1. Field of the Invention.
The present invention pertains to modification of qualities of a dielectric surface, in particular to render a dielectric surface to be an electric conductor. The invention may be used e.g. in various areas of industry for the preparation of dielectric surfaces of electroplating, especially for nickel-plating. Dielectric items which are metal-coated by processes of the invention may be used e.g. where a decorative or protective function is required, in manufacturing press-forms using the galvanoplastic method, or for assemblies for shielding electromagnetic emission.
2. Background.
One existing certain method of producing conductor surfaces is known (USSR patent No 980858, B 05 D 5/12, 1982), in which the dielectric surface is treated by amine-salt solution of copper, then immersed in water, then in sulphide solution and then rinsed with water.
Using this method, a quality conductor surface can be obtained by repeating the operation sequence described in the USSR patent at least three times. This increases the duration of the entire process, increases the consumption of water and chemicals and makes the use of automated production lines more difficult.
In addition, another certain existing process for producing conductor surfaces of cupric sulphide is known (USSR patent No 1762454, H 05 K 3/42, 3/18, 1991), in which dielectric items are immersed into solution of univalent cupric salt, then into a solution of 0.0025/0.025 of potassium persulphate, iodine or potassium nitrite solution, then rinsed with water and immersed into sulphide solution of an alkaline metal. This process is carried out at room temperature and each operation is repeated twice.
The shortcomings of the method described in USSR patent No 1762454 are the same as the shortcomings described in the preceding patent. In addition, through application of both existing methods, it is impossible to leave an isolated part of the item uncovered, i.e., it is impossible to achieve selective conductor coating.
See also the patent application of the Republic of Lithuania No 98-161, published in the official bulletin of Lithuanian State Patent Bureau (VPB) No 5 in 2000. That reported method also has notable shortcomiongs, including problematic use of a cobalt solution.
It would be desirable to have new methods to render a dielectric surface electrically conductive.
The present invention enables selective production of quality conductor metal sulphide coatings on a dielectric surface without the use of ammonia hydroxide or amine, and to make the process shorter.
The invention also enables producing conductor coatings with the use of stable ionic solutions.
More particularly, methods of the invention include treating a dielectric surface with a composition comprising bismuth and with sulfur (sulfide) treatment. Preferably, the dielectric substrate is treated with a solution that contains bismuth ions, particularly trivalent bismuth ions. The treated substrate is then preferably treated with a sulfide solution. A water rinse can be suitably employed between the treatment with bismuth composition and the sulfur treatment.
Prior to the treatment with a bismuth composition, a dielectric substrate is suitably etched. A variety of etchants may be employed. Generally preferred is an acidic aqueous solution that comprises e.g. KIO4, or K2S2O8 and CrO3.
A variety of sulfur compositions also may be employed. An aqueous solution containing a sulfur salt is generally preferred, e.g. a sodium or potassium sulphide water solution.
Methods of the invention enable selective production of quality metal sulphide conductor coating on dielectric surface without using ammonia hydroxide, amines or other compounds that form strong complex compounds with heavy metals. In addition, methods of the invention produce conductor coatings uses metal ion solution which are highly stable. That is, in distinction from prior systems, bismuth treatment compositions of the invention are highly stable for extended periods. See, for instance, the results set forth in Table 1 below.
The invention also includes articles having a metal plate thereon produced in accordance with the disclosed methods.
Methods and articles of the invention are useful for a wide variety of applications, including for forming electrical circuits and conductors such as present on a printed circuit board or other electronic packaging substrate, a metal finish, and other applications such as producing electromagnetic shielding. The methods of the invention are particularly useful to deposit a decorative or protective nickel plate, or other decorative or protective metal layer.
Other aspects of the invention are disclosed infra.
The invention provides new methods for plating dielectric substrates, particularly polymer substrates such as e.g. ABS (acrylonitrile butadiene styrene) copolymer substrates, epoxy resin substrates, polyetherimide substrates, and the like.
The methods of the invention generally include use of a bismuth treatment step. Subsequent treatment with a sulfide material or composition enables quality metallization of the substrate, e.g. with an electrolytic nickel, copper, gold, silver, platinum or other metal plating composition solution. In contrast to other prior systems, plating catalysts such as Pd, or Pd/Sn, platinum or other metal need not be employed to deposit the metal layer. Also, unstable treatment solutions such cobalt need not be employed.
To enhance the metal deposits on the dielectric substrate, preferably the dielectric substrate is first treated with an etchant solution that can provide a chemically and physically modified surface that is optimized for subsequent conditioning and plating. Such materials typically contain a strong oxidant and include the acidic compositions discussed above as well as alkali permanganate compositions.
Treatment compositions employed in accordance with the invention may be applied to a dielectric substrate by a variety of methods, including by spray application as well as immersion. A treatment composition is typically applied as a solution to a substrate.
A variety of bismuth compositions may be employed in accordance with the invention. A trivalent bismuth species is particularly preferred. Both inorganic and organic bismuth materials are suitably employed such as Bi(NO3)3, BiCl2 or Bi(CH3COO)3. Those bismuth compositions are preferably present in an acidic aqueous solution, such as a HCl, HNO3 or acetic acid solution. The solution may contain relatively small amount of the bismuth material, e.g. the solution may suitably be from 0.0001 to about 1 molar in bismuth ions, preferably a bismuth ion concentration of about 0.005 to about 0.5 molar, even more preferably a bismuth ion concentration of about 0.005 to about 0.3 molar in an aqueous treatment solution.
The bismuth composition may be applied to a dielectric substrate at room temperature to achieve good results, although the bismuth solution also may be at an elevated temperature.
After treatment with the bismuth composition, the substrate is then treated with a sulfur composition, preferably an aqueous solution that contains a sulfide species, e.g. a sulfur salt such as Na2S or K2S, or an organic sulfide, such as an alkyl sulfide. N2S or K2S is generally preferred.
The sulfide composition also may be applied to a dielectric substrate at room temperature to achieve good results, although a sulfide composition solution also may be at an elevated temperature.
Times of treatment of a dielectric substrate with the above compositions can vary rather widely. In general, treatment times of from 0.25 to 10 minutes are suitable, more typically from 0.5 to about 1, 2, 3, 4 or 5 or more minutes.
Preferably, a treated dielectric substrate is rinsed with water between treatment steps, i.e. after etching, after bismuth treatment and then after sulfur treatment. The substrate may be suitably dried before plating.
A variety of metals may be plated onto a dielectric substrate. Plating compositions are commercially available. For instance, suitable electrolytic copper, nickel and gold plating compositions are available from the Shipley Company (Marlborough, Mass.). A preferred nickel electroplating plating composition and method for use thereof is set forth in the examples below. See also, Coombs, Printed Circuits Handbook, (3rd Edition, McGraw Hill), incorporated herein by reference, for additional suitable plating compositions and uses thereof.
Preferred electrolytic copper plating compositions for use in accordance with the invention include an aqueous composition that contains an aqueous solution of CuSO45H2O at a concentration of 60 g/l; H2SO4 at a concentration of 225 g/l; and Cl ions at a concentration of 50 ppm. The treated substrate to be plated is suitably immersed in an air-agitated plating tank outfitted with multiple cathode rails and one rectifier and charged with such a copper plating solution. During plating, the following deposition conditions are suitably employed: current density of 14.5 mA/cm2; DC waveform was DC; and plating bath temperature of 25xc2x0 C.
References herein to solutions of materials are inclusive of fluid materials where all solid components are dissolved therein, as well as fluid compositions where one or more added components are dispersed or otherwise not fully dissolved in the fluid. Preferably, most or all added components are fully dissolved in the fluid carrier (typically water).
The following non-limiting examples are illustrative of the invention. All documents mentioned herein are incorporated herein by reference.
General Comments to Examples
In the following examples, products made of dielectric-plates made of ABS (a copolymer of vinyl cyanide, divinyl and styrene) plastic substance are etched for 5 minutes at room temperature in solution which contains 13M H3PO4 and 0.5M K2S2O8 or etched for 5 minutes in 60xc2x0 C. temperature solution which contains 3.8M H2SO4 and 3.8M CrO3 and rinsed with water.
In the following examples, products of dielectric-shock-resistant polystyrene (SAPS) are etched for 5 minutes in solution containing 17M H2SO4 and 0.5M KIO4 under room temperature and rinsed with water.
After etching, the products are treated for 2 minutes in solution which contains 0.005/0.300 M Bi(NO3)3 or BiCl3 or Bi(CH3COO)3 and 0.01/0.35 M HNP3 or HCl or CH3COOH, under room temperature. After this, products are rinsed with water and for further 30 seconds treated in solution containing 0.01/0.25 M Na2S or K2S at room temperature.
When the process is completed, the items are rinsed with distilled water, dried and nickel plated for 15 minutes in Watts electrolyte which contains 1/1.2M NiSO4; 0.15/0.2M NiCl2 and 0.4/0.5M H3BO3, initial flow density 0.3A/dm3, which, along the progress of nickel coating from the point of contact, increases to 3A/dm3, under electrolyte temperature of 40xc2x0 C.
Stability of the metal ion solution is assessed, examining the presence of sediment in solution (which means that the solution is unstable) or their absence (which means that the solution is stable).
The smoothness of the conductive sulphide coating is assessed visually immediately after the treatment, in daylight, using two parameters: smooth, not smooth.
Electrical conductivity of the coating is assessed by the chemical nickel-plating expansion speed from the point of contact, in centimeters per minute.
The possibility of selectively producing conductor coating on a dielectric item is assessed by examining whether isolated part of the item is covered in nickel or not.
In the following Examples 1 through 7, Examples 1, 3 and 6 are for control purposes, while Examples 2, 4, 5 and 7 have been prepared in accordance with the proposed method, under different concentrations of bismuth component ions, and using different technological means.
In Examples 3, 4, 5, 6 and 7, subjected to treatment are ABS plastic plates with surface area of 50 cm2, while in Example 2 there are profiled items made of shock-resistant polystyrene (SAPS) with surface area of 70 cm2.