Note: Descriptions are shown in the official language in which they were submitted.
CA 02326049 2000-09-26
Method for coating surfaces on copper or on a copper alloy with a tin or tin
alloy
layer
Description:
The invention relates to a method for coating surfaces on copper or on a
copper
alloy, for example on brass, with a tin or tin alloy layer, furthermore to a
layer
combination including a tin or tin alloy layer as well as applications of the
method for producing surfaces on copper or on a copper alloy which are
suitable for soldering even after thermal treatment, and for producing layers
to
protect such surfaces against corrosion. Moreover the invention also relates
to
a bath solution for the electroless deposition of a tin layer or a tin alloy
layer
For surface treatment of workpieces on copper or copper alloys with the aim of
creating corrosion-resistant surfaces, tin layers have, for a long time now,
been
applied in an electroless method by the basic metal decomposing in favour of
the deposited tin ions. The tin layers can in principle also be deposited on
other
metals, such as iron. In order to be able to form a sufficiently thick layer,
the
workpiece must here be treated for two to three hours at 90 to 100°C.
For example, in US 2,891,871 A a method is described for the direct coating of
workpieces on a copper or on a copper alloy, in which method a tin layer is
formed on the workpiece through charge exchange by dipping it into a solution
composed of a tin salt, a carboxylic acid and thiourea or a derivative of
thiourea.
In US 2,282,511 A a method for coating copper surfaces is described in which a
solution is used which contains compounds containing tin-II ions, dissolved
thiocarbamide and a small amount of an alkali metal carbonate. With this
solution, too, a tin layer can be directly formed on the copper surfaces. For
example, solutions of this kind can be used for the electroless deposition of
tin
on the inner surfaces of copper pipes, in order to prevent the decomposition
of
copper.
Also, in US 2.369,620 A, an electroless tin-plating method for the coating of
copper surfaces is described. The aqueous coating solution described in this
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document is acid and contains besides a tin-II salt, preferably SnCl2,
thiourea in
addition.
According to DE-AS 1 521 490, an aqueous dipping bath for the deposition of
tin also contains hypophosphorous acid or its alkali salts in order to improve
the
stability of the depositing solution and in order to obtain purer, brighter,
denser
tin layers which are more resistant to etching agents than with the previously
known depositing solutions. Furthermore, it is stated that the depositing
solution can also contain an organic acid, such as for example ethanoic acid,
citric acid, malic acid, malefic acid and similar aliphatic mono-, di- and
tricarboxyiic acids. Thiourea is also contained in this solution and in
addition a
wetting agent. Layers with a thickness of several Nm could be deposited with
this bath.
In DE 30 11 697 A1 is described an acid chemical tin- plating bath, for
example
for coating copper surfaces, which contains a source for tin-II ions,
thiourea,
and an inhibitor as a further component, an organic sulphonic acid being
preferably used as the inhibitor. The pH value of the solution is kept under
1.
Moreover, this solution also contains a hypophosphite.
For application in the manufacture of printed circuit boards, a method is
described in US 4,657,632 A in which a portion of the copper surface on the
base material is removed by etching, by an etch-resist being applied to the
copper surfaces which are not to be removed, and the portions of the copper
surfaces which are to be removed remaining free. The etch-resist layer is
formed by electroless deposition of a tin layer on the copper layers. The
depositing solution used for this contains besides the tin-II salt and
thiourea or a
thiourea derivative, urea or a derivative of urea in addition. Furthermore,
the
solution can also contain a chelating agent, for example amino- and
hydroxycarboxylic acids, a reducing agent, e.g. an aldehyde, and an acid. In
addition, a wetting agent can be contained in the solution.
In EP 0 503 389 A2 is described a method for the electroless coating of
workpieces with surfaces on copper or on a copper alloy with tin or a tin/lead
alloy. The acid coating bath contains, in addition to the metal salts, a
reducing
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agent such as hypophosphite or its acid, and a complexing agent, for example
organic carboxylic acids or thiourea or its derivatives.
In EP 0 521 738 A2 is described a solution for the electroless coating with
tin or
with a tin/lead alloy of copper surfaces preferably contained on printed
circuit
boards, which solution contains, besides tin salts, thiourea, an acid and a
reducing agent, such as hypophosphite, in addition one or more nonionic
wetting agents, preferably a polyoxyalkyl ether, for example
polyoxynonylphenol
ether.
In DE 40 01 876 A1 is described a composition for a bath for electroless
coating
with tin or with a tin/lead alloy, which contains alkane or alkanol sulphonic
acids
as well as their tin and lead salts, and in addition thiourea and thiourea
derivatives as well as mono-, di-, tricarboxylic acids or their salts as a
chelating
agent for tin and lead. The composition serves to coat copper or copper
alloys.
Contained in the composition can be, amongst other things also nonionic
wetting agents, for example polyoxyalkylene alcohol ether.
The use of tin layers as the solderable layer has been tested again and again
for the manufacture of printed circuit boards. The conductor tracks obtained
after the structuring of the copper layers has to be solderable for the
fitting of
components, the requirement existing that the exposed metal surfaces must
have, even after lengthy storage time of several days to weeks, good
wettability
with the solder, mostly tin/lead solder.
However, it has emerged that the wettability of the tin layers during
soldering is
generally substantially worse than with the use of tin/lead alloys. Through
storage of printed circuit boards with conductor tracks which are covered with
tin layers, it emerged that large regions of the printed circuifi board could
not be
wetted after its storage, whilst other regions did not present any problems of
this
kind. The electroless deposition of tin/lead layers is expensive, however,
such
that the use of purely tin layers seems advantageous as a soldering aid.
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An attempt has been made, for example, to avoid this phenomenon through the
application of temporarily applied organic protective varnishes. This
protective
coating is intended to dissolve within the first minutes in the tinllead
solder.
Varnishes of this kind are expensive however. In addition, they cause problems
during processing since, during the soldering, they reach the soldering bath
and
contaminate it. In addition to this, tin layers on printed circuit boards
provided
with a temporary protective varnish have disadvantageous properties.
Moreover, it has also emerged that with the known methods no sufficiently high-
quality layer surfaces can be obtained from tin or a tin alloy, such that, in
the
subsequent processing steps, problems arose for the printed circuit boards.
The problem underlying the present invention, therefore, is to avoid the
disadvantages of prior art and especially to find a method and a bath solution
with which it is possible to form tin and, if necessary, also tin alloy layers
on
surfaces on copper or on a copper alloy, which surfaces are, even after a
lengthy storage time, easily wettable with liquid tin/lead solder.
This problem is solved by the features of claims 1 and 8, 9, 12 and 13.
Preferred embodiments of the invention are quoted in the sub-claims.
It has been shown that. for the inadequate solderability of tin layers on
printed
circuit boards, which had been stored for several days to weeks, the formation
of an intermetallic phase is responsible. Through the storage, at the boundary
surface between the copper substrate and the tin layer, a copper/tin alloy
forms
which is the thicker, the longer the printed circuit board is stored and the
higher
the temperature is during storage. These intermetallic phases are formed very
quickly. The capacity for soldering sinks rapidly if the whole tin layer,
which
has for example a thickness of 0.7 Nm, is converted into the intermetallic
phase.
If the electroless deposition of tin is observed accurately in the first
seconds of
the depositing process, it can be ascertained that "states" which were visible
before tin plating on the copper surface, are "frozen" by the tin layer. On
specific morphological structures, which are located on the copper surface,
the
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tin or respectively tin alloy layer is more quickly deposited than at points
on the
copper surfaces at which these structures are not located. For example, at
rinsing water lines or oxidised portions of the copper surfaces, structures
are
formed which are distinguished at least optically from adjacent regions. If
the
printed circuit board is tempered after the tin plating of such points, very
quickly
intermetallic phases form at these points. These are more easily oxidised in
air
than pure tin layers or copper surfaces. During oxidation, these intermetallic
phases lose the capacity for wetting with liquid solder.
The problems described above were solved by the method according to the
invention which has the following essential method steps:
a. the copper or copper alloy surfaces are treated with a solution which
contains at least one noble metal compound, in order to deposit noble
metal in a first step. The thickness of this metal can be extremely small.
It is for example sufficient to form a noble metal layer which is not visible
with the naked eye. The solution of the above-mentioned problems in
carrying out this first method step and a subsequent tin plating alone
shows that the noble metal treatment leads to the formation of a noble
metal layer on the surfaces on copper or on a copper alloy.
b. the surfaces coated with noble metal as per step a. are then treated with
a solution which contains at least one tin compound and, if necessary, at
least one compound of a further metal to be deposited, at least one acid
and at least one complexing agent for copper from the group consisting
of thiourea and its derivatives. During this treatment, the tin or tin alloy
layer is formed.
The bath solution according to the invention for the electroless deposition of
a
tin layer or a tin alloy layer is characterised in that it
a. contains at least one tin compound and, if necessary, at least one
compound of a further metal to be deposited,
b. at least one acid,
c. at least one complexing agent for copper from the group consisting of
thiourea and its derivatives, and
CA 02326049 2000-09-26
d. if necessary, at least one wetting agent.
Just like the tin or tin alloy layer, the noble metal layer is formed by
charge
exchange, as a result of which copper ions go into solution at the same time
as
the metal deposition. It is assumed that the noble metal coating formed
prevents the further oxidation of copper and the formation of the
intermetallic
phase between copper and tin. By this means, the further oxidation of the tin
layer is probably suppressed, such that the wettability of the surfaces with
solder is preserved even after lengthy heat treatment.
During the soldering process itself, the tin and the noble metal layers are
extremely quickly decomposed by the liquid solder. This process advances
very quickly, such that sufficient time remains for the desired intermetallic
tin/copper phase, typical for the soldering process, to form between the
liquid
tin/lead solder and the copper.
The following method steps are preferably carried out:
1. The copper or copper alloy surtaces are cleaned or etched.
2. The treated surfaces are thoroughly rinsed.
3. Noble metal is then deposited by cementation.
4. Thereafter the workpieces are brought into contact with an aqueous acid
solution.
5. Following this, tin is deposited in an electroless manner, preferably at a
temperature of roughly 60°C; the treatment time is preferably roughly 4
minutes to roughly 30 minutes.
To clean the copper or copper alloy surfaces, conventional cleaning and
etching
solutions are used, for example solutions containing wetting agents, and which
can additionally contain, for example, hydrogen peroxide and sulphuric acid.
In the bath used for the noble metal deposition, one or more noble metal
compounds from the group consisting of silver, gold, platinum, palladium,
ruthenium, rhodium, osmium and iridium are used. The concentration of the
noble metals in the solution is preferably roughly 0.1 to 2000 ppm (parts by
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weight noble metal per 1 million parts by weight solution), preferably roughly
1
to roughly 100 ppm.
The printed circuit boards are only treated for a short time in the noble
metal
bath, for example within a period of roughly 60 seconds to roughly 120
seconds.
Longer treatment times can naturally also be chosen.
The treatment temperature during the deposition of noble metal is preferably
roughly 20 to roughly 30°C.
After the deposition of noble metal, the boards are brought into contact with
an
acid solution. An acid contained in the tin-plating bath can preferably be
used
as the acid. Naturally, a different acid can also be used. During this
treatment,
the copper or respectively copper alloy surfaces are prepared for tin-plating;
simultaneously, the acid forms a protection for the subsequent tin bath before
dilution.
The electroless tin bath must have as high stability as possible against
decomposition. In particular, with known baths there exists the tendency for
precipitates to form within a very short time (within days).
As acids contained in the tin bath, preferably mineral acids, organic acids
and
sulphonic acids are selected.
The tin-plating bath is operated at a temperature of roughly 50 to roughly
70°C.
Under these conditions, adhesive and evenly bright tin layers with a thickness
of
between roughly 0.6 and roughly 1.4 Nm can be deposited on copper or
respectively on a copper alloy.
As the tin alloy, a tin/lead alloy can be deposited for example. The
deposition
bath contains in this case in addition a lead-II salt, for example PbCl2 or
Pb(OCOCH3)2.
To carry out the process, the printed circuit boards are dipped in the
standard
method one after the other in containers in which the individual treatment
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solutions are contained. Since the treatment times are extraordinarily short,
the
printed circuit boards can also be treated in a continuous treatment plant,
through which the boards are led in horizontal or vertical alignment and in a
horizontal conveying direction.
The method according to the invention is particularly well suited for the
production of surfaces on copper or on a copper alloy which are suitable for
soldering even after thermal treatment, as well as for the manufacture of
layers
to protect surfaces on copper or on a copper alloy against corrosion. The
method can also serve to form the layer combination according to the invention
on workpieces other than printed circuit boards, for example for coatihg pipes
against corrosion.
The following examples serve to explain the invention further:
Example 1:
A wiring board, provided with copper structures and especially with connection
places for electrical components, was, after cleaning and etching of the
copper
with an aqueous solution of sodium peroxodi- sulphate in a silver complexing
solution, which contained 50 ppm silver, treated for a minute at room
temperature. After the board had been rinsed with water and subsequently
treated with a solution of 2% by wt. acid in water, it was tin-plated in an
electroless tin-plating bath for 15 minutes at 60°C.
The tin-plating bath had the following composition:
Tin-II fluoroborate 15g tin
Fluoboric acid 100 ml
Thiourea 100 g
Sodium lauryl sulphate 2 mg
Fill up with water to 1 I.
The tin layer obtained after treatment with the bath was bright metallic and
had
a thickness of 1.05 Nm. The plate was thereafter tempered for 8 hours at
155°C
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in air and finally subjected to a wave soldering process. As the fluxing
agent, a
low-solids (2%) no-clean fluxing agent (Kester~ product of the company Litton-
Kester, USA) was used.
The soldering result on the connection places was excellent, since the wetting
behaviour was perfect on the tin-plated copper surfaces. The rise of the
solder
into the bores contained in the printed circuit board was 80 - 90% perfect
(the
printed circuit board was not fitted with components).
Example 2:
A printed circuit board pre-treated as in Example 1, was coated with platinum
in
a platinum solution which contained 15 ppm platinum, for 1 minute at room
temperature, and thereafter treated further as in Example 1.
The bright tin layer obtained had, after a dipping time of 30 minutes in the
tin-
plating bath at 55°C, a thickness of 1 pm.
Then the board was tempered for 4 hours at 155°C and, analogously
to
Example 1, subjected to a solder test. Neither faults during the wetting with
the
liquid solder nor problems as the solder rose in the bores were apparent; this
was 100%.
Example 3:
A printed circuit board, pre-treated as in Example 1, was treated for 2
minutes in
a ruthenium solution which contained 50 ppm ruthenium. The copper layers
which were coated with the extremely thin ruthenium layer, were tin-plated at
50°C in a chemical tin bath of the composition:
Tin (II) chloride 5 g
N-methyl thiourea 55 g
Conc. sulphuric acid20 g
Isopropyl alcohol 500 ml
Water 500 ml
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The tin layer was subjected to a solder test (solder spread test). The
soldering
capability was excellent with 9° contact angle.
