GB2167610A

GB2167610A - Electrical spring test prod

Info

Publication number: GB2167610A
Application number: GB08523911A
Authority: GB
Inventors: Gustav Kruger
Original assignee: Feinmetall GmbH
Current assignee: Feinmetall GmbH
Priority date: 1984-09-27
Filing date: 1985-09-27
Publication date: 1986-05-29
Also published as: NL188186C; IT1185395B; GB2167610B; GB8523911D0; NL8502616A; ATA275985A; CH669465A5; IT8522294A0; FR2570830A1; FR2570830B1; AT401204B

Abstract

A coating (25, 31) on a piston (21) or cylinder (15) of a test prod, for improved contact therebetween, comprises a layer less than 2 mu m thick containing precious metal over an anti-diffusion layer. The coating may extend over the whole test pin (18). The cylinder may be made of bronze optionally Ag-coated. The piston may be of Cu/Be alloy. The anti-diffusion layer may be galvanically or chemically applied, be greater than 2 mu m thick, and consist of Ni, Ni alloyed with 4-5 wt% B or 8 wt.% P, or an alloy of at least 50 wt.% Ni. The diffusion layer may have a hardness of 400 Vickers. The outer layer may contain at least 50% precious metal and contain at least one of Rh, Pd, Ru, Os, Ir, Pt (e.g. Pd/Ni alloy containing 1-10 wt.%), or pure Au, or Au/Co alloy containing 3-8% Co. <IMAGE>

Description

SPECIFICATION Spring contact rod The invention relates to a spring contact rod for use in a device used for testing electrical components.

Devices of this type are used by electrically contacting the test piece with the spring contact rods of the device on a plurality of points simultaneously. The testing points may be very close together, and are obviously closer the narrower the conductors and the smaller the conductor spacings of the test piece. Devices of this type may also be used for measuring purposes, possibly for measuring already fitted printed-circuit boards or other electronic component parts.

In testing a test piece, the free ends of the contact pins, which may form tips or heads, are pressed onto the relevant points of the respective test piece with a good electrical contact. The pressing-on force may be in the region of approximately 0.8-5N and is applied by the spring of the relevant spring contact rod. The spring contact rod is used in this manner in order to provide good electrical contact with the relevant point to be tested and to form an electrical conductor with a low currentflow resistance.In the device, the rear end or the rear end region of the spring contact rod may be connected to a further electrical conductor, leading to an evaluator or evaluator circuit capable of evaluating the electrical currents or electrical voltages conducted from the spring contact rod following the application of an electrical voltage or electrical voltages during the testing of the test piece, thereby determining whether or not the test piece is electrically free of faults.

In order that the evaluator may operate perfectly, it is important that the conductors which connect it to the respective test pieces have low electrical resistance values, usually lying within relatively narrow limits; the narrower these limits and the lower the resistance values, the more favourable it is.

These resistance values are directly dependent on the current-flow resistance of the spring contact rods which also form the electrical conductors. The electrical current-flow resistance of a spring contact rod is its ohmic resistance from the free end of its contact pin to its rear connection end. This current-flow resistance is directly dependent upon the ohmic resistance across the contact surface between the piston of the contact pin and the cylinder of the cylinder member.

In the case of a known spring contact rod of this type the cylinder is made of bronze. The base of the contact pin is made of steel and is coated with a 0.5 Am nickel layer. A 2 Fm gold layer made of hard gold (gold-cobalt alloy) is then applied to the nickel layer. The nickel layer acts as a diffusion barrier, which prevents the diffusion of gold into the base. The current-flow resistance of such known spring contact rods measured under operational conditions, increases, probably as a result of the wear of the piston, with an increase in the number of load alternations. Load alternation is understood to mean a single operational loading of the contact pin of the spring contact rod.In the case of spring contact rods with this design which were examined, the electrical current-flow resistance thereof increased constantly after several hundred thousand load alternations with an increasing number of further load alternations, and at 1.6 x 106 load alternations was far above the initial current-flow resistance, more particularly 50% or more above. Although the current-flow resistance of these known contact rods was relatively low, it is desirable to obtain even lower currentflow resistances.

The invention provides an improved spring contact rod having a particularly low electrical currentflow resistance and/or does not vary its electrical current-flow resistance over a maximum number of load alternations.

The invention provides a spring contact rod as claimed in claim 1.

The electrical current-flow resistance of the spring contact rod may be reduced by means of a large reduction in the thickness of the outer layer, in which case a large increase in the thickness of the inner layer may be provided.

By implementing the above changes in thickness of the coating layers, the invention allows for a considerable reduction of the electrical current-flow resistance. Despite the reduction of the thickness of the outer layer, the wear of this layer is extremely slight and may be greatly reduced.

The invention also allows the electrical currentflow resistance of the spring contact rod to vary less with an increasing number of load alte;iia- tions.

The outer layer may be provided according to any one of the claims 3 to 7, 15, 23 or 25 to 44.

It is important that the periphery of the piston of the contact pin and/or the piston path of the cylinder of the cylinder member is provided with a new type of coating. If the entire surface of the contact pin is coated according to the invention, the outer layer may wear away at the free end of the contact rod relatively quickly, which does not however have an effect in practice on the electrical currentflow resistance of the spring contact rod.

The invention thus provides a plurality of effects.

Although the invention is particularly advantageous where the outer layer is thinner than the inner layer, the invention is not restricted to this, since it is also conveivable in a few cases for the inner layer to be thinner than the outer layer.

The thickness of the outer layer is preferably a maximum of 1.5 lim, advantageously a maximum of 1 IJLm, more particularly 0.3 to 1.0 Fm. It is particularly favourable when the thickness of the outer layer is a maximum of 0.7 Fm. The thickness of the outer layer may also expediently be at least 0.2 lim. It is particularly advantageous when the thickness of the outer layer is approximately 0.3 to 0.6 pWm, preferably in the region of 0.4 to 0.5 Fm, more particularly approximately 0.5 lim. The thickness of the outer layer may also be less than 0.2 lim. A particularly favourable thickness of the outer layer is 0.4 to 0.6 Fm.

The thickness of the diffusion barrier layer, ie the inner layer, may advantageously be equal to or greater than 2 lim. It is particularly favourable if the thickness of this inner layer is a maximum of approximately 7 lim. Preferably values for the thickness of the inner layer are 3 to 6 Wm, more particularly approximately 5 ilm.

The inner layer, forming a diffusion barrier for at least the precious metal atoms of the outer layer, may in many cases advantageously be pure nickel or nickel with admixtures, preferably a nickel alloy or nickel based alloy, particularly expediently hard nickel. Where this inner layer is applied galvanically to the contact pins or in the cylinder member, it may expediently be pure nickel. Pure nickel layers are also called mat nickel layers, since they comprise so-called serrated surfaces.

The inner layer may also be a chemically applied layer, in which case other atoms may be interposed between the nickel, eg boron or phosphorus. The boron may be present to the extent of 4 to 5% by weight and the phosperous to the extent of approximately 8% by weight. Methods of applying layers of this type and the corresponding baths used are known. A nickel layer may be formed with the admixtures of boron or phosphorus in a cauliflower structure or a matrix structure. Layers of this type are particularly suitable for the inner layers of the spring contact rods according to the invention.

The inner layer may also be made of other substances containing nickel as a constituent or a predominant constituent. The inner layer may advantageously comprise at least 50% by weight of nickel, preferably at least 90% by weight of nickel.

It has proved particularly advantageous for the inner layer to comprise a relatively high degree of hardness. Its hardness may be at least 300 Vickers.

Even more favourable are higher degress of hardness, preferably hardnesses of at least 400 Vickers, particularly favourably at least 600 Vickers. Hardnesses in the region of 800 to 1000 Vickers are particularly advantageous. Higher degrees of hardness, up to approximately 1200 Vickers, may also be favourable. The hardness of the nickel layer may be increased by additions to the nickel, more particularly by adding the above-mentioned boron atoms or phosphorus atoms.

Spring contact rods of this type, to which the invention relates, have very small diameters, in order that they may be disposed in a very dense arrangement. The maximum diameter of the cylinder member may be approximately 0.4 to 3 mm and the diameter of the piston of the contact pin may be 0.2 to 2 mm.

The base of the contact pin may be made of steel or copper-beryllium. The cylinder of the spring contact rod may be made of bronze, and possibly also be coated on the inside and/or on the outside, preferably with a thin layer of silver. It is also conceivable to use other metals for the base of the contact pin and for the base of the cylinder.

It is usually sufficient, and particularly favourable for low manufacturing costs, if only the contact pin has a coating according to the invention. This coating may cover the entire contact pin or only one or more areas of the contact pin. Thus it is advantageous and sufficient, at least in many cases, to provide only the piston of the contact pin or only the periphery of the piston of the said contact pin with a coating according to the invention.

It is however also possible and similarly advantageous to provide the cylinder, preferably the piston path thereof, with a coating according to the invention. In this case, a coating according to the invention of the contact pin is then not necessarily required, but may be similarly advantageous provided in many cases.

An embodiment of the invention is shown in the drawing, in which: Figure 1 shows a spring contact rod according to an embodiment of the invention partially in fragmented form and partially in longitudinal section.

Figure 2 is a greatly enlarged section of the contact pin of the spring contact rod shown in Figure 1.

In Figure 1, the reference 10 represents the flat plate of a mounting indicated by a dot-dash line, in which plate a plurality of spring contact rods may be rigidly inserted, one spring contact rod 11 being shown in longitudinal section. The spring contact rod 11 comprises a cylinder member 14, a contact pin 18 arranged inside the cylinder 15 and a prestressed compression spring 19 acting upon the contact pin 18 in an outward direction, and resting against the interior space base 16. A metal extension rod 12, being somewhat smaller in diameter, is also integrally attached to the closed rear end of the tube-shaped cylinder 15.

All parts of the spring contact rod 11 are advantageously made of metal or metal alloys. The cylinder member 14, of which the extension rod 12 may often be dispensed with, may be advantageously integrally formed and made of a metal which is a good electrical conductor, preferably bronze or silver-plated bronze.

The contact pin 18 comprises a piston 21, a shaft 22 and an enlarged head 24 being pointed at its free end, and used for contacting testing points of test pieces, one of which is shown in section by a dot-dash line 23. The contact pin 18 comprises an integrally formed metal base 20, which may be made of steel or copper-beryllium and is provided in this embodiment with a coating 25 according to the invention over its entire surface, which is more particularly also present on the periphery of the piston 21.

The coating 25 comprises a thin diffusion barrier layer (inner layer) 26, directly applied to the base 20 in a chemical, galvanic or other manner, and a thin outer layer 27, similarly directly applied to the inner layer 26 in a chemical, galvanic or other manner. The inner layer 26 comprises pure nickel, or a substance comprising nickel as the main component, preferably a nickel alloy, more particularly hard nickel. The inner layer 26 preferably has a relatively high degree of hardness. The hardness may be at least 300 Vickers, more particularly at least 400 Vickers, and particularly expediently 600 Vickers and above. Where the said inner electrically conductive layer 26 is not made of pure nickel, its proportion of nickel may be at least 50% by weight. The outer layer 27, which is similarly electrically conductive, is made of precious metal or an alloy comprising a precious metal.The thickness of this outer layer 27 is less than 2 lim.

The thicknesses and the materials of the outer layer 27 and the inner layer 26 are such that the electrical current-flow resistance of the spring contact rod 11, measured from the free end 32 of the extension rod 12, is considerably less than if the inner layer 26 were to have a thickness of 0.5 im and the outer layer 27 a thickness of 2 mm. The coating according to the invention may be formed such that the electrical current-flow resistance of the spring contact rod is a maximum of 0.7, preferably a maximum of 0.4, and particularly expediently only approximately 1/3 to 1/5 or less of the electrical current-flow resistance of a similarly formed spring contact rod, having an inner layer thickness of 0.5 am and an outer layer thickness of 2 slum.

In this embodiment, the test piece 23 comprises as a testing point 29 part of a conductor path or the like, connected by means of the spring contact rod 11 for the purpose of electrical connection with an evaluator (not shown) of the testing device comprising the spring Contact rod 11. An electrical conductor 30 leads from the end of the extension rod 12 to the evaluator. During a testing procedure, the plate 10 of the printed-circuit board 23 is displaced such that, via the point 29 to be tested of the conductor plate 23, with which the contact head 24 comes into contact, the contact pin 18 is pressed a few millimetres to the left compressing the spring 19, the circular cylindrical piston 21 sliding on the circular cylindrical piston path 31 of the cylinder 15.As soon as this movement of the plate 10 or the printed circuit board 23 has ended, an electrical testing voltage is applied and a testing current flows over the entire length of the spring contact rod and is evaluated in the evaluator. The electrical current-flow resistance is essentially determined by the electrical resistance across the contact surface between the piston 21 and the cylinder 15.

Due to the coating 25 according to the invention, the spring contact rod 11 has an extremely low electrical current-flow resistance, which remains substantially constant after a large number of load alternations. Furthermore, the wear of the outer layer 27 on the peripheral surface of the piston 21 is extremely slight, so that the invention also makes it possible to obtain very long service lives of the spring contact rod, which was not possible with the known spring contact rods described above.

Due to the axial loading of the piston 21 by the spring 19, the piston rests against the piston path 31 of the cylinder 15 with little force. Inspite of the resulting low bearing pressure, the invention allows extremely low current-flow resistances of the spring contact rod 11, which were previously not possible.

On account of the small amount of precious metal required for the outer layer 27, the coating 25 is also economical.

An alloy is understood to be a substance comprising a plurality of components having the properties of metals, ie also having electrical conductivity. The alloy may be made exclusively of metal. It may however be expediently provided that the alloy comprises, in addition to at least one metal, at least one half-metal or non metal. The non-metallic or half-metallic components or at least one of the non-metallic or half-metallic components of the alloy, in cases where it comprises at least one component of this type, may preferably also be an element or elements, such as phosphorus, boron, hydrogen or the like, or in some cases a chemical compound or compounds. The outer layer 27 of the coating 25 may preferably comprise at least 50% by weight of precious metal, particularly expediently at least 90% by weight of precious metal.

The inner and also the outer layer may often be expediently free of impurities, but may also contain small quantities of impurities.

Claims

1. A spring contact rod for use in a device for testing electrical components comprising a cylinder member, having a cylinder, a spring-loaded contact pin having a piston slidable in the cylinder, an electrically conductive coating between the piston and the cylinder, the coating comprising an inner layer, acting as a diffusion barrier, and an outer layer comprising at least one precious metal and having a thickness of less than 2 r*m, the outer layer being applied to the inner layer.

2. A spring contact rod as claimed in claim 1, wherein the thickness of the inner layer of the coating is greater than 0.5 wm.

3. A spring contact rod as claimed in claim 1 or 2, wherein the thickness of the inner layer and the outer layer are such that the electrical current-flow resistance of the spring contact rod is considerably less than if the thickness of the inner layer were 0.5 ijm and the thickness of the outer layer were 2 Fm, with the composition remaining the same in each case.

4. A spring contact rod as claimed in any one of the preceding claims, wherein the outer layer is thinner than the inner layer.

5. A spring contact rod as claimed in any one of the preceding claims, wherein the thickness of the outer layer is a maximum of 1.5 Fm, preferably a maximum of 0.7 Fm.

6. A spring contact rod as claimed in any one of the preceding claims, wherein the thickness of the outer layer is at least 0.2 Fm, preferably approximately 0.5 Fm.

7. A spring contact rod as claimed in claim 5 or 6, wherein the thickness of the outer layer is approximately 0.3 to 0.6 ijm, preferably approximately 0.4 to 0.5 Fm.

8. A spring contact rod as claimed in any one of the preceding claims, wherein the thickness of the inner layer is at least 2 Fm, preferably greater than 2 Fm.

9. A spring contact rod as claimed in any one of the preceding claims, wherein the thickness of the inner layer is a maximum of 7 lim.

10. A spring contact rod as claimed in claims 8 and 9, wherein the thickness of the inner layer is approximately 3 to 6 Fm, preferably approximately 5 Fm.

11. A spring contact rod as claimed in any one of the preceding claims, wherein the inner layer is made of hard nickel.

12. A spring contact rod as claimed in any one of claims 1 to 10, wherein the inner layer is made of pure nickel (mat nickel).

13. A spring contact rod as claimed in any one of the preceding claims, wherein the outer diameter of the piston of the contact pin of the spring contact rod is 0.2 to 2 mm.

14. A spring contact rod as claimed in any one of the preceding claims, wherein the base of the contact pin is made of steel or copper-beryllium.

15. A spring contact rod as claimed in any one of the preceding claims, wherein the inner layer and/or the outer layer of the coating is an alloy or are alloys.

16. A spring contact rod as claimed in any one of the preceding claims, wherein the cylinder of the cylinder member is made of bronze or silverplated bronze.

17. A spring contact rod as claimed in any one of the preceding claims, wherein the inner layer is entirely or substantially made of nickel and phos phorus.

17. A spring contact rod as claimed in any one of the preceding claims, wherein the hardness of the inner layer is at least 300 Vickers, preferably at least 400 Vickers.

18. A spring contact rod as claimed in claim 17, wherein the hardness of the inner layer is at least 600 Vickers, preferably 800 to 1000 Vickers.

19. A spring contact rod as claimed in claim 11, wherein the hard nickel of the inner layer is entirely or substantially made of nickel and boron.

20. A spring contact rod as claimed in claims 1 to 11 or 13 to 19, wherein the inner layer is entirely or substantially made of nickel and phosphorus, and is preferably a hard nickel layer.

21. A spring contact rod as claimed in claim 19, wherein the proportion of boron is 4 to 5% by weight.

22. A spring contact rod as claimed in claim 20, wherein the proportion of phosphorus is approximately 8% by weight.

23. A spring contact rod as claimed in claim 5, wherein the thickness of the outer layer is equal to or less than 1 Fm, preferably less than 0.6 lim.

24. A spring contact rod as claimed in any one of the preceding claims, wherein the inner layer comprises at least 50% by weight, preferably 90% by weight of nickel.

25. A spring contact rod as claimed in any one of the preceding claims, wherein the outer layer comprises at least 50% by weight, preferably at least 90% by weight of precious metal or precious metals.

26. A spring contact rod as claimed in any one of claims 1 to 25, wherein the outer layer comprises silver.

27. A spring contact rod as claimed in claim 26, wherein the outer layer is a silver-gold alloy.

28. A spring contact rod as claimed in claim 26, wherein the outer layer is made of silver.

29. A spring contact rod as claimed in claim 26, wherein the outer layer is a silver-palladium alloy.

30. A spring contact rod as claimed in claim 26, wherein the outer layer is a silver-copper alloy.

31. A spring contact rod as claimed in any one of claims 1 to 26, wherein the outer layer comprises at least one platinum metal.

32. A spring contact rod as claimed in claim 31, wherein the outer layer comprises a single platinum metal.

33. A spring contact rod as claimed in claim 31 or 32, wherein the platinum metal or at least one platinum metal is rhodium.

34. A spring contact rod as claimed in claim 31, 32 or 33, wherein the platinum metal or at least one platinum metal is palladium.

35. A spring contact rod as claimed in any one of claims 31 to 34, wherein the platinum metal or at least one platinum metal is ruthenium.

36. A spring contact rod as claimed in any one of claims 31 to 35, wherein the platinum metal or at least one platinum metal is osmium.

37. A spring contact rod as claimed in any one of claims 31 to 36, wherein the platinum metal or at least one platinum metal is iridium.

38. A spring contact rod as claimed in any one of claims 31 to 37, wherein the platinum metal or at least one platinum metal is platinum.

39. A spring contact rod as claimed in any one of the claims 31 to 34, wherein the outer layer is a palladium-nickel alloy.

40. A spring contact as claimed in claim 39, wherein the nickel content of the palladium-nickel alloy is 1 to 10% by weight.

41. A spring contact rod as claimed in any one of claims 1 to 27, 31 or 33 to 38, wherein the outer layer comprises gold.

42. A spring contact rod as claimed in claim 41, wherein the outer layer is substantially made of gold, preferably pure gold.

43. A spring contact rod as claimed in claim 41, wherein the outer layer is made of hard gold, preferably a gold-cobalt alloy with preferably 3 to 8% by weight of cobalt.

44. A spring contact rod as claimed in any one of the preceding claims, wherein the thickness of the outer layer is approximately 0.3 to 1.0 ,um, preferably approximately 0.4 to 0.6 ,ugm.

45. A spring contact rod as claimed in any one of the preceding claims, wherein the base of the contact pin is integrally formed.

46. A spring contact rod substantially as herein described with reference to the accompanying drawings.

Amendments to the claims have been filed, and have the following effect: (a) Claims 1, 11, 12, 17, 20 above have been deleted or textually amended.

(b) New or textually amended claims have been filed as follows: (c) Claims 13 - 16, 18, 19, 21-46 above have been re-numbered as 11-14, 15, 16, 18-43 and their appendancies corrected.

1. A spring contact rod for use in a device for testing electrical components comprising a cylinder member, having a cylinder, a spring-loaded contact pin having a piston slidable in the cylinder, an electrically conductive coating between the piston and the cylinder, the coating comprising an inner layer made of hard nickel, the inner layer acting as a diffusion barrier and having a hardness of at least 400 Vickers, and an outer layer comprising at least one precious metal and having a thickness of less than 2 Am, the outer layer being applied to the inner layer.