GB2168381A

GB2168381A - Gold plated electrical contacts

Info

Publication number: GB2168381A
Application number: GB08431280A
Authority: GB
Inventors: Ronald Arthur Neale
Original assignee: STC PLC
Current assignee: STC PLC
Priority date: 1984-12-12
Filing date: 1984-12-12
Publication date: 1986-06-18
Also published as: GB8431280D0; GB2168381B

Abstract

The contact has a substrate layer of a substantially non-corrodible palladium-nickel alloy on a base metal, the substrate layer having a thickness in the range of 1.5 to 5 micrometres, and carries a layer of gold, which is thin compared with the palladium-nickel layer. This is particularly useful for printed circuit board connectors and board edge contacts. The use of a thin gold layer reduces costs.

Description

SPECIFICATION Gold plating This invention relates to electrical contacts, especially but not solely for electrical connections.

The use of pluggable edge connections on printed wiring boards is well-known, and they need a reliable low contact resistance connection which can operate for decades. During this time the connection is made and unmade many hundreds of times, and has to resist the effects of aggressive environments. Hence careful selection of the materials used for the contacts is necessary. Hitherto relatively thick layers of gold have been used to seal off the substrate with no porosity, and to present a non-corrodible surface which has the low contact resistance and wear-resistant properties which would allow frequent sliding during matings.

An object of the invention is to provide contacts which meet the above indicated needs, but in which the amounts of precious metals such as gold are reduced compared with the known arrangements.

According to the invention there is provided an electrical contact, which includes a substrate layer of a substantially non-corrodible alloy on a base metal surface, which substrate has a thickness in the range of 1.0 to 5 micrometres, and a layer of a non-corrodible precious metal on the substrate, which precious metal layer is thin compared with the substrate layer and provides the contact with good contact resistance and good wear-resistant properties.

Thus we provide a system in which the substrate, which is deposited directly onto the basic metal of the contact is a reasonably thin layer of a non-corrodible, non-porous, deposit to achieve the desired resistance to aggressive environments.

This is then covered with a very thin, possibly porous, coating of a non-corrodible precious metal having the required contact reistance and wear properties. This latter layer, which is gold in the preferred arrangement, can be thought of as acting as a lubricant.

Such an arrangement provides a finish which when used on edge-connector contacts on a printed wiring board has a contact reistance of a few milliohms. This contact resistance does not change by more than 5 milliohms when subjected to a mechanical endurance of 500 matings with an approved socket connector, followed by exposure tp 21 days of Industrial Atmosphere Test KC of IEC 68/BS2011. The contact resistance also does not change by more than 5 milliohms when subjected to ten days of Test Ca of IEC/BS2011 or to 2000 hours of Dry Heat Test at 1250C.

When using gold as the outer, and "ultra thin" layer the substrate layer is an alloy of palladium and nickel with a palladium content of 70% to 85%, the thickness of this layer lying in the range of 1.5 to 5 micrometres. The gold layer is an electroplated layer of 98% minimum pure gold of hardness 120-200 VPN. In one specific example the palladium-nickel layer is 2 micrometres thick and the gold layer is 0.25 micrometres thick.

The use of a thin gold layer, although micro-porous, presents a large equipotential surface for the mating contact to touch, and results in the low contact resistance of the intrinsic metal being manifested. This does not occur when mating directly on to the non-corrodible (Pd/Ni) substrate layer.

The micro-porosity of the gold layer, while it is extensive, is rendered harmless by the non-corrodibility of the substrate.

Thus corrosion products, which would adversely affect contact resistance are not produced. The choice of c/a layer of 2 micrometres of Pd/Ni enables a completely pore free substrate to be obtained. If this thickness is reduced too much, there would be an increase in porosity, which with the porosity of the top finish increases the risk that a corrosion spot, which could cause contact failure, would result.

Thus by using the palladium-nickel alloy layer having a thickness in the range specified above, it is possible for the actual contacting layer of gold to be thin compared with what is normally used. In view of the high price of gold, it will be seen that a useful price saving results.

1. An electrical contact, which includes a substrate layer of a substantially non-corrodible alloy on a base metal surface, which substrate has a thickness in the range of 1.0 to 5 micrometres, and a layer of a non-corrodible precious metal on the substrate, which precious metal layer is thin compared with the substrate layer and provides the contact with good contact resistance and good wear-resistant properties.

2. An electrical contact which includes a substrate layer of a substantially non-corrodible palladium-nickel alloy on a base metal, which substrate layer has a thickness in the range of 1.5 to 5 micrometres, and a layer of gold which is also non-corrodible on the palladium-nickel alloy layer, which gold layer is thin compared with the substrate layer and provides the contact with good contact resistance and good wear-resistant properties.

3. A contact as claimed in claim 2, and in which the palladium-nickel alloy layer has a thickness of 2 micrometres and the gold layer has a thickness of 0.25 micrometres.

4. An electrical contact substantially as described herein.

New claims filed on 27 March 1985 New or amended claims: 5. An electrical contact, which includes a substrate layer of a substantially non-corrodible palladium-nickel alloy on a base metal substrate, the palladium content of the alloy layer being in the range of 70% to 85% and the thickness of the alloy layer being in the range of 1.5 to 5 micrometres, and a layer of gold which is also non-corrodible on

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. SPECIFICATION Gold plating This invention relates to electrical contacts, especially but not solely for electrical connections. The use of pluggable edge connections on printed wiring boards is well-known, and they need a reliable low contact resistance connection which can operate for decades. During this time the connection is made and unmade many hundreds of times, and has to resist the effects of aggressive environments. Hence careful selection of the materials used for the contacts is necessary. Hitherto relatively thick layers of gold have been used to seal off the substrate with no porosity, and to present a non-corrodible surface which has the low contact resistance and wear-resistant properties which would allow frequent sliding during matings. An object of the invention is to provide contacts which meet the above indicated needs, but in which the amounts of precious metals such as gold are reduced compared with the known arrangements. According to the invention there is provided an electrical contact, which includes a substrate layer of a substantially non-corrodible alloy on a base metal surface, which substrate has a thickness in the range of 1.0 to 5 micrometres, and a layer of a non-corrodible precious metal on the substrate, which precious metal layer is thin compared with the substrate layer and provides the contact with good contact resistance and good wear-resistant properties. Thus we provide a system in which the substrate, which is deposited directly onto the basic metal of the contact is a reasonably thin layer of a non-corrodible, non-porous, deposit to achieve the desired resistance to aggressive environments. This is then covered with a very thin, possibly porous, coating of a non-corrodible precious metal having the required contact reistance and wear properties. This latter layer, which is gold in the preferred arrangement, can be thought of as acting as a lubricant. Such an arrangement provides a finish which when used on edge-connector contacts on a printed wiring board has a contact reistance of a few milliohms. This contact resistance does not change by more than 5 milliohms when subjected to a mechanical endurance of 500 matings with an approved socket connector, followed by exposure tp 21 days of Industrial Atmosphere Test KC of IEC 68/BS2011. The contact resistance also does not change by more than 5 milliohms when subjected to ten days of Test Ca of IEC/BS2011 or to 2000 hours of Dry Heat Test at 1250C. When using gold as the outer, and "ultra thin" layer the substrate layer is an alloy of palladium and nickel with a palladium content of 70% to 85%, the thickness of this layer lying in the range of 1.5 to 5 micrometres. The gold layer is an electroplated layer of 98% minimum pure gold of hardness 120-200 VPN. In one specific example the palladium-nickel layer is 2 micrometres thick and the gold layer is 0.25 micrometres thick. The use of a thin gold layer, although micro-porous, presents a large equipotential surface for the mating contact to touch, and results in the low contact resistance of the intrinsic metal being manifested. This does not occur when mating directly on to the non-corrodible (Pd/Ni) substrate layer. The micro-porosity of the gold layer, while it is extensive, is rendered harmless by the non-corrodibility of the substrate. Thus corrosion products, which would adversely affect contact resistance are not produced. The choice of c/a layer of 2 micrometres of Pd/Ni enables a completely pore free substrate to be obtained. If this thickness is reduced too much, there would be an increase in porosity, which with the porosity of the top finish increases the risk that a corrosion spot, which could cause contact failure, would result. Thus by using the palladium-nickel alloy layer having a thickness in the range specified above, it is possible for the actual contacting layer of gold to be thin compared with what is normally used. In view of the high price of gold, it will be seen that a useful price saving results. CLAIMS

4. An electrical contact substantially as described herein.

New claims filed on 27 March 1985 New or amended claims:

5. An electrical contact, which includes a substrate layer of a substantially non-corrodible palladium-nickel alloy on a base metal substrate, the palladium content of the alloy layer being in the range of 70% to 85% and the thickness of the alloy layer being in the range of 1.5 to 5 micrometres, and a layer of gold which is also non-corrodible on the palladium-nickel alloy layer, which gold layer is thin compared with the palladium-nickel alloy layer, the arrangement of said layers being such as to provide the contact with good contact resistance and good wear-resistant properties.

6. A contact as claimed in claim 5, and in which the palladium-nickel alloy has a thickness of 2 micrometres and the gold layer has a thickness of 0.25 micrometres.