Stock material or miscellaneous articles – All metal or with adjacent metals – Composite; i.e. – plural – adjacent – spatially distinct metal...
Reexamination Certificate
2000-07-06
2002-05-14
Barr, Michael (Department: 1762)
Stock material or miscellaneous articles
All metal or with adjacent metals
Composite; i.e., plural, adjacent, spatially distinct metal...
C427S125000, C427S098300, C427S437000, C427S443100, C106S001230
Reexamination Certificate
active
06387542
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to electroless plating of silver onto a substrate. More particularly, this invention pertains to an aqueous silver plating bath, a process for plating a uniform coating of silver onto various substrates using an electroless plating composition, and a silver plated article formed therefrom.
2. Description of the Related Art
Plating of metals is a well known process employed to alter the existing surface properties or dimensions of a substrate. For example, a substrate may be plated for decorative purposes, to improve resistance to corrosion or abrasion, or to impart desirable electrical or magnetic properties to a substrate. Plating is a common practice many industries, including the manufacture of a variety of electronic packaging substrates, such as printed circuit boards.
There are various methods of plating known in the art, including electroplating and electroless plating. Electroplating involves the formation of an electrolytic cell wherein a plating metal represents an anode and a substrate represents a cathode, and an external electrical charge is supplied to the cell to facilitate the coating the substrate.
Electroless plating involves the deposition of a metallic coating from an aqueous bath onto a substrate by a controlled chemical reduction reaction which is catalyzed by the metal or alloy being deposited or reduced. This process differs from electroplating in that it requires no external electrical charge. One attractive benefit of electroless plating over electroplating is the ability to plate a substantially uniform metallic coating onto a substrate having an irregular shape. Frequently, electroplating an irregularly shaped substrate produces a coating having non-uniform deposit thicknesses because of varying distances between the cathode and anode of the electrolytic cell. Electroless plating obviates this problem by excluding the electrolytic cell. Another benefit of electroless plating over electroplating is that electroless plating is autocatalytic and continuous once the process is initiated, requiring only occasional replenishment of the aqueous bath. Electroplating requires an electrically conductive cathode and continues only while an electric current is supplied to the cell. Also, electroless coatings are virtually nonporous, which allows for greater corrosion resistance than electroplated substrates.
In general, an electroless plating bath includes water, a water soluble compound containing the metal to be deposited onto a substrate, a complexing agent that prevents chemical reduction of the metal ions in solution while permitting selective chemical reduction on a surface of the substrate, and a chemical reducing agent for the metal ions. Additionally, the plating bath may include a buffer for controlling pH and various optional additives, such as bath stabilizers and surfactants. The composition of a plating bath typically varies based on the particular goals of the plating process. For example, U.S. Pat. No. 6,042,889, teaches an electroless plating bath having a hypophosphite reducing agent and employing one of several different “mediator ions”, including silver ions, for the purpose of converting a non-autocatalytic metal-reduction reaction into an autocatalytic reaction to plate a substrate with copper.
Silver is a desirable plating metal for its high electrical conductivity, corrosion resistance and good friction and wear properties, but present coating techniques are very expensive. Additionally, known efforts to plate silver onto substrates have been imperfect because they use aqueous plating baths containing extremely toxic cyanide compounds and other compounds that contaminate the silver plating.
For example, Japanese patent JP55044540 teaches a process for the electroless plating of silver onto a substrate using an aqueous plating bath comprising silver cyanide, sodium hydroxide and potassium boron hydride as a reducing agent. This bath composition is disadvantageous because of the high toxicity of silver cyanide. Also, it is undesirable because boron hydride derivatives generate extremely flammable gaseous hydrogen and also contaminate the silver metal plating, degrading its appearance.
The present invention solves the problems of the prior art by employing a process for electroless plating of silver using a composition comprising an aqueous solution comprising a water soluble silver salt such as silver nitrate, ammonium hydroxide as a complexing agent, ammonium carbonate and/or bicarbonate as a stabilizer and hydrazine hydrate as a reducing agent. The composition of this aqueous solution is substantially free of non-volatile components that cause impure plating, allowing for improved appearance and properties of the plated silver. Further, the process generates essentially no hazardous substances and the absence of non-volatile components avoids the accumulation of byproducts that degrade the plating bath, allowing for virtually unlimited replenishment of the bath. Moreover, the unique composition of the plating bath allows metallic silver to be precipitated from the plating bath by boiling without undesirable contaminants.
This invention provides a simple low-cost method of a deposition of ultra pure silver coatings on virtually any material of any geometrical shape, including fibers and powders, by electroless autocatalytic plating. The method involves the controlled autocatalytic chemical reduction of a silver salt by a chemical reducer with the formation of a dense uniform metallic silver coating of unlimited thickness selectively on the substrate surface which is contacted with a silver plating bath.
SUMMARY OF THE INVENTION
The invention provides an electroless plating composition comprising an aqueous solution comprising:
a) a silver salt;
b) ammonium hydroxide;
c) ammonium carbonate and/or bicarbonate; and
d) hydrazine hydrate.
The invention also provides a process for plating a substrate comprising:
A) providing a plating composition comprising an aqueous solution comprising:
i) a silver salt;
ii) ammonium hydroxide;
iii) ammonium carbonate and/or bicarbonate; and
iv) hydrazine hydrate; and
B) contacting a substrate with the plating composition for a sufficient time and under conditions sufficient to plate metallic silver onto the substrate.
The invention further provides a process for plating a substrate comprising:
A) providing a plating composition comprising an aqueous solution comprising:
i) a silver salt;
ii) ammonium hydroxide;
iii) ammonium carbonate and/or bicarbonate; and
iv) hydrazine hydrate;
B) immersing a substrate into the plating composition for a sufficient time and under conditions sufficient to plate metallic silver onto the substrate; and
C) removing the substrate from the plating composition.
The invention still further comprises an article comprising a substrate immersed in a composition comprising an aqueous solution comprising:
a) a silver salt;
b) ammonium hydroxide;
c) ammonium carbonate and/or bicarbonate; and
d) hydrazine hydrate.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The present invention teaches a process for uniformly plating various substrates with metallic silver using an electroless plating bath. Initially, an aqueous plating bath comprising water, a water soluble silver salt, an ammonium hydroxide complexing agent, an ammonium carbonate and/or bicarbonate stabilizer and a hydrazine reducer is formed in a suitable container.
Once all of the components are combined in a suitable container, the water soluble silver salt dissolves, releasing silver ions into the bath. The ammonium hydroxide complexing agent forms a strong complex with the silver ions and prevents chemical reduction of the silver ions in the bath while permitting selective chemical reduction on a substrate surface. The hydrazine reducer allows reduction of the silver ions to metallic silver which is deposited selectively on a substrate surface because of catalytic action of a substrate surface. In particular, after a substrate is immersed i
Kozlov Alexander S.
Narasimhan Dave
Palanisamy Thirumalai
Barr Michael
Fredrick Kris T.
Honeywell International , Inc.
LandOfFree
Electroless silver plating does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Electroless silver plating, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Electroless silver plating will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2872281