ammonium salts in the plating 
solution is gradually liberated to produce a stronger ammonium odor, 
resulting in a worse working condition. 
BEST MODE OF EXECUTING THE INVENTION 
Representative examples of the invention will be described below.

EXAMPLE 1 
Composition of plating solution: 
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Palladium (as Pallaadous 
15 g/l 
ammine chloride) 
Nickel (as Nickel chloride) 
2 g/l 
Ammonium chloride 
50 g/l 
Ammonium sulfamate 
80 g/l 
Ammonium sulfate 
40 g/l 
Sodium 1 g/l 
formylbenzenesulfonate 
Plating conditions: 
By adding ammonium or 
pH 7.0 
sulfuric acid 
Temperature 50.degree. C. 
Agitation Intermediate, by 
magnetic stirrer 
Anode Insoluble anode consisting of 
titanium plated with platinum 
Object to be plated 
Test piece obtained by bright- 
plating a brass plate (2 .times. 4 cm) 
with nickel in a thickness of 
about 10 .mu.m, followed by specular 
gloss finishing and gold strike 
plating 
Current density 1 A/dm.sup.2 
Time 20 min 
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In the above composition, ammonium chloride and ammonium sulfamate are used 
for imparting electrical conductivity to the plating solution. Other than 
these salts, one or more of inorganic salts such as ammonium phosphate and 
organic acid salts such as ammonium citrate may be used singly or in 
combination. The amount of these salts is suitably 50 to 200 g/l. Ammonium 
sulfate is added for supplying sulfate ions into the plating solution, and 
by providing sulfate ions in the solution in an amount of about 10 to 100 
g/l, the gloss of the deposit can be improved. Sodium 
formylbenzenesulfonate functions to ensure uniform appearance of the 
deposit and to prevent "unevenness" or "stain" from being generated in the 
gloss, and the amount thereof is suitably 0.1 to 5 g/l. As has been 
mentioned above, it is essential to maintain the pH of the plating 
solution in a substantially neutral range of 6 to 8. If the pH is less 
than 6, the deposit will be blackish in color and brittle. Although a 
favorable deposit can be obtained at a pH of more than 8, as the pH 
increases, ammonium of the ammonium salts in the plating solution is 
gradually liberated to produce a stronger ammonium odor, resulting in a 
worse working condition. 
The deposit obtained by the palladium-nickel alloy plating under the 
abovementioned composition and conditions had a specular gloss comparable 
or superior to that of the appearance (gold strike plated appearance) of 
the test piece, was 3 .mu.m in thickness and had good adhesion. Also, when 
plating was conducted using the same plating solution as above using 
current density of 3 A/dm.sup.2 for 7 min, a favorable glossy deposit of 3 
.mu.m in thickness was obtained. 
EXAMPLE 2 
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Composition of plating solution: 
the same as in 
Example 1 
Plating conditions: 
pH, temperature, 
agitation and anode 
were the same as in 
Example 1 
Object to be plated 
Test piece obtained by 
bright-plating a brass plate 
(2 .times. 4 cm) with nickel in a 
thickness of about 10 .mu.m, 
followed by specular gloss 
finishing (not subjected to 
gold strike plating) 
Current density 1 A/dm.sup.2, 3 A/dm.sup.2 
Time 35 min, 12 min 
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In this example, as shown above, the same plating solution as that in 
Example 1 was used, and the test piece prepared by subjecting the brass 
plate to nickel bright plating but not to gold strike plating was plated 
with the palladium-nickel alloy using a current density of 1 A/dm.sup.2 
for 35 min and a current density of 3 A/dm.sup.2 for 12 min. The absence 
of gold strike plating in this example was intended for making the 
nickel-plated surface of the test piece somewhat inactive, thereby 
intentionally making poor the adhesion of the palladium-nickel alloy 
plating so that the alloy plating film can be easily released for analysis 
thereof after plating. The deposits obtained in this example had a uniform 
glossy appearance which was slightly inferior to the original 
nickel-plated surface. When the four side faces of the two test pieces 
thus plated were ground by sand paper, four palladium-nickel alloy films 
(sheets) were released from the face and back sides of each of the test 
pieces. The thickness of each of the films was determined by a micrometer 
to be about 5 .mu.m. Parts of the plating films were dissolved in aqua 
regia, and the nickel contents of the deposits were analyzed to be 3.5% 
and 4.3%, respectively. To evaluate the chemical resistance of the 
deposits, a plating film with a nickel content of 20% obtained by using a 
plating solution LADEX VIII (commercial name) produced by 
Electroplating Engineers of Japan Limited and a nickel-free pure palladium 
plating film obtained by using a plating solution LADEX IV (commercial 
name) were prepared, separately from the plating films obtained according 
to the present invention. Concentrated nitric acid was dropped onto the 
plating films by a squirt, and the state of the films after 2 min was 
observed. The pure palladium plating film and the plating films according 
to the present invention showed little change, while the film with the 
nickel content of 20% turned yellowish brown and was dissolved to such a 
degree that the substrate was exposed. 
EXAMPLE 3 
Plating was carried out in the same manner as in the above examples except 
that the amount of nickel added to the plating solution of Example 1 was 
changed to 1 g/l, 3 g/l, 5 g/l and 10 g/l, then the relationship between 
the amounts of nickel added and the nickel content of the resultant 
deposit films was investigated, and the chemical resistance of the films 
was comparatively evaluated. Every one of the thus obtained films showed a 
favorable glossy appearance. The nickel contents of the films were 
analyzed to be 1.5%, 5.5%, 15% and 17.5%, respectively. When concentrated 
nitric acid was dropped onto the films by a squirt as in Example 2, only 
the film with the nickel content of 17.5% turned yellowish brown after 2 
min, and was dissolved to such a degree that the substrate was exposed. 
EXAMPLE 4 
When a connector which is an electronic part was plated with the 
palladium-nickel alloy by using the abovementioned plating solution under 
the abovementioned plating conditions, a connector having excellent 
electrical properties and chemical resistance could be obtained. When the 
connectors thus plated with the palladium-nickel alloy were plated 
respectively with gold and a gold alloy, the resultant connectors showed 
excellent electrical properties and chemical resistance comparable to 
those of a pure gold plated connector. 
POSSIBLE INDUSTRIAL USE OF THE INVENTION 
Since the high-purity palladium-nickel alloy plating solution and process, 
the article plated with said alloy, and the article plated with said alloy 
and further with gold or a gold alloy according to the present invention 
are as explained above, they have the intrinsic feature of 
palladium-nickel alloy plating, namely, the feature of obtaining a glossy, 
malleable deposit which occludes little hydrogen and can have a large 
thickness with few cracks and favorable adhesion properties, and 
simultaneously, they have the effect that the deposit is resistant to 
chemicals such as nitric acid, and the plating is extremely advantageous 
as plating for electronic parts (first invention) (second invention). 
In addition, an article plated with the palladium-nickel alloy obtained as 
a deposit according to the present invention is excellent in electrical 
properties, chemical resistance, etc.; therefore, such articles can be 
widely used for not only electronic parts such as connectors, contacts and 
printed circuit boards but also necklaces, brooches, spectacle frames, 
knives, forks, etc. 
Further, an article plated with the abovementioned palladium-nickel alloy 
and further with gold or a gold alloy has good appearance, is excellent in 
electrical properties, in chemical resistance, etc., like a pure gold 
plating, and is more economical than the pure gold plating because of the 
smaller amount of gold plating required (fourth invention).