U.S. patent application number 10/494217 was filed with the patent office on 2004-12-09 for regeneration method for a plating solution.
Invention is credited to Beck, Thomas, Lamprecht, Sven, Matejat, Kai-Jens, Schoder, Rolf, Schreier, Hans-Jurgen.
Application Number | 20040245108 10/494217 |
Document ID | / |
Family ID | 7690626 |
Filed Date | 2004-12-09 |
United States Patent
Application |
20040245108 |
Kind Code |
A1 |
Beck, Thomas ; et
al. |
December 9, 2004 |
Regeneration method for a plating solution
Abstract
The invention relates to a method of depositing a layer of metal
and to a method of regenerating a solution containing metal ions in
a high oxidation state. To regenerate tin ions consumed from a tin
plating solution by metal deposition, it has been known in the art
to carry the plating solution over metallic tin to cause tin (II)
ions to form. However, the amount of tin contained in thus
regenerated baths slowly and continuously increases. The solution
to this problem is to utilize an electrolytic regeneration cell
that is provided with at least one auxiliary cathode and with at
least one auxiliary anode. Tin serving for regeneration is
electrolytically deposited from the solution onto the at least one
auxiliary cathode in the electrolytic regeneration cell. The
solution is carried over the tin serving for regeneration in order
to reduce formed tin (IV) ions to tin (II) ions.
Inventors: |
Beck, Thomas; (Vehlefanz,
DE) ; Schreier, Hans-Jurgen; (Velten, DE) ;
Lamprecht, Sven; (Eichstadt, DE) ; Schoder, Rolf;
(Burgthann, DE) ; Matejat, Kai-Jens; (Vehlefanz,
DE) |
Correspondence
Address: |
Frank J Bonini Jr
86 The Commons at Valley Forge East
1288 Valley Forge Road
P O Box 750
Valley Forge
PA
19482-0750
US
|
Family ID: |
7690626 |
Appl. No.: |
10/494217 |
Filed: |
May 3, 2004 |
PCT Filed: |
June 17, 2002 |
PCT NO: |
PCT/EP02/06654 |
Current U.S.
Class: |
205/85 |
Current CPC
Class: |
C23C 18/1617
20130101 |
Class at
Publication: |
205/085 |
International
Class: |
C25D 005/54 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 3, 2001 |
DE |
101 32 478.2 |
Claims
1. A method of depositing a metal layer, comprising the following
method steps: a. Preparing a metal plating bath containing metal
ions in a low oxidation state; b. Depositing a metal layer from the
metal plating bath onto a work piece; c. Bringing the metal plating
bath in contact with the metal serving for regeneration in order to
reduce metal ion in a high oxidation state contained in the metal
plating bath to metal ions in a low oxidation state, wherein an
electrolytic regeneration cell comprised of at least one auxiliary
cathode and of at lease one auxiliary anode is provided and wherein
the metal serving for regeneration is electrolytically deposited
from the metal plating bath onto the at least one auxiliary
cathode.
2. The method of claim 1, wherein the method serves for depositing
tin containing layers, wherein the metal ions in the low oxidation
state are Sn(II) ions and the metal ions in the high oxidation
state are Sn(IV) ions and wherein the metal is metallic tin.
3. The method of claim 1, wherein the at least one auxiliary
cathode is made of copper or of a copper alloy.
4. The method of claim 1, wherein the at least one auxiliary
cathode is made of an inert material.
5. The method of claim 4, wherein the at least one auxiliary
cathode is made of platinized titanium.
6. The method of claim 1, wherein the metal is deposited in scales
onto the at lease one auxiliary cathode by adjusting the cathodic
current density.
7. The method of claim 1, wherein metal deposited onto the at least
one auxiliary cathode is mechanically removed and wherein, after
removal, the metal is contacted with the metal plating bath in
order to reduce metal ions in a high oxidation state contained in
the metal plating bath to metal ions in a low oxidation state.
8. The method of claim 1, wherein the at lease one auxiliary anode
is separated from the space surrounding the at least one auxiliary
cathode by a membrane.
9. The method of claim 8, wherein the membrane is configured such
that the metal ions may not permeate said membrane.
10. The method of claim 8, wherein said membrane is an anion
exchange membrane or a monoselective ion exchange membrane.
11. The method of claim 8, wherein an acid is provided to the space
surrounding the at least one auxiliary anode.
12. The method of claim 1, wherein at least one electrode
containing the metal to be deposited is contacted with the metal
plating bath and wherein the at least one electrode is polarized
anodically relative to at least one further electrode so that the
at least one electrode containing the metal to be deposited
dissolves at least partially.
13. The method of claim 1, wherein the workpiece is conveyed in
horizontal direction through a coating chamber for deposition of
the metal layer.
14. A method of regenerating a solution containing metal ions in a
high oxidation state, in which method the solution is brought into
contact with a metal serving for regeneration in order to reduce
the metal ions in a high oxidation state metal ions in a low
oxidation state, wherein an electrolytic regeneration cell
comprised of at least one auxiliary cathode and of at least one
auxiliary anode is provide and wherein the metal serving for
regeneration is electrolytically deposited from the solution onto
the at least one auxiliary cathode.
15. The method of claim 14, wherein the method serves for
regenerating a tin containing solution, wherein the metal ions in
the low oxidation state are Sn(II) ions and the metal ions in the
high oxidation state are Sn(IV) ions and wherein the metal is
metallic tin.
16. The method of claim 9, wherein an acid is provided to the space
surrounding the at least one auxiliary anode.
17. The method of claim 10, wherein an acid is provided to the
space surrounding the at least one auxiliary anode.
18. A method of depositing a metal layer, comprising the following
method steps: a. Preparing a metal plating bath containing metal
ions in a low oxidation state; b. Depositing a metal layer from the
metal plating bath onto a work piece; c. Bringing the metal plating
bath in contact with the metal serving for regeneration in order to
reduce metal ion in a high oxidation state contained in the metal
plating bath to metal ions in a low oxidation state; wherein an
electrolytic regeneration cell comprised of at least one auxiliary
cathode and of at least one auxiliary anode is provided and wherein
the metal serving for regeneration is electrolytically deposited
from the metal plating bath onto the at least one auxiliary
cathode; wherein the method serves for depositing tin containing
layers, wherein the metal ions in the low oxidation state are
Sn(II) ions and the metal ions in the high oxidation state are
Sn(IV) ions and wherein the metal is metallic tin; wherein the
metal is deposited in scales onto the at least one auxiliary
cathode by adjusting the cathodic current density; wherein the at
least one auxiliary anode is separated from the space surrounding
the at least one auxiliary cathode by a membrane; and wherein the
membrane is configured such that the metal ions may not permeate
said membrane.
19. The method of claim 18, wherein said membrane is an anion
exchange membrane or a monoselective ion exchange membrane; and
wherein an acid is provided to the space surrounding the at least
one auxiliary anode.
20. The method of claim 19, wherein at least one electrode
containing the metal to be deposited is contacted with the metal
plating bath and wherein the at least one electrode is polarized
anodically relative to at least one further electrode so that the
at least one electrode containing the metal to be deposited
dissolves at least partially.
Description
[0001] The invention relates to a method of depositing a layer of
metal, more specifically a layer containing tin, above all 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.
[0002] 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.
[0003] 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.
[0004] 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 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.
[0005] 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 0 545 216 A2)) before the inner layers are pressed
together by action of heat and pressure.
[0006] 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.
[0007] A plating bath of this type is described in U.S. Pat. No.
4,715,894. In addition 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. Pat. No. 4,715,894 also contains a complexing agent, a
reducing agent and an acid. Accordingly, the Sn(II) compound used
is SnSO.sub.4 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.
[0008] EP 0 545 216 A2 reports that the Cu(I) thiourea complex
enriches in the solution. Sn(IV) 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 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, 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.
[0009] 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 with the
metallic tin to form the double quantity of Sn(II) ions according
to the reaction equation (4) set forth below.
[0010] 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(II) ions
and permitting or obviating contact between the plating solution
and 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.
[0011] 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.
[0012] 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.
[0013] 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:
[0014] a. Providing a metal plating bath, more specifically a tin
plating bath; containing metal ions in a low oxidation state, more
specifically Sn(II) ions,
[0015] b. Depositing a metal layer from the metal plating bath onto
a work piece;
[0016] c. Providing an electrolytic regeneration cell comprised of
at least one auxiliary cathode and of at least one auxiliary
anode;
[0017] d. Electrolytically depositing, in the electrolytic
regeneration cell, metal serving for regeneration, more
specifically metallic tin, from the metal plating bath onto the at
least one auxiliary cathode;
[0018] 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 metal ions in a low oxidation state,
more specifically Sn(II) ions.
[0019] 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:
[0020] a. Providing an electrolytic regeneration cell comprised of
at least one auxiliary cathode and of at least one auxiliary
anode;
[0021] 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;
[0022] 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.
[0023] 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.
[0024] 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.
[0025] 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 for the reduction to the oxidation state
zero, said substrate metal concurrently oxidizing and being
preferably dissolved thereby.
[0026] The method of the invention more specifically serves to coat
copper surfaces on printed circuit boards or other circuit carriers
with tin containing layers.
[0027] 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:
[0028] When tin is deposited from an electroless, cementation or
electrolytic tin bath, the following reaction takes place:
Sn.sup.2++2e.sup.---------------->2Sn (1)
[0029] In electrolytic deposition, the electrons originate from an
external source of electric current and are delivered to the Sn(II)
ions via the cathode. In the case of electroless tin 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:
2Cu---------------->2Cu.sup.++2e.sup.- (2)
[0030] In an interfering side reaction, Sn(II) ions oxidize in
these baths, through the oxygen from the air, to form Sn(IV)
ions:
Sn.sup.2++1/2O.sub.2+H.sub.2O--------->Sn.sup.4++2OH.sup.-
(3)
[0031] The Sn(IV) ions formed tend to precipitate tinstone
(SnO.sub.2). 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(IV) 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.
[0032] 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.sup.4++Sn---------->2Sn.sup.2+ (4)
[0033] 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.
[0034] 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(II) 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.
[0035] The method of the invention therefore permits to avoid the
detrimental 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.
[0036] 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.
[0037] The Sn(II) compound is preferably a Sn(II) 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 (SnSO.sub.4 and Sn(OSO.sub.2NH.sub.2).sub.2). The
solution may furthermore contain alkali metal stannates such as
sodium stannate or potassium stannate.
[0038] 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.
[0039] With respect to the Sn(II) compounds and to the compounds of
other alloying metals, reference is made to U.S. Pat. No.
4,715,894. The compounds disclosed therein are incorporated herein
by reference as a disclosure.
[0040] 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.
[0041] 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. Pat. No. 4,715,894 indicates suitable derivatives of
thiourea and of urea. The derivatives disclosed therein are
incorporated herein by reference as a disclosure.
[0042] The tin plating solution can also contain complexing agents,
those indicated in Kirk-Othmer, Encyclopedia of Chemical
Technology, 3.sup.rd 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.
Pat. No. 4,715,894 discloses certain examples of suitable
compounds. The complexing agents disclosed therein are incorporated
herein by reference as a disclosure.
[0043] 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. Pat. No.
4,715,894. The reducing agents disclosed therein are incorporated
herein by reference as a disclosure.
[0044] 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.
[0045] 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.
[0046] 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(II) ions.
[0047] 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.
[0048] 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.sup.2), 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 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.
[0049] 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.
[0050] 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. 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.
[0051] 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.
[0052] 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
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.
[0053] 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 SnSO.sub.4 and H.sub.2SO.sub.4 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
H.sub.2SO.sub.4. 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):
2H.sub.2O--------------->2H.sup.++2e.sup.-+O.sub.2 (5)
[0054] As the Sn(II) ions are prevented from contacting the
auxiliary anode, oxidation of Sn(II) ions according to the
following equation:
Sn.sup.2+------------------->Sn.sup.4++2e.sup.- (6)
[0055] cannot take place.
[0056] 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 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 separated from
the cathode space, both spaces communicating through a pipe whose
diameter is relatively small.
[0057] 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 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.
[0058] 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.
[0059] 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.
[0060] 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(II) ions.
[0061] 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.
[0062] 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.
[0063] 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 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.
[0064] 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.
* * * * *