Stock material or miscellaneous articles – All metal or with adjacent metals – Composite; i.e. – plural – adjacent – spatially distinct metal...
Reexamination Certificate
2000-07-06
2002-05-21
Barr, Michael (Department: 1762)
Stock material or miscellaneous articles
All metal or with adjacent metals
Composite; i.e., plural, adjacent, spatially distinct metal...
C427S125000, C427S437000, C427S443100, C106S001210, C106S001280
Reexamination Certificate
active
06391477
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to electroless plating of platinum onto a substrate. More particularly, this invention pertains to an aqueous platinum plating bath, a process for plating a uniform coating of platinum onto various substrates using an electroless plating composition, and a platinum 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 in 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 of 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 a non-uniform deposit thickness 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 and having a hypophosphite reducing agent and employing one of several different “mediator ions”, including platinum ions, for the purpose of converting a non-autocatalytic metal-reduction reaction into an autocatalytic reaction to plate a substrate with copper.
Platinum is a desirable plating metal for its excellent physical and chemical properties, but present techniques for electroless autocatalytic plating with platinum and other non-ferrous or precious metals require a catalytic substrate material. Non-catalytic substrate materials, such as lead, cadmium, bismuth, tin, glass, and ceramics can only be electroplated in an autocatalytic bath after the substrate surface has been catalytically activated. (See, for example,
Modern Electroplating
, Third Edition, Edited by Fla. Lowenheim, John Wiley & Sons, New York, 1974, pp. 710, 720, 721). A non-conductive substrate can be activated by using methods such as firing, or sputtering, or immersion in a tin-palladium solution to place a thin layer of a catalytic metal onto the substrate. Conductive substrate can also be activated by electroplating with a catalytic metal like copper, nickel, gold or silver. However, all these methods of catalytic activation (except the tin-palladium method) can not provide uniform activation of irregular surface. Also, the catalytic activation step significantly adds to the cost of the plating process.
Also, prior art processes for the electroless plating of platinum onto a catalytically active base metal substrate has not been very efficient. According to U.S. Pat. No. 3,562,911, electroless platinum plating in a solution of hexachloroplatinic acid H
2
PtCl
6
in hydrochloric acid HCl with hydrazine dihydrochloride N
2
H
4
*2HCl can be done only on metals which are more noble than copper, typically on palladium, gold and silver. In the presence of less noble metals like iron, nickel and their alloys, the plating bath rapidly decomposes to platinum powder because of poor stability of hexachloroplatinic acid at these conditions. Plating on non-conductors in this bath requires a catalytic activation of a substrate.
Japanese Kokai Patent Publication No. 80764 teaches an electroless plating bath of platinum or a platinum-palladium alloy comprising a platinum nitro complex salt M
2
[Pt(NO
2
)
4
] (M=Na, K) and/or nitro ammine complex salt Pt(NH
3
)
2
(NO
2
)
2
(DNP salt), ammonium hydroxide as a stabilizer and pH regulator, hydroxylamine chloride NH
2
OH*HCl as a stabilizer, hydrazine hydrate N
2
H
4
*H
2
O as; a reducer and wherein the pH of the bath must remain from 10 to about 18 to plate a substrate with platinum. This process is based on a strong ammonia solution of a platinum salt and a hydrazine reducer at pH>10. At pH<9 the bath decomposes to Pt powder. Since many metals and metal alloys are passivated by ammonia solution, electroless deposition of Pt in the presence of ammonia is difficult. For instance, plating even on catalytically active metals like copper and nickel in this solution still requires a preliminary activation by a dip in a palladium or rhodium chloride acid solution. It is well known that such activation is based on an electrochemical displacement mechanism, and provides highly porous, non-adherent films. Plating on non-conductors by the process of Japanese Kokai Patent Publication No. 80764 also requires a preliminary catalytic activation.
It has been known that M
2
[Pt(NO
2
)
4
] (M=Na, K) and DNP salt are decomposed by 10-40% solutions of strong acids HCl, HNO
3
and H
2
SO
4
(JP08176175, JP06178936, GB1325818, Chem. Abstr. 1966, 66:34406). Surprisingly, it has been found that at a pH of about 7 or below and in the absence of the ammonia stabilizer, platinum salt M
2
[Pt(NO
2
)
4
and DNP salt are sufficiently stable, and in the presence of hydrazine reducer produce good coatings from solutions which are stable and clear. It has also been surprisingly found that the bath is effective not only on materials which are normally difficult to plate, but also on non-catalytic materials such as glass and non-wet graphite.
The present invention solves the problems of the prior art by providing a process for electroless autocatalytic deposition of platinum on catalytically inactive materials using a composition comprising an acidic aqueous plating bath comprising a water soluble platinum salt such as platinum nitrite or platinum ammine nitrite salt, hydrazine hydrate as a reducing agent and adjusting the bath, such as with an acid sufficient to maintain it at a pH of about 7 or less. The preferred pH of the inventive plating solution ranges from about 0.5 to about 7, while the pH of prior art method, Japanese Kokai Patent Publication No. 80764 ranges from about 10 to about 18, however, such require the presence of ammonia water. This process provides a substantial cost reduction over prior art methods.
The composition of this aqueous solution is free of non-volatile compon
Koslov Alexander S.
Narasimhan Dave
Palanisamy Thirumalai
Barr Michael
Frederick Kris T.
Honeywell International , Inc.
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