Classifications

• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
  • C25D7/00—Electroplating characterised by the article coated
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
  • C25D5/00—Electroplating characterised by the process; Pretreatment or after-treatment of workpieces
  • C25D5/10—Electroplating with more than one layer of the same or of different metals
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
  • C25D5/00—Electroplating characterised by the process; Pretreatment or after-treatment of workpieces
  • C25D5/48—After-treatment of electroplated surfaces
  • C25D5/50—After-treatment of electroplated surfaces by heat-treatment
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01B—CABLES; CONDUCTORS; INSULATORS; SELECTION OF MATERIALS FOR THEIR CONDUCTIVE, INSULATING OR DIELECTRIC PROPERTIES
  • H01B1/00—Conductors or conductive bodies characterised by the conductive materials; Selection of materials as conductors
  • H01B1/02—Conductors or conductive bodies characterised by the conductive materials; Selection of materials as conductors mainly consisting of metals or alloys
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01R—ELECTRICALLY-CONDUCTIVE CONNECTIONS; STRUCTURAL ASSOCIATIONS OF A PLURALITY OF MUTUALLY-INSULATED ELECTRICAL CONNECTING ELEMENTS; COUPLING DEVICES; CURRENT COLLECTORS
  • H01R13/00—Details of coupling devices of the kinds covered by groups H01R12/70 or H01R24/00 - H01R33/00
  • H01R13/02—Contact members
  • H01R13/03—Contact members characterised by the material, e.g. plating, or coating materials
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
  • C25D3/00—Electroplating: Baths therefor
  • C25D3/02—Electroplating: Baths therefor from solutions
  • C25D3/56—Electroplating: Baths therefor from solutions of alloys
  • C25D3/60—Electroplating: Baths therefor from solutions of alloys containing more than 50% by weight of tin
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
  • C25D3/00—Electroplating: Baths therefor
  • C25D3/02—Electroplating: Baths therefor from solutions
  • C25D3/56—Electroplating: Baths therefor from solutions of alloys
  • C25D3/64—Electroplating: Baths therefor from solutions of alloys containing more than 50% by weight of silver

Definitions

• the present invention relates to connectors and more particularly to metallic finishes for the contacts of such connectors.
• Electrodeposited gold has been traditionally used as a connector material because of its unique combination of properties including low wear rate and excellent corrosion resistance.
• the utilization of gold as a contact material has come under close scrutiny since the marked increase in gold prices during recent years and it has become evident that the connector industry must consider alternative cheaper coatings.
• Tin also suffers from the disadvantage that electrodeposited tin is prone to the formation of whiskers and is therefore not suitable for use on miniature connectors where the contact pitch is small and shorting could readily occur.
• Silver is also used as an alternative contact finish on commercial connectors but is prone to silver migration and is susceptable to tarnishing in sulphurous atmospheres and has relatively poor wear resistance.
• the present invention therefore provides a metallic. finish for connectors including a mixture of silver and tin in which the silver and tin are combined in part or in whole to form an intermetallic or intermediate compound.
• Intermediate and Intermetallic compounds are defined in Physical Metallurgy 2nd Edition edited by R. W. Cahn, North Holland, 1970 page 229.
• the final finish contains from 25 to 100% by volume of the intermetallic or intermediate compound.
• a layer of silver is deposited on connector contacts, a layer of tin is deposited on the silver and the resultant layers are diffused to produce a combined silver-tin intermetallic connector finish.
• the layer of tin may be deposited first the layer of silver deposited onto the tin with subsequent diffusion to produce the intermetallic connector finish.
• multiple layers of tin and silver may be deposited to the desired total thickness and composition. This would increase the rate of conversion to the intermetallic phase during diffusion.
• a layer of iron is deposited onto the connector contact prior to the deposition of the silver and tin to form a barrier between the contact material and the intermetallic or intermediate compound
• the intermetallic or intermediate compound may be directly deposited from a carefully selected solution containing ions of silver and tin providing temperature and rate of deposition is carefully controlled.
• a practical method for the preparation of the intermetallic containing contact finishes involves the successive electrodeposition of a layer of one pure metal over another followed by subsequent diffusion treatment.
• the diffusion may be achieved by heat treatment in a 90N 2 /10H 2 atmosphere, but glow discharge assisted diffusion is an alternative method which might be considered for production purposes.
• the resultant diffused structure consists of varying proportions of solid solution and hard intermetallic-compounds depending upon the alloy composition, the heat treatment employed and the presence or absence of interaction with the substrate.
• the finish may be directly deposited. by electrodeposition with or without a subsequent diffusion process.
• the choice of constituent elements is based upon the material cost, the ease of electrodeposition from commercially available solutions and the melting points of both the original metals the nature of the phase diagram and the melting point of the resultant intermetallic phases.
• Silver has been selected because it is a semi-noble metal.
• the combination with a low melting point material such as tin enables the use of relatively low diffusion temperatures if a diffusion process is to be used which should not cause any deterioration of the mechanical properties of the underlying substrate material. Diffusion may be conducted wholly in the solid state; or involving a transient liquid phase if the melting point of the lower melting point metal is exceeded. It is possible for the substrate material to diffuse into the above electroplated layers during the heat treatment; this may or may not have adverse effects upon the performance. A diffusion barrier may be employed to prevent this.
• intermetallics achieved by suitable diffusion treatments or electrodeposition have several properties which are required by contact finishes.
• the atomic ordering which is very common in intermetallic compounds gives them intrinsically greater hardness than a pure metal or solid solution thereby imparting improved wear resistance.
• the strong chemical bonding of such phases indicated low reactivity and therefore good corrosion resistance.
• the relatively high melting points of the intermetallic compounds result in improved ambient temperature mechanical properties (in particular creep resistance) which is essential when contact finishes are mated under stress.
• a preferred intermetallic contact finish is obtained using diffused layers in the Ag-Sn system. Utilization of different relative plating thickness, diffusion temperatures and diffusion times enables the formation of varying proportions of Ag 3 Sn intermetallic and silver or tin (Ag or Sn) solid solution as determined form the phase diagram. A range of these materials have been tested on model connector contacts. In the simplest case these materials were prepared by the diffusion of tin and silver layers deposited directly onto a copper based alloy substrate without an intermediate barrier layer. The best of these finishes exhibit consistent low contact resistance ( ⁇ 5mn) and low friction (60 grams per contact) during 500 operations with 100 and 150 gram contact loads.
• a layer of 5 microns of tin is electrodeposited over a 5 micron layer of electrodeposited silver on a bronze substrate, and the layers interdiffused for 1 hour at 250°C in a mildly reducing atmosphere.
• the composition homogeneity and microstructure of the diffused layer were examined by standard metallographic sectioning and by X ray diffraction scanning electron microscopy and electron microprobe analysis.
• the resulting layer consisted of the Ag 3 Sn intermetallic and a smaller proportion of pure tin.
• the intermetallic Ag 3 Sn comprised the major proportion of the surface regions of the diffused layers.
• a layer of 2.5 microns of tin is electrodeposited over a 7.5 micron layer of electrodeposited silver on a bronze substrate and the layers interdiffused for one hour at 250 0 C in a mildly reducing atmosphere.
• the resulting layer consisted of the Ag 3 Sn intermetallic and a proportion of pure silver.
• the intermetallic Ag 3 Sn comprised the major proportion of the surface regions of the diffused layers.
• the electrodepositions of the silver and tin layers may be in the reverse order with the silver being deposited on top of the layer of tin which is initially deposited on the bronze substrate.
• the quantities of silver and tin will be the same.
• the material largely comprises a layer of Ag 3 Sn whilst a small amount of tin rich material remains at the surface and some silver rich material remains beneath the Ag 3 Sn layer.
• the contact resistance of these samples is consistant over 1,000 wipes under 100 g contact load, the values being ⁇ 8m ⁇ . The corresponding friction is also consistent and below 100 g force/contact.
• the wear of 10 ⁇ m coating is 4 - 6pm after 500 wipes and 7 - 8 f m after 1,000 wipes.
• Silver migration was monitored using the so called "Water Drop” test. In this test a drop of deionised water is placed so as to bridge the gap between two conductor lines and the migration of silver is observed upon applying a bias between the two conductors. The diffused tin silver layer shows no evidence of silver migration after 30 minutes at 5, 10 or 15 volts for a 1mm gap. Under the same test conditions silver shows clear evidence of migration after only 2 minutes at 5 volts.
• Silver Tin Alloys can also be directly electrodeposited from solutions containing silver and tin ions.
• solutions containing silver and tin ions Several formulations are possible.
• One example of such a solution has the following composition:
• the deposit produced from this solution consists mainly of the intermetallic compound Ag 3 Sn.
• a second example of a solution of electrodepositing silver tin alloys has the following composition:
• This solution produces a deposit containing 88% silver and 12% tin.
• a third example has the composition:
• the relative concentrations of silver and tin ions in solution determine the composition and structure of the electrodeposit. Under certain conditions a deposit containing intermetallic or intermediate compounds and free tin can be produced.
• Deposits have been prepared from the Ag CN K 2 Sn 0 3 solution described above.
• the deposit plated at a temperature of 55 0 C and at a current density of 6m A mps/cm 2 was shown by X ray diffraction to contain the Ag 3 Sn intermetallic with traces of free tin and silver.
• the contact resistance values varied between 3 and 6.5 ohms during 500 wipe cycles under a 100gr contact load and the corresponding friction rose from 50 to 100 grammes force during the test.
• the coating had worn through approximately 10 ⁇ m during the 500 wipe operations.

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Cited By (10)

Citations (7)

• 1981
  • 1981-01-30 EP EP81300397A patent/EP0033644A1/de not_active Withdrawn
  • 1981-01-30 GB GB8102987A patent/GB2069005A/en not_active Withdrawn
  • 1981-02-05 JP JP1628481A patent/JPS56123365A/ja active Pending

Patent Citations (7)

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