U.S. patent application number 12/587816 was filed with the patent office on 2010-02-18 for tin electroplating solution and tin electroplating method.
This patent application is currently assigned to Rohm and Haas Electronic Materials LLC. Invention is credited to Masaaki Imanari, Yasushi Takizawa.
Application Number | 20100038254 12/587816 |
Document ID | / |
Family ID | 37459566 |
Filed Date | 2010-02-18 |
United States Patent
Application |
20100038254 |
Kind Code |
A1 |
Takizawa; Yasushi ; et
al. |
February 18, 2010 |
Tin electroplating solution and tin electroplating method
Abstract
To provide a tin electroplating solution devoid of harmful lead
and having excellent solder wettability, and a method for
depositing a tin film on electronic parts using such a tin
electroplating solution. The tin electroplating solution includes
organic acids, a naphtholsulfonic acid and, as needed, an
antioxidant and a surfactant is disclosed.
Inventors: |
Takizawa; Yasushi;
(Ageo-shi, JP) ; Imanari; Masaaki; (Misato-shi,
JP) |
Correspondence
Address: |
John J. Piskorski;Rohm and Haas Electronic Materials LLC
455 Forest Street
Marlborough
MA
01752
US
|
Assignee: |
Rohm and Haas Electronic Materials
LLC
Marlborough
MA
|
Family ID: |
37459566 |
Appl. No.: |
12/587816 |
Filed: |
October 14, 2009 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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11507265 |
Aug 21, 2006 |
|
|
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12587816 |
|
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Current U.S.
Class: |
205/205 ;
205/300; 205/302; 205/303 |
Current CPC
Class: |
C25D 3/30 20130101; C25D
3/32 20130101; H05K 3/244 20130101 |
Class at
Publication: |
205/205 ;
205/300; 205/302; 205/303 |
International
Class: |
C25D 3/30 20060101
C25D003/30; C25D 3/32 20060101 C25D003/32; C25D 5/34 20060101
C25D005/34 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 19, 2005 |
JP |
2005-238921 |
Claims
1-8. (canceled)
9. A tin electroplating solution comprising one or more compounds
for supplying stannous ions selected from the group consisting of
stannous salts of sulfuric acid, alkane sulfonic acids, and alkanol
sulfonic acids, one or more organic acids selected from the group
consisting of alkane sulfonic acids and alkanol sulfonic acids and
one or more of 2-naphthol-7-sulfonic acid and alkali metal salts
thereof in amounts of 0.3 g/L to 5 g/L.
10. The tin electroplating solution of claim 9, wherein the alkali
metal salt of 2-naphthol-7-sulfonic acid is sodium
2-naphthol-7-sulfonate.
11. The tin electroplating solution of claim 9, further comprising
one or more antioxidants.
12. The tin electroplating solution of claim 11, wherein the one or
more antioxidants is chosen from substituted and unsubstituted
dihydroxybenzene compounds.
13. The tin electroplating solution of claim 9, further comprising
one or more surfactants.
14. A method for electroplating a tin film on a substrate
comprising: a) providing a tin electroplating solution comprising
one or more compounds for supplying stannous ions selected from the
group consisting of stannous salts of sulfuric acid, alkane
sulfonic acids and alkanol sulfonic acids, one or more organic
acids selected from the group consisting of alkane sulfonic acids
and alkanol sulfonic acids and one or more of 2-naphthol-7-sulfonic
acid or alkali metal salts thereof in amounts of 0.3 g/L to 5 g/L;
b) contacting the substrate with the tin electroplating solution;
and c) electroplating the tin film on the substrate.
15. The method of claim 14, further comprising a step of subjecting
the substrate to an activation step prior to contacting the
substrate with the tin electroplating solution.
16. The method of claim 14, wherein the alkali metal salt of
2-naphthol-7-sulfonic acid is sodium 2-naphthol-7-sulfonate.
Description
[0001] The present invention relates to a tin plating solution and
tin electroplating method. In further detail, the present invention
relates to a virtually lead-free tin electroplating solution that
is used for joining electronic parts and the like, and a method for
forming a tin film.
[0002] Tin-lead alloy plating has excellent bonding properties, low
cost, electric properties, and solderability, and is therefore
widely used in chips, crystal oscillators, lead frames, printed
circuit boards, and other electronic parts that require electrical
contact; and as an etching resist in steps used for the manufacture
of semiconductor devices and printed substrates.
[0003] However, the use of lead has been restricted in recent years
in order to protect the workplace environment and the natural
environment, and lead-free plating baths have come to be regarded
as a desirable substitute for tin-lead alloy plating materials.
Tin-silver alloy plating, tin-copper alloy plating, and tin-bismuth
alloy plating have been proposed as substitute materials.
Nevertheless, difficulties are presented when handling tin-silver
plating solutions due to electronic substitution of the metallic
silver, as well as other problems. Tin-copper alloy plating has the
drawback that the melting point thereof tends to increase and
solder wettability decreases, and tin-bismuth alloy plating has the
drawback in that the resulting film is brittle.
[0004] Tin plating solutions have also been investigated, but the
resulting precipitate is rough, and problems are presented with
regard to solder wettability. Gloss plating has been used in the
past; however, the organic matter precipitate derived from the
brightener in the glossy plating film clearly causes a reduction in
solder joint strength and cracking. In other words, the use of an
aldehyde compound or other organic brightener, or an ammonium salt
or other amine-based brightener, results in the formation of a fine
tin precipitate and improved solderability, but solder wettability
deteriorates over time due to an increase in the amount of organic
precipitate in the film. Moreover, a glossy plating film is
generally brittle and not appropriate for use in lead frames and
other applications where the plating film is required to be bent.
Consequently, a demand has existed for semi-glossy plating in
applications where gloss is not required.
[0005] Naphtholsulfonic acids have been used for years as an
additive for tin electroplating solutions. For instance, in U.S.
Pat. No. 2,407,579 it is disclosed that naphtholsulfonic acids or
salts thereof; e.g., 2-naphthol-6-sulfonic acid,
2-naphthol-7-sulfonic acid, and 1-naphthol-4-sulfonic acid, can be
added to fluorinated tin chloride baths consisting of tin chloride
and an alkali fluoride and having a pH of 1 to 5. This patent does
not disclose a combination of an organic acid bath and
naphtholsulfonic acid.
[0006] An electrodeposition method is disclosed in JP (Kokoku)
49-16176 involving an electro-deposition bath wherein a complex
salt of an aromatic sulfonic acid consisting of
.beta.-naphtholsulfonic acid and a metal compound is used as a
base. This patent does not disclose the addition of
.beta.-naphtholsulfonic acid to an organic acid bath as an
additive.
[0007] In JP (Kokai) 2002-17478 a tin-lead alloy plating bath is
disclosed, consisting of a soluble stannous salt, a soluble lead
salt, and one or more specific naphtholsulfonic acids. This patent
does not disclose the use of 2-naphthol-7-sulfonic acid in a tin
plating bath.
[0008] It is an object of the present invention to provide a tin
electroplating solution and a tin electroplating method that do not
involve the use of lead, yield a film whose appearance is uniform,
and provide good solder wettability with parts that require
electrical contact.
[0009] As a result of having conducted diligent research into
lead-free tin electroplating solutions consisting of organic
additives in order to achieve the above-mentioned object, the
present inventors perfected the present invention by discovering
that 2-naphthol-7-sulfonic acid and alkali salts thereof have
selective capability when compared to other naphtholsulfonic acids.
The plating solution of the present invention is capable of forming
a tin film of good appearance, and of providing a good semi-glossy
film with which the good solder wettability of the deposited tin
film does not deteriorate, particularly after pressure cooker
treatment.
[0010] In a first aspect, the present invention provides a tin
electroplating solution consisting of one, two, or more components
selected from the group consisting of stannous ions, organic acids,
and 2-naphthol-7-sulfonic acid or salts thereof, and comprising
virtually no lead ions.
[0011] In a second aspect, the present invention provides the
above-mentioned tin electroplating solution, wherein the organic
acid is one, two, or more acids selected from the group consisting
of alkane sulfonic acids and alkanol sulfonic acids.
[0012] In a third aspect, the present invention provides a plating
solution comprising a tin salt of an organic acid as tin ions; one,
two, or more acids selected from alkane sulfonic acids; one, two,
or more compounds selected from the group consisting of
2-naphthol-7-sulfonic acid and salts thereof; one, two, or more
antioxidants selected from the group consisting of substituted or
unsubstituted dihydroxybenzene compounds; and one, two, or more
nonionic surfactants; and consisting of virtually no lead ions.
[0013] In a fourth aspect, the present invention provides a tin
electroplating solution consisting of a tin salt of an organic
acid; one, two, or more alkane sulfonic acids; one, two, or more
compounds selected from a group comprising 2-naphthol-7-sulfonic
acid and an alkali salt thereof; one, two, or more antioxidants
selected from substituted or unsubstituted dihydroxybenzene
compounds; one, two, or more nonionic surfactants; and water.
[0014] In a fifth aspect, the present invention provides a method
for electrodepositing a tin film on a substrate using any of the
tin electroplating solutions described above.
[0015] In a sixth aspect, the present invention provides a method
for manufacturing electronic components having a tin film on a
substrate, including the step of electroplating a substrate using
an acidic tin electroplating solution consisting of a tin salt of
an organic acid; one, two, or more alkane sulfonic acids; one, two,
or more compounds selected from a group consisting of
2-naphthol-7-sulfonic acid and an alkali salt thereof; one, two, or
more antioxidants selected from a group consisting of substituted
and unsubstituted dihydroxybenzene compounds; one, two, or more
nonionic surfactants; and water.
[0016] The tin electroplating solution of the present invention is
composed of stannous ions, an organic acid, and
2-naphthol-7-sulfonic acid or a salt thereof.
[0017] The abbreviations used throughout the present specification
have the following denotations unless otherwise indicated: g=gram,
mg=milligram; .degree. C.=degrees Celsius; min=minute, m=meter;
cm=centimeter; L=liter; mL=milliliter; A=ampere, and
dm.sup.2=square decimeter. All of the numeric ranges include the
boundary values and can be combined in any order.
[0018] The terms "plating solution" and "plating bath" in the
present specification have the same meaning and are used
interchangeably. The terms "alkane" and "alkanol" refer to a linear
or branched alkane or alkanol.
[0019] Examples of organic acid are acids selected from alkane
sulfonic acids and alkanol sulfonic acids or salts thereof.
Preferred alkane sulfonic acids or alkanol sulfonic acids include
substituted or unsubstituted alkane sulfonic acids and alkanol
sulfonic acids. Examples include methanesulfonic acid,
ethanesulfonic acid, propanesulfonic acid,
2-hydroxyethane-1-sulfonic acid, 2-hydroxypropane-1-sulfonic acid,
and 1-hydroxypropane-2-sulfonic acid. Methanesulfonic acid is
particularly preferred. One or a mixture of two or more of these
acids can be used.
[0020] The organic acid content of the plating solution is at least
the stoichiometric equivalent of the bivalent tin ions present in
the plating bath. For instance, the free acid content in the
plating bath is 30 g/L to 500 g/L, particularly 50 g/L to 300 g/L,
and ideally 70 g/L to 250 g/L.
[0021] Stannous ions are bivalent ions. A variety of compounds can
be used as long as they are capable of supplying such ions to the
plating bath. Examples include stannous salts of sulfuric acid,
hydrochloric acid, methanesulfonic acid, citric acid, malic acid,
or other organic acids. An example of a preferred source of
stannous ions is a tin salt selected from the above-mentioned
organic acids. Particularly preferred examples are compounds
selected from an alkane sulfonic acid stannous salt or an alkanol
sulfonic acid stannous salt. It is generally preferred that a tin
salt of an organic acid be used in the plating solution. One or a
mixture of two or more of these stannous salts can be used.
[0022] The stannous ion content of the plating bath in terms of
stannous ions is for instance, 10 g/L to 150 g/L, preferably 30 g/L
to 120 g/L, and particularly 50 g/L to 100 g/L.
[0023] 2-naphthol-7-sulfonic acid can be used as a free acid or in
the form of a salt. Examples of ideal salts are soluble salts of
potassium, sodium, ammonium, tin, or the like. Potassium and sodium
salts are preferred, with sodium 2-naphthol-7-sulfonate being
particularly preferred. One or a mixture of two or more of these
salts can be used.
[0024] The amount of 2-naphthol-7-sulfonic acid or salt thereof
contained in the plating bath is, e.g., 0.01 g/L to 20 g/L,
preferably 0.2 g/L to 10 g/L, and particularly 0.3 g/L to 5
g/L.
[0025] A surfactant can be used as needed in the plating solution
of the present invention. A variety of surfactants can be used, but
nonionic surfactants are more suitable. Examples of preferred
nonionic surfactants include polyoxyethylene lauryl ether,
polyethylene glycol, polyoxyethylene alkyl ether, polyoxyethylene
polyoxypropylene glycol, polyoxyethylene nonylphenyl ether,
polyoxyethylene polyoxypropylene alkylamine, and polyoxyalkylene
adducts of ethylene diamine, with
tetrakis-(polyoxyalkylene)ethylene diamine or polyoxyethylene
polyoxypropylene (C.sub.8-C.sub.18) alkyl amines being particularly
preferred. Such surfactants can be obtained commercially from Lion
Akzo Co., Ltd. under such brand names as Ethopropomeen C18/18, or
from Asahi Denka Co., Ltd. under the brand name Adekanol
TR-704.
[0026] The appropriate surfactant concentration of the plating bath
is, e.g., 0.01 g/L to 50 g/L, preferably 0.1 g/L to 20 g/L, and
particularly 1 g/L to 15 g/L.
[0027] An antioxidant can be used as needed in the plating solution
of the present invention. An antioxidant is used in order to
prevent oxidation of the tin ions from divalent to tetravalent
form, with examples including hydroquinone, catechol, resorcinol,
fluoroglycine, pyrogallol, and hydroquinone sulfonic acid, as well
as salts thereof.
[0028] The plating bath should have an antioxidant concentration
of, e.g., 0.01 g/L to 10 g/L, preferably 0.1 g/L to 5.0 g/L, and
particularly 0.5 g/L to 2.0 g/L.
[0029] The tin plating solution of the present invention is
adjusted to the acidic range. The preferred pH of the plating bath
is, e.g., less than 7, and preferably 3 or less. Brighteners,
lubricants, conductors, anode dissolving agents, or other
conventional additives can be added to the present invention as
needed.
[0030] An example of an especially preferred composition for the
tin electroplating solution of the present invention is an acidic
tin electroplating solution consisting of a tin salt of an organic
acid; one, two, or more alkane sulfonic acids; one, two, or more
compounds selected from a group consisting of 2-naphthol-7-sulfonic
acid and alkali salts thereof; one, two, or more anti-oxidants
selected from substituted or unsubstituted dihydroxybenzene
compounds; one, two, or more nonionic surfactants; and water.
[0031] The composition of the present invention consists of
virtually no lead. "Consists of virtually no lead" indicates that
no lead is added to the plating bath as an additional component,
but does not rule out the possibility of a lead component being
present in the compound components. Moreover, the composition of
the present invention preferably contains no metallic stabilizers,
such as bismuth.
[0032] The method for electroplating using the plating solution of
the present invention can be a conventional method. The
concentration of each component of the plating solution is selected
as needed according to the plating method; e.g., barrel plating,
through-hole plating, rack plating, or high-speed continuous
plating.
[0033] Electroplating using the electroplating solution of the
present invention can be performed at a plating bath temperature
of, e.g., 10.degree. to 65.degree. C., and preferably room
temperature to 50.degree. C.
[0034] The cathode current density can be selected from within a
range of, e.g., 0.01 to 100 A/dm.sup.2, and preferably 0.05 to 70
A/dm.sup.2.
[0035] It is not necessary to stir the plating bath during plating,
but a method such as stirrer-assisted stirring or pump-assisted
circulation can also be selected.
[0036] As will be evident from the Working Examples, specific
naphtholsulfonic acids wherein hydroxyl groups and sulfonic acid
groups bond at a specific position in the naphthalene ring, and
specifically 2-naphthol-7-sulfonic acid or an alkali salt thereof,
show an obvious effect in comparison to other naphtholsulfonic acid
or salts thereof. In other words, a tin film that is fine and not
brittle will precipitate from the plating solution of the present
invention, with the precipitated film showing good solder
wettability.
[0037] The plating solution of the present invention can be used
for a variety of plated articles as a substitute for conventional
tin plating and tin-lead alloy plating for soldering or etching
resists. The article to be plated should have a conductive
component capable of being electroplated, and includes composites
made from conductive materials such as copper or nickel and
insulating materials such as ceramics, glass, plastics, ferrite,
and the like. Before being electroplated, the articles can be
pretreated by a conventional method depending on the material used.
According to the electroplating of the present invention, tin films
can be deposited on the surface of conductive materials on
substrates or a variety of electronic parts, including chip
capacitors, chip resistors, and other chip parts, crystal
oscillators, pumps, connector pins, lead frames, printed circuit
boards, and the like.
WORKING EXAMPLE 1
[0038] A tin plating solution was prepared with the following
composition.
TABLE-US-00001 Stannous methanesulfonate (as tin ions) 70 g/L
Methanesulfonic acid (as a free acid) 175 Sodium
2-naphthol-7-sulfonate 0.5 Polyoxyethylene polyoxypropylene
(C.sub.8-C.sub.18) alkyl amine 10 Potassium hydroquinone sulfonate
2 Distilled water Remainder
[0039] A Hull cell test was performed using a plating bath with the
above-mentioned composition and a 6.7 cm.times.10 cm copper sheet,
with electrolysis performed at 5 A.times.1 minute and a bath
temperature of 50.degree. C., and stirring by cathode vibration at
a rate of 4 to 6 m/minute. Table 1 shows the results obtained by
macroscopically evaluating the appearance of the plating film;
i.e., precipitation anomalies, uniformity, and gloss.
[0040] The term "precipitation anomalies" generally refers to
unplated areas that tend to form where the current is low, and
precipitation regions known as "burns" or "burnt deposits" that
tend to form where current density is too high. "Uniformity" means
that the state of a precipitated film surface was observed in the
current density region generally used by the electroplating
method.
COMPARATIVE EXAMPLE 1
[0041] A Hull cell test was conducted under the same conditions as
in Working Example 1 using a plating bath wherein the 0.5 g/L of
sodium 2-naphthol-7-sulfonate in Working Example 1 was substituted
for 0.5 g/L of sodium 2-naphthol-6-sulfonate. The appearance of the
plating film was observed macroscopically, with the results shown
in Table 1.
COMPARATIVE EXAMPLE 2
[0042] A Hull cell test was conducted under the same conditions as
in Working Example 1 using a plating bath wherein the 0.5 g/L of
sodium 2-naphthol-7-sulfonate in Working Example 1 was substituted
for 0.5 g/L of disodium 2-naphthol-3,6-sulfonate. The appearance of
the plating film was observed macroscopically, with the results
shown in Table 1.
COMPARATIVE EXAMPLE 3
[0043] A Hull cell test was conducted under the same conditions as
in Working Example 1 using a plating bath wherein the 0.5 g/L of
sodium 2-naphthol-7-sulfonate in Working Example 1 was substituted
for 0.5 g/L of disodium 1-naphthol-3,6-sulfonate. The appearance of
the plating film was observed macroscopically, with the results
shown in Table 1.
WORKING EXAMPLE 2
[0044] A Hull cell test was conducted under the same conditions as
in Working Example 1 using a plating bath wherein the amount of
sodium 2-naphthol-7-sulfonate in Working Example 1 was changed from
0.5 g/L to 0.2 g/L. The appearance of the plating film was observed
macroscopically, with the results shown in Table 1.
WORKING EXAMPLE 3
[0045] A Hull cell test was conducted under the same conditions as
in Working Example 1 using a plating bath wherein the amount of
sodium 2-naphthol-7-sulfonate in Working Example 1 was changed from
0.5 g/L to 0.3 g/L. The appearance of the plating film was observed
macroscopically, with the results shown in Table 1.
COMPARATIVE EXAMPLE 4
[0046] A Hull cell test was conducted under the same conditions as
in Working Example 1 using a plating bath wherein the amount of
disodium 2-naphthol-3,6-disulfonate added in Comparative Example 2
was changed from 0.5 g/L to 1.0 g/L. The appearance of the plating
film was observed macroscopically, with the results shown in Table
1.
COMPARATIVE EXAMPLE 5
[0047] A Hull cell test was conducted under the same conditions as
in Working Example 1 using a plating bath wherein the 0.5 g/L of
sodium 2-naphthol-7-sulfonate in Working Example 1 was changed to
0.2 g/L of 2-naphthol. The appearance of the plating film was
observed macroscopically, with the results shown in Table 1.
COMPARATIVE EXAMPLE 6
[0048] A hull cell test was conducted under the same conditions as
in Working Example 1 with a plating bath wherein the 0.5 g/L of
sodium 2-naphthol-7-sulfonate in Working Example 1 was changed to
10 mg/L of salicylic aldehyde. The appearance of the plating film
was observed macroscopically, with the results shown in Table
1.
TABLE-US-00002 TABLE 1 Plating film appearance Working Example 1
Some precipitation anomalies, uniform surface, semi-glossy
Comparative Example 1 Some precipitation anomalies, irregular
surface, semi-glossy Comparative Example 2 Some precipitation
anomalies, irregular surface, semi-glossy Comparative Example 3
Some precipitation anomalies, irregular surface, semi-glossy
Working Example 2 Some precipitation anomalies, virtually uniform
surface, semi-glossy Working Example 3 Some precipitation
anomalies, uniform surface, semi-glossy Comparative Example 4 Some
precipitation anomalies, uniform surface, dull to semi-glossy
Comparative Example 5 Some precipitation anomalies, irregular
surface, semi-glossy Comparative Example 6 Many precipitation
anomalies, irregular surface, irregular gloss
[0049] The plating films obtained in the working examples all
exhibited some precipitation anomalies but were uniform and had a
semi-glossy appearance. By contrast, the film obtained in
Comparative Example 4 had some precipitation anomalies and a
uniform appearance, but its gloss was diminished compared to those
of the working examples. A uniform film could not be obtained in
the other comparative examples.
Electrolysis Test
[0050] Two liters of each plating bath in Working Example 1 and
Comparative Example 4 were prepared and electrolysis was performed
with each plating bath under a current of 10 A for four hours. A
tin plating film was formed as in Working Example 1 using the bath
before electrolysis and the bath after electrolysis, with the
appearance thereof being macroscopically evaluated.
[0051] Although no changes in appearance were seen with the plating
film formed from the plating baths before and after electrolysis
when the plating bath in Working Example 1 was used, the plating
film obtained when the plating bath in Comparative Example 4 was
used was dull after electrolysis.
Solder Wettability Test
[0052] Two liters of each plating bath used in Working Example 1
and Comparative Example 4 were prepared, and a tin plating film of
10 .mu.m was deposited on a copper-lead frame. The bath temperature
was 27.degree. C., stirring was performed by vibrating the cathode
at a rate of 4 m/minute and also with a stirrer, and the current
density was varied between 5 and 40 A/dm.sup.2.
[0053] Each of the resulting tin plating films was subjected to a
moisture-resistance test at 105.degree. C. and 100% RH for eight
hours (PCT treatment (105.degree. C., 100% RH, 8 hours)), and the
solder wettability of the plating films after the
moisture-resistance test was evaluated by measuring the zero cross
time in accordance with the meniscograph method using a solder
checker. The measurement conditions were as shown below:
[0054] Zero cross time measurement
[0055] Solder cell: Sn/Pb=63/37
[0056] Bath temperature: 235.degree. C.
[0057] Immersion depth: 1 mm
[0058] Immersion speed: 10 mm/second
[0059] Immersion time: 5 seconds
[0060] Flux: Inactive rosin
[0061] Table 2 shows the results obtained in the above-mentioned
tests.
TABLE-US-00003 TABLE 2 Solder wettability Plating current after
moisture resistance test density (A/dm.sup.2) Working Example 1
Comparative Example 4 5 1.6 seconds 5 seconds or longer 10 1.4
seconds 5 seconds or longer 15 -- 5 seconds or longer 20 1.2
seconds 5 seconds or longer 25 -- 5 seconds or longer 30 1.1
seconds 5 seconds or longer 35 -- Not measured 40 1.7 seconds Not
measured
[0062] The tin plating time was adjusted for each current density
so that the plating film thickness would be 10 .mu.m. A uniform
film could not be obtained at a current density of 35 A/dm.sup.2 or
40 A/dm.sup.2 with the plating solution of Comparative Example 4;
therefore, the test was not conducted.
[0063] The plating film obtained in Working Example 1 showed
excellent results when its solder wettability after the
moisture-resistance test was compared to that of Working Example 4.
These data clearly show that the plating solution of the present
invention has excellent solder wettability.
[0064] Lead, which has been problematic, is not used; therefore,
the tin electroplating solution of the present invention is very
safe. Moreover, the plating solution is easily handled because
plating is not conducted with an alloy, and a uniform plating film
can be obtained. The resulting tin plating film is uniform,
semi-glossy, and has excellent solder wettability; and is therefore
useful as a joining material.
* * * * *