|
(11) | EP 0 531 099 A2 |
| (12) | EUROPEAN PATENT APPLICATION |
|
|
|
|
|||||||||||||||||||||||||||
| (54) | Corrosion resistant high temperature contacts or electrical connectors and method of fabrication thereof |
| (57) The invention provides a composite material specifically adapted for use as high
temperature corrosion resistant electrical connectors. A conductible nickel-base substrate
alloy having corrosion resistance, strength and creep resistance is used to hold shape
at 200°C. An interlayer of substantially pure nickel is electrolytically plated over
the nickel-base substrate. A noble metal surface is diffusion bonded to the electrolytic
nickel interlayer. The noble metal surface cannot be hardened by organic additives
which accelerate corrosion at 200°C. Alternatively, if a copper-base substrate is
used, a layer of wrought pure nickel is bonded to the substrate then a layer of electroplated
nickel is used to overplate the wrought nickel and the copper-base substrate. A noble
metal surface is then diffusion bonded to the electrolytic nickel layer. |
DISCUSSION OF THE PRIOR ART AND PROBLEM
[0001] Electrical contacts or connectors for electronic applications is an old but still evolving art. Generally these contacts are manufactured by roll bonding of wrought alloys or by electrolytic plating methods. The roll bonding of wrought alloys is well taught by Robert J. Russell "Properties of Incoloy Clad Wrought Gold Alloy," Solid State Technology. In the method of manufacture taught by Russell, a strip of gold or gold alloyed with Ni, Cu, Ag, Co, Pt or Pd alloyed with Ag is roll bonded onto a wrought nickel alloy, slit to width, then roll bonded as an inlay into a copper-base alloy such as CA725 (Cu-10Ni-4Sn), annealed to achieve the precipitation hardening of the composite and rolled to finish gauge. As will be shown in the body of this disclosure this manufacturing process has technical shortcomings in some high temperature, polluted environments. It is also costly to produce contacts using the process of Russell.
[0002] Most of the contacts used in the electronics industry today are electrolytically plated mainly because electroplating provides cost advantages. In electroplating processes, the base metal, usually CA725 or beryllium copper is typically electrolyticcally plated with nickel to a thickness of 1 to 10 micrometers usually 3.5 micrometers, then overplated only in the actual contact area with "hard" gold to a thickness of 0.5 to 1.5 micrometers. The contacts are strung together and mechanically formed into the final or semi-final shape prior to the hard gold plating because the "hard" gold is brittle. The "hard" gold bath is different than a "soft" gold bath in that either Ni or Co from.5 to 1 percent and some organic hardeners are incorporated in the plated gold. While these contacts have several desirable features, i.e. a hardened gold surface for wear and low contact resistance, and a barrier nickel underlayer to the copper alloy, we have discovered that they have several undesirable features which preclude their use in anything but the least hostile environments.
[0003] Other mixed roll bonding - plating methods of manufacturing have been devised. For instance AT&T produces a contact known as DGR-156. DGR-156 is produced by plating Ni on CA725 copper base alloy, then overplating with a thick layer of 60% Pd - 40% Ag alloy, and then overplating with a thin flash of hard gold to form a semi-finished composite sheet. The semi-finished composite sheet is then rolled to wherein thickness of the Pd-Ag layer is around 0.5 micrometers and thickness of the Au flash end is about 0.1 to 0.2 micrometers. This rolled material is usually heat treated to diffuse the Au into the Pd-Ag so that the surface is around 75% Au to form finished DGR-156. The co-plating of Pd and Ag is described by Cohen et al. in U.S. Patent No. 4,269,671. As will be shown, this contact material also has limited usefulness in severe service.
[0004] Another new contact material made by the electrolytic process is palladium-nickel as described by Abys et al. in "Metal Finishing" of July 1991 and in U.S. Patent Nos. 4,427,502, 4,468,296, 4,486,274 and 4,911,798. This Pd-5 to 20 wt% Ni alloy is usually plated to a thickness of 0.25 to 1.5 micrometers overtop of 3.75 micrometers of nickel which was plated overthe copper base CA725 material. Acobalt hard gold cap is plated overtop of the Pd-Ni to a thickness of 0.125 micrometers. This newer plated contact material has several advantages, the nickel in the palladium reduces the undesirable hydrogen embrittlement of pure Pd material and increases the hardness of the contact to a Knopp hardness with a 50 gm load of 430 KHN at 16 wt% Ni. These connectors have about the same performance as hard gold connectors of the same thickness and are generally cheaper. However, as will be shown in this disclosure, this material also has poor high temperature performance characteristics.
[0005] Yet another method to making connectors is disclosed by Bell et al. in U.S. Patent No. 4,956,026. This method electroplates nickel over an age hardenable base and 0.3 microinches (0.76 micrometers) of soft gold on the surface. The composite is then heat treated in such a manner as to precipitation harden the age hardenable substrate and at the same time diffuse Ni in to the surface Au layer. The diffusion of the Ni into the soft pure Au to a level of 2 to 10% Ni as disclosed by Bell et al. in U.S. Patent No. 4,505,060, hardens the Au so as to improve its wear characterisrics. Note that the Knopp hardness KHN50 is 300 for 10 wt.% Ni in Au (Russell, et al.) compared to KNOgograms of 70 for pure gold.
[0006] It has been discovered that all of common connections tested were unsuitable for use in high temperature or corrosive gas atmospheres. Specifically, it has been discovered that the degradation of contacts in a mixed gas corrosion test including industrial atmospheres shows that the deleterious oxidation (or sulphidization) may occur by copper migration through the electrolgically plated nickel underlayer or by migration of the Cu corrosion product from an exposed edge. It has also been discovered that all the electrolgically prepared contacts, i.e., hard gold, gold flash Pd-Ni, and gold flash Pd-Ag contacts have poor high temperature oxidation resistance.
[0007] It is an object of this invention to provide a composite material for use in electrical contacts resistant to corrosion.
[0008] It is a further object of this invention to provide a composite material for manufacturing of electrical contacts that is resistant to oxidation at elevated temperatures.
DESCRIPTION OF THE DRAWING
[0009]
Figure 1 is a schematic chart comparing circuit resistance of various contacts after an exposure of 100 °C for 1000 hours;
Figure 2 is a schematic chart comparing circuit resistance of various contacts after an exposure of 150°C for up to 1000 hours; and
Figure 3 is a schematic chart comparing circuit resistance of various contacts after an exposure of 200°C for up to 1000 hours.
SUMMARY OF INVENTION
[0010] The invention provides a composite material specifically adapted for use as high temperature corrosion resistant electrical connectors. Aconductible nickel-base substrate alloy having corrosion resistance, strength and creep resistance is used to hold shape at 200°C. An interlayer of substantially pure nickel is electrolgically plated over the nickel-base substrate. A noble metal surface is diffusion bonded to the electrolytic nickel interlayer. The noble metal surface cannot be hardened by organic additives which accelerate corrosion at 200°C. Alternatively, if a copper-base substrate is used, a layer of wrought pure nickel is bonded to the substrate then a layer of electroplated nickel is used to overplate the wrought nickel and the copper-base substrate. A noble metal surface is then diffusion bonded to the electrolytic nickel layer.
DESCRIPTION OF PREFERRED EMBODIMENT
Description of Testing
[0011] Various forms of NIGOLDTM alloy were compared to contact materials having general acceptance in the industry. A summary of materials tested is given below in Table 1.
[0012] NIGOLD™ alloy - Ni. These coupons were prepared by plating soft pure gold to a thickness of 0.5 micrometers on solid pure nickel and heat treating the coupons in a reducing atmosphere according to the teaching of 4,505,060 to diffuse 5-10% Ni to the surface of the gold. All elemental amounts are expressed in weight percent unless specifically indicated otherwise.
[0013] NIGOLD™ alloy - Cu coupons were prepared by plating 1.4,2.5 and 10 micrometers of pure Ni on a copper base substrate of alloy CA725 (a copper alloy widely used as a connector spring material). The nickel was overplated with 0.4 and 0.6 micrometers of pure soft gold and heat treated the same as the NIGOLDTM alloy - Ni samples.
Hard Gold
[0014] 1.4 and 10 microinches of nickel was plated on top of alloy CA725 and then overplated with 0.4 and 0.6 micrometers of conventional cobalt hardened gold. This is the standard Hard Gold connector.
Gold Flash Palladium Nickel - G.F. Pd-Ni
[0015] 0.125 micrometers of cobalt hardened gold was plated on 0.625 micrometers of 80/20 palladium/nickel on top of 2.5 micrometers of nickel on top of alloy CA725 by AT&T.
DGR - 156
[0016] A clad inlay material of 0.225 micrometers of pure gold on 2.28 micrometers of 60 % Pd/40 % Ag on 8. 1 micrometers of nickel on top of CA725. Contact was heat treated so the surface was 75% Au, ± 15% Au. DGR-156 was tested as a 4mm wide strip.
Exposure
[0017] All coupons were cut along at least one edge and exposed to a Class III Battelle flowing mixed gas test. This test exposed the coupon to an atmosphere of 20 C12, 200 N02, 100 H2S expressed in parts per billion at 30°C and 70% relative hunudity for periods of 2 to 10 days. The Class III Battelle mixed gas test is widely accepted in the industry as the simulating long term exposure in severe industrial environments.
Test Results
[0019] The corrosion products show up on the surface as dark spots. As the length of the exposure increased from 2 to 10 days the degradation at any particular pore or spot increased. Dark corrosion products also were found to creep over the surface from the edges.
[0020] NIGOLDTM alloy - Ni. In this specimen, which notably contains no copper, had no corrosion spots and no corrosion products creeping around the edge (edge creep). This material passed a 10 day Banelle Test.
[0021] NIGOLDTM alloy - Cu. All of the NIGOLD specimens on copper substrates exhibited extensive corrosion and edge creep (3mm in 6 days). Generally, there was little difference in corrosion with increase in thickens of the Au layer. Generally, the thicker the nickel underlayer, the less corroded the specimen. All of these specimens failed this test.
Hard Gold
[0022] Generally the hard gold was superior to NIGOLD™ alloy - Cu. The thickness of the Au layer had little effect on the results. Again, the thicker the nickel interlayer, the less corroded the specimen. All specimens showed extensive edge creep (3mm in 6 days). All of the specimens failed the Battelle test.
DGR-156
[0023] Since this contact strip was only 4mm wide, the entire surface was covered by edge creep and failed the test.
G.F. Pt-Ni
[0024] Generally, these results were about the same as the hard gold. Again all coupons eventually failed the test.
[0025] These results were entirely unexpected. The only suitable material was on a copper free substrate. This may indicate that enough copper is migrating through a 10 micrometer thick layer of electroplate Ni to cause the formation of deleterious copper corrosion products on the surface. Also, uncoated copper from cut edges of a contact may migrate over the surface up to 3mm in 10 days at 30°C.
[0026] These results have profound importance in design of new connectors for high temperature, corrosive atmosphere service.
Contact Resistance on Thermal Aging
[0027] To measure the high temperature stability of the samples of NIGOLDTM alloy - Ni, NIGOLDTM alloy - Cu, Hard Gold, DG156, and G.F. Pd-Ni were placed in an oven for 100, 500 and 1000 hours at 100°C, 150°C and 200°C. The contact resistance was determined by probing with a 50g load orwithoutwiping according toASTM B-667.
[0028] The results are summarized in Figures 1, 2 and 3 which show the resistance in millivolts for the various materials after exposure at 100, 150, and 200°C respectively. Exposure times are for 1000 hours except where a high resistance reading was noted and a shorter exposure time is noted. In power applications (greater than 10amps) contact resistance values higher than 5 milliohnms can result in significant contact heating and accelerated failure. In signal (low power) circuits a contact resistance below 5 milliohms is generally acceptable.
[0029] These results show that where only oxygen and nitrogen is in the atmosphere that NIGOLDTM alloy on pure nickel is the most thermally stable material. NIGOLD™ alloy on electroplated Ni in copper is the next best and has a service temperature capability in excess of 200°C for over 1000 hours. Hard Gold becomes unacceptable in services between 100 to 150°C presumably because of the degradation in the organic hardeners in the deposit. DGR is the second best material compared to NIGOLD™ alloy, but DGR fails somewhere from 150 to 200°C. The G.F. Pd-Ni also fails somewhere between 100 and 150°C.
[0030] From these unexpected results and the observations the design for connectors specifically adapted for high temperature and corrosive gaseous atmospheres has been ascertained. Generally, the requirements of a connector are passing a 10 day Battelle Class III test without spotting visible to the unaided eye while simultaneously maintaining a contact resistance of less than 5 milliohms with a 50g load after 1000 hours over 200°C as specified in test ASTM B-667.
[0031] The contact surface should be a noble metal. The noble metal surface advantageously is Au, Pd, Ni or an alloy of any combination therof. Most advantageously, gold is used as the contact metal. Preferably for good wear resistance, the gold should be alloyed with up to 10% Ni to harden it. The thickness of the contact surface layer is most advantageously less than 0.4 micrometers. Hard gold containing organics are not desirable for use in plating surfaces. The noble metal alloy advantageously is formed from strip and roll bonded. The precious metal can be electroplated, but the deposit must be free from organics. If pure gold is deposited, it can be heat treated to diffuse Ni from the substrate into the Au as taught by Bell et al. The least cost method for thin surface layers is probably electroplating and annealing as taught by Bell et al in U.S. Pat. No. 4,505,060 ('060). The entire specification of the'060 patent is hereby incorporated. The porosity of the surface layer does not appear to be critical.
[0032] The preferred high temperature copper-base contacts are made with no direct contact between the copper-base substrate and the gold plating. Pure wrought nickel is bonded to the copper-base sprig material to prevent direct copper diffusion. Furthermore, to prevent edge creep of copper, the entire clad wrought Ni nickel/copper- base sprig material is preferably enveloped with electrolytic nickel prior to the gold plating. Gold is most advantageously plated only over regions wherein electroplated nickel is covering the wrought nickel. Alternately, gold is preferably bonded to a copper-free spring material like PERMANICKEL® alloy 300 (nickel-titanium alloy) or DURANICKEL® alloy 301 (nickel-aluminum-titanium alloy) with an interlayer of pure electrolytic nickel between the gold and copper-free spring material. (PERMANICKEL and DURANICKELare registered trademarks of the Inco family of companies.) The interlayer of nickel is used to prevent elements which could adversely affect corrosion properties from diffusing into the gold. For both nickel-base and copper-base materials the contacts are preferably heat treated to interdiffuse the nickel and gold. The interdiffusion of nickel into the gold provides increased wear resistance. This discovery also shows that the fabrication method of 4,956,026 is also applicable if a PERMANICKEL or DURANICKEL base alloy is used.
[0033] Nominal composition specifications by weight percent for PERMANICKEL alloy 300 and DURANICKEL alloy 301 are provided below in Table 3:
[0034] It has been discovered that wrought nickel interlayers for supporting noble metal surfaces may be used over copper containing substrates in hostile environments. Electroplating over other substrates containing no copper is acceptable. Most advantageously, wrought nickel has a thickness of at least 10 microns and electroplated nickel interlayers have a thickness of 1 to 10 microns. For instance PERMANICKEL® alloy or DUR-ANICKEL® alloy each have acceptable properties. Advantageously, the copper-free substrate provides corrosion resistance, strength and creep resistance to hold shape at 200°C in an air atmosphere. The copper-free substrate most advantageously must also possess the required spring properties and stress relaxation requirements associated with alloy CA725.
[0035] If a copper containing substrate is used, a substantially pure wrought nickel interlayer such as Nickel 290 is required. For purposes of this specification, substantially pure is defined as at least 98% nickel. Most advantageously, 99.9% pure wrought nickel is used.
[0036] Inlays of wrought pure Ni 290 onto CA725 strip are not acceptable because of edge creep. If any inlay or roll bonding of pure nickel sheet onto CA725 is used then the roll bonded bimetallic must be electrolytically overplated with 3 to 10 micrometers of Ni to prevent edge creep, the Au must then be overplated and interdised with the Ni as taught by Bell et al.
[0037] While in accordance with the provisions of the statute, there is illustrated and described herein specific embodiments of the invention. Those skilled in the art will understand that changes may be made in the form of the invention covered by the claims and the certain features of the invention may sometimes be used to advantage without a corresponding use of the other features.
1. A composite material for use in electrical connectors comprising:
a) a nickel-base conductible substrate alloy, the substrate alloy having suitable corrosion resistance, strength and creep resistance to hold shape at 200°C;
b) an interlayer of substantially pure electrolytic nickel having a thickness bonded to substrate (a); and
c) a noble metal diffusion bonded to the interlayer of substantially pure electrolytic nickel for increasing hardness of the noble metal and the noble metal surface being free of organic hardeners.
2. The composite material of claim 1 wherein the noble metal is selected from the
group consisting of gold and gold-nickel alloys containing from 1 to 10% Ni and the
noble metal has a thickness less than 0.4 f..lm and the thickness of the interlayer
is 1 to 10 micrometers.
3. The composite material of claim 1 wherein the noble metal containing surface includes
a gold-nickel alloy produced by interdiffusion of pure soft gold plate and the interlayer
of substantially pure electrolytic nickel substrate.
4. The composite material of claim 1 wherein the noble metal surface is made from
an alloy formed from the group consisting ofAu, Pd and Ni and the nickel-base substrate
is selected from the group consisting of nickel-aluminum-titanium alloys and nickel-titanium
alloys.
5. A method of producing a composite material useful to form electrical contacts comprising:
a) providing a nickel-base conductible substrate alloy having sufficient corrosion resistance, strength and creep resistance to hold shape at 200°C;
b) bonding an interlayer of substantially pure electrolytic nickel to the substrate; and
c) diffusion bonding noble metal alloy to the interlayer of substantially pure electrolytic nickel to harden the noble metal and the noble metal being free of organic hardeners.
6. The method of claim 5 wherein step b) includes electrodepositing nickel over the
conductible substrate and step c) includes electrodepositing soft gold to the interlayer
and further including step d) of heat treating the composite material to diffuse nickel
into the soft gold.
7. A composite material for use in electrical connectors comprising:
a) a conductible copper-base substrate alloy, the copper-base substrate alloy having spring properties and creep resistance properties;
b) an interlayer of substantially pure wrought nickel having a thickness of at least 10 microns mechanically bonded to the copper-base substrate to form a clad intermediate product of the copper-base substrate and the wrought nickel; and
c) an electrodeposited layer of nickel 3 to 10 microns in thickness enveloping the clad intermediate product of the copper-base substrate and the wrought nickel;
d) a noble metal surface diffusion bonded to the electrodeposited nickel adjacent to the interlayer of substantially pure wrought nickel for increasing hardness of the noble metal and the noble metal being free of organic hardeners.
8. The composite material of claim 7 wherein the noble metal is selected from the
group consisting of gold and gold-nickel alloys containing from 1 to 10% Ni and the
noble metal has a thickness less than 0.4 µm and the interlayer is wrought nickel
having a purity of at least 99.9 %.
9. The composite material of claim 7 wherein the noble metal containing surface includes
a gold-nickel alloy produced by interdiffusion of pure soft gold plate and the pure
nickel substrate and the noble metal surface is made from an alloy formed from the
group consisting of Au, Pd and Ni.
10. A method of producing a composite material useful to form electrical contacts
comprising:
a) providing a conductible copper-base substrate alloy having spring properties and creep resistance properties;
b) mechanically bonding a 3 to 10 microns interlayer of wrought nickel to the copper-containing substrate to form a clad intermediate product of copper-base substrate and wrought nickel;
c) electroplating nickel to cover the clad intermediate product of copper-base substrate and wrought nickel;
d) diffusion bonding a noble metal surface to the electroplated nickel adjacent the wrought nickel to harden the noble metal and the noble metal being free of organic hardeners; and
e) heat treating to harden the noble metal containing surface with nickel.