Note: Descriptions are shown in the official language in which they were submitted.
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TREATMENT FOR ELECTROPLATING RACKS
TO AVOID RACK METALLIZATION
FIELD OF THE INVENTION
The present invention relates generally to a method of treating electroplating
racks used
for supporting non-conductive substrates during a metallization step.
BACKGROUND OF THE INVENTION
For many years, processes have been available to facilitate the deposition of
electrodeposited metals onto plastic substrates. Typically, the process
involves the steps of:
1) Etching the plastic in a suitable etching solution such that the surface of
the plastic
becomes roughened and wetted so that the subsequently applied deposit has good
adhesion;
2) Activating the surface of the plastic using a colloidal or ionic solution
of a metal
(usually palladium) capable of initiating the deposition of an
autocatalytically applied
metal coating (e.g., copper or nickel);
3) Depositing a thin layer of autocatalytically applied metal; and
4) Carrying out electrodeposition of metal onto the metallized plastic
substrate.
Typically, layers of copper, nickel and/or chromium will be applied to produce
the finished
article.
The most widely used plastic substrates are acrylonitrile/butadiene/styrene
copolymers
(ABS) or ABS blended with polycarbonate (ABS/PC). These materials are readily
formed into
components by the process of injection molding. ABS comprises a relatively
hard matrix of
acrylonitrile/styrene copolymer and the butadiene polymerizes to form a
separate phase. It is this
softer phase of polybutadiene (which contains double bonds in the polymer
backbone) which
may be readily etched using various techniques.
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Traditionally, the etching has been carried out using a mixture of chromic and
sulfuric
acids operated at elevated temperature. The chromic acid is capable of
dissolving the
polybutadiene phase of the ABS by oxidation of the double bonds in the
backbone of the
polybutadiene polymer, which has proven to be reliable and effective over a
wide range of ABS
and ABS/PC plastics. However, the use of chromic acid has become increasingly
regulated
because of its toxicity and carcinogenic nature. For this reason, there has
been considerable
research into other means of etching ABS plastics and a number of approaches
have been
suggested to achieve this.
For example, acidic peinianganate is capable of oxidizing the double bonds in
the
polybutadiene. Chain scission can then be achieved by further oxidation with
periodate ions.
Ozone is also capable of oxidizing polybutadiene. However, ozone is extremely
dangerous to
use and highly toxic. Likewise, sulfur trioxide can be used to etch ABS, but
this has not been
successfully achieved on a typical plating line. Other examples of techniques
for etching ABS
plastics are described in U.S. Pat. Pub. No. 2005/0199587 to Bengston, U.S.
Pat. Pub. No.
2009/0092757to Sakou et al., and U.S. Pat. No. 5,160,600 to Gordhanbai et al.,
the subject
matter of each of which is herein incorporated by reference in its entirety.
More recently, it has been discovered that ABS and ABS/PC plastic can be
etched in a
solution containing manganese(III) ions in strong sulfuric acid as described
in U.S. Pat. Pub. No.
2013/0186774 to Pearson et al., the subject matter of which is herein
incorporated by reference
in its entirety.
In order to plate plastic components, they are attached to plating racks which
transmit the
electrical current to the sensitized and metallized plastic components. The
racks are typically at
least partially coated with a non-conductive material to prevent the rack from
being entirely
covered with metal during the electroplating process, and the most common rack
coating is a
PVC plastisol. The use of chromic acid in the etching stage prior to
activation is effective in
modifying the surface of the plastisol coating so that it is resistant to
metallization after being
coated with a palladium activator (usually a colloid of palladium and tin).
When chromic acid is
replaced with other etching techniques, for example, using processes
containing permanganate or
manganese (III), the plastisol coating of the plating rack becomes coated with
the activator and
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subsequently becomes coated with a layer of either nickel or copper in the
electroless plating
stage. Thus, a major problem with all of the currently known methods that do
not utilize
chromic acid is that rack coatings tend to become plated in the subsequently
electroless plating
stage. This phenomenon is known as "rack plate up" and is a major problem with
any form of
chrome-free etching technology.
SUMMARY OF THE INVENTION
It is an object of the present invention to inhibit rack plate up in the
process of
electroplating non-conductive substrates.
It is another object of the present invention to inhibit rack plate up in the
process of
electroplating non-conductive substrates in which the non-conductive
substrates are etched using
a chrome-free etchant.
It is still another object of the present invention to provide a treatment for
electroplating
racks used for supporting non-conductive substrates during the electroplating
process.
To that end, in one embodiment, the present invention relates generally to an
electroplating rack for supporting non-conductive substrates during an
electrodeposition process,
wherein the electroplating rack is at least partially coated with a non-
conductive material;
and
wherein the electroplating rack is treated with a non-aqueous solution
comprising a
metallization inhibitor.
In another embodiment, the present invention relates generally to a method of
treating an
electroplating rack used for supporting non-conductive substrates during an
electrodeposition
process, wherein the electroplating rack is at least partially coated with a
non-conductive
material, the method comprising:
contacting the electroplating rack with a non-aqueous solution comprising a
metallization
inhibitor.
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DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The present invention allows for the treatment of electroplating racks used
for the
purpose of supporting non-conductive substrates during a metallization step.
The method
described herein allows for the effective activation of plastics that have
been etched without the
use of chromic acid while avoiding the common problem of rack "plate up" which
occurs when
chromic acid free etchants are used for the initial roughening of the plastic.
In addition, the
present invention relates generally to the catalysis and subsequent
metallization of plastics such
as ABS and ABS/PC plastics that have been etched in process solutions that do
not contain
chromic acid and without problems of "plate up" on at least partially coated
racks.
In one preferred embodiment, the method generally comprises the steps of:
1) Immersing a rack partially coated with non-conductive material in a
solution
containing a metallization inhibitor;
2) Rinsing and drying the rack;
3) Mounting the parts to be metallized on the rack;
4) Etching the plastic components mounted on the treated racks in etching
solutions that
do not contain chromic acid (including, for example, etching solutions based
on
permanganate or manganese (III);
5) Activating the surface of the plastic by immersing the plating racks in a
solution
comprising colloidal palladium/tin or ionic palladium;
6) Immersing the rack in an accelerating process to remove protective tin
oxides from
the surface (in the case of colloidal palladium/tin activation) or immersing
the rack in
a reducing process to form palladium metal on the surface (in the case of
ionic
palladium);
7) Immersing the racks containing the etched and activated parts in a
metallization bath
to chemically deposit either nickel or copper onto the surface of the
activated part;
and
8) Electroplating the parts, typically by plating copper, nickel and/or
chromium.
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Thus, in one embodiment, the present invention relates generally to an
electroplating rack
for supporting non-conductive substrates during an electrodeposition process,
wherein the electroplating rack is at least partially coated with a non-
conductive material;
and wherein the electroplating rack is treated with a non-aqueous solution
comprising a
metallization inhibitor.
As described herein, the electroplating rack is typically coated with a PVC
plastisol, or
another non-conductive material.
The non-aqueous solution generally comprises about 5 g/L to about 40 g/L of
the
metallization inhibitor, more preferably about 15 g/L to about 25 g/L of the
metallization
inhibitor, and most preferably about 10 g/L to about 20 g/L of the
metallization inhibitor.
The non-aqueous solution is preferably maintained at a temperature of between
about
C and about 75 C, more preferably a temperature of between about 35 C and
about 65 C,
during the time that the electroplating rack is immersed in the non-aqueous
solution. In addition,
the electroplating rack is immersed in the non-aqueous solution for a period
of time sufficient to
treat the PVC plastisol coated rack to avoid rack plate on. That is the
electroplating rack is
20
preferably immersed in the non-aqueous solution for between about 1 minute and
about 60
minutes, more preferably for between about 2 minute and about 30 minutes.
The inventors of the present invention have found that metallization
inhibitors that are
substantially soluble in aqueous media are unsuitable for the process
described herein because
they tend to slowly leach into subsequent process solutions and prevent
metallization of the
25
parts. Preferably, the metallization inhibitor is at least essentially
insoluble in aqueous media.
Thus, the solution containing the metallization inhibitor is a non-aqueous
solution.
Suitable water insoluble metallization inhibitors are generally organic
compounds
comprising sulfur in a -2 valency and include, but are not limited to,
transition metal salts of di-
substituted dithiocarbamates and tetra-substituted thiuram sulfides.Suitable
dithiocarbamates
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include, for example, zinc dimethyl-dithiocarbamate (ZDMC), zinc
diethyldithiocarbamate
(ZDEC), zinc dibutyldithiocarbamate (ZDBC), zinc ethylphenyldithiocarbamate
(ZEPC), zinc
dibenzyldithiocarbamate (ZBEC), zinc pentamethylenedithiocarbamate (Z5MC),
tellurium
diethyldithiocarbamate, nickel dibutyl dithiocarbamate, nickel
dimethyldithiocarbamate, and zinc
diisononyldithiocarbamate. Preferred tetra-substituted thiuram sulfides
include, for example,
tetrabenzylthiuram disulfide, mercaptobenzothiazoles,
mercaptothiazolines,
mercaptobenzimidazoles, mercaptoimidazoles, mercaptobenzoxazoles,
mercaptothiazole,
mercaptotriazole, dithiocyanuric acid, and trithiocyanuric acid. Combinations
of one or more of
the metallization inhibitors may also be sued. In a preferred embodiment, the
metallization
inhibitor comprises nickel dibutyl dithiocarbamate or tetrabenzylthiuram
disulfide.
Suitable non-aqueous solvents include, but are not limited to butylene
carbonate,
propylene carbonate, ethylene carbonate, dimethyl carbonate, diethyl
carbonate, ethylmethyl
carbonate, propyl lactate, gamma-butyrolactone, ethyl 3-ethoxypropionate and
diethyleneglycol
monomethyl ether acetate, ethyleneglycol monomethyl ether acetate,
ethyleneglycol monoethyl
ether acetate, diethyleneglycol monoethyl ether acetate, diethyleneglycol mono-
n-butyl ether
acetate, propyleneglycol monomethyl ether acetate, propyleneglycol monoethyl
ether acetate,
propyleneglycol monopropyl ether acetate, propyleneglycol monobutyl ether
acetate,
dipropyleneglycol monomethyl ether acetate, dipropyleneglycol monoethyl ether
acetate, glycol
diacetate, by way of example and not limitation.
The solvent mixture must be capable of dissolving an effective amount of the
metallization inhibitor, be readily rinsed off treated racks, and be
preferably non-volatile and safe
to handle with regards to its toxicology and flammability. In addition, the
solvent mixture
should cause no damage to the rack coating. It has been found that solvents
that are very readily
water-soluble can have difficulty in dissolving water-insoluble metallization
inhibitors and thus
do not give an effective inhibition of metallization. However, substantially
water insoluble
solvents that readily dissolve the inhibitors and provide a better degree of
inhibition can cause a
greater degree of attack on the rack coating and are also more problematic to
rinse off the rack
after treatment.
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The degree of attack on the rack coating is related to the degree of diffusion
of the
metallization inhibitor into the surface of the rack coating, and the choice
of solvents is therefore
critical to the success of the process.
The metallization inhibitor described herein can be readily applied to racks
during the
normal treatment cycle to remove unwanted metallic deposits from the tips of
the contact points.
The invention will now be illustrated with reference to the following non-
limiting
examples:
Comparative Example 1:
An ABS test panel and a new PVC plastisol coated test piece were processed
through a
standard pretreatment sequence comprising the following stages:
1) Etch the test pieces in a solution containing 400 g/L of chromic acid, 350
g/L of sulfuric
acid and 0.1 g/L of perfluorooctylsulfonic acid (8 minutes at 65 C);
2) Rinse;
3) Subject the test pieces to a reducing stage comprising an aqueous solution
of
hydroxylamine hydrochloride and hydrochloric acid;
4) Rinse;
5) Immerse the test pieces in a solution of 30% hydrochloric acid as a pre-dip
before
activation;
6) Immerse the test pieces in a conventional palladium/tin activator
(MacDermid Macuplex
D34C) for 3 minutes at 30 C;
7) Rinse;
8) Immerse the test pieces in a conventional accelerator solution (MacDermid
Ultracel
9369) for 2 minutes at 50 C;
9) Rinse; and
10) Immerse the test pieces in an electroless nickel process designed for
plating on plastic
applications (MacDermid Macuplex J64) for 7 minutes at 30 C.
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Following this treatment, the test pieces were examined. It was found that the
ABS test
panel was fully covered in electroless nickel with no apparent voids.
Subsequent electroplating
of this test panel gave full coverage and good adhesion. The PVC plastisol
coated test piece
showed no coverage of the electroless nickel. Repeated cycling of ABS and PVC
test pieces
through steps 1-10 continually showed full electroless nickel coverage of ABS
and no electroless
nickel coverage of the PVC.
Comparative Example 2:
An ABS test panel and a new PVC plastisol coated test piece were processed
through a
pretreatment sequence comprising the following stages:
1) Immerse the test pieces in a solvent predip comprising 100 mL/L of
propylene carbonate
and 50 rnL/L of gamma-butyrolactone for 2 minutes at 35 C;
2) Rinse;
3) Etch the test pieces in a solution comprising 12.5 M sulfuric acid
containing 0.04 M
manganous sulfate and 0.02 M of manganese(III) ions at 68 C for 20 minutes;
4) Rinse;
5) Subject the test pieces to a reducing stage comprising an aqueous solution
of ascorbic
acid; and
6) Carry out stages 4 to 10 of Comparative Example 1.
Following this treatment, the test pieces were examined. It was found that the
ABS test
panel was fully covered in electroless nickel with no apparent voids.
Subsequent electroplating
of this test panel gave full coverage and good adhesion. The PVC plastisol
coated test piece
showed significant coverage of the electroless nickel which was observed to
cover between 10%
and 50% of the surface area. This would be expected to cause considerable
problems in
commercial practice. Repeated cycling of ABS and PVC plastisol coated test
pieces through
steps 1-6 continually showed full electroless nickel coverage of ABS and
increasing amounts of
electroless nickel coverage of the PVC plastisol coated test piece.
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Comparative Example 3:
An old PVC plastisol coated test piece which had been cycled hundreds of times
in a
production facility using hexavalent chromium treatment solutions, was leached
for several hours
in hot water to remove any remaining hexavalent chromium on the surface (the
inventors have
determined that this leaching effectively eliminates any metallization
inhibition provided by
hexavalent chromium in the PVC plastisol).
An ABS test panel and the old PVC plastisol coated test piece were processed
through
stages 1-6 of Comparative Example 2.
Following this treatment, the test pieces were examined. The ABS test panel
was fully
covered in electroless nickel with no apparent voids. Subsequent
electroplating of this test panel
gave full coverage and good adhesion. The PVC plastisol coated test piece
showed full coverage
of the electroless nickel over the entire surface of the plastisol test piece.
This would be totally
unacceptable in commercial practice.
Comparative Example 4:
A new PVC plastisol coated test piece was treated as follows:
A. Immerse the plastisol coated test piece in gamma butyrolactone for 10
minutes at
65 C;
B. Rinse and dry the test piece.
An ABS test panel and the treated PVC plastisol coated test piece were
processed through
the following stages:
1) Immerse the test pieces in a solvent predip comprising 100 mL/L of
propylene carbonate
and 50 mL/L of gamma-butyrolactone for 2 minutes at 35 C;
2) Rinse;
3) Etch the test pieces in a solution comprising 12.5 M sulfuric acid
containing 0.04 M
manganous sulfate and 0.02 M of manganese(III) ions at 68 C for 20 minutes;
4) Rinse;
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5) Subject the test pieces to a reducing stage comprising an aqueous solution
of ascorbic
acid; and
6) Carry out stages 4 to 10 of Comparative Example 1.
Following this treatment, the test pieces were examined. It was found that the
ABS test
panel was fully covered in electroless nickel with no apparent voids.
Subsequent electroplating
of this test panel gave full coverage and good adhesion. The PVC plastisol
coated test piece
showed significant coverage of the electroless nickel which was observed to
cover between 10%
and 50% of the surface area. There was no apparent difference observed between
the PVC
plastisol coated test piece that had been treated in a solvent versus a PVC
plastisol coated test
piece that had not been treated in a solvent.
These comparative examples illustrate the problems associated with rack plate-
up when
chrome-free pretreatment sequences are utilized and demonstrate that old used
PVC plastisol
surfaces are more prone to metallization than new PVC plastisol surfaces when
hexavalent
chromium is absent. Comparative Example 4 demonstrates that a solvent
treatment without the
inhibitor has no effect.
Example 1:
A new PVC plastisol coated test piece was treated as follows:
A. Immerse the plastisol coated test piece in a solution of gamma
butyrolactone
containing 20 g/L of nickel dibutyldithiocarbamate for 10 minutes at 65 C;
B. Rinse and dry the test piece.
An ABS test panel and the treated PVC plastisol coated test piece were
processed through
the following stages:
1) Immerse the test pieces in a solvent predip comprising 100 mL/L of
propylene carbonate
and 50 mL/L of gamma-butyrolactone for 2 minutes at 35 C;
2) Rinse;
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3) Etch the test pieces in a solution comprising 12.5 M sulfuric acid
containing 0.04 M
manganous sulfate and 0.02 M of manganese(III) ions at 68 C for 20 minutes;
4) Rinse;
5) Subject the test pieces to a reducing stage comprising an aqueous solution
of ascorbic
acid; and
6) Carry out stages 4 to 10 of Comparative Example 1.
Following this treatment, the test pieces were examined. It was found that the
ABS test
panel was fully covered in electroless nickel with no apparent voids.
Subsequent electroplating
of this test panel gave full coverage and good adhesion. In addition, the PVC
plastisol coated
test piece showed no coverage of the electroless nickel.
Repeated cycling of ABS and the treated PVC plastisol test piece through steps
1-6
continually showed full electroless nickel coverage of ABS and no electroless
nickel coverage of
the treated PVC plastisol coated test piece up to 3 cycles. After 3 cycles,
approximately 10% of
metallization was visible on the PVC plastisol. At this stage, the PVC
plastisol coated test piece
was treated in the inhibitor solution for a second time and then repeatedly
cycled through stages
1 through 6 again. No metallization was found on the treated PVC plastisol for
at least another 3
cycles, while full electroless nickel coverage was obtained on the ABS test
piece. The
appearance of the PVC plastisol was still satisfactory with little or no
change from its original
appearance.
Example 2:
A new PVC plastisol coated test piece was treated as follows:
A. Immerse the plastisol coated test piece in a solution of ethyl 3-
ethoxypropionate
containing 20 g/L of nickel dibutyldithiocarbamate for 30 minutes at 42 C;
B. Rinse and dry the test piece.
An ABS test panel and the treated PVC plastisol coated test piece were
processed through
stages 1-6 as described in Example 1.
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Following this treatment, the test pieces were examined. It was found that the
ABS test
panel was fully covered in electroless nickel with no apparent voids.
Subsequent electroplating
of this test panel gave full coverage and good adhesion. The treated PVC
plastisol coated test
piece showed no coverage of the electroless nickel.
Repeated cycling of the ABS and treated PVC plastisol coated test pieces
through steps
1-6 as described in Example 1 continually showed full electroless nickel
coverage of ABS and
no electroless nickel coverage of the treated PVC plastisol up to 4 cycles.
The appearance of the PVC plastisol was still satisfactory but was softer than
the original
coating.
Example 3:
A new PVC plastisol coated test piece was treated as follows:
A. Immerse the plastisol coated test piece in a solution of n-
propyl lactate and ethyl
3-ethoxypropionate containing 10 g/L of tetrabenzylthiuram disulfide for 10
minutes at 40 C;
B. Rinse and dry the test piece.
An ABS test panel and the treated PVC plastisol coated test piece were
processed through
stages 1-6 as described in Example 1.
Following this treatment, the test pieces were examined. It was found that the
ABS test
panel was fully covered in electroless nickel with no apparent voids.
Subsequent electroplating
of this test panel gave full coverage and good adhesion. The treated PVC
plastisol coated test
piece showed no coverage of the electroless nickel.
Repeated cycling of the ABS and treated PVC plastisol coated test pieces
through steps
1-6 described in Example 1 continually showed full electroless nickel coverage
of ABS and no
electroless nickel coverage of the treated PVC plastisol up to 6 cycles. After
6 cycles, a small
amount of metallization was visible on the PVC plastisol. At this stage, the
PVC plastisol coated
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test piece was treated in the inhibitor solution for a second time and then
repeatedly cycled
through steps 1 to 6 as described in Example 1 again. No metallization was
found on the PVC
plastisol for at least another 6 cycles, while full electroless nickel
coverage was obtained on the
ABS test piece.
The appearance of the PVC plastisol was still satisfactory, with little or no
change from
its original appearance.
Example 4
A new PVC plastisol coated test piece was treated as follows:
A. Immerse the plastisol coated test piece in a solution of ethyl 3-
ethoxypropionate
containing 10 g/L of tetrabenzylthiuram disulfide for 30 minutes at 40 C;
B. Rinse and dry the test piece.
An ABS test panel and the treated PVC plastisol coated test piece were
processed through
stages 1-6 as described in Example 1.
Following this treatment, the test pieces were examined. It was found that the
ABS test
panel was fully covered in electroless nickel with no apparent voids.
Subsequent electroplating
of this test panel gave full coverage and good adhesion. The treated PVC
plastisol coated test
piece showed no coverage of the electroless nickel.
Repeated cycling of the ABS and treated PVC plastisol coated test pieces
through steps A
and B above and then through steps 1-6 as described in Example 1 (i.e. with
the PVC plastisol
being treated in the inhibitor solution prior to each etch and metallization
cycle) continually
showed full electroless nickel coverage of ABS and none or minimal electroless
nickel coverage
of the treated PVC plastisol up to 10 cycles. After 10 cycles, a small amount
of metallization was
visible on the PVC plastisol.
The appearance of the PVC plastisol was satisfactory, but was softer than the
original
coating.
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Example 5
An old plastisol test piece which had been cycled hundreds of times on a
production
facility using hexavalent chromium treatment solutions, was leached for
several hours in hot
water to remove any remaining hexavalent chromium on the surface.
The old PVC plastisol coated test piece was treated as follows:
A. Immerse the plastisol coated test piece in a solution of n-propyl
lactate and ethyl
3-ethoxypropionate containing 10 g/L of tetrabenzylthiuram disulfide for 5
minutes at 40 C;
B. Rinse and dry the test piece.
An ABS test panel and the treated PVC plastisol coated test piece were
processed through
stages 1-6 as described in Example 1.
Following this treatment, the test pieces were examined. It was found that the
ABS test
panel was fully covered in electroless nickel with no apparent voids.
Subsequent electroplating
of this test panel gave full coverage and good adhesion. The treated PVC
plastisol coated test
piece showed no coverage of the electroless nickel despite being a very well
used and aged
coating with a cracked and roughened surface.
Repeated cycling of the ABS and treated PVC plastisol coated test pieces
through step A
and B above and then through steps 1-6 as described in Example 1 (for this
example, the plastisol
coating was treated in the inhibitor solution prior to each etch and
metallization cycle)
continually showed full electroless nickel coverage of ABS and no electroless
nickel coverage of
the treated PVC plastisol up to 25 cycles.
Example 6
A new PVC plastisol coated test piece was treated as follows:
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A. Immerse the plastisol coated test piece in a solution of n-propyl
lactate and ethyl
3-ethoxypropionate containing 10 g/L of tetrabenzylthiuram disulfide for 2
minutes at 40 C;
B. Rinse and dry the test piece.
An ABS test panel and the treated PVC plastisol coated test piece were
processed through
stages 1-6 as described in Example 1.
Following this treatment, the test pieces were examined. It was found that the
ABS test
panel was fully covered in electroless nickel with no apparent voids.
Subsequent electroplating
of this test panel gave full coverage and good adhesion. The treated PVC
plastisol coated test
piece showed no coverage of the electroless nickel.
Repeated cycling of the ABS and treated PVC plastisol coated test pieces
through steps A
and B above and then through steps 1-6 as described in Example 1 (for this
example, the plastisol
coating was treated in the inhibitor solution prior to each etch and
metallization cycle)
continually showed full electroless nickel coverage of ABS and no electroless
nickel coverage of
the treated PVC plastisol up to 25 cycles.
The appearance of the PVC plastisol was still satisfactory, with little or no
change from
its original appearance.
