US3495061A - Contacts for reed switches - Google Patents

Description

Feb. 10, 1970 s. s. COFFIN ETAL 1 CONTACTS FOR REED SWITCHES Filed July 11, 1968 48 VOLTS,165 MILLIAMP LOAD O WITH ARC SUPPRESSION FAILURE TYPICAL TIN -N|CKEL 10 5 1 0 2o 5o 55 5o so s5x1o INVENTORS SPENCER S. COFFIN ROBERT J. NANCE ATTORNEY United States Patent 3,495,061 CONTACTS FOR REED SWITCHES Spencer S. Collin and Robert J. Mance, Poughkeepsie,

N .Y., assignors to International Business Machines Corporation, Armonk, N.Y., a corporation of New York Filed July 11, 1968, Ser. No. 744,110 Int. Cl. H01h 1/02 US. Cl. 200--166 4 Claims ABSTRACT OF THE DISCLOSURE Switch contacts consisting of a thin electroplated layer of an inter-metallic alloy of tin-nickel are provided in a reed switch of the type where the contacts are enclosed in a sealed envelope containing an inert atmosphere. The inter-metallic alloy consists of substantially 65% tin and 35% nickel to give improved life in use under high current and arcing conditions.

The invention relates to reed switches, and more particularly, to improved contacts for reed switches which provide a longer life under high current and high arcing conditions.

Reed switches and reed relays are well known in the prior art. They consist essentially of thin reeds located in an inert atmosphere provided within a sealed glass envelope. The reeds extend from opposite ends of the glass envelope and are sufficiently long that the tips thereof overlap to form contacting surfaces. The reeds are made of a resilient magnetic material such that a magnetic field will cause movement of the reeds to close the contacts.

The contacts of reed switches or relays are generally made of a layer or layers of materials such as rhodium, gold, silver, etc. The main advantage of these contact materials is that they afford a stable contact resistence throughout their life. However, in high current high areing applications the known contact materials tend to transfer material from one contact to another at a particular point on the contacts. This results in the building of a narrow pit and opposing long spire at the point which eventually causes failure of the contacts. Another disadvantage of the known contact materials is that they generally cannot be cheaply electroplated. Some materials, such as tungsten, are very difiicult to electroplate. In the miniature reed relay, the area over which the electroplating of a layer of contact material is to be provided is exceedingly small. It is also necessary to provide very close control of the plating operation so that a uniform thin layer can be obtained. It is necessary to maintain the layer thin so that the magnetic operation and response time of the reed switches is not interfered with.

Accordingly, it is the main object of the present invention to provide a contact material for a reed switch which gives longer life and more reliable operation under high current and high arcing conditions.

It is another object of the present invention to provide an improved contact material which can be relatively cheaply electroplated in thin uniform layers.

Briefly, the invention provides improved contacts in a switch of the type having a pair of relatively movable magnetic reed elements positioned to overlap at a portion thereof. The overlapping portions of the magnetic reed are enclosed in an envelope containing an inert atmosphere. The contacts consist of a thin, uniform coating of tin-nickel applied to the facing surfaces of the overlapping portions of the relatively movable reed elvments. This provides a more reliable longer life switch for use under relatively high current and arcing conditions.

The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of a preferred embodiment of the invention, as illustrated in the accompanying drawings.

FIG. 1 is a vertical cross sectional diagram of a reed switch.

FIG. 2 is a blown up perspective view of one of the contacts of the switch.

FIG. 3 is a graph of the percent failure versus number of operations for switches having tin-nickel and rhodiumon-gold contacts.

Referring to FIG. 1, there is shown a cross sectional representation of a reed switch. The switch consists essentially of a sealed glass envelope 12 within which is contained an inert atmosphere 14. One atmosphere utilized in reed switches and relays is a mixture of helium and nitrogen i.e., helium, 20% nitrogen. A pair of reeds 16 extend into the glass enclosure, one from each end thereof. The reeds 16 are sealed at one end in the glass 12 with their inner ends overlapping slightly. The reeds 16 are made of a magnetic material such as nickeliron. One of the important features of the reeds is that they be sufficiently resilient to respond to a magnetic field. The nickel-iron reed is generally utilized since it has the same co-eificient of expansion as readily available glass so that no problem arises from the sealing of the reed in the glass envelope. The nickel-iron makes a poor contact surface. The overlapping contact portion 18 of the reed 16 made of the nickel-iron alloy produces a rather high contact resistance.

Another difiiculty with the use of nickel-iron is its inherent quality of easily establishing fine bridging in switching a current from on to off." Consequently, the high resistance and fine bridging occurs in a comparatively early state of operation and will culminate in the inability of the contacts 18 to separate. Accordingly, it has been the practice to provide materials on the overlapping portions of the reeds to form contacts 18 which have a low contact resistance and which minimize the fine bridging.

The present invention provides a thin, uniform layer of tin-nickel 20 on the opposiing overlapping portions of the reeds 16. It has been found by experimentation, that a 65% tin and 35% nickel compound or inter-metallic alloy is best suited to give a higher reliability and a long life characteristic to the switch especially under high current and arcing conditions. The percentages of the materials of the composition may vary from the above amounts somewhat without interferring with the operating characteristics. The thickness limitations of the layers 20 are dictated by the requirements of the switch. Since the switch is operated in response to magnetic energization, the non-magnetic material contact must be sufficiently thin as to not interfere with the flux path to any significant degree. Where the magnetic characten'sics are less critical, the tin-nickel layer may be thicker, however, the thickness is limited by intertia characteristics which affect the response time.

For good switch operation, it is necessary to have uniform contact surfaces. Thus, it is necessary to control the thickness of the layer of material applied to form uniform contacts 18. One way the necessary uniformity can be obtained is thin electroplating. Various materials such as tungsten, rhodium, etc. cannot be easily and/or cheaply electroplated. This is especially true, when the small surface area over which the electroplating is to take place is considered. In FIG. 2, the lengthwise dimension y, which is the length of the overlapping portions of the reed, is about 15 mils for the miniaturized reed relay.

The contacts 18 of the reed switch, in operation, transfer material from one contact to the other. In the prior art contact materials, the transfer from one contact to the other during operation tends to take place at a single point. At this single point between the contacts, a pit and opposing spire or bridge tends to build up by means of which the material is transferred. This bridging or spire continues to grow until the contacts weld or are short circuited, thus, producing unreliable operation and an early failure of the switch. In the present invention, the tin-nickel inter-metallic alloy contacts 18 transfer the material from one contact to the other during operation, however, the transfer does not appear to take place at a single point, as was the case in the prior art, but the point seems to move around the contact, thus, making a wider pit and mound rather than a thin spire. This characteristic is believed to be the cause of the longer life and better reliability of the switch.

The tin-nickel inter-metallic alloy contacts 18 provide the above advantages in operation as well as being applicable by electroplating. The process which is used to electroplate the tin-nickel layer 20 onto the overlapping areas of the reeds 16 of the reed switches consists generally of thoroughly cleaning the area to be plated. This is done by electrochemical cleaning. After the area is thoroughly rinsed, it is subject to a chemical etch cleaning, after which it is again thoroughly rinsed. Once the area has been thoroughly cleaned, it is immersed in an electroplating bath. The bath used is the standard Parkinson bath for electroplating tin-nickel as reported by N. Parkinson, The Electrodeposition of Bright Tin-Nickel Alloy, Electrodepositors Technical Society Journal, 1950-51.

In order to provide an example of the improvement obtained with the tin-nickel inter-metallic alloy contacts 18, a graph of millions of operations versus cumulative failure in percent for reed switches using tin-nickel contacts and prior art rhodium-on-gold contacts is shown in FIG. 3. As can be seen from the graph, the cumulative failure in percentage of the switches having tin-nickel contacts is per 65 million operations while the failure of the switches having the prior art rhodium-on-gold contacts is 40% per 10 million operations. Somewhat the same comparatively high failure rates are obtained with various other known prior art contact materials when used in high energy applications. The graph was obtained using a 48 volt, 0.165 ampere inductive circuit.

Neither the tin or nickel alone provides the above advantages, but only the combination of the tin and nickel in an inter-metallic alloy 20 forming the contacts 18 in a reed switch or relay has been found to provide the improved reliability and long life characteristics.

While the invention has been particularly shown and described with reference to a preferred embodiment thereof, it will be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the invention.

What is claimed is:

1. In a switch having a pair of relatively movable magnetic reed elements positioned to overlap at a portion thereof and enclosed in an envelope containing an inert atmosphere, a thin uniform coating of tin-nickel applied to the facing surfaces of said overlapping portions of said relatively movable reed elements forming contacts thereon, whereby the life and reliability of the switch is increased in use under relatively high current and arcing conditions.

2. A switch according to claim 1, wherein said tinnickel contacts contain approximately tin and 35% nickel.

3. A switch according to claim 1, wherein said tinnickel coating thickness is primarily limited by the magnetic operating characteristics of said switch.

4. A switch according to claim 1, wherein said tinnickel contacts are applied by an electroplating process to obtain the uniform desired thickness.

References Cited UNITED STATES PATENTS 2,882,648 4/1959- Hovgaard et al.

3,007,855 11/1961 Ellwood.

3,222,486 12/1965 Moriyama et al.

3,251,121 5/1966 Prival.

3,426,302 2/1969 Lindenberger 29622 X 3,431,377 3/1969 Barlow et al.

HERMAN O. JONES, Primary Examiner U.S. Cl. X.R. 29622

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