GB2092383A - Electrical contacts - Google Patents

Abstract

An electrical contact assembly comprises a spring carrier (11) on which a contact (12) is mounted, the latter comprising a conductive body formed by sintering a conductive ink composition. The ink may contain a precious metal e.g. Ag, Au, Pd; alumina; transition metal borides, carbides or nitrides e.g. TiC, TiB, TiN; glass compositions; ceramics; and semiconducting metal oxides e.g. SnO2, In2O3, ZnO, CdO. The materials in the ink may be in the form of fibres which may be parallel and normal to the contact surface. An apparatus for making the contact may include a laser or electron beam for sintering the ink. <IMAGE>

Description

SPECIFICATION Electrical contacts This invention relates to electrical contacts, and in particular to contacts formed from conductive ink compositions.

There is no single metal contact material which meets all requirements in terms of electrical, mechanical, chemical and wear performance. Most practical contacts are therefore combinations of materials in the form of alloys or composites which contain silver or gold as the main component for providing low contact resistance. The other constituents contribute to the contact properties by their inherent material properties or structure.

The important material properties of these other constituents include, high heatofvapourisation, low work function, hardness and chemical stability.

Alloys used for this purpose are normally obtained from the melt. Composites may be obtained from alloys by internal oxidation; by infiltration of liquid conductor into ceramic matrices or by cold pressing and sintering. Considerable further processing is involved before the final contact is in place and most of these processes are outside the control of the component manufacturer. Alloy contacts can also be formed directly by electroplating but limitations are imposed on the choice of materials available.

The object of the invention is to minimise or to overcome these disadvantages.

According to one aspect of the invention there is provided an electrical contact comprising a conductive body formed from a sintered electrically conductive ink composition.

According to another aspect of the invention there is provided an electrical contact unit, including first and second matable contacts each disposed on a respective contact carrier, and wherein one or each said contact comprises a conductive body formed from a sintered conductive ink composition.

According to another aspect of the invention there is provided a method of making electrical contacts, including depositing a quantity of a conductive ink composition on a selected region of a contact carrier, and sintering the ink to form a solid conductive body.

According to a further aspect of the invention there is provided an apparatus for manufacturing electrical contact assemblies, including means for applying a conductive ink to contact carriers, means for sintering the ink applied to each carrier to form a solid conductive contact, and means for directing the carriers sequentially to the ink applying and covering means.

Embodiments of the invention will now be described with reference to the accompanying drawings in which Figure 1 is a cross-section of an electrical contact assembly in which the contact comprises a body of a sintered conductive ink; Figure 2 shows an alternative contact construction; and Figure 3 is a schematic diagram of an apparatus for manufacturing contact of the type shown in Figure 1 or Figure 2.

Referring to Figure 1, the contact assembly shown comprises a conductive spring carrier 11 on which a contact 12 is mounted. Typically the carrier 11 has a domed portion 13 pressed out of the plane of the carrier and defining the contact region.

The contact 12 comprises a conductive body formed by sintering a conductive ink composition deposited on the contact region. Various inks may be employed for this purpose, but to minimise contact resistance, it is preferable that the ink should contain a precious metal such as gold, silver or palladium.

The ink may also contain a hard insulating powder, e.g. alumina, which improves the mechanical properties of the finished contact and in particular enhances its abrasion resistance.

The ink is sintered by the application of local heating to the contact region. In some applications the entire carrier and contact may be heated in an oven to effect sintering, but we prefer to am ploy local heating of the contact region only. This may be achieved by laser or electron beam heating on the contact region.

In the arrangement shown in Figure 2 the contact spring carrier 11 is provided with a rivet 14 located in an opening 15 in the carrier. The faces of the rivet are coated each with a layer 16 of a conductive ink composition which is subsequently sintered to form a contact surface. Such an arrangement is suitable for use e.g. as a changeover contact.

As the finished contact comprises a sintered mass of metal particles, the technique permits the use of metal weight ratios that cannot be obtained from alloy compositions and also allows the incorporation of non metallic materials. The contact composition can thus be 'tailored' according to the particular application in which the contact is to be used.

The ink composition may contain a variety of materials either as primary contact metals or as additives. Thus it may include one or more materials having a high heat of vapourisation, a high hardness or a high chemical stability. It may also incorporate one or more materials having a relatively low work function thus ensuring that the ohmic resistance of a made contact is low. It will be appreciated by those skilled in the art that various combinations of these material properties may be found in a single material or a plurality of materials. Suitable materials for incorporation in such ink compositions include inter alia transition metal borides, carbides or nitrides; e.g. TiC, TiB orTiN; glass compositions, ceramics, and semiconducting metal oxides e.g.SuO2, In203, ZnO or CdO. It will be clear that this list is in no way limiting and that other materials may also be incorporated.

The ink may contain such or other materials in fibreous form. This provides in effect a composite structure in which the fibres reinforce other relatively soft contact metals e.g. gold. The fibres may be randomly distributed in the ink or, advantageously, they may be aligned parallel to one another and e.g.

disposed normal to the contact surface. Such an arrangement has a high degree of wear resistance, whilst providing a high electrical conductivity along the fibre direction. This alignment of the fibres may be achieved e.g. by forcing the ink through a relatively narrow tube immediately prior to applica tion to the contact carrier followed by rapid sintering to prevent loss ofthe ordered structure.

Referring now to Figure 2, an apparatus for manufacturing sintered contact is shown in schema tic form. Contact carriers mounted on a bandolier 21 are fed through an inking station 22 where a body 23 of a conductive ink is applied to each carrier. The inked carriers are then fed to a curing station 24 where the ink bodies 23 are heated and sintered to form the finished contacts. Preferably this heating is effected by an electron beam or a high power laser, e.g. a Q-switched YAG laser.

After curing has been effected the bandolier is fed to S cropping station 24 where the contact carriers 9nrremoved for subsequent assembly e.g. in the construction of switches or relays.

In some applications provision may be made between the inking and curing stations for coining the ink droplet, i.e. for forming the ink body into a desired shape.

It will be clear that there is a variety of methods whereby the conductive ink may be applied to a contact carrier. Thus the ink may be applied from a metered jet, orfrom a daisy wheel having circum ferential openings each of which contains a quantity of the ink. In a further embodiment a plurality of contacts may be deposited simultaneously by silk screen printing.

Claims (23)

1. An electrical contact comprising a conductive body formed from a sintered conductive ink com position.

2. A contact as claimed in claim 1, wherein said sintered composition includes one or more precious metals.

3. A contact as claimed in claim 1 or 2, wherein said sintered composition includes one or more further solid materials.

4. A contact as claimed in claim 3, wherein said sintered composition includes alumina.

5. A contact as claimed in claim 1 or 2, wherein said sintered composition includes solid materials having a high heat ofvapourisation, a hardness and chemical stability, and low work function.

6. A contact as claimed in claim 3, 4 or 5, wherein said solid materials are fibrous in form.

7. A contact as claimed in claim 6, wherein the fibrous solid constituents are aligned normal to the contact surface.

8. A contact as claimed in any one of claims 3to 7, wherein said sintered composition includes one or more electrically conducting transition metal borides, carbides or nitrides.

9. A contact as claimed in claim 8 containing TiB, TiC, TiN, or mixtures thereof.

10. A contact as claimed in any one of claims 3 to 9, wherein said sintered composition includes one or more semiconducting metal oxides.

11. A contact as claimed in claim 10 and includ ing SnO2, ln203, ZnO, or CdO.

12. An electrical contact unit, substantially as described herein with reference to Figure 1 or 2 of the accompanying drawings.

13. An electrical contact unit, including first and second suitable contacts each disposed on a respective contact carrier, and wherein one or each said contact comprises a conductive body formed from a sintered conductive ink composition.

14. An electrical contact unit provided with one or more contacts as claimed in any one of claims 1 to 10.

15. Aspring contact carrier provided with a contact as claimed in any one of claims 1 to 10.

16. A method of making electrical contacts including depositing a quantity of a conductive ink composition on a selected region of a contact carrier, and sintering the ink to form a solid conductive body.

17. A method as claimed in claim 9, wherein said sintering is effected by laser or electron beam heating.

18. A method as claimed in claim 16 or 17, wherein said ink contains a fibrous material.

19. A method as claimed in claim 18, wherein said fibrous material is aligned along a predetermined direction.

20. A method of making electrical contacts substantially as described herein with reference to the accompanying drawings.

21. An electrical contact made of the method of any one of claims 16 to 20.

22. An apparatus for manufacturing electrical contact assemblies, including means for applying a conductive ink to contact carriers, means for sintering the ink applied to each carrier to form a solid conductive contact, and means for directing the carriers sequentially to the ink applying and curing means.

23. A contact assembly manufacturing apparatus substantially as described herein with reference to Figure 3 of the accompanying drawings.