Compositions: coating or plastic – Coating or plastic compositions – Metal-depositing composition or substrate-sensitizing...
Reexamination Certificate
2002-10-24
2004-08-17
Klemanski, Helene (Department: 1755)
Compositions: coating or plastic
Coating or plastic compositions
Metal-depositing composition or substrate-sensitizing...
C106S001260, C427S098300, C427S304000
Reexamination Certificate
active
06776828
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates generally to the field of electroless metal plating. In particular, the present invention relates to the field of electroless gold plating.
Immersion or displacement plating is an electroless plating process, but is given a separate classification in the art. In immersion plating, deposition is by displacement of an elemental metal from a substrate by metal ions in a plating solution. In electroless plating deposition takes place primarily by autocatalytic reduction of metal ions from solution. Such electroless plating requires the presence of a reducing agent.
Immersion plating does not employ an external electric current but rather is an electrochemical displacement reaction which is driven by the position of the substrate metal in the electromotive series relative to the metal to be deposited from solution. Plating occurs when the dissolved metal ions in a plating bath are displaced by a more active (less noble) metal that is contacted with the plating bath.
In the manufacture of printed wiring boards, solderable finishes are typically applied to printed wiring board substrates having pads and/or through holes exposed through a mask, such as a soldermask. Such solderable finishes are often applied by immersion plating as electroless plating can also deposit metal on the surface of the mask, which is undesirable. As an immersion plating reaction is driven by the difference in electrochemical potentials, plating will only occur at areas of exposed metal. For example, U.S. Pat. No. 5,143,544 (lantosca) discloses a solution for immersion plating a tin-lead alloy suitable as a solderable finish on a printed wiring board substrate. However, there is a growing demand for more environmentally acceptable alternatives to lead for use in printed wiring board manufacture. Thus, the use of lead and lead alloys in electronic components faces an uncertain future. See, for example, U.S. Pat. No. 5,536,908 (Etchells et al.).
Silver is a more environmentally acceptable alternative to lead and has been suggested for use as a solderable finish. As discussed above, the preferred method of depositing such a solderable finish is by immersion plating. For example, U.S. Pat. No. 5,955,141 (Souter et al.) discloses certain immersion silver plating baths suitable for depositing a layer of silver on a printed wiring board. Silver has many drawbacks, such as poor adhesion of immersion silver deposits and silver's tendency to tarnish, thus requiring the use of anti-tarnish coatings.
Gold has long been used in the electronics industry as a metal for contact surfaces because of its low electrical resistivity and its inertness to attack by corrosive substances. Such gold deposits have typically been plated using electroless or immersion gold plating baths. In particular, gold has long been used over a nickel undercoat to provide a solderable finish. Typically, the nickel undercoat is electrolessly applied while the gold is immersion deposited. Such processes are referred to as electroless-nickel-immersion-gold or “ENIG.”
One conventional form of electroless gold plating bath is thiosulfate ion based, stabilized with sulfite ions. Such baths are typically unstable when operated at a pH of 6 or below as sulfur dioxide is liberated from the bath under these pH conditions. It is known that the thiosulfate ion decomposes in acid solution to give elemental sulfur and sulfite ions. When an aqueous solution of sodium thiosulfate is adjusted to a pH of about 4 to 5, the solution will turn cloudy due to the formation of elemental sulfur. However, if sodium sulfite is also added to the above solution, elemental sulfur will not form and the solution will be stable and clear. Sodium sulfite has, therefore, been used in prior art metal plating solutions and sodium thiosulfate to stabilize the solution. The problem with using sodium sulfite, however, is that the sulfite ion itself is not stable in mildly acidic solutions, such that sulfur dioxide is slowly formed and liberated from the solution. The more acidic the solution, the faster the rate of sulfur dioxide formation will be. This leads to high consumption of sodium sulfite and instability of the metal thiosulfate complex in acidic solutions.
U.S. Pat. No. 5,302,278 (Nobel et al.) discloses metal electroplating solutions, including gold electroplating solutions, containing thiosulfate where the thiosulfate is stabilized by an organic sulfinic acid salt. Neither electroless nor immersion plating solutions are disclosed in this patent.
Electroless gold plating baths contain a reducing agent. Typical reducing agents are thiourea and alkyl thiourea derivatives, enol-containing compounds such as ascorbic acid (see U.S. Pat. No. 4,481,035 to Andrascek et al.), and boron containing compounds such as alkylboranes and borohydrides. These conventional plating baths have certain drawbacks. For example, baths containing thiourea as the reducing agent must be heated to about 80° to 90° C. in order to achieve acceptable deposition rates. Such temperatures are too high for use with some electronics packaging materials. Also, at such temperatures the plating solutions can become unstable and spontaneously form fine particles of gold throughout the solution instead of producing gold deposits only on the desired substrate. When boron containing compounds are used as the reducing agent, such compounds first undergo a hydrolysis reaction whose rate increases with temperature. Much of the boron containing reducing agent is consumed in undesired side reactions making control of its concentration quite difficult.
International patent application WO 99/18254 (Scheel et al.) discloses a solution for electroless gold plating which may contain certain reducing agents, such as oxalic acid. This patent application fails to disclose sulfinic acid or sulfinic acid salts as stabilizers.
Immersion gold plating baths avoid many of the above reducing agent-derived drawbacks. However, such immersion plating baths typically require high plating temperatures, such as about 70° C. or greater, for proper operation. Such high temperatures are often incompatible with some electronics packaging materials.
Thus, there is a need for electroless gold plating solutions that are stable and that work at lower temperatures than conventional plating solutions.
SUMMARY OF THE INVENTION
In one aspect, the present invention provides an electroless gold plating composition including: a) one or more water soluble gold compounds; b) one or more gold complexing agents; c) one or more organic stabilizer compounds of the formula R—SO
2
—Y wherein R is (C
1
-C
18
)alkyl, aryl, or heteroaryl and Y is hydrogen or a monovalent cation; and d) one or more uniformity enhancers.
In another aspect, the present invention provides method of electrolessly depositing gold on a substrate including the step of contacting a substrate with the composition described above.
The present invention further provides a method for depositing gold on a metal that is less electropositive than gold including contacting a substrate containing a metal that is less electropositive than gold with the composition described above.
In a still further aspect, the present invention provides a method of manufacturing an electronic device including depositing a gold layer including the step of contacting an electronic device substrate with a composition including: a) one or more water soluble gold compounds; b) one or more gold complexing agents; c) one or more organic stabilizer compounds of the formula R—SO
2
—Y wherein R is (C
1
-C
18
)alkyl, aryl, or heteroaryl and Y is hydrogen or a monovalent cation; and d) and one or more uniformity enhancers; for a period of time sufficient to deposit the desired gold layer.
DETAILED DESCRIPTION OF THE INVENTION
As used throughout this specification, the following abbreviations shall have the following meanings, unless the context clearly indicates otherwise: ° C.=degrees Centigrade; ° F.=degrees Fahrenheit; g=gram; L=liter; mN&equ
Kanzler Miriana
Toben Michael P.
Cairns S. Matthew
Klemanski Helene
Shipley Company L.L.C.
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