NICKEL-BASED ELECTRICAL CONTACT DEVICE
Technical Field
The invention is concerned with devices having an electrically conducting member having electrical contact surface of nickel-based materials. Background of the Invention
Typically, the manufacture of high-quality electrical contacts has involved the usage of gold whose properties of low contact resistance and high chemical stability are key advantages in such usage. However, as the price of gold remains high, efforts continue at finding alternative materials for contact manufacture. Prominent among such alternatives are precious metals other than gold; e.g., silver-palladium alloys have been found suitable for certain applications.
While such alternate alloys are less expensive than gold, still further cost reduction is desired, and nonprecious metal alloys such as, e.g., copper-nickel alloys have been investigated for contact resistance and stability over time. See S. M. Garte et al., "Contact Properties of Nickel-Containing Alloys", Electrical Contacts, 1972, Illinois Institute of Technology. Summary of the Invention
It has been discovered that a material consisting essentially of nickel and a controlled amount of hydrogen has contact properties comparable to those of gold such as, in particular, low and stable contact resistance. Preferred amounts of hydrogen in nickel are regarded to be such as to associate atoms of hydrogen with nickel atoms on dislocations, thus blocking oxidation at critical sites. Typically, surface contact resistance of the material is significantly less than 100 milliohms even after prolonged exposure to an oxidizing ambient.
Brief Description of. the Drawing
FIG. 1 is a perspective view of an electrical connector device in accordance with the invention; and
FIG. 2 is a schematic cross-sectional view of a portion of a device in accordance with the invention. Detailed Description
The electrical connector device shown in FIG. 1 comprises housing 11 and contact pins 12. Housing 11 ismade of an electrically insulating material, and contact pins 12 have contact surfaces in accordance with the invention.
Shown in FIG. 2 are, in cross section, an electrically conducting member 21 on which layer 22 is situated. Member 21 may consist of a copper conductor material, and surface layer 22 is a nickel material which comprises hydrogen at least in a surface region 23. The incorporation of controlled amounts of hydrogen into nickel material results in enhanced contact properties such as low contact resistance and long-term stability of such resistance.
Hydrogen may be incorporated in a nickel material in a variety of ways such as, e.g., in the course of electroplating, by sputtering in an argon-hydrogen atmosphere, and by indiffusion at a bulk surface which, preferably, has been subjected to plastic deformation by cold working. Preferred concentrations of hydrogen depend on conditions under which layers or bodies of nickel are produced and processed, and it is postulated that preferred concentrations increase in direct relationship with the cumber of nickel atoms on dislocations. In particular, greater amounts of hydrogen are beneficial for cold worked material, preferred amounts being directly related to level of cold working. In the case of electrodeposited layers, preferred amounts are in the range of from 0.0004 to 0.0009 atom concentration of hydrogen in nickel; when severe cold work is applied up to 0.01 atom concentration is preferred.
Fortuitously, as dislocation slip bands produced by cold working also facilitate indiffusion of hydrogen, contact properties of cold-worked bulk nickel material are most favorably affected by hydrogen indiffusion. Accordingly, applications are preferred in which nickel material is plastically deformed by a significant amount, such as, e.g., corresponding to at least 50 percent reduction of cross-sectional area prior to hydrogen diffusion, the latter being carried out at a temperature which is less than the recrystallization temperature of Ni. Hydrogen indiffusion is typically effected over a time of a few minutes, and indiffusion is facilitated by heating at a temperature below the recrystallization temperature of Ni. Among applications of cold-worked material are those involving the use of microscopic flakes dispersed or embedded in a non-conductive matrix material as, e.g., in electrically conducting inks, pastes, and adhesives.
Conveniently, hydrogen can be incorporated in nickel layers by electroplating out of a suitable nickel bath, solutions of nickel salts being considered most suitable where the anion is but weakly oxidizing.
While a contact material of the invention may be free or essentially free of elements other than nickel and hydrogen, impurities may be present and additional elements may be included such as, e.g., boron, silicon, germanium, phosphorus, arsenic, antimony, cr bismuth. When present in solid solution or, in other words, when incorporated in the nickel structure, impurities and additives are considered not to interfere with the beneficial effect of hydrogen in nickel. Amounts of at least 70 atom percent nickel-hydrogen are preferred in the contact material.
Contacts of the invention may receive a final coating of "flash" comprising a significant amount of a coating material such as gold, one or several platinum-group elements, or gold and one or several platinum-group
elements, the amount being sufficient to impart to the coated surface the appearance of such coating material. The structure of such coating may be essentially homogeneous or layered, and coating thickness typically is in a range from 0.01 to 0.05 micrometer. For example, a cobalt-hardened gold coating may be electro-deposited from a slightly acidic solution (pH 5) comprising potassium gold cyanide, cobalt citride, and a citric buffer. (The presence of cobalt, nominally in a range of from 0.2 to 0.5 percent by weight, enhances surface hardness especially in the case of thicker coatings.) Preferred temperature of the plating bath is approximately 35 degrees C, and a plating current of approximately
5 milliamperes per cm2 is convenient. Typical plating times are of the order of half a minute. Prior to plating, a surface may be cleaned, e.g., by electrolytic scrubbing in an alkaline solution, rinsing in de-ionized water, and dipping in dilute hydrochloric acid at elevated temperature. Example 1. A layer having a thickness of approximately 1.68 micrometer and having approximately 0.005 atom concentration of hydrogen in nickel was deposited on a copper substrate by sputtering from an essentially pure nickel target in an atmosphere of approximately 10 percent by volume hydrogen, remainder essentially argon. The layer was exposed to atmospheric test conditions at 75 degrees C and 95 percent relative humidity for 65 hours. After such exposure contact resistance was determined to be in the range of from 7 to 10 milliohms. Example 2. A layer having a thickness of approximately 0.48 micrometer was deposited as further described in Example 1 above. Ultimate contact resistance was in the range of from 10 to 13 milliohms. Example 3. A layer having a thickness of approximately 4.5 micrometers was deposited on a copper substrate by electroplating from a 2-molar nickel chloride solution at a temperature of approximately 75 degrees C, pH of the
solution was approximately 3 as obtained by the addition of ammonium hydroxide, and current density during deposition was approximately 150 milliamperes/cm2. The layer was exposed to atmospheric test conditions as described in Example 1 above, and contact resistance was determined to be in the range of from 1 to 10 milliohms.
Example 4. A layer was deposited as described in
Example 3 above except that a 2-molar nickel citrate solution was used at a pH of approximately 6. Contact resistance of the layer was found to be in the range of from 0.3 to 10 milliohms.
Example 5. A layer was deposited as described in Example 3 above except that a 1/2-molar nickel acetate solution was used at a pH of approximately 8. Contact resistance of the layer was in the range of from 2 to 15 milliohms.