Electrolysis: processes – compositions used therein – and methods – Electrolytic coating – Depositing predominantly alloy coating
Reexamination Certificate
2002-10-03
2004-06-01
Wong, Edna (Department: 1753)
Electrolysis: processes, compositions used therein, and methods
Electrolytic coating
Depositing predominantly alloy coating
C205S259000, C205S265000, C106S001280
Reexamination Certificate
active
06743346
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to an electrolysis bath for the electrochemical deposition of palladium or its alloys and to a process for the electroplating of palladium or one of its alloys.
The electrical contacts and the connectors used in the field of electronics receive, as a finish, thin layers of electroplated precious metals which have to be suitably bright, have good ductility, be non-porous and have corrosion resistance, frictional resistance and low contact resistance. Industry started by using deposits of gold hardened with small amounts of codeposited nickel or cobalt, often referred to as hard gold. Palladium is a precious metal whose deposits have a lower density (12 g/cm
3
) than those of hard gold (17.3 g/cm
3
); it also has a greater hardness and a lower porosity. Being less expensive, palladium and its alloys were considered suitable gold substitutes for the majority of applications. As a finish in a wide variety of applications, industry uses thin deposits (also called flash deposits) of gold on palladium or palladium alloys. The main palladium alloys used are palladium-nickel or palladium-silver alloys. Techniques commonly used for the electroplating of palladium and its alloys are the barrel, the vibrating basket, the rack, batch metallization, high-speed continuous metallization (or jet plating) or pad metallization. Industry is constantly in search of more efficient electrolysis baths and processes. Palladium and its alloys are also used for decorative applications as an undercoat or finish.
State of the Art Concerning Ammoniacal Baths
The majority of palladium and palladium alloy baths currently on the market are ammoniacal baths most frequently containing chloride ions. These baths nevertheless have a high nuisance factor, both in terms of the operators' health and in terms of corrosion of the equipment, and they require a large number of maintenance operations.
Aqueous ammonia tends to evaporate at ambient temperature and many commercial baths, particularly “high-speed” baths, operate at between 40 and 60° C. These baths emanate large quantities of gas in the treatment plants; these vapors not only irritate the operators' respiratory tracts, but also are corrosive towards all surrounding cuprous metals, including the parts of pieces not immersed in the electrolyte.
Furthermore, the intense evaporation of aqueous ammonia causes a rapid drop in the pH and the volume of these electrolytes and obliges the users to make incessant and expensive additions of aqueous ammonia and pH adjustments. This maintenance is essential, including after every period in which the electrolyte is not in use.
Ammoniacal baths are conventionally alkaline baths operating in a pH range of between 8 and 13. In the case of metallization on nickel, for example, when the piece is immersed the alkalinity of the electrolyte favors passivation of the nickel, which can cause a lack of adhesion of the palladium alloy deposits.
When chlorides are present, they are the cause of yet more trouble:
The corrosion of stainless steel equipment is facilitated, resulting in electrolyte contamination.
During electrolysis, an insoluble yellow palladium salt is generated on the surface of platinized titanium anodes, resulting in multiple difficulties for all applications of the jet plating or continuous selective pad metallization type.
State of the Art Concerning Non-Ammoniacal Baths
The first baths of this type to have been described were pure palladium baths in very acidic media free of organic amines. They were difficult to use. In fact, at pH values of between 0 and 3, the substrates are attacked too strongly. Furthermore, many of these formulations contain chlorides.
A second type consists of pure palladium or palladium alloy baths containing organic amines, which operate at 40 to 65° C., typically in a pH range of 9 to 12, i.e. under strongly alkaline conditions. At these high pH values and these temperatures, polyamines tend to evaporate appreciably and to rapidly become carbonated and produce crystals. Furthermore, under these conditions, the passivation of nickel-plated substrates is even greater than in ammoniacal baths. To overcome the lack of adhesion, it is necessary to palladium-plate the substrates beforehand in a preliminary step, thereby increasing the cost price of these deposits accordingly.
A third type of pure palladium baths containing organic amines is described in particular in patent U.S. Pat. No. 4,278,514. These baths, whose pH values are intermediate at between 3 and 7, generally contain phosphates and use a compound of the imide type, such as succinimide, as a brightening agent. In such baths, the allowable current densities are below 4 A/dm
2
. Furthermore, these baths contain pure palladium and are therefore mainly intended for decorative purposes.
These baths generally use phosphate buffers effective for the intended alkaline pH values. In certain cases, however, the incorporation of traces of phosphorus in the deposits can influence their quality and, in particular, can detract from their brightness.
On the other hand, compounds of the imide type are capable of improving the brightness of these pure palladium baths at low current densities, but the maximum current densities giving bright deposits do not exceed 4 A/dm
2
. Furthermore, to obtain this brightening action, imides are added in large amounts. Now, imides are strong complexing agents and their concentration therefore has a substantial influence on the complexation of any secondary metal incorporated. This makes it too difficult to control the composition of alloys under conditions of suitable brightness.
There is therefore a need for a novel process which excludes the use of aqueous ammonia, chlorides, phosphates and imides and which makes it possible to deposit stable alloys of bright appearance, optionally at very high speed, to give adherent ductile deposits without palladium plating beforehand. The pH of these baths should remain in the weakly acidic range. These baths should also be able to be associated with a metal reloading process capable of avoiding rapid concentration of the salts so as to obtain a long life.
None of the processes currently on the market is fully satisfactory.
An optimal formulation capable of meeting all these demands is precisely what the present invention proposes.
A problem which arises particularly in the case of electronic applications is that of finding a brightening agent which is effective at very high current density in a non-ammoniacal medium. In fact, as explained above, many brightening agents—and this applies particularly to those of the imide type—only enable bright deposits to be obtained at moderate or low current densities. In non-ammoniacal baths, the known commercial brightening agents, such as nicotinamide or compounds of the sulfonate type, are incapable of extending the brightness of the deposits to high current densities, particularly those of between 15 and 150 A/dm
2
that are desirable in “high-speed” electroplating baths.
The present invention is aimed particularly at solving this problem by proposing the use of well-defined brightening agents capable of being used under the ideal conditions mentioned above.
U.S. Pat. No. 4,767,507 describes gold electroplating baths which use two specific brightening agents, namely 3-(3-pyridyl)acrylic acid or 3-(3-quinolyl)acrylic acid.
In the gold baths described in said document, these brightening agents exhibit a very good stability, even when used in very small amounts. They make it possible to extend the brightness to high current densities.
SUMMARY OF THE INVENTION
It has now been established that these brightening agents can also be used in electrolysis baths for the electrochemical deposition of palladium or its alloys in the presence of ethylenediamine acting as a palladium complexing agent. It has been demonstrated in particular that, in such baths, these brightening agents prove particularly active at high current densities, even in very low concentration.
It has thus
Chalumeau Lionel
Gonzalez Jose
Limayrac Michel
Dennison Schultz Dougherty & MacDonald
Metalor Technologies France Sas a French Simplified Joint Stock
Wong Edna
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