Note: Descriptions are shown in the official language in which they were submitted.
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Regeneration Method For A Plating Solution
Specification:
The invention relates to a method of depositing a layer of metal, more
specifically a layer containing tin, above alt for fabricating printed circuit
boards
and other electrical circuit carriers, and to a method of regenerating a
solution
containing metal ions in a high oxidation state, more specifically Sn(IV)
ions.
The plating method is mainly intended for utilization in the production of
solderable layers and etch-resist layers as well as in the deposition by
cementation of layers of tin onto conductive patterns made of copper more
specifically on the inner layers of printed circuit boards in order to bond
said
inner layers together.
For fabricating printed circuit boards, layers of tin and tin alloys, more
specifically tin-lead coatings are deposited onto the copper surfaces to serve
diverse purposes.
On the one side, tin-lead alloy coatings serve as solder pads on the surface
of
~ the printed circuit board at the places at which electronic component parts
are
to be soldered. In this case, such layers are deposited locally in those
regions
in which leads or other connecting elements of the component parts are to be
electrically connected to the copper surface. After the solder regions are
formed on the copper surfaces, the components are mounted on the solder
pads where they are bonded. Next, the solder is remelted in an oven to allow
the electrical interconnections to form.
Layers of tin may also be used as etch-resist layers, e.g., to form metal
patterns on the surfaces of the printed circuit boards. For this purpose, a
negative image of the conductive pattern is at first formed on the copper
surfaces by means of a photo-patternable resist. Then, the layers of tin or of
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tin-lead alloy are deposited in the canals of the resist layer. After the
resist is
removed, bare copper may be removed by etching so that it is only the circuit
traces and all the other metal patterns on the surfaces of the printed circuit
board that remain below the layer of tin or tin-lead.
Furthermore, tin layers are also utilized as intermediate layers between the
copper surfaces of the inner layers of multilayered circuit boards and the
areas
of the dielectric (usually glass fiber reinforced layers of resin). For to
provide
tight bonding of the copper areas with the dielectric, it is necessary to
roughen
the copper surfaces prior to pressing in order to achieve sufficient bonding
strength between copper and resin. To accomplish this, the surfaces have
heretofore been superficially oxidized by a so called black oxide treatment.
However, the thereby formed oxide layer is not sufficiently resistant to acids
so
that the inner layers, which have been cut in the process of drilling the PCB
material, are delaminated from the resin of the PCB material, forming
delaminations. This problem is avoided when tin layers are used instead of the
black oxide layers. For production, the tin layers are directly deposited by
cementation onto the copper surfaces of the circuit traces. In post-treatment,
if
necessary, further bonding compounds are applied to the tin layers (e.g., a
mixture of an ureidosilane and a disilane cross-linking agent
(EP 0 545 2'16~A2)) before the inner layers are pressed together by action ~of
heat and pressure.
Whereas, in the second application mentioned, the layers of tin or tin-lead
alloy, respectively, can be electrolytically deposited as no electrically
isolated
metal regions have to be tin-plated, in the first and in the last mentioned
case
tin cannot be deposited by means of an electrolytic method since the copper
areas to be metal plated usually are electrically mutually isolated so that it
is
hardly possible to establish an electric contact. For this reason, so called
cementation baths are at hand for tin-plating.
A plating bath of this type is described in U.S. Patent No. 4,715,894. In
addition
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to a Sn(II) compound, this bath also contains a thiourea compound and an urea
compound. According to EP 0 545 216 A2, thiourea, urea and the derivatives
thereof may also be used as alternatives. Furthermore, the solution in
accordance with U.S. Patent No. 4,715,894 also contains a complexing agent,
a reducing agent and an acid. Accordingly, the Sn(11) compound used is SnS04
for example. According to EP 0 545 216 A2, the bath contains Sn(II)
compounds of inorganic (mineral) acids, for example compounds of acids
containing sulfur, phosphorus and halogen, or of organic acids such as Sn(II)
formiate and Sn(II) acetate for example. According to EP 0 545 216 A2, the
Sn(II) salts of the acids containing sulfur are preferred i.e., the salts of
sulfuric
acid and of sulfamic acid. Furthermore, the bath may also contain alkali metal
stannates such as sodium stannate or potassium stannate. Moreover, the
thiourea and the urea compounds are, in the simplest case, the unsubstituted
derivatives of thiourea and urea, respectively. According to EP 0 545 216 A2,
Cu(I) ions complexed with thiourea are to form onto the copper surfaces when
tin is deposited. Concurrently, metallic tin is deposited by reduction of
Sn(II)
ions. In this reaction, copper is dissolved, a tin coating being
simultaneously
formed on the copper surfaces.
EP 0 545 216 A2 reports that the Cu(I) thiourea complex enriches in the
_ _.. __...
'solution. Sn(lV)~ions also enrich in the solution through oxidation of Sn(II)
ions
as oxygen from the air is carried into the solution. However, the
concentrations
of the Cu(I) thiourea complex and of the Sn(IV) ions do not exceed stationary
concentration values when the printed circuit boards are merely immersed into
the solution for treatment, since the bath solution is permanently drained
away
f
by the boards and diluted with water that has been carried over. If however,
the
bath fluid is sprayed onto the copper surfaces by way of spray nozzles, the
rate
of substance turnover, related to the volume of the bath, is considerably
higher.
Under these conditions, the concentration of the Cu(I) thiourea complex
increases to such an extent that the limit of its solubility is reached and
the
complex precipitates as a deposit. The deposit clogs the nozzles and causes
problems in the movable mechanical parts of the plant. In the plating bath,
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Sn(IV) compounds are also increasingly formed by oxidation of the Sn(II) ions
through oxygen from the air as air is carried to a greater extent into the
bath
solution by spraying the latter onto the printed circuit boards.
To mitigate these problems, the following provisions have been described in
the publication mentioned: to reduce the concentration of the Cu(I) thiourea
complex, part of the solution of the plating bath is taken from the treatment
container to another tank where it is left to cool down so that a large part
of the
complex precipitates and can thus be separated. The solution, which is now
largely freed from the complex, may then be returned to the treatment
container. To further lower the concentration of the Sn(IV) ions in the
plating
solution, there is provided a reservoir for the plating solution that contains
metallic tin. The solution contained in said reservoir is sprayed onto the
copper
surfaces, the Sn(II) ions being reduced according to the reaction equation (1)
set forth below, and metallic copper simultaneously oxidizing to form Cu(I)
ions
according to reaction equation (2) which is also set forth below. A complex
with
thiourea or with the derivatives thereof, respectively, is formed thereby.
Simultaneously, through the oxygen carried into the solution, part of the
Sn(II)
ions oxidizes to form Sn(IV) ions according to the reaction equation (3) set
forth
below. The sprayed solution is next returned to the reservoir. There, the
Sn(IV)
,.
ions react witli the metallic tin to form the double quantity of Sn(II) ions
according to the reaction equation (4) set forth below.
The method of regenerating tin-plating cementation baths described in
EP 0 545 216 A2 proved however to cause the concentration of tin contained in
the solution to rise continuously. Therefore, the concentration of Sn(II) ions
in
the solution must be subjected to permanent analytic control. This is often
not
easily possible under manufacturing conditions and often readily causes the
concentration to vary greatly. As a result thereof, the deposition of tin can
become uncontrollable. This is not acceptable. One approach to overcoming
this problem could involve automated monitoring of the concentration of Sn(ll)
ions and permitting or obviating contact between the plating solution and
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metallic tin in the reservoir when a predetermined range of reference values
is
exceeded or not reached, respectively. This is very complicated though and
requires quite complicated devices.
5 It is therefore an object of the present invention to overcome the problems
mentioned and to find means permitting tin plating of copper surfaces by
cementation without variations of the Sn(II) ions content affecting the
deposition of tin. It is aimed at making this possible without the use of
complicated devices.
A solution to this object is the plating method of claim 1 and the
regeneration
method of claim 14. Preferred embodiments of the invention are indicated in
the subordinate claims.
The plating method in accordance with the invention serves to produce layers
of metal, more specifically layers containing tin and preferably layers of
pure
tin. The method can also be utilized for depositing layers consisting of a tin
alloy. It involves the following method steps:
a. Providing a metal plating bath, more specifically a tin plating bafih;
containing metal ions in-a low oxidation state, more specifically Sn(ll)
ions,
b. Depositing a metal layer from the metal plating bath onto a work
piece;
c. Providing an electrolytic regeneration cell comprised of at least one
auxiliary cathode and of at least one auxiliary anode;
d. Electrolytically depositing, in the electrolytic regeneration cell, metal
serving for regeneration, more specifically metallic tin, from the metal
plating bath onto the at feast one auxiliary cathode;
e. Bringing the metal plating bath into contact with the metal serving for
regeneration in an effort to reduce metal ions in a high oxidation state
contained in the metal plating bath, more specifically Sn(IV) ions, to
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metal ions in a low oxidation state, more specifically Sn(II) ions.
The regeneration method of the invention serves to regenerate solutions
containing metal ions in a high oxidation state, more specifically Sn(IV) ions
in
order to reduce the metal ions in the high oxidation state to metal ions in a
low
oxidation state, more specifically to Sn(II) ions. This comprises the
following
method steps:
a. Praviding an electrolytic regeneration cell comprised of at least one
auxiliary cathode and of at least one auxiliary anode;
b. Electrolytically depositing, in the electrolytic regeneration cell, metal
serving for regeneration, more specifically metallic tin, from the solution
onto the at least one auxiliary cathode;
c. Bringing the solution into contact with the metal serving for
regeneration in an effort to reduce metal ions in the high oxidation state,
more specifically Sn(IV) ions, to metal ions in the Low oxidation state,
more specifically Sn(II) ions.
When hereinafter layers containing tin, a tin plating bath or a tin plating
solution, metallic tin, Sn(II) ions, Sn(IV) ions and a tin electrode or an
electrode
containing tin, respectively, are referred to, this should also apply
generally and
in lieu of to metal layers, a metal plating bath, metal, metal ions in a low
oxidation state, metal ions in a high oxidation state, a metal electrode or an
electrode containing metal, respectively.
The methods of the invention may more specifically be utilized for electroless
deposition of tin or tin alloys utilizing a reduction agent, for the
electrolytic
deposition of tin and tin alloys and for the deposition by cementation of tin
or tin
alloys.
By a method of deposition by cementation a method is meant by which the
metal to be deposited receives from the substrate metal the electrons needed
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r
for the reduction to the oxidation state zero, said substrate metal
concurrently
oxidizing and being preferably dissolved thereby.
The method of the invention more specifically serves to coat copper surfaces
on printed circuit boards or other circuit carriers with tin containing
layers.
In the method described in EP 0 545 216 A2, metallic tin is added to the
plating
solution contained in the reservoir in order to convert Sn(IV) ions to Sn(II)
ions.
By contrast, with the method of the invention, the metallic tin utilized for
regeneration is produced by electroplating it from the very tin plating bath.
The
method of the invention thus permits to avoid variations in the concentration
of
Sn(II) ions contained in the plating bath. This can be explained as follows:
When tin is deposited from an electroless, cementation or electrolytic tin
bath,
the following reaction takes place:
Sn2+ + 2 e' _______________> 2 Sn (1 )
In electrolytic deposition, the electrons originate from an external source of
electric current and are delivered to the Sn(ll) ions via the cathode. In the
case
of electroless fin plating, the electrons needed for depositing the metal are
provided by a reduction agent. In deposition by cementation, the electrons
originate from the dissolving base metal, in the present case copper, onto
which tin is deposited:
2 Cu ________________> 2 Cu+ + 2 a (2)
In an interfering side reaction, Sn(II) ions oxidize in these baths, through
the
oxygen from the air, to form Sn(1V) ions:
Sn2+ + ~/2 02 + H20 _________> Sn4+ + 2 OH- (3)
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The Sn(IV) ions formed tend to precipitate tinstone (snow). The problems
related therewith are, inter alia that spray nozzles for delivering the
plating
solution to the copper surfaces may clog and that the function of movable
parts
in the processing plant may be impaired or the parts may even be damaged by
precipitating solid matter. Furthermore, the Sn(1V) ions also have the
disadvantageous property that the layer of tin, freshly deposited according to
the reaction equation (1 ), is attacked by the Sn(IV) ions according to the
reaction equation (4) set forth herein below, so that it may be dissolved
again,
at least partially.
In contacting the plating solution with metallic tin, Sn(IV) ions contained in
the
solution are reduced to Sn(II) ions according to the equation set forth below
for
this reaction, metallic tin being dissolved in the process
(comproportionation):
Sn'~~ + Sn ----------> 2 Sn2~ (4)
This means that for each Sn(IV) ion formed, two Sn(II) ions are formed. As a
result thereof, the concentration of tin contained in the plating solution
increases gradually when the regeneration method according to EP 0 545 216
A2 is applied.
By contrast, in carrying out the method of the invention, the metallic tin
used for
reducing the Sn(IV) ions originates through electrolytic deposition from the
very
tin plating solution. As a result thereof, the tin balance of the bath is not
disturbed by the regeneration according to equation (4). As the metallic tin
used
for regeneration is also formed from Sn(II) ions according to equation (1 ),
and
hence the concentration of the Sn(II) ions being lowered at first by
electrolytic
deposition, the Sn(11) ions consumed both through this reaction (1 ) and
through
the side reaction (3) are produced again by the regeneration reaction (4). The
Sn(II) ions content therefore remains constant.
The method of the invention therefore permits to avoid the detrimental
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consequences resulting from the formation of Sn(IV) ions and to concurrently
regenerate the Sn(II) ions from the Sn(IV) ions without complicated devices
and
analytic expenditure.
The plating solution substantially contains at least one Sn(II) compound, at
least one compound from the group comprising thiourea, urea and the
derivatives thereof as well as at least one acid. If a tin alloy is deposited,
the
solution additionally contains at least one salt of the metal to be deposited
additionally, e.g., one or more nickel, lead, mercury and/or gold salts.
Furthermore, the tin plating solution may also contain complexing agents,
reducing agents as well as other component parts, like stabilizing agents for
controlling deposition and for making sure that the plating solution be stable
to
decomposition, as well as surface-active agents. Usually, the solution is
aqueous, i.e., the solvent contained in the solution consists of at least
50 percent by volume of water. It may also contain organic solvents like for
example alcohols and ether esters.
The Sn(II) compound is preferably a Sn(Il) salt of an inorganic (mineral)
acid,
e.g., of an acid containing sulfur, phosphorus and/or halogen; hydrogen
halides
however should be avoided because of their corrosive effect and their tendency
~ ' to incorporate tin halides into the deposited tin. Furthermore, the Sn(II)
compound may also be the Sn(II) salt of an organic acid, e.g., of Sn(II)
formiate, Sn(II) acetate and the homologues thereof and the salt of an
aromatic
acid, more specifically of Sn(II) benzoate. The preferred salts are the Sn(II)
salts of the acids containing sulfur, i.e., the salts of the sulfuric acid and
of the
sulfamic acid (SnS04 and Sn(OSOZNH2) 2). The solution may furthermore
contain alkali metal stannates such as sodium stannate or potassium stannate.
If a tin alloy is deposited, the tin plating solution additionally contains at
least
one compound of the other alloying metals, for example a nickel, lead, mercury
and/or gold salt; the anions of these salts can be the same as those utilized
for
the tin salts.
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With respect to the Sn(I I) compounds and to the compounds of other alloying
metals, reference is made to U.S. Patent No. 4,715,894. The compounds
disclosed therein are incorporated herein by reference as a disclosure.
5 The acid contained in the tin plating solution preferably is a mineral acid
but
may also be an organic acid, the anion of the acid being generally identical
with
that of the tin salt and, if necessary, with that of the salts of the other
alloying
metals.
10 The compounds of thiourea and urea used are more specifically the
unsubstituted derivatives (thiourea, urea), the solution generally containing
only
thiourea and/or the derivatives thereof. U.S. Patent No. 4,715,894 indicates
suitable derivatives of thiourea and of urea. The derivatives disclosed
therein
are incorporated herein by reference as a disclosure.
The tin plating solution can also contain complexing agents, those indicated
in
Kirk-Othmer, Encyclopedia of Chemical Technology, 3'd Edition, Volume 5,
pages 339 - 368 being particularly suited. The complexing agents disclosed
therein are incorporated herein as a disclosure. More specifically, amino
carboxylic acids and hydroxy carboxylic acids may be used. U.S. Patent No.
4,715,894 discloses certain examples of suitable compounds. The complexing
agents disclosed therein are incorporated herein by reference as a disclosure.
The solution may also contain reducing agents, aldehydes, e.g., formaldehyde
and acetaldehyde being more specifically utilized. Further reducing agents are
indicated in U.S. Patent No. 4,715,894. The reducing agents disclosed therein
are incorporated herein by reference as a disclosure.
Anionic, cationic and amphoteric surface-active agents may be used alike. It
only matters that the surface-active agents are suited to reduce the surface
tension of the plating solution sufficiently.
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The metallic tin used for regeneration may be deposited onto an inert
auxiliary
cathode. By inert cathode, a separate electrode is meant which consists of a
material that resists dissolution in the tin plating solution when the
electrode is
subjected to anodic polarization. More specifically, the auxiliary cathode can
be
made of platinized titanium.
The auxiliary cathode can be configured as a plate, a tube, expanded metal or
as a formed body like for example a plate provided with ribs. The auxiliary
cathode may also be shaped in smaller pieces, e.g., in the shape of spheres
having for example a diameter of some few millimeters to some few
centimeters. In the latter case, these pieces may be accommodated in a
separate container for example, the plating solution flowing through said
container. For this purpose, the pieces may for example be placed on a
perforated bottom plate accommodated in a tower, the plating solution entering
through said bottom plate and flowing through said tower. Configuring the
auxiliary cathode in the form of smaller pieces permits to considerably
increase
the conversion rate of the Sn(IV) ions to Sn(ll) ions.
If an inert auxiliary cathode is made use of, the maximum quantity of tin that
can be dissolved again in the regeneration reaction according to reaction
_~
equation (4) is that amount that had been previously deposited from the bath.
As a result thereof, the bath can be regenerated continuously without
complicated analytical bath monitoring and, by contrast to the method
according to EP 0 545 216 A2, the concentration of tin in the bath does not
rise.
If, for depositing tin onto platinized titanium for example, the cathodic
current
density set for the auxiliary cathode is sufficiently high (e.g., 8 A/dm~), a
tin
coating in the form of flat scale crystals is obtained. This crystal shape has
a
very large surface which is well suited for the regeneration reaction
according
to equation (4) since it provides a very large surface referred to the weight
of
tin. As a result thereof, a large surface of deposited tin can be provided in
a
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predetermined volume of plating solution. A similar scale deposition is also
observed when a high current density is produced on the auxiliary cathode
when said auxiliary cathode is made of copper or of a copper alloy, for
example
with silver. The advantage of copper over inert materials, for example
platinized
titanium, is that copper is less expensive. The durable life of this material
in a
chemical tin plating solution is limited though.
The auxiliary cathode is in electric contact with the plating solution. An
auxiliary
anode, which is in direct electric contact with the plating solution or which
is in
electric contact with the plating solution via another solution, is also
provided.
By application of voltage between the auxiliary cathode and the auxiliary
anode, a flow of current can be generated between these two electrodes, the
auxiliary cathode being polarized cathodically and the auxiliary anode being
polarized anodically when tin is to be deposited onto the auxiliary cathode.
If tin
deposited onto the auxiliary cathode is directly utilized to regenerate the
tin
plating solution, the auxiliary cathode is not to be polarized cathodically
during
the actual regeneration process in order to allow the tin to dissolve from the
auxiliary cathode. Therefore, with this method, the auxiliary cathode is only
polarized cathodic intermittently each time tin is to be deposited onto the
auxiliary cathode. As soon as enough tin has been deposited onto the auxiliary
cathode, the electrical~connection between the auxiliary cathode and the
auxiliary anode is interrupted in order to halt the deposition process. Then,
the
dissolution reaction according to equation (4) of this reaction takes place
under
these conditions, the plating solution having to be contacted with the
auxiliary
cathode. As soon as but a small amount of tin or no tin at all is left at the
auxiliary cathode, tin can again be deposited onto said electrode.
For the regeneration reaction, metallic tin formed on the auxiliary cathode
may
either be used directly in contacting the plating solution with the auxiliary
cathode coated with metallic tin or be removed mechanically from said
electrode and be contacted with the tin plating solution after removal
thereof.
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To mechanically remove the tin deposited onto the auxiliary cathode, the
auxiliary cathode is preferably taken out of the plant and the scales of metal
that have grown thereon are stripped off. The removed tin may then be placed
into the container for treating the printed circuit boards or into a reservoir
that
contains the tin plating solution. In the treatment container or in the
reservoir,
the tin dissolves to form Sn(II) ions, Sn(IV) ions being consumed in the
process. As soon as the whole quantity or at least almost the whole quantity
of
tin placed in the container or in the reservoir has dissolved, further tin
that has
deposited onto the auxiliary cathode may be added.
The rate at which tin from the very auxiliary cathode or metallic tin removed
from the auxiliary cathode and placed into the treatment container or into a
reservoir dissolves in the plating solution depends on a plurality of
parameters:
the dissolution rate of tin depends inter alia on the composition and on the
temperature of the plating bath, on the morphology of electrolytically
deposited
tin, on the geometrical surface of the auxiliary cathode and on the flow
conditions in immediate proximity to the dissolving tin. The rate may thus be
optimized. A maximum dissolution rate is permanently aimed at since under
these conditions Sn(IV) ions are actually quantitatively reduced to Sn(II)
ions.
This makes it possible to minimize the concentration of Sn(IV) ions contained
in
the (plating solution. The dissolution rate is the higher the higher the
concentration of acid in the tin plating solution, the higher the temperature
of
the bath, the larger the surface of tin deposited onto the auxiliary cathode,
referred to the weight of the tin, the larger the geometrical surface of the
auxiliary cathode and the higher the convection of the plating solution in
immediate proximity to the dissolving tin.
To optimize the method of the invention, the space surrounding the auxiliary
anode (anode space) in the electrolytic regeneration cell can be separated
from
the space surrounding the auxiliary cathode (cathode space) by a membrane.
The membrane is preferably configured in such a manner that cations (Sn(II)
ions and Sn(IV) ions) cannot pass through. Therefore, the membrane may
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more specifically be an anion exchange membrane or a monoselective ion
exchange membrane. In a particularly preferred embodiment of the method in
accordance with the invention, there is an acid in the anode space. The acid
contained in the plating solution in the cathode space and the acid contained
in
the anode space may be identical. However, a very good regeneration result is
also obtained when the acid contained in the tin plating solution differs from
the
acid in the solution contained in the anode space. For example a tin plating
solution containing methane sulfonic acid and a sulfuric acid solution
contained
in the cathode space yield good results. There is transfer of fluid between
the
cathode space and the region in which layers containing tin are deposited onto
the printed circuit boards.
These further improvements of the method in accordance with the invention
permit to prevent the tin plating bath from directly contacting the auxiliary
anode. Sn(IV) ions are thus prevented from forming at the auxiliary anode,
which would otherwise lower the efficiency of regeneration. The auxiliary
anode
may for example be immersed into an anode space that is separated from the
cathode space surrounding the auxiliary cathode by an anion exchange
membrane. The plating solution in the cathode space, which more specifically
contains SnS04 and H2S04 for example, cannot get near the auxiliary anode
since the membrane prevents Sn(II) ions from passing through. A solution of
the acid which is also contained in the cathode space is preferably also
filled
into the anode space. In the present example, the acid would be H2SO4. When
the current flows between the two spaces, electroneutrality is guaranteed by
the
transfer of sulfate anions and by the corresponding electrode reactions, i.e.,
by
the tin plating reaction at the auxiliary cathode according to equation (1 )
of this
reaction and by an oxidation reaction at the auxiliary anode, in which oxygen
is
formed from water according reaction equation (5):
2 H20 _______________> 2 H+ + 2 a + 02 (5)
As the Sn(II) ions are prevented from contacting the auxiliary anode,
oxidation
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of Sn(II) ions according to the following equation:
Sn2+ ___________________> Sn4+ + 2 a (6)
5 cannot take place.
Alternatively, the auxiliary anode can also contact the tin plating solution
directly. In order to also prevent in this case oxidation of the Sn(II) ions
according to reaction equation (6), the concentration overvoltage must be high
10 enough for this reaction. This may be realized by an appropriate
geometrical
arrangement of the auxiliary anode relative to the auxiliary cathode for
example:
a depletion of the Sn(II) ions in the solution in the immediate proximity to
the
auxiliary cathode, which may lead to the concentration overvoltage, may also
be achieved in that the anode space is accommodated in a container which is
15 separated from the cathode space, both spaces communicating through a pipe
whose diameter is relatively small.
Concentration overvoltage in the above mentioned sense may also be achieved
in considerably increasing the current density at the auxiliary anode so that
Sn(II) ions are virtually no longer available in the immediate proximity of
the
i ' ~~ i
-' auxiliary anode. Under these conditions, Sn(II) ions do not oxidize to form
Sn(IV) ions, but water oxidizes to form oxygen. The current density at the
auxiliary anode may for example be increased by reducing the surface of the
auxiliary anode relative to the surface of the auxiliary cathode.
In another embodiment of the invention, at least one electrode containing the
tin to be deposited, i.e., an electrode of metallic tin for example, can be
contacted with the tin plating bath. This tin electrode is polarized
anodically
relative to another electrode so that the tin electrode dissolves at least
partially.
Such a soluble tin electrode may for example consist of poured balls which are
located in a suitable container, e.g., in a titanium basket.
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In this case, the tin electrode is at least intermittently polarized anodic
relative
to the other electrode so that metallic tin dissolves to form Sn(II) ions.
In using the soluble tin electrode, it is possible to produce the Sn(II) ions
by
dissolution consumed in the electrolytic deposition reaction so that the total
amount of tin contained in the plating solution is kept constant. As soon as
the
desired concentration of Sn(II) ions contained in the solution is achieved in
the
process of anodic dissolution, the anodic dissolution reaction at the tin
electrode can be halted by interrupting the flow of current. After the current
is
no longer supplied to the soluble tin electrode, Sn(IV) ions may also be
reduced
at this electrode in causing them to react with the metallic tin of the
electrode to
form Sn(ll) ions.
When using tin electrodes, the concentration of tin contained in the plating
solution, namely the concentration of Sn(II) ions, must however be
analytically
monitored with accuracy since otherwise, the dissolution of the tin electrodes
may cause the concentration of tin contained in the plating solution to exceed
the reference value. In this case, dissolution of metallic tin of the tin
electrode is
not automatically limited which is the case when an inert auxiliary cathode is
exclusively used.
The tin plating solution may be contacted with the work in different ways:
with
conventional methods, the work is immersed into a bath of the plating
solution,
which is filled in a container. In this case, the arrangement with auxiliary
cathode and auxiliary anode is located either in the same container in a free
space or in a separate container through which the plating solution flows.
Fluid
conduits in which the plating solution can be circulated between the treatment
container and the regeneration container are provided for this purpose between
the treatment container and this other regeneration container.
Furthermore, the work can be treated in a so called horizontal plant with a
coating chamber. In this horizontal plant, the work is conveyed in horizontal
CA 02450258 2003-12-10
WO 03/004725 PCT/EP02/06654
17
direction of transport through said chamber. In this case, the plating
solution is
delivered to the copper surfaces of the work by way of nozzles, e.g., spray
nozzles, flow nozzles, jet nozzles or the like, while the work is conveyed
through
the chamber. For this purpose, the solution is kept in a reservoir from where
it is
delivered to the nozzles by means of pumps. After the plating solution has
contacted the copper surfaces, it is drained into collecting tanks from where
it is
returned to the reservoir via fluid conduits. In this case, the arrangement
with
auxiliary cathode and auxiliary anode is accommodated either in the reservoir
or in a separate regeneration container.
Thus a method of depositing a layer of metal and a method of regenerating a
solution containing metal ions in a high oxidation state, especially a
solution
containing Sn(IV) ions, is described. Although specific embodiments, including
specific equipment, method steps, method parameters, materials, solutions
etc., have been described, various modifications to the disclosed embodiments
will be apparent to those skilled in the art upon reading this disclosure.
Therefore, it is to be understood that such embodiments are merely
illustrative
of and not restrictive on the broad invention and that this invention is not
limited
to the specific embodiments described, but only by the scope of the appended
claims.
