GB1573751A - Silver preplating solution and process for plating - Google Patents

Description

(54) SILVER PREPLATING SOLUTION AND PROCESS FOR PLATING (71) We, HITACHI LIMITED, a Corporation organised and existing under the laws of Japan, of 5-1, 1-chome, Marunouchi, Chiyoda-ku Tokyo, Japan, do hereby declare the invention, for which we pray that a patent may be granted to us and the method by which it is to be performed to be particularly described in and by the following statement:- This invention relates to silver preplating solutions containing a thiocyanic com- pound as a main component, and to silver plating with such solution.

In a silver-plating process when the substrate metal is a pure noble metal, a good adhesion is obtained between the substrate metal and the resulting plating film, but in other cases the adhesiveness is sometimes poor. When a thick silver plating film is formed, the resulting plating film is sometimes peeled off during the plating, if the adhesiveness is poor. As a means for enhancing the adhesiveness, a preplating is applied in the present process. When a more readily ionizable metal than silver is placed in a silver plating solution, the metal is dissolved into the silver plating solution, while silver is deposited instead. That is, the so-called substitution plating takes place. The silver plating film formed by the substitution plating has a low mechanical strength. To prevent the substitution plating, the preplating is carried out in the present invention.

In the present invention, the preplating is carried out in a solution having a low concentration of silver at an extremely low cathode current density to deposit a thin silver film. The ordinary strike plating is carried out at a high current density, whereas the present process is carried out characteristically at a low current density.

The plating solution for the preplating has a silver concentration of 0.001 to 0.02 moles/l and contains a large amount of silver complexing agent. The composition of plating solution for the preplating is 0.001 to 0.02 moles/l of silver and 0.1 to 5 moles/l of thiocyanic ion, and at least one film-improving agent selected from bromide, iodide and selenocyanic ions may be contained or not in the plating solution for the preplating. The preplating can be carried out at a cathode current density of 0.1 to 80 mA/dm2, preferably 5 to 50 mA/dm2 at room temperature for a period of 10 seconds to 10 minutes. The plating film to be formed by the preplating must have a thickness large enough to substantially prevent any occurrence of the substitution plating, that is, usually a thickness of at least several hundred .

According to the invention there is provided a process for silver electro-plating, which comprises plating a metallic-material in a preplating solution comprising 0.001 to 0.02 moles/1 of silver, and 0.1 to 5 moles/l of thiocyanic ion, at a cathode current density of 0.1 to 80 mA/dm2, and then plating the preplating substrate material in a silver plating solution.

For use in the invention there is provided a preplating solution being an aqueous solution containing 0.001 to 0.02 moles/l of silver ions and 0.1 to 5 moles/l of thiocyanic ions.

In regard to the subsequent silver plating almost all of the silver plating solutions so far in practice are solutions containing cyan ions, that is, the solutions containing the so-called cyanides as the main component. Plating films having very good elongation and luster can be formed from such plating solutions. However, as is well known, the cyan ions have a strong toxicity, and thus such plating solutions have many problems in maintenance of safe working atmosphere, treatment of waste effluent solution, etc. Thus, development of a silver plating solution containing no cyan ions is also desirable.

Typical silver plating solutions containing no cyan ions include a silver nitrate bath (F. C. Mathers, J. R. Kuebler: Trans. Amer. Electro, Soc., 29, 417 (1916)), a silver sulfamate bath (R. Pionteppi: Rorr. u. Metalschutj. 19, 110 (1943)), a silver chloride bath (A. K. Graham, S. Heiman, H. L. Pinkerton: Plating, 36, 47 (1949)), and a bath containing thiocyanate ions (L. Domnikov: Metal Finishing, 64, (4), 57 (1966)).

These baths have been carefully tested and compared, and it has been found that all the plating films obtained from these baths have considerably poorer properties than those of the plating films obtained from the bath containing cyan ions.

That is, the former plating films have such disadvantages as poor luster, fragile plating films, or dendritic or rugged surfaces, lacking in smoothness, etc., and thus have not been utilized on an industrial scale. Furthermore, when a substrate metal is other than silver there is such a common disadvantage that an adhesiveness between the substrate metal and the plating film is poor.

There is therefore in addition to the main invention a need for a silver plating solution having a lower toxicity and forming a plating film having a high toughness, an excellent appearance and good throwing power.

The present invention additionally provides for use in the silver plating step a method employing plating solutions being prepared by adding a small amount of a film-improving agent to a silver electro-plating solution containing a thiocyanate as a main component and having a lower toxicity and no fear of environmental pollution, and forming a silver plating film having a good toughness and an excellent appearance, and also provides a process for electro-plating with the same silver plating solution.

The present inventors have found that, when at least one of bromide, iodide, selenocyanic ions, such as a mixture of bromide or iodide with selenocyanic ion is added as a film-improving agent to a plating solution of silver thiocyanate-thiocyanic compound system, a silver plating film having a toughness and appearance as good as those of the silver plating film obtained from the cyan bath can be obtained. When said film-improving agent is added to the plating solution containing a thiocyanate as a main component, stable complex ions of silver or sparingly soluble silver compounds are formed in the plating solution. Thus, silver deposition overvoltage is increased, as the complex ions or the sparingly soluble silver compounds are absorbed selectively at the active locations on a cathode, and consequently silver deposition on its locations are suppressed.That is, it seems that flatness of the plating film is ensured thereby, and consequently luster and mechanical properties of the plating film are improved.

In carrying out this aspect of the present invention practical composition of the silver plating solution can be in the following range: Silver 0.04-0.8 moles/l- Thiocyanic ions calculated as SCN- 0.5-10 moles/l The silver in the plating solution is dissolved from anode silver during the plating, and exists as complexes of silver and thiocyanic ion, but its composition is not always constant.

The thiocyanic ion exists as a free ion and complexes in the solution.

As the film-improving agent, Suitable ranges Bromide 0.001-0.1 mole/l Iodide 1201200 amole/l Selenocyanic ion 500-10,000 amole/l When the film-improving agent is added to the plating solution in a mixture form such as a mixture of bromide and selenocyanic ion or a mixture of iodide and selenocyanic ion, they each should be added to the plating solution within said ranges.

The ranges for the film-improving agents as described above have been determined by experiments, and if the amount of the film-improving agents is less than the lower limits of the ranges, the resulting silver plating films generally have a poor luster and unsatisfactory mechanical properties. When the amount exceeds the upper limits of the ranges, the grains in the plating films can become coarse, and the plating films can become brittle. It seems that the film-improving agents each exist in the form of a silver compound in the solution.

The improvement of the toughness of the silver plating films is most remarkable when iodide is added to the solution, and is decreased in the order of selenocyanic ion to bromide, but the differences in the improvement among these agents are not so large. However, the improvement of the throwing power is far greater when selenocyanic ion is added to the solution than when the other two agents are each added thereto. Therefore, an addition of a mixture of bromide or iodide with selenocyanic acid to the solution can greatly improve both the properties of the resulting silver plating films and the throwing power.

Besides said film-improving agents, ammonia has a capacity to form complex compounds of silver, and thus it is preferable to use ammonia to control pH of the present plating solution or add ammonium thiocyanate as an electrolyte in place of alkali thiocyanate. Especially, a silver plating film having a good luster can be obtained from the present plating solution having a pH elevated by the addition of ammonia.

When effective additives for silver cyanide plating solutions, such as urea, alcohol, gelatin or amines, for example, ethylenediamine tetraacetic acid, are added to the present plating solution, similar effects can be obtained. In the addition of these additives, it is necessary to determine relations between the amounts of the additives and other effects by carrying out tests in advance, for the effects of the addition sometimes depend upon the kind of plating solution and plating conditions employed.

The pH range of the plating solution to be used in the present invention is less than 11, generally in the range 0.5 to 10.5, which is very much broader than the pH range of more than 10 in the conventional silver cyanide bath. When the plating solution is in an acidic zone of less than pH 0.5, the luster of the resulting plating film is deteriorated, losing the effect of the addition of the film-improving agent.

On the other hand, when pH is higher than 10.5, black silver compounds are suspended in the plating solution, and tone of the resulting plating film becomes blackish.

However, if the plating is carried out only for a short time even at pH of about 11, the plating solution deteriorates and no more silver plating film is formed. The optimum pH range for the present plating solution is 3 to 9. When ammonia is contained in the present plating solution, a preferable condition is obtained for forming the plating film at pH of 8 to 11, as described earlier.

The broadness of the allowable pH range for the plating solution offers not only such an advantage that the bath can be easily controlled, but also such another advantage that impurities contaminating the plating solution can be precipitated by adjusting pH to the desired value and removed by filtration, and the resulting filtrate can be used immediately as the plating solution. Furthermore, if there is a solder layer or an insulating material attached to a material to be plated, and if the solder layer or the insulating material is readily attacked by the plating solution, such a pH range as to sparingly attack the solder layer or the insulating material can be selected to carry out the plating. This is another advantage of the present plating solution.

As materials to be plated, nickel, copper, silver, gold, metals of platinum group, and their alloys can be used. Surfaces of these substrate metals are thoroughly cleaned in the manner as usually practised by those skilled in the art, and then after the preplating the plating is carried out by connecting the cleaned substrate metals to a cathode.

Plating is desirably carried out by stirring the plating solution or keeping the plating solution in a flowing state at a cathode current density of 0.5 to 10 A/dm2 and a bath temperature of room temperature to 50 C, using silver as an anode.

Electric source may be a DC current or AC current-superposed DC current. Of course, either a constant voltage source or a constant current source can be used.

Besides the plating in the ordinary plating tank, the so-called local plating method can be carried out by making the plating solution flow along parts of the material to be plated or providing the plating solution soaked in a sponge or the like on the parts of the material to be plated, and plating films can be thereby formed, because the present plating solution is hardly toxic.

Now, the present invention will be described in detail below, referring to an Example and the accompanying drawing.

The drawing 1 is a schematical view showing a bending test to evaluate an adhesiveness of silver plating film.

Procedures for determining the property of adhesiveness of plating films shown in the Example are described below.

Adhesiveness: A silver plating film having a thickness of 12 am is formed on a copper plate having a thickness of 6 mm, a width of 12 mm, and a length of 100 mm, and the resulting plate is used as a test piece. The side of the plated plate to be tested is placed on two fulcra having a distance of 60 mm, and a cylindrical pressing plate having a radius of curvature of 3.5 R at its tip is pressed against the back side of the plated plate at a center between the fulcra. State of crack development and state of peeling of the plating film on the plated side are observed. Pressing speed is 5 mm/min. Adhesiveness is classified into 5 grades, whose standards are given in Table 1. The outline of testing apparatus is shown in the drawing, wherein numeral 1 is a test piece, 2 fulcra, and 3 a pressing plate.

Thickness of plating film: measured according to procedure for electrolytic measurement of thickness (JIS H 8618) and microscopic measurement of crosssection.

Appearance: White light is irradiated onto the plating film, and the film is visually observed.

TABLE t

Grade of adhesiveness State of plating film 1 No cracks are developed 2 Slight cracks are developed at edge parts 3 Small cracks are developed at both edge parts and center part 4 Large cracks are developed at both edge parts and center part 5 Plating film is peeled off EXAMPLE.

Effect of preplating upon adhesiveness is shown in this Example.

Surfaces of copper plates having a thickness of 6 mm, width of 12 mm and length of 100 mm were polished to smooth by Emery abrasive paper and buffing, then defatted in acetone, and washed with water, and then the copper plates were subjected to preplating. Then, silver plating films were formed to a thickness of 12 am, using plating solutions containing 0.2 moles/l of silver thiocyanate, 3 moles/l of potassium thiocyanate, and film-improving agent. The resulting plating films were deformed on a bending testing machine shown in Figure 1, and the adhesiveness of the films was evaluated in view of the states of crack development on the plating films. The bending angles of less than 60C were obtained by forcedly bending the plate outside the testing machine. Conditions for preplating, film-improving agents used in the silver plating, and results of the bending tests are given in Table 1.

Preplating solution (mole/l) Current Silver plating Bending test (a) density Time film-improving agent No. AgSCN KSCN Br-, I-, or SeCN- (mA/dm) (min.) (mole/l) 135 90 45 20 1 Comparative 6 x 10-4 0.8 0 10 5 Br- 1 x 10-2 1 3 4 4 2 1 x 10-3 0.8 0 10 5 " 1 1 1 1 3 1 x 10-3 0.8 0 10 3 " 1 1 1 2 4 1 x 10-2 0.8 0 10 5 " 1 1 1 2 5 Comparative 5 x 10-2 0.8 0 10 5 " 1 1 2 2 6 Comparative - - - - - " 2 3 5 5 7 1 x 10-3 0.1 Br- 1 x 10-2 10 5 " 1 1 2 2 8 1 x 10-3 1 " 10 3 " 1 1 1 2 9 1 x 10-3 4 " 10 3 " 1 2 2 3 10 1 x 10-3 1.5 I- 2 x 10-4 0.1 5 I- 2 x 10-4 1 1 2 3 11 1 x 10-3 1.5 " 5 5 " 1 1 1 2 12 1 x 10-3 1.5 " 30 5 " 1 1 1 2 13 1 x 10-3 1.5 " 50 5 " 1 1 1 2 14 Comparative 1 x 10-3 1.5 " 100 5 " 1 2 3 3 15 1 x 10-3 1.5 0 20 5 SeCN- 5 x 10-4 1 1 2 2 15 1 x 10-3 1.5 SeCN- 5 x 10-4 20 5 " 1 1 1 2

As is apparent from the results shown in Table 1, the preplating solutions contain 0.001 to 0.02 moles/l of silver thiocyanate and 0.1 to 4 moles/l of thiocyanic ion as essential components, and the plates were treated in the preplating solutions at a cathode current density of 0.1 to 80 mA/dm2, preferably 5 to 50 mA/dm2.

Test piece No. Comparative 6 was the one directly subjected to the silver plating without any preplating, and small cracks developed at edges and center part at the bending angle of 90".

When a silver plating film obtained from the conventional silver cyanide solution was subjected to the same bending test as above, the grades judged were 1 to 2 when bent to 200. Thus, the adhesiveness was almost equal to that of the plating film obtained according to the present invention.

WHAT WE CLAIM IS:- 1. A process for silver electro-plating, which comprises plating a metallicmaterial in a preplating solution comprising 0.001 to 0.02 moles/l of silver, and 0.1 to 5 moles/l of thiocyanic ion, at a cathode current density of 0.1 to 80 mA/dm2, and then plating the preplated substrate material in a silver plating solution.

2. A process according to Claim 1, wherein the silver plating solution contains silver ions and thiocyanic ion.

3. A process according to Claim 2, wherein said plating is effected by supplying an electric current to said preplated material connected as cathode, in a solution containing a thiocyanic ion, silver, and at least one of bromide, iodide and selenocyanic ion in amount sufficient to act as a film-improving agent.

4. A process according to Claim 3, wherein said plating step is effected using a solution comprising 0.04 to 0.8 moles/l of silver ions, 0.5 to 10 moles/l thiocyanic ion and at least one of 0.001 to 0.1 mole/l of bromide ion, 120 to 1,200 amoles/l of iodide ion, 500 to 10,000 amoles/l of selenocyanic ion as a film-improving agent at a cathode current density of 0.5 to 10 A/dm2.

5. A process according to Claim 4, in which the plating is effected by supplying an electric current to said preplated material connected as cathode in a solution containing silver, thiocyanic ion, and 120 to 1,200 .omoles/l of iodide and 500 to 10,000 ,amoles/l of selenocyanic ion.

6. A process according to any one of Claims 3 to 5, wherein the solution for plating has a pH adjusted to 8 to 11 with ammonia, at a cathode current density of 0.5 to 10 A/dm2.

7. A preplating solution suitable for use in a process as claimed in any one of Claims 1 to 6, being an aqueous solution containing 0.001 to 0.02 moles/l of silver ions and 0.1 to 5 moles/l of thiocyanic ions.

8. A method of preplating prior to silver plating which comprises treating a metallic-material with a solution according to claim 7, at a current density of 0.1 to 80 mA/dm2.

9. A preplating solution as claimed in Claim 7, substantially as hereinbefore described.

10. A process according to Claim 1, for silver electro-plating substantially as hereinbefore described.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (10)

**WARNING** start of CLMS field may overlap end of DESC **. As is apparent from the results shown in Table 1, the preplating solutions contain 0.001 to 0.02 moles/l of silver thiocyanate and 0.1 to 4 moles/l of thiocyanic ion as essential components, and the plates were treated in the preplating solutions at a cathode current density of 0.1 to 80 mA/dm2, preferably 5 to 50 mA/dm2. Test piece No. Comparative 6 was the one directly subjected to the silver plating without any preplating, and small cracks developed at edges and center part at the bending angle of 90". When a silver plating film obtained from the conventional silver cyanide solution was subjected to the same bending test as above, the grades judged were 1 to 2 when bent to 200. Thus, the adhesiveness was almost equal to that of the plating film obtained according to the present invention. WHAT WE CLAIM IS:-

1. A process for silver electro-plating, which comprises plating a metallicmaterial in a preplating solution comprising 0.001 to 0.02 moles/l of silver, and 0.1 to 5 moles/l of thiocyanic ion, at a cathode current density of 0.1 to 80 mA/dm2, and then plating the preplated substrate material in a silver plating solution.

2. A process according to Claim 1, wherein the silver plating solution contains silver ions and thiocyanic ion.

3. A process according to Claim 2, wherein said plating is effected by supplying an electric current to said preplated material connected as cathode, in a solution containing a thiocyanic ion, silver, and at least one of bromide, iodide and selenocyanic ion in amount sufficient to act as a film-improving agent.

4. A process according to Claim 3, wherein said plating step is effected using a solution comprising 0.04 to 0.8 moles/l of silver ions, 0.5 to 10 moles/l thiocyanic ion and at least one of 0.001 to 0.1 mole/l of bromide ion, 120 to 1,200 amoles/l of iodide ion, 500 to 10,000 amoles/l of selenocyanic ion as a film-improving agent at a cathode current density of 0.5 to 10 A/dm2.

5. A process according to Claim 4, in which the plating is effected by supplying an electric current to said preplated material connected as cathode in a solution containing silver, thiocyanic ion, and 120 to 1,200 .omoles/l of iodide and 500 to 10,000 ,amoles/l of selenocyanic ion.

6. A process according to any one of Claims 3 to 5, wherein the solution for plating has a pH adjusted to 8 to 11 with ammonia, at a cathode current density of 0.5 to 10 A/dm2.

7. A preplating solution suitable for use in a process as claimed in any one of Claims 1 to 6, being an aqueous solution containing 0.001 to 0.02 moles/l of silver ions and 0.1 to 5 moles/l of thiocyanic ions.

8. A method of preplating prior to silver plating which comprises treating a metallic-material with a solution according to claim 7, at a current density of 0.1 to 80 mA/dm2.

9. A preplating solution as claimed in Claim 7, substantially as hereinbefore described.

10. A process according to Claim 1, for silver electro-plating substantially as hereinbefore described.