The present invention relates to electroplating solutions for reducing
the amount of oxidation of stannous tin ions in an electroplating solutions containing
Electroplating baths containing divalent tin are used widely in industry
for plating tin and/or tin alloys onto basis metals. These baths are acidic and
are mainly based on acids such as sulfuric, phenolsulphonic, fluoroboric, methane
sulfonic, or a combination of hydrochloric and hydrofluoric. In all of these baths,
a common problem has been the formation of a sludge during operation that results
in a loss of divalent tin and excessive clean-up costs. This sludge occurs because,
during the plating process, divalent tin has a tendency to become oxidized to
tetravalent tin by oxidation at the anode or by oxygen which is introduced into
the bath from the surrounding air. Tetravalent tin thus becomes soluble stannic
acid which accumulates in the bath to eventually form β stannic acid which
is not soluble and which precipitates to form the undesirable sludge. In order
to prevent the formation of this sludge, tin must remain in the divalent state.
When plating tin from these solutions onto strip steel using insoluble
anodes, the problem is multiplied even further. Oxygen is liberated at these insoluble
anodes to further oxidize divalent tin to its tetravalent state. U.S. Patent No.
4,181,580 describes a process for plating strip steel using insoluble anodes and
a method for replenishing tin. Divalent tin is replenished in these plating installations
by separately dissolving metallic tin granules in a fluidized bed of acidic plating
bath into which oxygen is fed to dissolve the metallic tin. The tin enriched solution
is returned to the plating bath thereby replenishing the tin which has been plated
out. Excess oxygen in the tin dissolving cell described in this patent can also
react with divalent tin to form tetravalent tin; therefore, tin plating machines
of this type are particularly subject to formation of tin sludge.
In normal plating installations using soluble anodes and cathode
rod agitation, the sludge problem can be minimized. However, when rapid pumping
of the solution is used in high speed plating machines, the inclusion of substantial
amounts of air into the bath accelerates the oxidation of divalent tin by the oxygen
which is present in the air. The sludge problem therefore exists somewhat in normal
tin plating installations, is worsened in high speed plating installations, and
is further worsened in strip steel machines that use insoluble anodes and tin
Attempts have been made in the art to minimize sludge formation in
these divalent tin baths. A paper by J. McCarthy entitled "Oxidation Characteristics
of Tin-Plating Electrolytes," which appeared in the July 1960 issue of
Plating magazine, discussed studies of tin oxidation by bubbling oxygen
into various tin solutions. U.S. Patent Nos. 5,094,726 and 5,066,367 disclose methods
and solutions for limiting sludge using alkyl sulfonic acid based tin solutions
in combination with reducing agents or antioxidants to prevent a buildup of tin4+.
Dihydroxybenzene reducing agents were disclosed to be very effective for this
purpose. A recent paper by Chi Pong Ho of the Nanfang Metallurgical Institute appearing
in Vol. 24 #1 of Materials Protection (January 1991) describes the use
of reducing agents based on vanadium pentoxide in divalent tin sulfate-sulfuric
acid solutions to limit sludge formation.
Tin plating onto steel strip using acid solutions also results in
a continual build-up of iron in the plating bath. The iron content can continue
to build until its concentration reaches as high as about 30 g/l. Although the
iron interferes only slightly in the tin deposition process, it causes a rapid
acceleration of tin sludge formation and a decrease in rate of dissolution of
metallic tin in the dissolving cell described above. Any antioxidant used to prevent
tin sludge formation in strip plating installations should maintain its usefulness
in the presence of this iron buildup in the bath.
Chemical Abstract number 88:81098e discloses a tin plating solution
comprising Sn(II) ions, sulphate ions, and at least one additive selected form
fluoro complexes, Fe ions, Ti ions, Cr ions, Sb ions, V ions and C6-18
The additives are present in order to stabilize the solution against air oxidation.
Chemical Abstract number 91:99172w discloses a tin plating solution
comprising Sn(II) ions, sulphate ions, BF4- ions and a small
amount of a salt of Ti. The salt of Ti is present as a stabilizer.
Chemical Abstract No. 84:113539k discloses a tin electroplating bath
comprising SnSO4, cresolsulfonic acid, K2TiO(C2O4)2.H2O,
beta-napthol, sulphuric acid and gelatin.
Summary of the Invention
The present invention provides a solution for use in the electroplating
of tin or tin alloys comprising: a basis solution of an organic sulfonic acid or
a salt thereof; divalent tin ions; and an antioxidant compound in an amount effective
to assist in maintaining the tin ions in the divalent state,
characterised in that the antioxidant compound is a vanadium,
tantalum, zirconium or tungsten compound.
The preferred amount of antioxidant compound ranges from about 0.025
to 5 g/l. Generally, the antioxidant compound is added to the solution as an oxide
or a solution soluble compound.
These antioxidant compounds are highly effective when used in a basis
solution which comprises an alkane sulfonic acid, an alkanol sulfonic acid, an
alkane sulfonate, an alkanol sulfonate, phenol sulfonic acid or a phenol sulfonate.
If desired, these solutions may also contain at least one or more of a wetting
agent, a brightener, or divalent lead ions to improve or enhance electroplating
performance or the resultant deposit characteristics.
The antioxidant compound is added in an amount effective to assist
in maintaining the tin ions in the divalent state. Also, this compound may be added
to an electroplating solution which contains iron ion contamination.
Detailed Description of the Invention
It has been found that the addition of certain multivalent metal
compounds into divalent tin or tin alloy alkyl or alkylol sulfonic acid plating
baths results in a substantially reduced rate of tin sludge formation. This is
particularly true in high speed plating installations that pump the solution rapidly
to provide a high agitation rate thereby introducing air into the plating bath.
The improvement caused by the above combination is very significant, particularly
in those installations that use insoluble anodes and a tin metal dissolving cell.
The multivalent compounds that are effective are those based on vanadium, tantalum,
zirconium and tungsten.
The preferred metal compounds are those that are readily soluble
in the plating bath, are relatively inexpensive, and readily available in commercial
quantities. Typical of the preferred compounds are those of vanadium whose valences
are 5+, 4+, 3+, and 2+. Any vanadium
compound can be used provided it can form the required ions in solution and is
not harmful to the bath. Examples of useful vanadium compounds are vanadium pentoxide
(V2O5), vanadium sulfate VOSO4, and sodium vanadate.
If vanadium pentoxide (V2O5), previously dissolved in acid,
is added to a tin plating bath, the existing V5+ reacts with tin2+
and becomes reduced to V4+, V3+, and V2+, primarily
by reacting with tin2+ and metallic tin anodes. The dominant ions in
solution are believed to be V4+, V3+ and V2+.
If tin2+ becomes oxidized to tin4+, it quickly reverts back
to tin2+ by reacting with V2+ and V3+ which then
becomes V4+. V4+ then reacts with tin anodes to regenerate
V2+ and V3+.
The other components of the electroplating baths are generally known
to one of ordinary skill in the art.
The tin compounds useable are those which are soluble in the basis
solution. The desired alloying metals can be added in any form which is soluble
in or compatible with the basis solution. When sulfonic acids are used, the metals
are preferably added in the form of sulfonate or sulfonic acid salts.
The acids which can be used in the invention are mentioned above
and illustrated in the following examples. Alkane sulfonic acids containing 1-7
carbon atoms; alkylol sulfonic acids containing 1-7 carbon atoms, aromatic sulfonic
acids, such as phenol sulfonic acid, alone or in combination, are suitable for
use as the basis solution. Methane sulfonic acid, and "Ferrostan" (i.e., phenol
sulfonic acid) are the most preferred. Salts or other derivatives of these acids
can also be used, provided that the solution is sufficiently acidic and can retain
all necessary components in solution. The pH range of these solutions will generally
be less than 5, preferably 2-3 or less.
Any of a wide variety of surfactants can be included in the electroplating
solutions of the invention. Since much of the electrodeposited tin is accomplished
using high speed electroplating processes and equipment, it is preferred to utilize
wetting agents or surfactants which are substantially non-foaming. Typical surfactants
of this type can be found in U.S. Patents 4,880,507 and 4,994,155.
When high speed electroplating is not necessary, any of the wetting
agents or surfactants of U.S. Patent 4,701,244 can be used. Of those surfactants,
the higher cloud point materials are preferred. In addition, the solutions of
the invention can contain brighteners, leveling agents or any other additives (such
as bismuth compounds or acetaldehyde) which are known to those persons skilled
in the art to improve the performance of the electroplating process or the properties
of the resulting electrodeposit. The '244 patent discloses such surfactants and
The amounts of these surfactants or other additives are not critical
and optimum amounts will vary depending on the particular agent selected for use
and the particular bath in which it is used. Generally, about 0.05 to 10 ml/1
of the wetting agents give excellent results with pure tin and 60/40 tin-lead alloy
baths. Higher amounts could be used but there is no particular reason to do so.
As the lead content of the bath is increased, additional amounts of these wetting
agents may have to be employed.
The electroplating solution can be prepared by placing tin or tin
and lead compounds in an excess of the selected acid, adjusting the acid content
to the required pH, adding the appropriate wetting agent and antioxidant compound,
removing undissolved matter by filtration, and then diluting with water to the
final desired volume. The electroplating solution is generally operated at ambient
temperatures, although agitation and elevated temperatures are desirable for high
speed electroplating. When the electroplating step is conducted under high speed
conditions, the agitation and solution turnover due to pumping action maintains
the oxygen content of the solution at or near its maximum concentration, thus
promoting the tendency of to oxidize tin2+ to tin4+. Under
these conditions, the use of the present antioxidants is most important to maintain
tin as tin2+.
Various alloys can be produced depending on the relative tin and
alloying metal ratios employed in the solutions. For plating a 60-40 tin-lead alloy,
for example, 20 g/l of tin metal and 10 g/l of lead metal can be used. Other ratios
can be routinely determined by one of ordinary skill in the art.
The scope of the invention is further described in connection with
the following examples which are set forth for the purposes of illustration only
and are not to be construed as limiting the scope of the invention in any manner.
In order to determine whether a material is capable of reducing sludge
in a given tin solution, a laboratory setup of the tin dissolving cell utilizing
oxygen with a fluidized bed of tin granules described in U.S. Patent 4,181,580
was constructed. The solution containing the antioxidant is pumped at a rapid rate
through a bed of metallic tin granules and oxygen is fed into the solution. The
rate of pumping was adjusted to a level capable of keeping the bed fluid with no
settling of the metallic tin granules. The result is very rapid mixing of the
oxygenated solution with the tin. This method of test is similar to that used by
J. McCarthy described above except that a vastly increased oxygen flow is used
with very thorough mixing of the oxygenated solution.
The tin solutions used in the above apparatus were the following:
Free acid g/l
All tests were made at ambient temperature, with a constant oxygen
flow of 283 × 103 cm3/hr. at 345 kPa (10 cu. ft./hr.
at 50 psi), and the same level of pumping to produce the same fluidized bed. The
same amount and size of tin granules was used to begin each test, the same volume
of solution was used each time in the same apparatus, and the time for each test
was 16 hours.
A number of electrolytes containing various antioxidants were prepared
and tested as noted above. The sulfate baths, those baths which did not contain
an antioxidant, and those baths which contained conventional antioxidants (i.e.,
examples 1-3 and 8-16) were included for comparison purposes. The test results
appear in Table 1.
Example - Bath
Dissolved Iron (g/l)
Tin IV rate of buildup (g/h)
V2O5 - 0.5 g/l
3 - MSA
4 - MSA
V2O5 - 0.5 g/l
5 - MSA
V2O5 - 0.5 g/l
6 - MSA
V2O5 - 0.5 g/l
7 - MSA
V2O5 - 0.5 g/l
8 - MSA
Catechol - 1 g/l
9 - MSA
Catechol - 1 g/l
10 - MSA
Catechol - 1 g/l
11 - MSA
Catechol - 1 g/l
The sulfate bath used by the Nanfang Metallurgical Institute described
earlier, developed tin4+ at a disastrous rate in this test and an unusually
high amount of sludge was formed. When 0.5 g/l of V2O5 was
added to this bath, there was an improvement; however, the amount of sludge and
amount of tin4+ generated was still completely unacceptable. Iron was
not added to the bath in this test since it would only have made matters worse,
as indicated by all other tests containing iron. Although the use of V2O5
in the Nanfang sulfate bath showed improvement in their tests, its use in strongly
oxygenated solutions was of little value. Note the extremely high rate of tin4+
build-up in the sulfate bath test results even with the addition of V2O5.
The results indicate that the sulfate bath would be impractical for use in high-speed
tin plating, even if V2O5 is added.
The combination of V2O5 with MSA (examples 4-7)
showed a remarkable improvement. This combination was capable of reducing the
amount of tin2+ buildup to essentially zero. When iron was added to
the bath, this build-up remained very close to zero even with an iron content
of 10 g/l. The bath containing a very high iron content of 20 g/l showed an increase
tin4+ build-up which shows the harmful effect of iron in the bath, even
with vanadium present.
The prior art tin baths containing MSA plus a catechol antioxidant
(examples 8-11) behaved similarly to the MSA bath with vanadium, but was much worse
than the MSA-vanadium bath when iron was added. These improved results with vanadium
compared with catechol in the iron-containing MSA baths proved the unexpected superiority
of vanadium as an antioxidant in the MSA bath.
The Ferrostan bath containing stannous sulfate and phenolsulfonic
acid does not normally contain an additional antioxidant since phenolsulfonic acid
is itself known to be a reducing agent or antioxidant. These baths behaved similarly
to the MSA plus catechol bath when iron was added in increasing amounts up to
10 g/l. When 20 g/l iron was present in the tests of both the MSA and Ferrostan
baths, the build-up of tin4+ in the Ferrostan bath became excessive
by comparison to the MSA. The Ferrostan bath thus remains commercially feasible
only when iron is periodically removed from production baths to minimize its harmful
effects relating to sludge formation.
Additional tests were performed using the method of McCarthy. In
these tests, the same amount of oxygen was bubbled into each flask under test containing
tin granules plus the solution being tested. The major difference between the
two test methods is the amount of oxygen bubbling into the test solutions and the
time of test. The McCarthy test was run for 7 days at ambient temperature and
oxygen flowed at 5.66 × 103 cm3/hr. (0.2 cu. ft./hr.).
To compare the utility of the antioxidants in organic sulfonic acid
solutions with that in other divalent tin acid solutions, tests were also performed
using tin in the "Halogen" tin bath based on hydrofluoric and hydrochloric acids,
used in present day production for high speed plating. Without an antioxidant,
the baths exhibited the same sludge problems exhibited by MSA and Ferrostan solutions.
Tantalum, tungsten, zirconium, chromium, and molybdenum were also used as additional
examples of multivalent ions which perform similar to vanadium and which demonstrate
antioxidant qualities when iron was present in the plating solution.
The Halogen bath contained the following components:
3.2 - 3.6
The bath was formulated with an antioxidant in accordance with the
present invention. Tantalum was added as tantalum chloride, vanadium as vanadium
sulfate, tungsten as sodium tungstate, zirconium as zirconium sulfate, chromium
as chromium sulfate, and molybdenum as molybdenum chloride. The amount of metal
used as an antioxidant in each solution was 0.28 g/l.
The Ferrostan bath was the same as that in the previous test.
Ten g/l of dissolved iron was added to some test solutions and 20
g/l of dissolved iron to others in order to simulate production baths containing
iron. The bubbling oxygen test results are shown in Table 2.
Results show that vanadium, tantalum, zirconium, and tungsten are
effective as antioxidants to reduce tin4+ buildup in the presence of
oxygen. Chromium and molybdenum are far less effective. The baths in which the
antioxidants are effective are organic sulfonic acid based baths such as methyl
sulfonic acid and phenolsulfonic acid. Results with the Halogen bath were poor,
showing that the antioxidants are not effective in these baths when they contain
iron. The Halogen baths are successful in production since iron, which accelerates
tin4+ buildup, is constantly being removed from solution and is not
permitted to build up to any appreciable amount.
The useful quantities of these multivalent metal antioxidants can
vary from about 0.025 g/l of metal in solution to about 5 g/l. Their effectiveness
is apparent in very low concentrations with increasing effectiveness with increasing
concentration until about 1 g/l. Above 1 g/l, there is only slight improvement.
Generally, the multivalent metals either do not co-deposit at all with the metal
being plated or they may only be detected in the deposit in trace amounts.
Example - Bath
Antioxidant (one g/l)
Dissolved Iron (g/l)
% Divalent Sn lost to form Sn IV
17* - MSA
18 - MSA
19 - MSA
24 - MSA
25 - MSA
26 - MSA
27* - MSA
28* - MSA