JPS6229516B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The method of electrodepositing metal of various thicknesses onto a substrate is known in the metallization industry as electrodeposition and electroplating.

In a typical plating bath consisting of ions of the metal to be plated and a suitable electrolyte, the article to be plated is immersed in or in contact with the bath, while the article to be plated is connected to a power source as a cathode, and a metal electrode is connected to the bath. Connect to the same power supply as an anode.

During operation, ions of the metal to be deposited are reduced to zero-valent metals in the bath and deposited on the surface of the treated article. There are various publications regarding methods for electrodepositing metallic palladium on substrate surfaces, particularly on metal surfaces.
In the case of these conventional methods, palladium electrodeposition baths tend to be unstable, and the baths cannot be used continuously for long periods of time unless one is prepared to suffer a significant decrease in bath efficiency. The bath efficiency here is the ratio of the actual bath plating rate to the theoretical bath plating rate mathematically calculated from Faraday's law, at the current density.

Palladium ions in the bath are oxidized to higher valence states during operation, making it more difficult for the palladium ions to be reduced to metallic palladium, and for this reason excessive current is supplied to the bath. It is believed that the plating layer will not precipitate unless it is done.

Although conventional palladium electrodeposition baths exhibit good bath efficiency initially and initially in operation, this bath efficiency often decreases rapidly within a few hours, and in some cases returns to its original value after only about 24 hours. Reduce to 50% or less. In order to maintain the palladium plating rate at or near the original level, it is necessary to supply additional current to the plating bath.
The cost will be high.

An object of the present invention is to provide a plating bath composition that allows metal palladium to be deposited with substantially constant bath efficiency over a long period of time. Another object is to provide a palladium electrodeposition replenishment solution composition that is effective in maintaining substantially constant bath efficiency. A further object of this invention is to provide an improved method for electrodepositing metallic palladium from an aqueous plating bath. These and other objects will become more apparent from the following description of embodiments of the invention.

The composition according to the invention comprises a water-soluble palladium compound which dissociates in water to produce palladium ions, an electrolyte and an amount sufficient to produce a stoichiometric excess of free nitrite ions relative to the palladium ions. Consisting of a nitrite ion source. These components are mixed in advance to form a form that can be sold externally, and this mixture is then added to water to bring the pH to 7.
Prepare the plating bath by adjusting the above or by dissolving each component separately in water. A feature of the above composition is that when metal palladium is electrodeposited on a substrate over a long period of time, there is no need to add an extra current to prevent a decrease in the plating speed. Moreover, the present invention provides a palladium bath replenishment composition that is effective in maintaining constant bath efficiency over long periods of time. Such a composition is electrolyte-free and consists of a mixture of a water-soluble palladium compound and a water-soluble nitrite compound, the latter in a constant stoichiometric excess over the palladium ions in the mixture when dissolved in water. It is characterized in that it is contained in an amount sufficient to generate a large amount of free nitrite ions.

This invention relates to a method for electrodepositing metallic palladium using a plating bath consisting of an aqueous solution of a palladium compound, preferably a divalent palladium compound, in which nitrite ions are present in the bath in a certain excess amount relative to the palladium ion source. The present invention provides an improved method for stabilizing bath efficiency over a long period of time.

Because bath efficiency begins to decline almost immediately after the plating operation begins, it is important to have an excess of nitrite ions present from the beginning of operation and maintain such bath composition throughout the plating cycle.

Although water-soluble palladium compounds such as dinitrodiamine palladium, which are suitable for supplying palladium ions into the bath, contain nitrite radicals, this nitrite radical exists as a complex ion in the bath. It is useless as a source of “free” nitrite ions. It is therefore necessary to initially include other dissociable nitrite compounds in the bath which produce free nitrite ions that do not take the form of complex ions. Suitable sources of free nitrite ions include water-soluble inorganic nitrite compounds, particularly sodium nitrite, potassium nitrite, other alkali metal nitrites, or ammonium nitrite.

The excess amount of nitrite ions to contribute to the stabilization of the bath can be sufficiently effective even in a very small amount, and is usually at least 0.05% by weight relative to the palladium ions in the bath. In many cases, the initial excess of free nitrite ions relative to palladium ions is approximately
Prepare the bath from 0.1 to about 50% by weight.

As the plating operation continues and palladium is consumed by precipitation, some of the free nitrite ions in the bath are simultaneously consumed by oxidation to nitrate ions.

Even if nitrite ions are initially present in considerable excess, free nitrite roots will eventually be consumed as the plating process progresses. Therefore, it is necessary to occasionally add new free nitrite sources to maintain an excess of free nitrite roots throughout the plating process. For this purpose, it is convenient to add the nitrite compound to the bath at the same time as periodically replenishing the palladium compound to replenish the consumed palladium. In any case, as a general rule at least about 10% by weight of nitrite ions relative to the palladium ions provided by the palladium compound.
Sufficient amount of nitrite must be added to produce excess.

For palladium, a water-soluble organic or inorganic secondary palladium compound is used both in the initial charge and in the replenishment, and these compounds can be selected from compounds commonly used in ordinary palladium electrodeposition baths. For example, dinitrodiammine palladium [Pd
(NH ₃ ) ₂ (NO ₂ ) ₂ ], palladium chloride (PdCl ₂ ), palladium sulfate, palladium chloride, palladium diammine hydroxide, palladium tetraammine chloride and palladium dichlorodiammine chloride. Among these, dinitrodiammine palladium and palladium chloride are particularly preferred.

As the electrolyte, a water-soluble compound is used that generates a conductive ionic medium when dissolved in water.
These can also be freely selected from commercially available products. Usually this is a water-soluble nitric acid compound, preferably ammonium nitrate or an alkali metal nitrate such as potassium or sodium nitrate.

Although the amounts of each component in the bath composition according to the invention can vary widely, an example composition of the mixture before addition to the water is as follows. Component amount , parts by weight Water-soluble divalent palladium compound, preferably dinitrodiammine palladium or palladium chloride
30-40 Water-soluble electrolyte, preferably alkali metal nitrate or ammonium nitrate 65-75 Water-soluble free nitrite ion source, preferably alkali metal nitrite 5-15 Composition of palladium electrodeposition baths prepared using each of these components And the plating operation conditions are as follows. Component amount , g/water-soluble divalent palladium compound, preferably dinitrodiammine palladium or palladium chloride
40-60g/water-soluble electrolyte, preferably alkali metal nitrate or ammonium nitrate 85-95g/water-soluble free nitrite ion source, preferably alkali metal nitrite 5-15g/water (prepared to 1) Temperature 50-70°C PH 8-9 Current Density 0.1075-53.75 A/Dm ² The pH of the bath is adjusted by adding an acid or base, such as nitric acid or ammonium hydroxide, in a conventional manner before and/or during the operation. Other components, usually added for other purposes, may also be present. Examples include brighteners, wetting agents or surfactants, palladium ion complexing agents, antioxidants, etc., all of which are additives known to those skilled in the art.

In the electrodeposition method of this invention, the bath temperature is room temperature;
That is, it can be operated at a wide range of temperatures from 25°C to below the boiling point of the bath, that is, 100°C. Plating time varies depending on current density, bath temperature and palladium film thickness. Bath temperature 50~70℃, current density 0.1075~53.75A/D
In the case of m ² , about 10 minutes or less is usually sufficient to obtain a palladium film of about 0.0025 cm.

According to this invention, a uniform and glossy adhesion layer is formed from palladium electrodeposited on a metal base. The metal materials to be plated include copper, nickel, silver, steel, etc., and alloys such as brass, bronze, and stainless steel are also mentioned. Bath efficiencies with the practice of this invention are greater than 90%, and bath efficiencies of 80-95% are typically obtained even after long periods of operation. Moreover, the high bath efficiency mentioned above is 32.25~37.62A/
This is achieved even in operation in the current density range such as Dm ² . For example, 48.37~
Even at high current densities like 53.75A/ ^Dm2
Bath efficiencies of 60-70% are obtained.

Example 1 A small smooth copper plate was pretreated, cleaned of dirt and grease, weighed and then immersed in a palladium bath having the following composition.

Dinitrodiammine palladium Pd( _NH3 ) ₂
(NO ₂ ) ₂ 50g / ammonium nitrate 90g / sodium nitrite 10g / water (prepared to 1) A small piece of tantalum/titanium coated with platinum is immersed in the bath and connected to a DC constant current power source as an anode, and a copper sample is connected as a cathode. I connected it and started playing. The nitrite ions to palladium ions in the bath were initially in a 1.5 to 1 weight ratio. bath
The pH was between 8 and 9 at the beginning. Bath temperature 70
It was kept at ℃. The current was adjusted to deposit palladium at a current density of 16.12 A/Dm ² . At this current density, metallic palladium was deposited on the copper sample at a plating deposition rate of 30 milligrams/A-minute. Fresh dinitrodiammine palladium compound was added from time to time to maintain the initial palladium concentration in the bath. Sodium nitrite corresponding to a 10% excess of nitrite ions relative to the palladium ions of the dinitrodiammine compound was added at each replenishment stage of the palladium compound. After about 6 minutes
A 0.0025 cm thick palladium film was deposited. The bath efficiency at this time was 95%, calculated from the current density, time, and thickness of the electrodeposited palladium film. Thereafter, the plating operation was resumed and consumable components in the bath were regularly replenished.
The plating bath was thus used continuously for several weeks, during which time the bath efficiency could be maintained constant at around 95%.

Comparative Example 1 For comparison, the procedure of Example 1 was repeated except that no new nitrite compound was added after the start of operation. Within a few hours of operation, bath efficiency decreased and palladium electrodeposition decreased significantly.

Comparative Example 2 A plating bath having the following composition and conditions was prepared according to Example 1 described in US Pat. No. 4,092,225.

10 g/palladium ion 150 g/potassium pyrophosphate Bath temperature 54.4°C Adjusted to pH=9 with pyrophosphate or potassium hydroxide This bath was used to test copper coupons at various current densities while stirring. The result of cross-plating is as follows.

Table A Current density Bath efficiency (A/Dm ² ) (%) 1.08 94 2.15 94 3.21 94 4.30 94 5.37 77 Example 2 Example 1 at various current densities for comparison
repeated. The results are shown in the table below.

Table B Current density Bath efficiency (A/Dm ² ) (%) 2.68 97 5.37 97 10.75 94 21.50 86 32.25 80 37.67 77 43.00 73 48.37 67 53.75 60 As is clear from the table, the composition and method of the present invention can be used for a long time. It can be seen that not only does the present invention provide high bath efficiencies throughout the process, but the current densities at which such high bath efficiencies are obtained are at least seven times higher than those of previously known baths and methods.

It will be apparent that various changes and modifications can be made to this invention without departing from its spirit and scope. The specific examples disclosed herein are for the purpose of clarifying the invention, and are not intended to limit the invention itself.

Claims (1)

[Scope of Claims] 1. An improved electrodeposition method for maintaining a substantially constant bath efficiency in a method for electrodepositing metallic palladium from a plating solution comprising an aqueous solution containing a water-soluble palladium compound as a source of palladium ions. An electrodeposition method in which free nitrite ions maintained in a constant excess amount relative to palladium ions coexist in the bath, and the electrodeposition is carried out at a cathode current density of 5 A/Dm ² or more. 2. The electrodeposition method according to claim 1, wherein the constant excess amount is maintained by periodic addition of a water-soluble nitrite compound as a source of free nitrite ions. 3. The electrodeposition method according to claim 2, wherein the water-soluble nitrite compound is an alkali metal nitrite. 4 Excess amount of free nitrite ions in the bath is 0.1 to 50
3. A method according to claim 2, characterized in that a sufficient amount of water-soluble nitrite compound is added to maintain the range of % by weight.