Note: Descriptions are shown in the official language in which they were submitted.
1~L5577
This application relates to tin-nickel alloys, to
aqueous electroplating baths capable of depositing such
alloys on various metallic substrates, to a method of elec-
trodepositing the tin-nickel alloy on such metallic sub-
strates, and to the resulting tin-nickel alloy coated metallic
substrates.
Gold and alloys of gold have long been favored as
coatings on various metallic substrates, both electrically
conducting and non-conducting, for the purpose o~ providing
a durable, smooth and bright ~inish ~lhich to~ether ~lith the
substrate material ls easily soldered. Drastic recent
increases in the price o~ gold have generated considerable
interest ln substitutes for gold or gold alloys and attention
has been dlrected to tin-nickel alloys as candidates in
view of the known durability of such alloys.
Howeverj when it was attempted to electrodeposit on a
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metallic substrate a known tin-nickel alloy containing 65%
tin and 35% nickel, it was foulld that the resulting article
was deficient in many properties, expecially brightness,
ductility, solderability, electrical conductivity and contact ;
resistance. The electrical properties of the coatlng are
highly critical when the tin-nickel plated article is to be
utiliæed as a component o~ an electronic system, especially
in aerospace electronics where high temperature stability is
a serious consideration.
Commercial acceptance of an alloy also requires versa-
tility of application For example, duplex coatings are now
widely used on metallic substrates to improve corrosion
reslstance and to act as barriers to diffusion of an under-
lying metal to t~e surface. Such duplex coatings includ
~L~455~7
copper-nickel-chrome, copper-gold, tln plate over nickel
or copper, gold over tin-nickel, and many other combinations.
Examples of duplex coatings having barrier properties are
copper under tin on brass components to prevent di~fusion
of zinc through the tin, and copper, or nickel under gold,
on zinc die castings for the same reason. In addition,
if the same alloy exhibits superior brightness, smoothness,
and durability, it can also be used where electrical proper-
ties are not as important, such as in the electroplating
of' watch parts, surgical instruments, drawing instruments
and balance weights. An alloy which also exhibits non-
magnetic propertles is especially useful in the electro-
plating of watch parts.
From the standpoint of solderability, the tin-nickel
alloy of U. S. Patent 3~573,008 would seem to be an out-
standing alloy for the applications discussed. However, it
was found that even when the tin-nickel alloy of that patent,
containing only 3% nickel, was electroplated on a metallic
substrate, the electrical properties of the plated article
were deficient in many respects. Nor did the plated
article have the re~uired brightness, even when the plating
bath contained the ammonium bi~luoride brightening agent
recommended in the patent.
OBJECTS A~D SUMMARY
Accordingly, an ob~ect of the invention is to provide
a new and improved tin-nickel alloy having great versatility
of application in the coating of metallic substrates f`or
the electronic and other industries.
Another ob~ect is to provide a new and improved tin-
nickel alloy, an electroplating bath and method of electro-
plating such alloy~ for the efficient electroplating Or
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metallic substrates, which electroplate exhibits brightness,
durability, smoothness and has a beneficial effect on the
electrical and soldering properties of the substrate material.
A further object is to provide an equal or superior
substitute for gold as an electroplate on various metallic
substrates in the manufacture of conducting and non-conducting
articles.
These and other objects, features and advantages of
the invention will be apparent from the specification which
follows.
In accordance with the invention, there is provided
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an aqueous electroplating bath capable of depositing an alloy
consisting essentially of 0.001 - 0.30 wt. % nickel and the
balance tin, said bath consisting essentially of (a) a nickel
sal-t selected from nickel sulfate, nickel fluoborate and nickel
sulfamate, (b) a corresponding tin salt7 (c) sulfuric acid,
sulfamic acid, benzene sulfonic acid or fluoboric acid in an -
amount providing a pH of not higher than l, and (d) an effective
amount of a brightening a~ent, wherein the concentration of ;
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said tin salt is at least about l oz./gal. and the weight ratio ;
of said nickel salt to said tin salt is at least 0.20 to 1.
The nickel salt is preferably nickel sulfate, the tin
salt is preferably stannous sulfate, the acid is preferably
sulfuric acid, the brightener is preferably N-benxyltrimethyl
ammonium bromide or hydroxide, and the concentration of the
stannous sulfate is preferably about 1-7 oz./gal.
Alternatively, the nickel salt may preferably be
nickel fluoborate, the tin salt may preferably be stannous
fluoborate, the acid may preferably be fluoboric acid, the
brightener may preferably be N-benzyltrimethyl ammonium bromide
or hydroxide, and the concentration of the stannous fluoborate
may preferably be about 1-7 oz./gal.
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From a different aspect, the invention relates to a
method of coating a metallic substrate so as to provide the pro~
perties of a gold coating thereon but without utilizing gold,
which comprises electrolytically contacting the said substrate
with an aqueous electroplating bath capable of depositing an
alloy consisting essentially of 0.001 - 0.30 wt. % nickel and
the balance tin, said bath consisting essentially of (a) a nickel
salt selected from nickel sulfate, nickel fluoborate and nickel
sulfamate, (b) a corresponding tin slat, (c) sulfuric acid,
sulfamic acid, benzene sulfonic acid or fluoboric acid in an
amount providing a pH of not higher than 1, and (d) an effective
amount of a brightening agent, wherein the concentration of said
tin salt is at least about 1 oz./gal. and the weight ratio of
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said nickel salt to said tin salt is at leastJl~to 1.
An anode rnay be utilized consisting essentially of
a metal resistant to excessi~e acid attack and to formation of
a coating thexeon.
Said anode may consist essentially of nickel, platinum,
or platinum-plated titanium.
It has beer~ found that certain combinations of metal
salts and electroplating conditions will give the foregoing
results. The electrolytic salts which are effective are the
sulfates~ sulfamates and fluoborates of tin and nickel.
With these salts must be used the corresponding acid, that
is, benzene sulfonic acid, sulfuric acid, sulfamic acid or
fluoboric acid, in amounts to provide a pH no higher than 1 in
the aqueous bath from which the metals are to be deposited.
The highly acidic pH enhances conductivity and permits the
utilization of certain brightening agents which contribute to
good throwing power and which would not otherwise be compatible
in the bath. The highly acid condition also prevents hydrolysis
of the tin salts.
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Other conditions important for attaining -the objects
of the invention are the concentrations and relative amounts of
the reactive ingredients in -the electrolytic bath, and the
selection of anodes for the electrodeposition. The bath must
contain the tin salts at a concentration of at least about 1 Z~9Q~
and the weight ratio of the nickel salt to the tin salt must be at
least 0.20 to 1. The upper limits on the concentrations of the
salts relate to economy and solubility since over about 7 oz./gal~, .
a tin salt tends to be insoluble.
The highly acidic condition of the bath requires careful
selection of anodes to avoid acid corrosion or interference with .
electrodeposition due to other forms of attack on the anode,
such as formation of a passivating film. It has been found that
one or more nickel anodes or their e~uivalents provide -the be~st
results. Equivalent anodes include platinum or platinum-plated :
titanium.
The versatility of the alloys of the invention is
perhaps most evident in that the alloys may be electrodeposited
on a great variety of metallic substrates in order to enhance
the properties of the substrates as finished articles of
manufacture. For the purposes of this specification, "metallic
substrate" means and includes a base ma-terial of a single
metal, a base material of a single alloy, a base material which
is formed of a non-metallic layer such as plastic, mineral fiber
fabric or non-mineral fiber fabric coated with a metallic layer ::
(a single metal or an alloy), or so-called "duplex" materials.
The latter usually comprises a single base metal such
as zinc, which is first coated with a "barrier" metal (pure .
metal or alloy) and then top coated with another metal ~pure .
alloy). The barriex layer prevents diffusion of the base metal ~ :
-to the surface and often contributes to other desirable properties,
such as solderability, smoothness, hardness, and the like. While
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normally the metallic substrate will be in a finished or semi-
finished state of manufacture, such as electrical contacts, medical
instruments or watch parts, the term is also intended to cover
materials requiring additional processing steps to place them in
finished form as articles of manufacture.
In its major aspects, the invention includes a novel
aqueous electrolytic bath, a method of coating a metallic sub-
strate with the alloy utilizing the electrolytic bath, and an
article of manufacture comprising a metallic substrate coated
with the novel alloy.
DETAILED DESCRIPI'IO~
The electroplating bath of the invention is a highly
acidic, aqueous solution containing stannous sulfate and nickel
sulfate, or stannous fluoborate and nickel fluoborate, or
stanr-ous sulfate and nickel sulfamate, as the alloy yenerating
constituents. The high acidity, of the order of pH 1 or lower,
preferably pH .4 or less, is provided by a corresponding acid,
that is, benzene sulfonic acid, sulfuric acid, fluoboric acid or
sulfamic acid. While certain other mineral or organic acids
will be useful, alone or in combination with the foregoing acids,
to provide the requisite acidity, such other acids when utilized
alone as the acidifying agent do not contribute as effectively
as the foregoing acids to the solubility and compatibility of
the metal generating salts and other ingredients and therefore
no advantage is had by their utilization. Benzene sulfonic acid
may also be used in
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1~55~7 :~
admixture ~ith sulfuric acid. Hydrochloric acid is to be
avoided due to its highly corrosive action on the elec-
trodes and electrodeposition apparatus, and the noxious
character of its fumes.
The plating bath preferably is formulat;ed to contain
a brightening agent or agents. These agents not only
enhance the brightness and color of the resulting alloy
but also, ~7hen carefully selected, enhance the throwing
power of the bath. The preferred brighteners for the su]famate,
sulfate or fluoborate baths are ketones, colloids, imida-
zolines and quaternary ammonium compounds.
The ketones include saturated and unsaturated
aliphatic, cycloaliphatic, heterocyclic and aromatic
ketones such as l-benzoyl acetone, trans-4-phenyl-3-buten-
2-one, catechol, and those described in U. S. Patent
3,429,790, diones and complex ketones such as the imida-
zol~ne derivatives of Belgian Patent 652,4Q5. Colloidal
brighteners include gelatin, alone or in combination with
tannins, particularly when used with benzene sul~onic acid,
and wood tars. Imidazolines include those of U. S. Patent
3,575,826.
The quaternary ammonium compounds include N-benzyl-
trimethyl ammonium bromide or hydroxide, l-hexadecyl
pyridinium bromide, and benzyl hexadecyl dimethyl ammonium
bromide.
The brightener may be present in the bath in an amount
of about .0001 percent to about 1.0% by weight of the
solution, depending upon the level of effectiveness of the
brightener and its solubility. Solubility of the brightener
and other ingredients as well as rate of solution can be
controlled in routine fashion by the addition of dlspersants
known in the electroplating art, such as the lignin derivative
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5577
sold under the brand name "Lignasoll' by Lignasol Chemical
Corporation, nonlonics such as alkylphenol-alkYlene oxide
condensates`, and lonics to the extent o~ compatibiliky with
other ingredients of the plating bath.
Methods of forming the electrolytic baths of the
invention are not critical. The usual precautions known in - -
the electroplating art in connection with the handling of
highly acid and reactive materials should be observed. One
convenient mode of formulating the solutions is to dissolve
each of the salts in a moderately acidic solution to ~orm a
concentrate, dilute the solution to the requisite strength,
and then add the brightening agents~ alone or in combination
with suitable dispersants. The acidity of the solution may
then be ad~usted, usually by the addition of a concentrated
acid. In some instances it may be preferable to separately
dissolve only the tin salt in the acid solution and then
add the acid solution of the tin salt to a solution of the
other ingredients~ in order to minimize any hydrolysis of
the tin. The solutions or pre-mixed components thereof,
may be agitated and/or heated somewhat to promote dissolvin~.
Generally, temperatures above abouk 90F. are unnecessar~
and should be avoided, particularly if it is lnconvenlent
or uneconomical to cool the solution to the preferred tempera~
ture for electrodeposition. Acid may be metered to khe bath
during the electrodeposition process in order to maintain
the requisite pH. A 50% acid solution is a suitable acid
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solution for this purpose.
The tin salt should be present in the aaueous solution
at a concentration of at least about 1 oz./gal. The tin salk
concentration may be higher but generally concentrations
over about 7 oz./gal. are uneconomical~ In the case of
kin sulfate, considerable dif~iculty in solubilization :ls
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experienced over 7 oz./gal. The ratio of the nickel
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salt to the tin salt should be at least e~ to 1. The
upper limit on the ratio is not important except from the
standpoint of economy and solubility. A preferred weight
ratio range is 0.25:1 to 1.75:1, nickel salt to tin salt.
Representative concentrations providing this range are
4 oz./gal. NiS04 . 6H2o and 1-7 o~./gal. SnS0~.
The ef~ective concentrations and proportions of metal
salts are also related to the pH and the concentration of
acid in the plating bath. For example, the more sulfuric
acid present in the bath the more difficult it is to
solubilize the higher concentrations of tin sulfate. ~on-
sequently, the results are best when the concentrations
o~ salts are kept wlkhin the stated ranges.
Any apparatus conventional in the metal electroplating
art may be employed in producing the metal alloys and metal
coatings o~ the invention. Normally the metallic substrate
upon which the alloy is to be electrodeposited will be the ~ ;
cathode. A plurality of substrates may be electroplated
simultaneously, if desired. The ahode should be nickel or
an equivalent metal, that is, a metal which is not excessively
attacked by the acid in the bath and which will not be
oxldized or coated by ingredients of the bath so as to
passivate the anode. For example, copper cannot be used as
an anode metal because it is attacked by the highly acidic
bath. Anodic materials equivalent to nickel are platinum
or platinum-plated titanium. A plurality o~ anodes may
be utilized, some of which may be tin anodes. Tin as an
anodic material permits plating ~or only a short period of
time without developing a resistive film and is normally
to be avoided, at least as the s~le anode metal.
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1~557~ ~
Current densities and electrodeposition times are not
critical and can vary widely depending upon other conditions
of the process, such as temperature of the bath, the
particular ingredients in the bath and their concentrations~
and the thickness desired in the electrodeposited alloy.
Generally, good results are achieved at room temperature
and at current densities of` about 10 - 30 amperes per
square foot over about 5 - 30 minutes treating time. The ~ ;
temperature during the electrodeposition may range up to about
lOO~F. but there is no particular advantage in going above
room temperature and some disadvantage for particular
ingredients. For example, 100F. should not be exceeded
when tln sulfate is an ingredient of the bath since at
about this temperature the stannous sulfate hydrolyses
and stannous ion is oxidized to a higher valence stake,
leading to an insoluble sludge of stannic hydroxide.
Agitation of the bath during preparation and plating, when
the temperature rises above room temperature, helps to
minimize formation of the sludge. The resulting deposits
generally may range in thickness of from about 30 to 300
microinches and are clear, smooth, firmly adherent and
silvery white in color.
Metallic substrates upon which alloys of the invention
may be electrodeposited include single base metals such as
iron, nickel, silver, surgical skeel, aluminum, copper,
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alloys of copper and the llke, and intermediate or top
coatings in the ~ormation of duplex coatings on certain
substrates. For~example, the alloy may replace the ;`
gold in the top coating in the case of nickel over zinc
die castings. Other top coating replacements include
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5~77
replacement of the chrome in copper-nickel-chrome coatings,
of the gold in nickel-gold coatings, and of the tin plating
ln tin plate-nickel or copper.
~he invention also holds promise for restoring silver
plating to its prior prominence. Silver pl~lting in the
electronic industry has substantially ~allen out of use
because of tarnishing and silver-migration. Although
tarnishing was mitigated somewhat by electrolytic alkaline
chromate treatment and by the application of organic
materials for improving the resistance o~ silver to
tarnishing, it was found that when silver connectors treated
in this manner were mated or so~t-soldered the pro~ection
was destroyed. It has now been discovered that by over~
coating silver-plated electronic components~ before
tarnishing and silver migration set in~ such effects are -
eliminated completely. A coating o~ about 50 millionths
o~ an inch of the tin-nickel alloy o~ the invention ~rill
provide this result.
The tin-nickel alloy of the invention also provides
a hlghly advantageous substitute for conventional nickel
plating utilized in the electronic industry as a protective
finish. Conventlonal nickel plating has two shortcomings.
First, the contact resistance of nickel is high. Secondly,
when nickel-plated electronic components require the --~
application o~ soft solder ~or connecting wires or for
Joining to other components, non-corrosive rosin flux.
The following ~xamples are intended as further illus-
trations o~ the inventi~n but are not necessarily limitative
except as set forth in the claims. All parts and percentages
are by we~ght unless otherwise indicated.
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45577
Exam~le l
In a suitable plating vessel equipped with mechanical
stirrer, an aqueous plating bath was prepared having the
following formulation: -
stannous sulfate50 grams per liter
nickel sulfate
(NiS04 . 6H2o)lO grams per llter
benzene sulfonic acidlO0 grams per liker
gelatin 2 grams per liter
tannin 2 grams per liter
After mechanical mixing for one hour at about 20 - 40C.,
the solution was allowed to cool to roorn temperature and one
nickel anode and one tin anode were immersed therein. Brass
plates which had previously been alkaline-cleaned, bright-
dipped in a sulfuric-nitric acid solution and well rinsed,
were then also immersed in the solution as cathodes. The
electrodes were then suitably connected to a power supply.
The current density ranged between 20 and 3 amperes per~ ;
square foot. After 5, lO, 20 and 30 minute intervals, the
brass plates were removed from the bath and inspected. It
was determlned that the brass plates had become plated with
a tin-nickel alloy containing about O.l~ and 0.3~ nickel and
the balance tin to depths of about 30 - 300 microinches ;
depending on the duration of plating. The electrodeposited
coatings were clear, smooth, firmly adherent and silvery
white in color. The pH of the plating bath ~ras under 0.4.
Certain of the electroplated coatin~s had a Knoop
hardness of 18.0 and 19.0 (as compared with about 22 for
tin) and a melting point of ~24F. (as compared with 449F.
~or bright tin electrop1ate). The contact resistance of
the alloy relative to gold was measured by comparing contacts
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electroplated with 20 millionths of an inch gold w~th the
same contacts plated with 100 millionths of an inch of the
new tin-nickel alloy. At a current of 1 ampere, the gold-
plated contacts showed a millivolt drop of 2.4 to 2.8
whereas the tin-nickel alloy plate showed a millivolt drop
of 2.2 to 2.6. The contact resistance after a 500 cycle
wear test indicated an average increase of 0.2~ millivolts
for the tin-nickel alloy as compared with an average-increase
of 0.41 millivolts for the gold plated contacts. m e contact
resistance a~ter 48 hours o~ salt spray testin~ increased on
an average of 0.30 millivolts for the tin-nickel alloy as
compared with 0.40 millivolts for the gold plated contacts.
It was concluded that the new tin~nlckel alloy provided
the same or better electrical properties as compared with
gold and there~ore was a highly practical substitute ~or
gold as a coating for electronic components.
Example ? :
Substantially as described in Example 1 an a~ueous ~;
electroplating bath was pre~ared having the follo~ing
~orrnulation: .
stannous sulfate50 grams per liter
nickel sulfate
(NiS04 . 6H20)10 grams per liter
s~lfuric acid (98%)100 grams per liter -
N-benzyltrimethyl5 grams per liter
ammonium bromide ~-
As the result of electroplating substantlally as
described in Example 1, brass plates were shown ko be coated
with firmly adherent tin-nickel alloy deposits having a bright
white and smooth appearance~ and a nickel content of about
~.~% and 0.3~ ith the balance tin. Optimum deposits
were obtained at a current density between 15 and 30 amperes
per square foot and at a temperature of 20 _ 40~C.
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Example 3
Substantially as described in Example 1 an aqueous
electroplating bath ~las prepared having the folloT~ing
formulation:
tin fluoborate100 grams per liter
nickel fluoborate20 grams per liter
fluoboric acid100 grams per liter
gelatin2 grams per liter
N-benzyltrimethyl
ammonlum hydroxide1 gram per liter
Under substantially the same electroplating conditions
as described in Example 1, smooth, bright, ~Jhite and firmly
adherent deposits of a tin-nickel alloy containing about
0.1% nickel, the balance tln, were obtained. ~bove about
25 amperes per square foot curren~ density, however, the
deposit evidenced hlgh porosity and low coverage on a bent
cathode.
Example 4
In the manner indicated below, an aqueous electroplating
20 bath was prepared having the following formulation:
stannous sulfate 50 grams per liter
nickel sulfamate50 grams ~er liter
sulfamic acid100 grams per liter
N-benzyltrimethyl
ammonium bromide5 grams per liter
l-benzoyl acetone1 gram per liter
Specifically, the platin~ solution ~as prepared by dissolving
the metal salts ln 900 ml. of 100 g./l. sulfamic acid. The
organlc components were dissolved in 100 ml. of the 100 go/l~ ,
sulfamic acid solution and the resulting solution was added
to the first solution.
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Plating was carried out under substantially the same
conditions as indicated in Example 1 except that the effects
of varying the current density from 5 to 100 aM~eres per
square foot were studied. It was determined that at a
current density of 5~50 amperes per square foot the current
e~ficiency was 90 ~ 5% and that movement of the cathode
permitted higher current densities without burning as well as
uniform corrosion of tin and nickel anodes without passiva-
tion, even up to a current density of 100 amperes per square
~oot. The alloy contained about ,007~ nickel, the balance
tin.
Other Ex~eriments
.
Generally, with respect to most of the experiments and ;
formulations, the N-benzyltrimethyl ammonium bromide
appeared to have a retarding action on the oxidation of the
stannous ~alts. For optimum results it was determined that
the anode area should be about twice the cathode area and
the anode current density should be kept below about 25
amperes per square foot in order to prevent passivation of
the tin anodes. Also, current densities above 25 amperes
per square foot tend to increase the brittleness of the
electrodeposited coating whereas deposits produced below
25 amperes per square foot were generally quite ductlle.
Levels of porosity of the deposits were minimum under these
conditions, and le~eling and throwing power of the baths
were excellent.
Above pH 1 the current effioiency falls radically,
The optimum temperature ~s about room temperature to about
30C, At a temperature higher than 50C. the current
e~iciency falls~ probably due to oxidation of stannous salts
to stannate.
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It is believed that the acids in the plating baths may
be used singly or in admixture.
In view of the foregoing description it will be apparent
that the invention is not limited to the speclfic details
set forth therein for the purposes of illustration, and that
various other modifications are equlvalent for the stated .
and illustrated functions without departing from the spirit
and scope of the invention.
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