BACKGROUND OF THE INVENTION
1. Technical Field
[0001] The present invention is related to electroplating and, more particularly, to an
electroplating salt, bath and process.
2. Art Background
[0002] Precious metals are used in a.variety of industrial applications including electrical
circuits as electrical contact surfaces, conducting paths and heat sinks. Various
properties of precious metals make their use in such applications highly advantageous.
These properties include physical and chemical stability, high electrical conductivity,
low porosity, and high thermal conductivity. Precious metals are often used in high-density
circuits such as integrated circuits because of one or more of the properties set
forth above.
[0003] One problem with precious metal electroplating processes involves the selection of
a suitable compound to supply the plating metal initially to the bath. The compound
should be stable, easily made in reasonably pure form and readily soluble in the electroplating
bath. The problem is particularly difficult where rapid, high quantity palladium electroplating
is being carried out. In this case, relatively large amounts of palladium metal are
being plated out and therefore large amounts of palladium must be added to the bath.
Under these circumstances, high solubility and high rate of solubility is extremely
important.
[0004] In addition, compatibility of the components of the palladium compound (complexing
species, anion, etc.) as well as the complexity of the bath are also of much practical
importance since these factors may limit the lifetime of the electroplating bath,
alter the electroplating characteristics of the bath as well as add contaminants to
the main plating tank.
SUMMARY OF THE INVENTION
[0005] In accordance with one embodiment, an electroplating bath includes a metal-sulfonic
acid complex wherein the metal is a precious metal, and free sulfonic acid. As used
herein, the term "precious metals" include Pd, Au, Hard Au (with Ni or Co as hardening
agents), Pt, Rh, Ru, Ag and alloys thereof.
[0006] The sulfonic acid may be an acid having the following structure:
R[SO
3]-X
wherein R = alkyl or aryl group and X = cationic species.
The sulfonic acid may for example be methane sulfonic acid (MSA). The resulting bath
may have a pH less than 2.5, particularly less than 1.0.
[0007] The electroplating bath may include only a single additive, such as a brightener
or brightening agent.
[0008] When a precious metal alloy is to be plated, the electroplating bath may also include
an alloying metal-sulfonic acid complex. For example, the bath for plating an Pd-Ni
alloy may include a Pd-sulfonic acid complex and a Ni-sulfonic acid complex. The concentration
of the two metal complexes is largely dictated by the desired percentage composition
of the alloy.
[0009] In accordance with a further embodiment, a process of electroplating a metal on an
object (e.g., a substrate), involves maintaining an object to be plated in a solution
including a metal-sulfonic acid complex having a precious metal, and free sulfonic
acid while providing current through the bath. The object may be pre-dipped in sulfonic
acid.
[0010] In one example, the precious metal may be Pd which is immersion deposited on a substrate,
such as Cu or an alloy thereof
[0011] In accordance with another embodiment, a metal salt for use in an electroplating
bath to electroplate Pd or Pd alloys is provided. The chemical formula for the Pd
salt may be expressed as follows:
Pd(NH
3)
xMSA
2
wherein x is in the range of 0 to 4.
[0012] In accordance with a further embodiment, a process of synthesizing a metal salt for
use in supplying a plating metal to an electroplating bath is provided. The process
may involve mixing sulfonic acid with a source of metal ions to form a slurry, the
metal being a precious metal; filtering the slurry to collect the precious metal-sulfonic
acid salt; washing the collected salt with alcohol; and drying the washed salt. The
mixing may occur at approximately room temperature.
[0013] Other and further aspects of the present invention will become apparent during the
course of the following description and by reference to the attached drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014]
Fig. 1 illustrates a flow chart of a general process for synthesizing a metal-sulfonic
acid complex or salt or electrolyte for use in supplying a plating metal to a bath
or solution to electroplate precious metals (including their alloys);
Fig. 2 illustrates a flow chart of a general process for employing a bath or solution
including a metal-sulfonic acid complex to electroplate precious metals; and
Figs. 3A-3D illustrate images obtained with a scanning electron microscope of a Pd
layer deposited on a substrate or object using the processes of Figs. 1 and 2 at various
thicknesses.
DETAILED DESCRIPTION OF THE SEVERAL EMBODIMENTS
[0015] The present invention is directed to a metal composition, particularly a metal complex
of methane sulfonic acid (MSA), for supplying a plating metal to an electroplating
bath and a process of synthesizing the metal-MSA complex.
[0016] The present invention is further directed to an electroplating bath including a metal
complex of sulfonic acid and a process of employing such a bath to electroplate precious
metals on an object (e.g., a substrate). The term "precious metals" as used herein
include Pd, Au, Hard Au (with Ni or Co as hardening agents), Pt, Rh, Ru, Ag and alloys
thereof.
[0017] It has been discovered that metal complexes of a sulfonic acid may generally be employed
to supply a plating metal to an electroplating bath to electroplate precious metals
onto a surface of an object. A process to form the metal-sulfonic acid complex is
provided herein which results in high yields of the metal salt with little or minimal
waste of the underlying metal.
[0018] Through the use of such metal-sulfonic acid complexes, it is possible to obtain a
very acidic bath for electroplating precious metals which is particularly suitable
in plating a strike or underlayer onto an object. Additionally, the metal-sulfonic
acid complex provides an electrolytic bath or solution which requires less chemicals
and additives, reduces contamination or the likelihood of contamination of the main
plating tank, produces quality-grade plated products, and may reduce the overall costs
in maintaining the main plating tanks.
[0019] In accordance with one embodiment, an electrolytic bath or solution is provided for
plating precious metals onto a surface of an object. The bath or solution comprises
a metal-sulfonic acid complex or salt wherein the metal is a precious metal, and free
sulfonic acid. The bath may also include an alloying metal-sulfonic acid complex or
salt for plating precious metal alloys. Such an arrangement provides for an acidic
bath with a pH level less than 2.5, preferably less than 1.0.
[0020] In one example, the bath or solution may be limited only to a single additive, such
as a brightening agent or brighteners or other well known plating additives. For example,
additive systems for Pd plating can typically be grouped in to two classes.
[0021] Class I - (brighteners) are generally unsaturated sulfonic compunds where unsaturation
is in the alpha- or beta -position with respect to the sulfonic group. Such compounds
have the formula A - SO
3 - B where A is an aryl or alkylene group, substituted or unsubstituted, and B may
be OH, OR, OM, NH2, etc.
[0022] Class II organic brighteners are generally unsaturated or carbonyl organic compounds.
Examples are coumpounds containing >C=O, >C=C<, >C=N-, (-N=N-), etc.
[0023] Accordingly, the reduction or minimization of chemicals in the bath or solution reduces
contamination or the potential for contamination of the main plating tank.
[0024] In accordance with another embodiment, a process of plating precious metals is provided
herein. The plating process generally involves maintaining an object (e.g., a substrate)
to be plated in a bath or solution containing metal-sulfonic acid complex wherein
the metal is a precious metal, while providing a current through the bath. For plating
precious metal alloys, the bath may further include an alloying metal-sulfonic acid
complex.
[0025] The process may be employed to deposit a strike or underlayer of a precious metal.
[0026] In accordance with a further embodiment, an electrolyte or salt and a process of
manufacturing thereof are provided for use in plating precious metals, such as Pd.
The Pd electrolyte or salt may comprise a palladium-amine methane sulfonic acid complex:
Pd(NH
3)
xMSA
2
where
x is in the range of 0 to 4,
MSA is methane sulfonic acid.
The Pd complex of sulfonic acid may generally be formed by mixing free sulfonic acid,
in this example MSA, with a solution containing an ion source of Pd, such as a Pd
salt.
[0027] The use of a metal-sulfonic acid salt provides various benefits and advantages over
current approaches. For example, the use of Pd-MSA salt provides a very acidic bath
with a pH below at least 2.5, preferably below 1.0, which is particularly useful to
plate a strike (e.g., a layer having an approximate thickness of 3-4 µ-inches) as
well as to plate other-types of metal layers having an approximate thickness of 10-20
µ-inches. The high acidity of the bath scrubs off the surface layer (e.g., an oxide
layer) on the substrate to be plated without requiring additional chemicals or additives
which typically increase the complexity and cost of the bath. The bath may be limited
to a single additive.
[0028] By requiring less chemicals and additives, this in effect reduces contamination or
the likelihood of contamination of the main plating tank due to substrate corrosion
and/or metal salts which are dragged into the tank from a previous plating operation.
The reduction in contaminants in the main plating tank ensures the production of quality-grade
plated products, and reduces the overall costs in maintaining the main plating tanks.
[0029] Furthermore, the Pd-MSA salt provides a plating bath which is particularly suitable
for depositing a strike. This results in additional benefits including improved adhesion
on easily passivated surfaces such as Ni, reduction of porosity of subsequent deposits
and further protects the main plating bath from contamination caused by substrate
corrosion and metal salts dragged-in from previous plating operations.
[0030] Additionally, the Pd-MSA salt provides a plating bath that is compatible with other
plating operations, such as non-halide and halide based chemistries, which may be
performed by the main plating tank.
[0031] An example of a reaction scheme involving the preparation of a Pd-MSA salt, in this
example Pd(NH
3)
x[MSA]
2, is shown below in equation (3).

where :
x preferably ranges from 0 to 4,
A is an anion.
In this example, the sulfonic acid is methane sulfonic acid and the metal ion source
for Pd is Pd(NH
3)A salt, where the anion A may be, for example, a sulfate, halide (e.g., fluorine,
chlorine, bromide and iodide), nitrate, nitrite, acetate, phosphate or sulfamate.
With regard to equation (3), NH
3 may be eliminated to provide a pure metal-sulfonic acid complex or replaced with
other organic amines to provide complexed species of Pd-MSA salt.
[0032] While the above describes a reaction scheme involving one example of an Pd-MSA salt,
the same or similar anion exchange reaction may be applied generally to obtain other
precious metal-sulfonic salts as well as to obtain alloying metal-sulfonic salts.
[0033] For example, a reaction scheme of an alloying metal-MSA salt may generally be expressed
as follows:

where:
M is an alloying metal, such as Ni, Co, Ag, Sn, etc.,
MSA is methane sulfonic acid.
As with equation (3), the anion A may be a sulfate, halide (e.g., fluorine, chlorine,
bromide and iodide), nitrate, nitrite, acetate, phosphate or sulfamate. M may be any
desired alloying metal, such as Ni, Co Ag or Sn, which may be selected according to
the desired application. The alloying metal M may be initially in the form of an alloying
metal ion source, such as a metal salt including the alloying metal. The concentration
of the alloying metal depends upon the alloy composition to be deposited (10% to 95%
Pd by weight).
[0034] In plating alloys, the bath or solution may be prepared by mixing a solution of the
alloying metal-MSA complex together with a Pd-MSA complex and free MSA.
[0035] For facilitating the plating of alloys, a mixed ligand system may also be employed
as desired. An appropriate ligand may be selected depending on the metals in the desired
alloy. For example, the ligand for Pd is ammonia.
[0036] While the above has been described with reference to methane sulfonic acid, other
sulfonic acids in the group having the structure
R[SO
3]-X where R = alkyl or aryl group and X = cationic species, e.g. H
+ or Na
+, etc. may be employed to provide a metal-sulfonic acid complex and bath thereof for
electroplating precious metals.
[0037] With reference to Fig. 1, the general process for synthesizing a precious metal complex
of sulfonic acid for use in supplying a precious metal plating metal to a bath, may
involve:
(a) adding sulfonic acid to a solution with a source of precious metal ions (e.g.,
precious metal salt) to form a slurry,
(b) stirring the slurry with precipitates for an appropriate amount of time,
(c) filtering the slurry and collecting the metal-sulfonic salt,
(d) washing the salt with an alcohol such as isopropanol, and
(e) air drying the washed metal-sulfonic salt.
[0038] The remaining filtrates and alcohol may be collected and recycled accordingly.
[0039] The above provides an efficient process for manufacturing metal-sulfonic salts, particularly,
Pd-MSA salts, at significantly high yields with minimal waste of the metal. This is
particularly important when dealing with precious metals which are costly.
EXAMPLE #1
[0040] An example of a process of synthesizing Pd-MSA salt is provided herein. In this example,
an amount of 250 ml of methane sulfonic acid (70% v/v solution) was added slowly to
1 liter of [Pd(NH
3)
4][SO
4] which contained 70 g/L of Pd (as metal), with stirring at room temperature. The
resulting pale yellow suspension was continuously stirred after the complete addition
of MSA for an additional 30 minutes. The contents of the reaction vessel were then
emptied into a large Büchner funnel. The slightly off-white, finely crystalline product
was washed three times with 40 mL portions of isopropanol (IPA) and allowed to air
dry. This resulted in a yield of 220 g of Pd-MSA salt (91% based on a 70 g/L Pd from
a [Pd(NH
3)
4][SO
4] solution).
[0041] The remaining filtrates were collected and sent to recycle for Pd recovery. If desired,
the alcohol used to wash the product may also be recycled by evaporation and condensation
using standard equipment.
[0042] With reference to Fig. 2, the general process by which a precious metal may be electroplated
onto an object (e.g., a substrate) through the use of a bath including metal-sulfonic
salt for supplying the precious metal preferably involves:
(a) pre-dipping an object whose exposed surface portions are to be plated with a precious
metal or alloys thereof, such as palladium or palladium alloy, in a sulfonic acid
compatible with the plating metal-sulfonic salt(s);
(b) rinsing the object;
(c) applying through the bath a current density greater than 5 mA/cm2 dependent on the type of process (e.g. rack or continuous), and
(d) maintaining in the bath the object to be plated for a time sufficient to develop
upon the exposed surface portions a precious metal layer of desired thickness.
[0043] The above electroplating process may be employed to immersion deposit Pd on a substrate,
such as Cu or an alloy thereof.
[0044] Examples of the plating bath or solution for Pd and Pd alloys are provided below.
The bath may be employed with various plating processes, such as low speed plating
which is typical for rack mounted articles or high speed plating which is typical
of reel-to-reel type continuous feeding for electronic components (e.g., connectors,
printed circuit boards, printed wiring boards, etc.).
EXAMPLE #2
[0045] In this example, a composition of a bath or solution for electroplating Pd is provided.
The single additive may be a class I or II brightening agent (brightener), as discussed
above. Pd may be deposited at a pH below 1.
[Pd(NH3)4]MSA2; |
0.5-20 g/L of Pd |
a single additive |
0.5-10mL/L |
Free MSA |
50-300 mL/L |
pH |
less than (<) 1 |
Temperature |
Room Temperature |
Current density |
5-50 ASF |
Rotation |
100-500 rpm |
EXAMPLE #3
[0046] In this example, a composition of a bath or solution for electroplating Pd-Ni alloy
is provided. The single additive may be a class I or II brightening agent (brightener),
as discussed above. Pd-Ni alloy may be deposited at a pH below 1.
[Pd(NH3)4]MSA2; |
0.5-20 g/L of Pd |
Ni[MSA] |
0.5-20 g/l of Ni |
a single additive |
0.5-10mL/L |
Free MSA |
50-300 mL/L |
pH |
less than (<) 1 |
Temperature |
Room Temperature |
Current density |
5-50 ASF |
Rotation |
100-500 rpm |
[0047] Various tests were performed to investigate the use of Pd-MSA to electroplate Pd
on an object or substrate, such as one formed of Cu, and are described below.
[0048] Figs. 3A-3D illustrate images obtained with a scanning electron microscope at different
thickness levels at 2200x of Pd layer deposited on a substrate or object using the
processes of Figs. 1 and 2. As shown, the deposited layer did not evidence any micro-cracking.
[0049] On copper substrates, immersion deposition with the use of the Pd-MSA bath resulted
in a bright, white Pd layer. This phenomenon may find particular use in the electronics
industry. More specifically, in printed wire board (PWB), printed circuit board (PCB)
manufacture and plating on plastics.
[0050] To test metallic contamination of the main plating tank, Ni and Cu were both added
up to 50 ppm with no negative impact on deposit quality. Regarding drag-in concerns
from other plating operations, up to 20 ml/l was deliberately added to both halide
and non-halide systems with no resulting problems.
1. An electroplating bath comprising:
a metal-sulfonic acid complex wherein the metal is a precious metal; and
free sulfonic acid.
2. The electroplating bath according to claim 1, further including only a single additive.
3. The electroplating bath according to claim 1, further comprising an alloying metal-sulfonic
acid complex.
4. The electroplating bath according to claim 2, wherein the precious metal is Pd and
the alloying metal is selected from the group consisting of Ni, Co, Ag and Sn.
5. The electroplating bath according to claim 1, wherein the pH is less than 2.5.
6. The electroplating bath according to claim 5, wherein the pH is less than 1.0.
7. The electroplating bath according to claim 1, wherein the sulfonic acid is an acid
having the following structure:
R[SO3]-X
wherein R = alkyl or aryl group and X = cationic species.
8. The electroplating bath according to claim 1, wherein the single additive is one of
a group I or II brightening agents.
9. The electroplating bath according to claim 1, wherein the sulfonic acid comprises
methane sulfonic acid, and the metal-sulfonic acid complex comprises a metal-methane
sulfonic complex.
10. A process of electroplating a metal on an object, comprising maintaining an object
to be plated in a solution including a metal-sulfonic acid complex having a precious
metal, and free sulfonic acid while providing current through the bath.
11. The process according in claim 10, further comprising pre-dipping the object in sulfonic
acid.
12. The electroplating bath according to claim 10, wherein the bath only includes a single
additive.
13. The electroplating bath according to claim 10, wherein the bath further comprises
an alloying metal-sulfonic acid salt.
14. The electroplating. bath according to claim 13, wherein the precious metal is Pd and
the alloying metal is selected from the group consisting of Ni, Co, Ag and Sn.
15. The electroplating bath according to claim 10, wherein the pH is less than 2.5.
16. The electroplating bath according to claim 15, wherein the pH is less than 1.0.
17. The electroplating bath according to claim 10, wherein the sulfonic acid is an acid
in the following structure:
R[SO3]-X
wherein R = alkyl or aryl group and X = cationic species.
18. The electroplating bath according to claim 10, wherein the sulfonic acid comprises
methane sulfonic acid and the metal-sulfonic acid complex comprises a metal methane
sulfonic complex.
19. The electroplating bath according to claim 10, wherein the precious metal is Pd which
is immersion deposited on the substrate.
20. The electroplating bath according to claim 19, wherein the object comprises at least
one of Cu or Cu alloy.
21. A metal salt for use in an electroplating bath to electroplate Pd or Pd alloys, comprising:
Pd(NH3)xMSA2
wherein x is in the range of 0 to 4.
22. A process of synthesizing a metal salt for use in supplying a plating metal to an
electroplating bath, comprising:
mixing sulfonic acid with a source of metal ions to form a slurry, the metal being
a precious metal; and
filtering the slurry to collect the precious metal-sulfonic acid salt.
23. The process according to claim 22, further comprising:
washing the collected salt with alcohol; and
drying the washed salt.
24. The process according to claim 22, wherein the mixing occurs at approximately room
temperature.
25. The process according to claim 22, wherein the precious metal is Pd and the sulfonic
acid is methane sulfonic acid.