Electroless nickel plating solutions

Details Electroless nickel plating solutions Electroless nickel plating solutions

2002-06-18

2004-10-05

Klemanski, Helene (Department: 1755)

Compositions: coating or plastic

Coating or plastic compositions

Metal-depositing composition or substrate-sensitizing...

C106S001270, C427S443100, C427S438000

Reexamination Certificate

active

06800121

ABSTRACT:

TECHNICAL FIELD OF THE INVENTION
This invention relates to aqueous electroless nickel plating solutions, and more particularly, to nickel plating solutions based on nickel salts of alkyl sulfonic acids as the source of nickel ions.
BACKGROUND OF THE INVENTION
Electroless nickel plating is a widely utilized plating process which provides a continuous deposit of a nickel metal or nickel/alloy coating on metallic or non-metallic substrates without the need for an external electric plating current. Electroless plating has been described as a controlled autocatalytic chemical reduction process for depositing metals. The process involves a continuous buildup of a nickel coating on a substrate by immersion of the substrate in a suitable nickel plating bath under appropriate electroless plating conditions. The plating baths generally comprise an electroless nickel salt and a reducing agent. Some electroless nickel baths use hypophosphite ions as a reducing agent, and during the process, the hypophosphite ions are oxidized to orthophosphite ions, and the nickel cations in the plating bath are reduced to form a nickel phosphorous alloy as a deposit on the desired substrate surface. As the reaction proceeds, the level of orthophosphite ions in the bath increases, and the orthophosphite ions often are precipitated from the plating solutions as insoluble metal orthophosphites. The precipitation of insoluble orthophosphites from the plating solutions may cause “roughness” on the plated article. Typically, the source of nickel ions in the electroless plating baths described in the prior art has included nickel chloride, nickel sulfate, nickel bromide, nickel fluoroborate, nickel sulfonate, nickel sulfamate, and nickel alkyl sulfonates.
SUMMARY OF THE INVENTION
This invention relates to electroless nickel plating solutions utilizing nickel salts of alkyl sulfonic acids, and to a method of plating substrates utilizing the electroless nickel plating solutions of the invention. The nickel plating solutions of this invention produce acceptable nickel deposits over an extended period of time and at a high plating rate. In particular, the plating baths of the invention exhibit longer plating lives and faster plating rates than conventional electroless nickel electrolytes based on nickel sulfate.
In one embodiment, the aqueous electroless nickel plating solutions of the invention comprise:
(A) a nickel salt of an alkyl sulfonic acid, and
(B) hypophosphorous acid or a bath soluble salt thereof selected from sodium hypophosphite, potassium hypophosphite and ammonium hypophosphite,
wherein the solution is free of added nickel hypophosphite, and free of alkali or alkaline earth metal ions capable of forming an insoluble orthophosphite.
In yet another embodiment, the aqueous electroless nickel plating solutions of the invention are prepared from:
(A) a nickel salt of an alkyl sulfonic acid, and
(B) hypophosphorous acid or a bath soluble salt thereof selected from sodium hypophosphite, potassium hypophosphite and ammonium hypophosphite,
wherein the solution is free of added nickel hypophosphite, and free of alkali or alkaline earth metal ions capable of forming an insoluble orthophosphite.
In yet another embodiment, the invention relates to a process for the electroless deposition of nickel on a substrate from a nickel plating solution which comprises contacting the substrate with a solution comprising:
(A) a nickel salt of an alkyl sulfonic acid, and
(B) hypophosphorous acid or a bath soluble salt thereof selected from sodium hypophosphite, potassium hypophosphite and ammonium hypophosphite,
wherein the solution is free of added nickel hypophosphite, and free of alkali or alkaline earth metal ions capable of forming an insoluble orthophosphite.
In another embodiment, the invention relates to a process for the electroless deposition of nickel on a substrate with a nickel plating solution which comprises:
(A) preparing a nickel plating solution comprising
(i) a nickel salt of an alkyl sulfonic acid characterized by the formula
wherein R″ is hydrogen, or a lower alkyl group that is unsubstituted or substituted by oxygen, Cl, Br or I, CF
3
or —SO
3
H
R and R′ are each independently hydrogen, Cl, F, Br, I, CF
3
or a lower alkyl group that is unsubstituted or substituted by oxygen, Cl, F, Br, I, CF
3
or —SO
3
H,
a, b and c are each independently an integer from 0 to 3,
y is an integer from 1 to 3, and the sum of a+b+c+y=4, and
(ii) hypophosphorous acid or a bath soluble salt thereof selected from sodium hypophosphite, potassium hypophosphite and ammonium hypophosphite,
wherein the nickel plating solution is free of added nickel hypophosphite, and free alkali or alkaline earth metal ions capable of forming an insoluble orthophosphite, and
(B) contacting the substrate with the plating solution prepared in (A).
DESCRIPTION OF THE EMBODIMENTS OF THE INVENTION
In one embodiment, the aqueous electroless nickel plating solutions of the invention comprise:
(A) a nickel salt of an alkyl sulfonic acid, and
(B) hypophosphorous acid or a bath soluble salt thereof selected from sodium hypophosphite, potassium hypophosphite and ammonium hypophosphite, wherein the solution is free of added nickel hypophosphite, and free of alkali or alkaline earth metal ions capable of forming an insoluble orthophosphite.
In one embodiment, the alkyl sulfonic acid of the nickel salt may be characterized by the formula
wherein R″ is hydrogen, or a lower alkyl group that is unsubstituted or substituted by oxygen, Cl, Br or I, CF
3
or —SO
3
H
R and R′ are each independently hydrogen, Cl, F, Br, I, CF
3
or a lower alkyl group that is unsubstituted or substituted by oxygen, Cl, F, Br, I, CF
3
or —SO
3
H,
a, b and c are each independently an integer from 0 to 3,
y is an integer from 1 to 3, and the sum of a+b+c+y=4.
In one embodiment the alkyl sulfonic acid is an alkyl monosulfonic acid or an alkyl disulfonic acid, (i.e., y=1 or 2). In another embodiment, each of the lower alkyl groups R, R′ and R′ independently contains from 1 to about 4 carbon atoms.
Representative sulfonic acids include the alkyl monosulfonic acids such as methanesulfonic, ethanesulfonic and propanesulfonic acids and the alkyl polysulfonic acids such as methanedisulfonic acid, monochloromethanedisulfonic acid, dichloromethanedisulfonic acid, 1,1-ethanedisulfonic acid, 2-chloro-1,1-ethanedisulfonic acid, 1,2-dichloro-1,1-ethanedisulfonic acid, 1,1-propanedisulfonic acid, 3-chloro-1,1-propanedisulfonic acid, 1,2-ethylene disulfonic acid and 1,3-propylene disulfonic acid.
Because of availability, the sulfonic acids of choice are methanesulfonic acid (MSA) and methanedisulfonic acid (MDSA). In one embodiment of the invention the entire nickel ion content of the electroless nickel plating bath can be supplied in the form of the alkyl sulfonic acid salts.
In the electroless nickel solutions of the invention the operating nickel ion concentration is typically from about 1 up to about 18 grams per liter (g/l). In some embodiments, concentrations of from about 3 to about 9 g/l are utilized. Stated differently, the concentration of nickel cation will be in the range of from 0.02 to about 0.3 moles per liter, or in another embodiment, in the range of from about 0.05 to about 0.1 5 moles per liter.
The nickel alkyl sulfonates which are utilized as the source of nickel cations in the plating solutions of the present invention may be prepared by methods known to those skilled in the art. In one method, a saturated solution of a nickel alkyl sulfonic acid such as nickel methane sulfonate can be prepared at room temperature by dissolving nickel carbonate in MSA. The reaction proceeds as follows:
 NiCO
3
+2CH
3
SO
3
H→Ni(CH
3
SO
3
)
2
+H
2
O+2CO
2
↑
Another chemical process for preparing a nickel alkyl sulfonate involves the reaction of nickel with, e.g., MSA. This reaction proceeds as follows:
A nickel alkyl sulfonate such as nickel methane sulfonate also can be produc

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