Electrodeposited precious metal finishes having wear resistant particles therein

Abstract: An electrical connector comprises connector contacts having a metal base and a surface finish layer over the base, the surface finish layer being a composite of an electroplated precious metal composition and wear resistant particles dispersed therein. In the preferred embodiment, the precious metal layer is a PdCo alloy and the wear resistant particles are sub-micron sized lubricating particles, e.g. polytetrafluoroethylene particles and a flash coat of gold is provided over the composite layer.

Description: The trend in today's electronic technology is for electronic connectors, e.g. connectors for integrated circuit devices to a printed circuit board, to keep getting smaller and have increasing numbers of contact elements. Consequently, the entry pressure when mating the contacts increases unless the spring forces are reduced. This can ultimately result in unwanted increased wear of the contacts as measured for example by either frictional forces or contact resistance as a function of wear cycles. Composite plated coatings of various types are known in the plating field and other fields. For example, Henry et al, in U.S. Patent No. 4,830,889 and Feldstein, in U.S. Patent No. 5,721,055 describe the co-deposition of fluorinated polyethylene and nickel from an electroless nickel plating bath. Historically, electroless or auto catalytic plating has always been as a separate and distinct technology from that of electroplating, the latter requiring an electric current for deposition to take place. The plated articles as disclosed in Feldstein are textile spinning machinery parts. Henry et al. sets forth several general uses for such co-deposits. It should be noted that Henry et al. state that "fluorinated carbon has been co-deposited directly on the substrate surface in electroplating processes", however, no details or description of the process or the metals being deposited are given. Itoh et al., U.S. Patent No. 5,103,637 is directed to a rocket engine combustion chamber with a surface comprising a metal matrix, such as nickel or copper alloys having heat resistant particles such as ceramic oxide particles dispersed therein. The surface is formed by electroplating. Further, Tsuchiya et al., U.S. Patent No. 5,124,007 describes a composite electroplated nickel having particles such as silicon nitride, silicon carbide or tungsten carbide dispersed therein for use on piston rings. SE-C-510 043 (ABB RES LID), 12 April 1999 (1999-04-12) & DATABASE WPI Section Ch, Week 199921 Derwent Publications Ltd., London, GB; Class L03, AN 1998-205933 XP002154543 discloses a contact surface partially or completely covered with a silver based composite material containing wear resistant materials. The silver further includes organic sulfur compounds to reduce the composite material's surface energy. US-A-3,620,839 describes surfaces of electrical contact members. which move relative to each other and are lubricated with discrete particles of graphite bonded to at least one of the surfaces. US-A-5,199,553 describes a moveable electrical contact with a mating connector, the surface being coated with a composite material in which particles of graphite are dispersed in a matrix of silver. According to one aspect of this invention there is provided an electrical connector as claimed in claim 1. According to another aspect of this invention there is provided an electroplating bath as claimed in claim 7. According to a further aspect of this invention there is provided an electroplating method as claimed in claim 9. A precious metal electroplating bath for plating a composite layer of the precious metal together with lubricating particles and the process of plating the composite is described. The invention is particularly directed to a method and plating bath for coating the surface of an electrical connector having a conductive base metal and the resultant electronic connector to provide a connector having improved wear characteristics without significant loss of contact conductivity. More particularly, the preferred process comprises the use of an electroplating bath having an anode and a cathode and comprising an aqueous solution of palladium ions and cobalt ions having a lubricating fluoropolymer dispersed therein. The article to be plated, such as an electronic connector, comprises a conductive base metal which is itself made or is in electrical contact with the cathode of the bath and is immersed in the plating bath. Plating is initiated by passing a current through the bath while the bath is agitated. The resultant deposit is a PdCo alloy having the fluoropolymer dispersed therein. Brief Description Of The Drawings Figure 1 is a schematic representation of an electrodeposited layer structure useful in a high reliability electrical connector and for measuring sliding wear. Figure 2 is a schematic drawing of an apparatus useful for measuring wear and frictional forces on a surface. Figure 3 is a chart of the coefficient of friction of various precious metal finishes as a function of the number of wear cycles as measured by a sliding wear test. Figure 4 is a graphic representation of the chart shown in Figure 3. Detailed Description Of The Invention Generally, modern electronic interconnects requiring high reliability connectors employ precious metal finishes to provide low electrical resistance and resistance to oxidation or other chemical degradation. If, instead of reducing the spring force, the entry force can be reduced such as by lowering the friction between mating parts, the spring force, and hence the reliability of the connection can be maintained. However, this must be accomplished without any significant increase in contact resistance. We have now found that by co-depositing lubricating particles, such as fluorinated polyethylene with an electro-deposited precious metal layer, the entry force friction can be significantly reduced without any significant gain in contact resistance and with an increased wear resistance of the electrical connector. The preferred plating bath of the invention is a sulfate bath comprising from about 35 to about 45 g/l of Pd, from about 6 to about 10g/l of Co and from about 2 to about 5 ml/l of Dupont's PTFE 30, a fluoropolymer dispersion in water having polytetrafluoroethylene (PTFE) particles of from about 0.05 to about 0.5 µm in diameter in a concentration of about 60% by total weight. It should be understood that other concentrations of bath ingredients can be employed as well, and the invention is not meant to be limited to the use of a bath wnh the above stated composition. Typical plating conditions for the pretend bath include a current density of from about 50 to about 700 ASF, a bath temperature of from about 45°C to about 60°C at a pH of from about 7 to about 7.5, with agitation of from about 0.25 to about 3 m/sec. The resultant deposit comprises a PdCo alloy containing 20-30 wt.% Co having the PTFE particles uniformly dispersed therein. However, PdCo alloys containing 5-35 wt% Co are suitable. Typically, for electronic connectors, the substrate upon which plating occurs is bulk Cu which preferably has a Ni layer deposited thereon. A suitable thickness for the Ni layer is in the order of 2-3 µm, but may be more or less. However, it should be understood that the substrate need not be copper and could be any other conductive material. The thickness of the electroplated precious metal composite which overlies the nickel, when and if it is present, is typically in the order of about 1 µm, but may be more or less than this thickness. Preferably, subsequent to electrodeposition, a thin flash coating of gold in the order of 0.1 µm thick is provided over the PdCo alloy-fluoropolymer composite layer. While one may expect a reduction in surface friction with a coating having nonconductive lubricant particles therein as compared with a coating without such particles, one would also expect that the presence of such particles in an amount that would significantly reduce the friction, would also significantly increase the contact resistance of the surface making it unsuitable for use in an electrical connector. We have found that this is not the case. In our recent article, "Palladium-Cobalt Makes a Superior Finish", Connector Specifier , pg. 12 et al., February 1999, we describe the PdCo system (without lubricant co-deposition) and compare it to PdNi and Hard-Au. In order to determine the long-term performance characteristics of the surface connector contacts, an important parameter is called sliding wear. The sliding wear test provides a measure of the hardness and lubricity of the electroplated finish. These are major factors, along with contact resistance, to be considered when designing high reliability electronic connectors. The sliding wear was measured using a rider and flat apparatus as shown in Figure 2. The rider is a rivet with a 1.75 mm radius and the flat was a 25 mm diameter disc, both fabricated from copper. Mated samples were plated with identical surface finishes. A load of 100 grams (about 1.0 N) is applied to the rider, and reciprocal movement with a 12.5 mm amplitude is imposed at a rate of 30 cycles/min. (0.5 Hz). A test sample for the sliding wear test, which sample depicts the layers one may find in a high reliability connector made in accordance with the present invention is shown with reference to Figure 1. As shown, a Ni layer, typically about 2.5 µm thick is formed, such as by electrodeposition, on a copper substrate. The novel electrodeposited PdCo alloy having lubricating particles dispersed therein is formed over the Ni layer. The PdCo layer is typically about 1 µm thick. A 0.1 µm gold flash layer is then preferably formed over the composite layer. The sliding wear performance is examined by measuring the friction force and contact resistance. The friction force is measured by a strain gauge, and the contact resistance was simultaneously determined employing a 25 mA current through the system while measuring the voltage drop. Results of the sliding wear test, (Figures 3 and 4), have shown that the friction force of a precious metal finish can be considerably reduced by incorporating nonconductive lubricating particles into the finish during electroplating without significantly increasing the electrical contact resistance. A comparison of the coefficient of friction as measured by the sliding wear test as a function of wear cycles for various finishes is shown in Figure 3. As can be seen from this Figure, the novel finish has the lowest coefficient of friction throughout the range of cycles tested. Further, the measured contact resistance during the 10,000 sliding wear cycles for the novel finish was only about 3-4 mΩ

Claims: An electrical connector having connector contacts which comprise a base metal and an electro-plated composite layer over said base metal, said composite layer comprising a PdCo alloy and sub-micron sized lubricating particles of PTFE dispersed therein. A connector as claimed in claim 1 wherein said alloy contains from about 5 to about 35 wt. % Co. A connector as claimed in claim 1 comprising a flash layer of Au over the electroplated composite layer. A connector as claimed in claim 1, comprising a thin layer of nickel disposed between the base metal and the composite layer. A connector as claimed in claim 4 wherein the base metal is copper. A connector as claimed in claim 4 comprising a flash layer of Au over the electroplated composite layer. An electroplating bath for depositing a composite layer of metal, the bath comprising an aqueous sulfate solution of palladium and cobalt having dispersed therein sub-micron sized lubricating particles of PTFE. An electroplating bath as claimed in claim 7 wherein the concentration of palladium ions in the bath is from about 35 to about 45 g/l, the concentration of cobalt ions in the bath is from about 6 to about 10 g/l and the pH of the bath is from about 7 to about 7.5. An electroplating method of forming a composite layer on an article having an exposed metal surface, comprising: placing said article in an electroplating bath comprising an aqueous sulfate solution of palladium and cobalt having dispersed therein sub-micron sized lubricating particles of PTFE, making or contacting the exposed metal surface of the article with a plating bath cathode, and passing a current, whilst agitating the bath, at an appropriate voltage between an anode of the bath and the cathode so as to deposit said composite layer comprising a PdCo alloy and said lubricating particles dispersed therein.

Inventors: ABYS JOSEPH ANTHONY, DULLAGHAN CONOR ANTHONY, FAN CHONGLUN, SMITH BRIAN THOMAS, ABYS, JOSEPH ANTHONY, DULLAGHAN, CONOR ANTHONY, FAN, CHONGLUN, SMITH, BRIAN THOMAS
Applicants: LUCENT TECHNOLOGIES INC, LUCENT TECHNOLOGIES INC.
Application year: 2000
Main classification: C25D3
IPC codes: C25D5, H01R13, C25D3, C25D15, H05K3