U.S. patent application number 14/352145 was filed with the patent office on 2014-08-28 for reducing electroless silver plating solution and reducing electroless silver plating method.
This patent application is currently assigned to C. UYEMURA & CO., LTD. The applicant listed for this patent is C.UYEMURA & CO., LTD.. Invention is credited to Daisuke Hashimoto, Kota Kitajima, Akira Okada.
Application Number | 20140242288 14/352145 |
Document ID | / |
Family ID | 48167607 |
Filed Date | 2014-08-28 |
United States Patent
Application |
20140242288 |
Kind Code |
A1 |
Hashimoto; Daisuke ; et
al. |
August 28, 2014 |
REDUCING ELECTROLESS SILVER PLATING SOLUTION AND REDUCING
ELECTROLESS SILVER PLATING METHOD
Abstract
Provided are a reducing electroless silver plating solution and
a reducing electroless silver plating method using the silver
plating solution, the reducing electroless silver plating solution
being capable of preventing decomposition of silver in the plating
solution thereby to maintain stability of the solution and also
being capable of preventing excessive roughening of an underlying
metal or the like thereby to form a plating film having good film
characteristics and a good appearance. The reducing electroless
silver plating solution according to the present invention
comprises a water-soluble silver salt and a reducing agent, wherein
cyanide ions in a concentration of 0.006.times.10.sup.-3 mol/L to
12.5.times.10.sup.-3 mol/L are contained.
Inventors: |
Hashimoto; Daisuke; (Osaka,
JP) ; Kitajima; Kota; (Osaka, JP) ; Okada;
Akira; (Tokyo, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
C.UYEMURA & CO., LTD. |
Osaka-shi, Osaka |
|
JP |
|
|
Assignee: |
C. UYEMURA & CO., LTD
OSAKA-SHI, OSAKA
JP
|
Family ID: |
48167607 |
Appl. No.: |
14/352145 |
Filed: |
October 9, 2012 |
PCT Filed: |
October 9, 2012 |
PCT NO: |
PCT/JP2012/076141 |
371 Date: |
April 16, 2014 |
Current U.S.
Class: |
427/443.1 ;
106/1.05 |
Current CPC
Class: |
C23C 18/44 20130101;
C23C 18/1879 20130101; C23C 18/30 20130101; C23C 18/1633
20130101 |
Class at
Publication: |
427/443.1 ;
106/1.05 |
International
Class: |
C23C 18/44 20060101
C23C018/44; C23C 18/16 20060101 C23C018/16 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 27, 2011 |
JP |
2011-235559 |
Claims
1. A reducing electroless silver plating solution, comprising: a
water-soluble silver salt and a reducing agent, wherein cyanide
ions in a concentration of 0.006.times.10.sup.-3 mol/L to
12.5.times.10.sup.-3 mol/L are contained.
2. The reducing electroless silver plating solution according to
claim 1, wherein the above-mentioned water-soluble silver salt is a
silver salt other than a cyanide, and the above-mentioned cyanide
ions are contained as an alkali metal cyanide.
3. The reducing electroless silver plating solution according to
claim 1, wherein the above-mentioned reducing agent is at least one
kind or more selected from hydroxylammonium sulfate and
hydroxylacetate ammonium.
4. The reducing electroless silver plating solution according to
claim 1, having a pH of 8 to 11.
5. A reducing electroless silver plating method, wherein, using the
reducing electroless silver plating solution according to claim 1,
electroless silver plating is applied to a plated material.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to a reducing electroless
silver plating solution and a reducing electroless silver plating
method, more specifically, relates to a reducing electroless silver
plating solution and a reducing electroless silver plating method
using the silver plating solution, the reducing electroless silver
plating solution being stable and being capable of forming a good
plating film without excessive roughening of an underlying metal or
the like.
[0002] The present application asserts priority rights based on JP
Patent Application 2011-235559 filed in Japan on Oct. 27, 2011. The
total contents of disclosure of the patent application of the
senior filing date are to be incorporated by reference into the
present application.
BACKGROUND OF THE INVENTION
[0003] Silver plating has been long used for ornamental purposes
and the like, and in recent years, with making use of its
electrical characteristics and high reflectivity, it has been
frequently used in the field of electrical industry, and the field
of optical industry, and also in other fields, such as the field of
electromagnetic wave shield and the field of sterilization coating.
Especially, electroless silver plating has been more frequently
used since it allows film thickness to be controlled and a plating
film having a necessary thickness to be easily formed.
[0004] Electroless silver plating is broadly divided into
substituting electroless silver plating and reducing electroless
silver plating. Substituting electroless silver plating is
relatively excellent in stability of a plating solution, and has
been frequently employed in the market (For example, refer to
Patent Literatures 1 and 2). However, since substituting
electroless silver plating is performed in such a manner that
silver plating is precipitated by a substitution reaction with an
underlying metal, there is a problem that a limited kind of
underlying material is used for substituting electroless silver
plating.
[0005] On the other hand, reducing electroless silver plating is
performed in such a manner that silver plating is precipitated on
an underlying metal by making a reducing agent to be contained in a
plating solution and thereby reducing a water-soluble silver
compound to metallic silver, and this reducing electroless silver
plating is capable of forming a good silver plating film without
roughening of an underlying material and also without limiting a
kind of underlying material.
[0006] Particularly, from a viewpoint of solution stability, a
reducing electroless silver plating solution is made to contain a
silver cyanide compound, such as silver potassium cyanide, as a
water-soluble silver salt. Where this silver cyanide compound is
used as a water-soluble silver salt, generally, an amount of
cyanogen contained in a plating solution is more than twice as that
of silver at a molar ratio.
[0007] Also, there has been proposed a method for improving the
stability of a reducing electroless silver plating solution by
adding a cyanide, such as potassium cyanide, wherein many free
cyanides are present in the plating solution (For example, Patent
Literature 3 and Non Patent Literature 1).
[0008] However, while excessive cyanogen in the prior plating
solution makes it possible to control decomposition of silver in
the plating solution and improve the stability as mentioned above,
the excessive cyanogen dissolves a plated material which is made
of, metal of nickel, copper, or the like, and excessively roughens
a surface of the material, whereby a plating film having good film
characteristics cannot be formed.
[0009] On the other hand, in recent years, a cyanogen-free reducing
electroless silver plating solution has been also proposed (For
example, Patent Literatures 4 and 5). In such cyanogen-free
electroless silver plating solution, cyanogen is not present in the
plating solution and therefore an underlying metal or the like is
not dissolved, and accordingly, a surface thereof is not
excessively roughened, however, with respect to the solution
stability, the cyanogen-free electroless silver plating solution
has been remarkably inferior to that of a plating solution
containing cyanogen. As disclosed in Patent Literatures 4 and 5,
there has been proposed a technique to improve the stability of a
cyanogen-free silver plating solution by adding an additive
thereto, but the cyanogen-free silver plating solution has not yet
had sufficient stability. Furthermore, in such cyanogen-free
electroless silver plating, the thicker a silver film becomes, the
more the silver film becomes yellowish, and thus a problem arises
also in view of appearance of a plating film.
PRIOR-ART DOCUMENTS
Patent Document
[0010] PTL 1: Japanese Patent Application Laid-Open No. 2000-309875
[0011] PTL 2: Japanese Patent Application Laid-Open No. 2002-180259
[0012] PTL 3: Japanese Patent Application Laid-Open No. H5-279863
[0013] PTL 4: Japanese Patent Application No. 3937373 [0014] PTL 5:
Japanese Patent Application Laid-Open No. 2003-268558
Non Patent Document
[0014] [0015] Non PTL 1: "Mudenkai Mekki--Kiso to Ohyo"
(Electroless Plating--Fundamentals and Applications) (edited by
Electroplating Research Society Japan, published by Nikkan Kogyo
Shimbun, Ltd., pp 176-177)
SUMMARY OF THE INVENTION
[0016] The present invention is proposed in view of such actual
circumstances, and aims at providing a reducing electroless silver
plating solution and a reducing electroless silver plating method
using the silver plating solution, the reducing electroless silver
plating solution being capable of preventing decomposition of
silver in the plating solution thereby to maintain the stability of
the solution and also being capable of preventing excessive
roughening of metal or the like as an underlying plating material
thereby to form a plating film having good film characteristics and
a good appearance.
[0017] The present inventors earnestly studied to achieve the
above-mentioned aim, and as a result, found that control of a
cyanogen concentration in a plating solution allows the stability
of the plating solution to be maintained and prevents an underlying
metal or the like from being excessively roughened, whereby a
plating film having good film characteristics and being excellent
in appearance can be formed, and the present inventors completed
the present invention.
[0018] In other words, a reducing electroless silver plating
solution according to the present invention comprises a
water-soluble silver salt and a reducing agent, wherein cyanide
ions in a concentration of 0.006.times.10.sup.-3 mol/L to
12.5.times.10.sup.-3 mol/L are contained.
[0019] Furthermore, in the reducing electroless silver plating
solution according to the present invention, the above-mentioned
water-soluble silver salt is preferably a silver salt other than a
cyanide, and the above-mentioned cyanide ions are preferably
contained as an alkali metal cyanide.
[0020] Furthermore, in the reducing electroless silver plating
solution according to the present invention, the above-mentioned
reducing agent is preferably at least one kind or more selected
from hydroxylammonium sulfate and hydroxylacetate ammonium.
[0021] Furthermore, the reducing electroless silver plating
solution according to the present invention preferably has a pH of
8 to 11.
[0022] Also, a reducing electroless silver plating method according
to the present invention is such that electroless silver plating is
applied to a plated material, using a reducing electroless silver
plating solution comprising a water-soluble silver salt and a
reducing agent, wherein cyanide ions in a concentration of
0.006.times.10.sup.-3 mol/L to 12.5.times.10.sup.-3 mol/L are
contained.
EFFECTS OF INVENTION
[0023] The reducing electroless silver plating solution and the
reducing electroless silver plating method according to the present
invention are capable of preventing decomposition of silver in the
plating solution thereby to make the stability of the solution good
and also capable of preventing excessive roughening of metal or the
like as an underlying plating material thereby to form a plating
film having good film characteristics and a good appearance.
BRIEF DESCRIPTION OF DRAWINGS
[0024] FIG. 1 illustrates graphs showing relationships between film
thickness of a plating film and plating time where different
reducing agents are employed.
DETAILED DESCRIPTION OF THE INVENTION
[0025] Hereinafter, a specific embodiment of the reducing
electroless silver plating solution according to the present
invention (hereinafter, referred to as the present embodiment.)
will be described in detail.
[0026] A reducing electroless silver plating solution according to
the present embodiment comprises a water-soluble silver salt and a
reducing agent, the silver plating solution being capable of
reducing the water-soluble silver salt to metallic silver with the
reducing agent and precipitating silver plating on a plated
material, such as an underlying metal, thereby forming a silver
plating film.
[0027] The reducing electroless silver plating solution according
to the present embodiment is characterized in that, in the plating
solution comprising a water-soluble silver salt and a reducing
agent, cyanide ions prepared to have a predetermined concentration
range are contained. Specifically, cyanide ions in a concentration
of 0.006.times.10.sup.-3 mol/L to 12.5.times.10.sup.-3 mol/L are
contained.
[0028] A reducing electroless silver plating solution, according to
the prior art, to which a water-soluble silver salt, such as silver
potassium cyanide, and a cyanide, such as potassium cyanide, are
added as additives is excellent in solution stability, but,
excessively dissolves and roughens metal or the like as an
underlying plating material composed of, for example, copper or
nickel, whereby a plating film having good film characteristics has
not been formed.
[0029] On the other hand, a cyanogen-free plating solution has been
also developed in order to avoid such excessive dissolution of an
underlying metal or the like, but, where a silver plating solution
having low stability of the solution, in particular, is made to be
cyanogen-free, then such plating solution causes rapid
decomposition of silver contained in the plating solution, whereby
the stability of the plating solution has been remarkably
decreased. Furthermore, such cyanogen-free plating solution causes
a yellowish plating film to be formed, whereby formation of a
plating film having a good appearance has not been realized. The
application of such silver plating film having a poor appearance to
a LED device, for, example, has caused a lower reflectivity.
[0030] To solve this problem, the reducing electroless silver
plating solution according to the present embodiment is made to
contain cyanide ions having a concentration of
0.006.times.10.sup.-3 mol/L to 12.5.times.10.sup.-3 mol/L in the
plating solution.
[0031] Such reducing electroless silver plating solution can
achieve only the advantages brought by both the prior electroless
silver plating solution using a cyanide and the prior cyanogen-free
electroless silver plating solution, that is, achieve higher
solution stability, and can form a plating film having excellent
film characteristics without excessive roughening of a plated
material, such as a underlying metal. Furthermore, this reducing
electroless silver plating solution allows a plating film to have a
beautiful white silver color and thus to have a good appearance,
and when the plating film is applied to, for example, a LED device,
reflectance characteristics thereof can be improved.
[0032] Here, when an amount of cyanide ions contained is less than
0.006.times.10.sup.-3 mol/L, silver in the plating solution is
decomposed, and thus the stability of the solution is decreased.
Furthermore, the thicker a plating film becomes, the more the film
becomes yellowish, and thus the plating film cannot be formed
having a good appearance. On the other hand, when an amount of
cyanide ions contained is more than 12.5.times.10.sup.-3 mol/L,
cyanogen in the plating solution dissolves and excessively roughens
a plated material which is composed of metal or the like as a
plating underlying material, and thus even if a plating film is
formed on the plated material, the plating film cannot have good
film characteristics.
[0033] A source of cyanide ions is not particularly limited, but,
for example, alkali metal cyanide, such as potassium cyanide or
sodium cyanide, may be used. Furthermore, a silver cyanide
compound, such as silver potassium cyanide, may be used as a
water-soluble silver salt and made to be contained as a part or a
whole of a source of cyanide ions so as to have the above-mentioned
cyanide ion content.
[0034] Among these sources of cyanide ions, alkali metal cyanide is
more preferably used. The use of alkali metal cyanide allows the
cyanide having a cyanide ion concentration within the
above-mentioned range to be appropriately and easily prepared, and
allows solution stability to be maintained and a plating film
having good film characteristics to be formed more efficiently and
more effectively. Furthermore, also when a plating solution is
continuously used, it is not necessary to add alkali metal cyanide
too often, and furthermore, unlike the case with adding a silver
cyanide compound, due to an increase in amount of silver contained
in the plating solution, the solution stability are not
decreased.
[0035] In the reducing electroless silver plating solution
according to the present embodiment, the water-soluble silver salt
is not particularly limited as long as it is soluble in the plating
solution, and examples of the water-soluble silver salt which may
be used include silver nitrate, silver oxide, silver sulfate,
silver chloride, silver sulfite, silver carbonate, silver acetate,
silver lactate, silver sulfosuccinate, silver sulfonate, silver
sulfamate, and silver oxalate. Moreover, as mentioned above, a
silver cyanide compound, such as silver potassium cyanide, may be
used as a water-soluble silver salt. These water-soluble silver
salts may be used alone or two or more kinds thereof may be used in
combination.
[0036] The water-soluble silver salt is preferably contained in a
silver concentration of 0.1 g/L to 10 g/L (0.9.times.10.sup.-3
mol/L to 90.times.10.sup.-3 mol/L), more preferably 0.1 g/L to 3.0
g/L (0.9.times.10.sup.-3 mol/L to 30.times.10.sup.-3 mol/L). When a
water-soluble silver salt is contained in a silver concentration
within a range of 0.1 g/L to 10 g/L, a precipitation rate of silver
plating can be higher, and also a plating solution having higher
stability can be achieved.
[0037] Moreover, where a silver cyanide compound is used as a
water-soluble silver salt, a silver cyanide compound to be added or
a cyanide as an additive contained together with the silver cyanide
compound are made to be contained in a cyanide ion concentration
within the above-mentioned range, that is a concentration of
0.006.times.10.sup.-3 mol/L to 12.5.times.10.sup.-3 mol/L.
[0038] The reducing agent is not particularly limited as long as it
is capable of reducing a water-soluble silver salt contained in a
plating solution to metallic silver and also is water-soluble, for
example, the reducing agent may be used including hydrazine and its
derivative, a formaldehyde compound, hydroxylammonium salt,
saccharides, Rochelle salt, a boron hydride compound,
hypophosphite, DMAB (dimethyl amine boranes), and ascorbic acid and
the like. These reducing agents may be used alone or two or more
kinds thereof may be used in combination.
[0039] Among these reducing agents, particularly hydroxylammonium
salts, such as hydroxylammonium sulfate and hydroxylacetate
ammonium, are preferably used. The reason for this is that these
reducing agents make it possible to easily change plating thickness
by controlling plating time, and to form a plating film having a
desired plating thickness and good film characteristics with a
film-thickening treatment or the like.
[0040] More specifically, in the reducing electroless silver
plating solution according to the present embodiment, as mentioned
above, in order not to excessively dissolve an underlying metal or
the like, cyanide ions are controlled to be in a predetermined
concentration range. On the other hand, where a reducing agent
having higher reducing power, such as DMAB, is used, it is possible
to cause decomposition of silver in a plating solution. In this
respect, the use of hydroxylammonium salt, such as hydroxylammonium
sulfate or hydroxylacetate ammonium, prevents decomposition of
silver in a plating solution, and thus can lead to a stable
reduction reaction.
[0041] Also, where a reducing agent having weaker reducing power,
such as hydrazine or formaldehyde, is used, a silver precipitation
rate is slower. Therefore, a portion of the surface of an
underlying metal or the like on which silver is not precipitated is
present for a long time, and thus cyanogen in a plating solution
may act on the portion and excessively dissolve it. Also in this
respect, the use of hydroxylamines, such as hydroxylammonium
sulfate or hydroxylacetate ammonium allows silver plating to be
certainly precipitated by a reduction reaction without dissolution
of an underlying metal or the like, and therefore, by control of
plating time, the underlying metal or the like can be coated with
silver plating with a desired film thickness, and a plating film
having good film characteristics can be formed.
[0042] Thus, the use of hydroxylammonium sulfate or hydroxylacetate
ammonium allows a plating film with a desired film thickness to be
easily formed and solution stability to be higher, and prevents
excessive dissolution of an underlying metal or the like due to
cyanogen, whereby a plating film having good film characteristics
can be more effectively formed.
[0043] A reducing agent is preferably contained in a concentration
of, for example, 0.006 mol/L to 0.12 mol/L, more preferably 0.006
mol/L to 0.03 mol/L. When a reducing agent is contained in a
concentration of less than 0.006 mol/L, it is possible that a
water-soluble silver salt in a plating solution cannot be reduced
to metallic silver and sufficient silver plating cannot be
precipitated. On the other hand, when a reducing agent is contained
in a concentration of more than 0.12 mol/L, the stability of a
plating solution is adversely affected and it is not economically
preferable.
[0044] The reducing electroless silver plating solution according
to the present embodiment may be used at a liquid temperature
within a range of 0 to 80 degrees C., and, particularly, the use of
the plating solution at a temperature of approximately 30 to 60
degrees C. allows the stability of the plating solution to be
higher. When the temperature of the plating solution is too low,
silver is precipitated at a slow rate, and it takes a long time to
obtain a predetermined amount of silver precipitate. On the other
hand, when the temperature of the plating solution is too high, it
is easily to cause the loss of a reducing agent due to its
autolysis reaction and the decrease in the stability in plating
solution.
[0045] Furthermore, the reducing electroless silver plating
solution may be used when the solution has a pH of 2 to 14, but,
since cyanide ions are contained in a predetermined concentration
as mentioned above, in particular, the reducing electroless silver
plating solution preferably has a pH of 8 to 11. When the plating
solution has a pH of not less than 8, generation of cyanogen gas
can be effectively controlled and the plating solution can be
safely used without adverse effects on environment. Furthermore,
the stability of the plating solution can be higher. Also, the
plating solution having a pH of not more than 11 allows the
stability of the plating solution and the film characteristics of a
plating film to be improved.
[0046] The pH adjustment for a plating solution is carried out in
such a manner that, when pH is lowered, an acid is usually used
which has the same kind of anion portion as an anion portion of a
water-soluble silver salt, for example, sulfuric acid in the case
of using silver sulfate as a water-soluble silver salt, or nitric
acid in the case of using silver nitrate as a water-soluble silver
salt. On the other hand, when pH is raised, alkali metal hydroxide,
such as sodium hydroxide, ammonia, or the like is used.
[0047] Moreover, with respect to the reducing electroless silver
plating solution according to the present embodiment, a complexing
agent may be added as needed. The complexing agent is not
particularly limited, and examples of the complexing agent include
sulfite, succinimide, hydantoin derivatives, ethylenediamine, and
ethylenediaminetetraacetic acid (EDTA). These complexing agents may
be used alone or two or more kinds thereof may be used in
combination.
[0048] An amount of additive of the complexing agent depends on its
kind and is not particularly limited, but preferably approximately
1 g/L to 100 g/L. The complexing agent having such concentration
allows a good precipitation rate of silver plating to be achieved
and the plating solution to have more excellent stability.
[0049] Moreover, an additive, such as a well-known surface active
agent, a pH adjuster, a buffer, a lubricant, and a stress
relaxation agent, may be mixed, as needed.
[0050] A plating method using the reducing electroless silver
plating solution having the above-mentioned characteristics is, for
example, such that a plated material is immersed in the reducing
electroless silver plating solution having the liquid temperature
and the pH value each adjusted as mentioned above, whereby
silver-plating is applied to the material. Also, the reducing
electroless silver plating solution is sprayed or applied to a
plated material, whereby the plated material is brought into
contact with the plating solution to undergo a plating
treatment.
[0051] As mentioned above, the electroless silver plating method
operates in such a manner that, mainly, a water-soluble silver salt
is reduced to metallic silver with the reducing agent contained in
the plating solution, and silver plating is precipitated on a
plated material, such as metal, as an underlying plating material,
whereby a plating film is formed.
[0052] The plated material to form an electroless silver plating
film is not particularly limited, and metallic materials, such as
copper and nickel, various kinds of other conductive materials and
non-conductivity materials, and the like may be applied. Where a
metallic material is used as a plated material, a pretreatment,
such as a degreasing treatment, is applied thereto in accordance
with a usual method, and then the plated material is directly
immersed in a plating solution.
[0053] Moreover, in the case of plating a nonmetallic material,
such as ceramics and plastics, a pretreatment, such as a degreasing
treatment, is applied to the plated material, and then an
activation treatment is applied thereto, followed by immersion
thereof in the plating solution. It is beneficial to perform the
activation treatment in accordance with a usual method, and, for
example, the activation treatment is performed using a palladium
catalyst (catalyst-accelerator process, sensitizer-activator
process, or the like), a silver catalyst, a copper catalyst, or the
like, in accordance with well-known conditions.
[0054] As mentioned above, the reducing electroless silver plating
solution according to the present embodiment comprises a
water-soluble silver salt and a reducing agent, wherein cyanide
ions are contained at a concentration of 0.006.times.10.sup.-3
mol/L to 12.5.times.10.sup.-3 mol/L. Such reducing electroless
silver plating solution is capable of effectively preventing the
decomposition of silver contained in the silver plating solution
thereby to achieve good stability of the solution and also capable
of preventing excessive roughening of a plated material, such as an
underlying metal, thereby to form a plating film having good film
characteristics and a good appearance.
EXAMPLES
[0055] Hereinafter, specific Examples of the present invention will
be described. It should be noted that the present invention is not
limited to any of the following Examples.
Plating Solution Stability and Plating Film Evaluation
[0056] As shown below, reducing electroless silver plating
solutions each were adjusted.
Example 1
[0057] There was prepared an aqueous solution containing a silver
nitrate having a silver concentration of 9.0.times.10.sup.-3 mol/L
(1.0 g/L), a hydroxylammonium salt (hydroxylammonium sulfate) in a
concentration of 1.24.times.10.sup.-3 mol/L as a reducing agent,
and EDTA in a concentration of 0.15 mol/L (50 g/L) as a complexing
agent, and furthermore, potassium cyanide in a concentration of 1
mg/L was added thereto, whereby the aqueous solution is made to
have a cyanide ion concentration of 0.006.times.10.sup.-3 mol/L in
a plating solution, and the aqueous solution is adjusted using
caustic soda to have a pH of 9.0, whereby a reducing electroless
silver plating solution was prepared.
Example 2
[0058] A reducing electroless silver plating solution was prepared
in the same manner as in Example 1, except that potassium cyanide
in a concentration of 300 mg/L was added, thereby allowing the
plating solution to have a cyanide ion concentration of
1.8.times.10.sup.-3 mol/L.
Example 3
[0059] A reducing electroless silver plating solution was prepared
in the same manner as in Example 1, except that potassium cyanide
in a concentration of 500 mg/L was added, thereby allowing the
plating solution to have a cyanide ion concentration of
3.0.times.10.sup.-3 mol/L.
Example 4
[0060] A reducing electroless silver plating solution was prepared
in the same manner as in Example 1, except that potassium cyanide
in a concentration of 1000 mg/L was added, thereby allowing the
plating solution to have a cyanide ion concentration of
6.5.times.10.sup.-3 mol/L.
Example 5
[0061] A reducing electroless silver plating solution was prepared
in the same manner as in Example 1, except that potassium cyanide
in a concentration of 2000 mg/L was added, thereby allowing the
plating solution to have a cyanide ion concentration of
12.5.times.10.sup.-3 mol/L.
Comparative Example 1
[0062] A reducing electroless silver plating solution was prepared
in the same manner as in Example 1, except that a silver potassium
cyanide having a silver concentration of 9.0.times.10.sup.-3 mol/L
(1.0 g/L) was added in place of silver nitrate, and furthermore a
potassium cyanide in a concentration of 300 mg/L was added, whereby
the plating solution had a cyanide ion concentration of
19.8.times.10.sup.-3 mol/L.
Comparative Example 2
[0063] A reducing electroless silver plating solution was prepared
in the same manner as in Example 1, except that a silver potassium
cyanide having a silver concentration of 9.0.times.10.sup.-3 mol/L
(1.0 g/L) was added in place of silver nitrate, and potassium
cyanide was not added, whereby the plating solution had a cyanide
ion concentration of 18.0.times.10.sup.-3 mol/L.
Comparative Example 3
[0064] A reducing electroless silver plating solution was prepared
in the same manner as in Example 1, except that potassium cyanide
was not added. In other words, a cyanogen-free reducing electroless
silver plating solution was prepared.
Comparative Example 4
[0065] A reducing electroless silver plating solution was prepared
in the same manner as in Example 1, except that a potassium cyanide
in a concentration of 2100 mg/L was added, thereby allowing the
plating solution to have a cyanide ion concentration of
13.0.times.10.sup.-3 mol/L.
[0066] Next, using each of the reducing electroless silver plating
solutions prepared as mentioned above, reducing electroless silver
plating was applied to a BGA substrate (manufactured by C. Uyemura
& Co., Ltd.) as a plated material.
[0067] It should be noted that, prior to the reducing electroless
silver plating treatment, each step shown in the following Table 1
was performed in order as a pretreatment. In other words, a cleaner
treatment (degreasing) using ACL-738 (manufactured by C. Uyemura
& Co., Ltd.) was applied to a BGA substrate as a plated
material, and then soft etching was performed using a sodium
persulfate solution (SPS) in a concentration of 100 g/L. Next,
etching residues were removed using a 10% sulfuric acid
(H.sub.2SO.sub.4) solution (acid pickling), and pre-dipping was
performed using a 3% sulfuric acid solution, and then a Pd catalyst
treatment was performed using MNK-4 (manufactured by C. Uyemura
& Co., Ltd.) (catalyst treatment). After that, by using an
electroless nickel solution NPR-4 (manufactured by C. Uyemura &
Co., Ltd.) and an electroless palladium solution TPD-30
(manufactured by C. Uyemura & Co., Ltd.), a metallic film was
formed as an underlying material.
TABLE-US-00001 TABLE 1 Treatment Step Chemicals Temperature time
(min) Cleaner ACL-738 50 5 Soft-etching SPS 100 g/L 25 1 Acid
pickling 10% H.sub.2SO.sub.4 r.t. 1 Pre-dipping 3% H.sub.2SO.sub.4
r.t. 1 Catalyst MNK-4 30 2 Electroless Ni NPR-4 80 25 Solution
Electroless Pb TPD-30 55 2 Solution
[0068] The electroless silver plating treatment was performed in
such a manner that a plated material was immersed in the
above-mentioned reducing electroless silver plating solution at 60
degrees C. for 20 minutes. After performing the plating treatment,
the plating solution was left for 100 hours with keeping the
temperature at 60 degrees C. Then, the stability of the plating
solution was evaluated by whether the plating solution was
self-decomposed or not, and also the solder joint strength and the
appearance of a plating film were evaluated. The following Table 2
shows evaluation results.
[0069] It should be noted that, in order to evaluate the solder
joint strength of a plating film, a reflow treatment was performed
once at 240 degrees C., and the joint strength was evaluated in
such a manner that, when a solder fracture mode was found in 16 or
more among 20 solder joints, the joint strength was evaluated to be
good (.largecircle.), on the other hand, when a solder fracture
mode was found in less than 16 solder joints, the joint strength
was evaluated to be poor (X). Furthermore, the appearance of a
silver plating film was evaluated by visually observing the
appearance of the plating film having a thickness of 0.5
micrometer.
TABLE-US-00002 TABLE 2 Solder Bath joint Cyanogen concentration in
solution stability strength Appearance Example 1 0.006 .times.
10.sup.-3 mol/L not .largecircle. white silver KCN: 1 mg/L(0.015
.times. 10.sup.-3 mol/L) decomposed Example 2 1.8 .times. 10.sup.-3
mol/L not .largecircle. white silver KCN: 300 mg/L(4.6 .times.
10.sup.-3 mol/L) decomposed Example 3 3.0 .times. 10.sup.-3 mol/L
not .largecircle. white silver KCN: 500 mg/L(7.5 .times. 10.sup.-3
mol/L) decomposed Example 4 6.5 .times. 10.sup.-3 mol/L not
.largecircle. white silver KCN: 1000 mg/L(16.0 .times. 10.sup.-3
mol/L) decomposed Example 5 12.5 .times. 10.sup.-3 mol/L not
.largecircle. white silver KCN: 2000 mg/L(31.0 .times. 10.sup.-3
mol/L) decomposed Comparative 19.8 .times. 10.sup.-3 mol/L not X
white silver Example 1 decomposed Comparative 18.0 .times.
10.sup.-3 mol/L not X white silver Example 2 decomposed Comparative
0 precipitated .largecircle. yellow Example 3 in beaker silver
Comparative 13.0 .times. 10.sup.-3 mol/L not X white silver Example
4 KCN: 2100 mg/L(33.0 .times. 10.sup.-3 mol/L) decomposed
[0070] As shown in Table 2, in the reducing electroless silver
plating solutions in Examples 1 to 5 each of which was controlled
to have a cyanide ion concentration of 0.006.times.10.sup.-3 mol/L
to 12.5.times.10.sup.-3 mol/L, silver contained in the plating
solution was not decomposed and good solution stability was
achieved. Furthermore, in each of Examples 1 to 5, a plating film
of higher solder joint strength and having good film
characteristics was formed. Furthermore, the formed plating film
was white-silver-colored and had a beautiful appearance.
[0071] On the other hand, in the reducing electroless silver
plating solutions in Comparative Examples 1, 2, and 4 which were
controlled to have a cyanide ion concentration of
19.8.times.10.sup.-3 mol/L, 18.0.times.10.sup.-3 mol/L, and
13.0.times.10.sup.-3 mol/L, respectively, excessive free-cyanide in
the plating solution prevented silver from being decomposed while a
plating film of lower solder joint strength and having poor film
characteristics was formed. The reason for this may be that, due to
the excessive free-cyanide present in the plating solution, an
underlying metal was excessively dissolved and roughened.
[0072] Also, in the reducing electroless silver plating solution in
Comparative Example 3 which was cyanogen-free, a plating film of
higher solder joint strength and having good film characteristics
was formed. On the other hand, silver in the plating solution was
decomposed due to the cyanogen-free, and the stability of the
plating solution was not secured. Furthermore, where the reducing
electroless silver plating solution in Comparative Example 3 was
used, a yellowish plating film, that is, a yellow silver plating
film was formed, thereby having an extremely poor appearance.
[0073] From the above-mentioned results, it was found that, when
the plating solution had a cyanide ion concentration in a range of
0.006.times.10.sup.-3 mol/L to 12.5.times.10.sup.-3 mol/L,
decomposition of silver was controlled and the stability of the
solution was maintained, and moreover, beautiful and
white-silver-colored silver was precipitated and thus a film having
an excellent appearance was formed. Furthermore, it was found that
a cyanide ion concentration in the above-mentioned range prevented
a metallic material or the like as an underlying plating material
from being excessively roughened, thereby allowing a plating film
having good film characteristics to be formed.
Reducing Agent
[0074] Next, it was examined what type of a reducing agent is
preferable for the reducing electroless silver plating solution
which is controlled to have a cyanide ion concentration in a range
of 0.006.times.10.sup.-3 mol/L to 12.5.times.10.sup.-3 mol/L, as in
the above-mentioned Examples 1 to 5. It should be noted that
experiments were conducted using the composition employed in
Example 2 as a basic composition.
Example 6
[0075] A reducing electroless silver plating solution having a
cyanide ion concentration of 0.006.times.10.sup.-3 mol/L was
prepared in the same manner as in Example 2, except that hydrazine
(hydrazine sulfate) in a concentration of 1.24.times.10.sup.-3
mol/L was added as a reducing agent.
Example 7
[0076] A reducing electroless silver plating solution having a
cyanide ion concentration of 0.006.times.10.sup.-3 mol/L was
prepared in the same manner as in Example 2, except that
formaldehyde in a concentration of 1.24.times.10.sup.-3 mol/L was
added as a reducing agent.
[0077] Using the reducing electroless silver plating solutions
prepared in the above-mentioned Examples 2, 6, and 7, the
correlation between thickness of a silver plating film and plating
time was examined. The graphs in FIG. 1 show the results of the
measurement of film thickness with respect to plating time when the
different reducing agents were used.
[0078] As shown in FIG. 1, it is understood that, in the
electroless silver plating solution of Example 2 using a
hydroxylammonium salt as a reducing agent, as a plating time is
extended, the resulting thickness of a silver plating film is
approximately linearly increased, and it is understood that a
reduction reaction allows silver to be precipitated, whereby a film
is formed. On the other hand, it is understood that, in Examples 6
and 7 using hydrazine and formaldehyde as reducing agents,
respectively, once a predetermined time passes, the thickness is
not much increased after that. The reason for this may be that
hydrazine and formaldehyde allow silver to be precipitated by a
reduction reaction while a factor resulting from a substitution
reaction greatly works, whereby an underlying metal is slightly
dissolved.
[0079] Hence, it was found that, in the reducing electroless silver
plating solution which is controlled to have a cyanide ion
concentration in a range of 0.006.times.10.sup.-3 mol/L to
12.5.times.10.sup.-3 mol/L, the use of a hydroxylammonium salt,
such as hydroxylammonium sulfate or hydroxylacetate ammonium, as a
reducing agent allows a reduction reaction to proceed more
effectively and dissolution of an underlying metal by a
substitution reaction not to be caused, whereby a plating film
having good film characteristics can be formed.
* * * * *