GB2152287A - Contact unit of a push button switch - Google Patents

Abstract

A contact unit of a push button switch comprises a fixed contact arrangement and a pushable button member carrying a movable contact (22). The fixed contact arrangement consists of an insulating substrate (34) to which a pair of electrically relatively low conductive layers (32,33) are applied. An insulating layer eg a soldering resist layer (35) is applied over the surface of the substrate (34) and the conductive layers (32,33) except for the central portions of the latter and electrically high conductive carbon layers (36,37) are applied substantially onto the central portions of the low conductive layers (32,33) so as to define a distance between the adjacent high conductive layers (36,37) which is larger than that between the adjacent low conductive layers (32,33) whereby the edge portions of the low conductive layers (36,37) are prevented from being exposed, a required low contact resistance can be achieved at the top surface of the low conductive layers, and a high wiring density of the switch and the circuit can be ensured. <IMAGE>

Description

SPECIFICATION Contact unit of a push button switch This invention relates to a contact unit of a push button switch.

A plurality of switches of this kind are frequently arranged as so-called key switches, for example, on the operating surface of key telephone sets and the like, in which case the contact unit can be closed to form a predetermined circuit in known manner when a push button ofthe switch is depressed.

A push button switch is disclosed in Japanese Utility Model Application Laid-Open Publication No. 37763/ 1974, in which electrically conductive layers of copper foil and forming fixed contacts are printed on an insulating substrate and a carbon layer is applied by a screen printing process all overthetop contacting surface ofthe respective conductive layers, whereby the electric conductive property at the top surface of the conductive layers is improved to reduce the contact resistance at the time of closing the contact unit. With such application of the carbon layer onto the top surface of the conductive layer, a well known buta highly expensive measureforthe contact resistance reduction by applying a gold layer onto the conductive layer could be replaced and an inexpensive contact unit could be provided.

However, the arrangement disclosed in this Japanese Laid-Open Publication involves a problem that, when the carbon layer applied onto the conductive layer is in lack of viscosity, the carbon layertends to deteriorate easily and even to be chipped off at its edge portions during operation ofthe unit, so that corresponding edge portions ofthe conductive layer will be exposed, due to the removal of the carbon layer, and subjected to oxidation or corrosion.Further, an insulation distance of 0.3 mm will be sufficient to be provided between adjacent ones ofthe conductive layers of copperfoil but the conductive carbon layers which are highly conductive require an insulation distance of more than 0.6 mm and, in an arrangement ofthe carbon layer applied all over the outer surfaces ofthe conductive layer including its all side end surfaces, it has been necessary to separate the copper layers from each other by at least0.6 mm, due to which the wiring density per unit area of insulating substrate on which the switch and circuit are printed is low. If, in this case the width of the conductive later itself is lessened, then the wiring density can be increased butthe contact resistance atthetime of closing the contact unit is correspondingly increased so that there arises a risk of breaking an associated circuit.

A primary aim of the present invention is, therefore, to provide a contact unit of a push button switch which ailows the exposure ofthe outersurfaces of conductive layers forming fixed contacts on an insulating substrate to be reliably avoided, thereby eliminating any possibility oftheoxidation of the conductive layers, and to remarkably increase the wiring density on the insulating substrate while attaining a sufficient width ofthe respective conductive layers.

Accordingly, the present invention provides a contact unit of a push button switch in which an electrically high conductive contact member is provided on the bottom surface of a pushable button member, a pair of electrically relatively lower conductive layers between which an electric path is formed through said contact member being brought into contact therewith are provided on an insulating substrate and spaced from each other by a predetermined first distance, and electrically relatively higher conductive carbon layers, wherein an insulating covering layer is provided on said insulating substrate to cover at least from an area between the pair of said lower conductive layers to edgewise top surface portions along opposing side edges of the respective lower conductive layers, and said carbon layers are provided at least on the top surfaces of the lower conductive layers not covered with said insulating covering layer so that the carbon layers are spaced from each other at their opposing side edges by a second distance which is larger than said first distance.

Preferably, the insulating covering layer is of a soldering resist. The soldering resist layer is desirably provided on said insulating substrate to overall of said edgewise top surfaces, except the central portions, of said pair of lower conductive layers which consist of a copper foil. The carbon layers are desirably provided to expand at their edges over the top surface of said resist layer on said lower conductive layers. In this case, because the soldering resist material has a mechanical strength sufficiently higher than carbon material, there will not occur any problem of being chipped off at the end edge portions of the resist layer disposed on the edgewise portions ofthe conductive layers.In addition, since the width of the applied carbon layer can be made smaller than that of the conductive layer, the distance between the adjacent conductive layers can be remarkably reduced to be less than 0.6 mm. Preferably,the first distance between the pair of conductive layers is less than 0.6 mm but greaterthan 0.3 mm.

The invention will now befurtherdescribed, byway of example, with reference to the drawings, in which: FIG. lisa schematic fragmental section ofthe operating surface part of a telephone set or the like, showing a known push button switch employing a contact unit ofthe kind referred to; FIG. 2 is a fragmental section, to an eniarged scale, of a fixed contact section in a contact unit according to the present invention, with a movable contact section schematically shown by chain-dotted lines; FIG. 3 is a fragmental plan view showing a pattern of the fixed contact section according to an embodiment of the present invention; and FIG. 4 is also a fragmental plan view showing a pattern of the fixed contact section according to another embodiment ofthe invention.

For a better understanding ofthe present invention, the entire arrangment of a known push button switch will first be described with reference to FIG.1 ofthe drawings. The known push button switch 10 comprises a fixed contact section 11 and a movable contact section 12. The fixed contact section 11 includes a pair offixed contacts formed by electrically conductive layers 14 and 15 respectively madeofa copperfoil adhered onto an insulating substrate 13.While in the drawing one of the fixed contacts is shown as divided into two as represented by the conductive layers 14, they are formed in practice in a single continuous layersurrounding the other fixed contact denoted by the layer 15 shown as centrally disposed, and the respective layers 14 and 15 are inserted in an electric path of an associated electric circuit (not shown) to be opened and closed by the switch.

On the other hand, the movable contact section 12 comprises a push button member 16 of an electrically insulating material and forming a key top of respective keys on the operating surface of a key telephone set or the like. Afollower 17 made of an insulating and resilient material such as a rubber is fixedly mounted on the lowersurface ofthe button member 16 disposed above the conductive layers 14and 15. The follower 17 is integrally coupled through a flexible portion 20 with a plate-shaped spacing member 19 made ofa rubberorthe like, which member is disposed between the insulating substrate 13 and a top plate 18forming the operating surface ofthe telephone set orthe like.With such an arrangement, the button member 16 is resiliently supported by the plate-shaped member 19 through the follower 17 and flexible portion 20, and projects out of an aperture 21 of th e top plate 18. Therefore, the button member 16 can be depressed to come into engagement with the conductive layers 14and 15. Secured to the lower surface of the follower 17 is a contact piece 22to be brought into direct contact with the conductive layers 14 and 15. More specifically, the contact piece 22 is made of a rubber material containing powdery carbon or the like to be electrically conductive and disposed to be engageable resiliently with the conductive layers l4and 15.

With the push button switch 10 arranged as shown in FIG. 1, the button member 16 is manually depressed to shift the button member 16 downwards, the contact piece 22 is resiliently brought into engagement with the pair of conductive layers 14and 15 across the respective layers so that an electric path is formed between them through the contact piece 22 which is sufficiently conductive.

In this type of push button switch, it is preferable that, in orderto remarkably reduce the contact resistance with respect to the contact piece of the movable contact section to be negligibly small, at least the top contact surface ofthe conductive layers ofthe fixed contact section is provided with a high electric conductivity, while it has been demanded to eliminate the foregoing problems in the prior artwith an inexpensive arrangement.

According to a uniquefeature ofthe present invention,a high electricconductivitycan be provided to the contacting surface zone of the conductive layers formingthefixed contact section forengagement therewith ofthe movable contact piece 22 at a lower costthan in the case of using gold orthe like, and a high wiring density can be achieved by making the width ofthis highly conductive zone to be smallerthan thatofthe conductive layers while reliably preventing the conductive layers from being exposed.Referring to FIG. 2, a fixed contact section 31 according to the present invention with which the contact piece 22 shown by an imaginary line ofthe movable contact section is to be contacted, comprises a pair of conductive layers 32 and 33 which are formed on an insulating substrate 34 with a copperfoil orthe like preferably as printed through an etching process, providing between the layers 32 and 33 an insulation distance less than 0.6 mm even at the extremity.

Substantially all over the top surface of the insulating substrate 34, a soldering resist layer 35 is provided preferably by a screen printing process to the extent that the resist layer covers up to the top surface edgewise portions of the layers 32 and 33, leaving their central portions exposed. Carbon layers 36 and 37 are provided on the exposed central portions of the layers 32 and 33 preferably also by the screen printing process so that respective edge portions 36a, 36b and 37a, 37b ofthe carbon layers 36 and 37 expand over the resist layer 35.

With the aforementioned arrangment of FIG. 2, the carbon layers do not cover the entire top surface of the conductive layers in contrast to the prior art, and the resist layer 35 is applied onto the top surface of the insulating su bstrate 34 so that the resist layer 35 covers only the edgewise portions ofthe conductive layers 32 and 33 leaving their central portions uncovered, whereby the edgewise portions and side edge surfaces of the conductive layers 32 and 33 can be reliably protected bythe resistlayer35 having a relatively high mechanical strength, without being exposed.Furthermore, the carbon layers 36 and 37 are provided sothattheywill integrally join at the bottom surface with the top surfaces ofthe conductive layers 32 and 33 and the edge portions 36a, 36b, 37a, and 37b of the carbon layers 36 and 37 will ride on peripheral edge portions of the resist layer 35, wherebythe contact surface between the carbon layers and the contact piece 22 of the movable contact section can have a high electric conductivity with a sufficient contacting area, and the contact resistance can be reduced substantiallyto the same level as in the case oftheforegoing Japanese Utility Model Application Laid-Open Publication.In FIG. 2, the dimensions of the respective layers are shown as somewhat exaggerated in orderto clarify the relationship between the respective layers, andthetop surfacesofthe carbon layers 36 and 37 are shown as if they are largely stepped, but practical level difference is negligibly small so that the contact piece 22 which is resilient can come into complete and intimate contact at least with the top surfaces of the carbon layers 36 and 37, as depressed through the button member 16.

More specifically, the arrangement of FIG. 2 makes it possible, as seen in the drawing, to reduce the width of the carbon layers 36 and 37 to be smallerthan that of the conductive layers 32 and 33 and, accordingly, a distance 12 between the carbon layers 36 and 37 becomes largerthan a distance 17 between the conductive layers 32 and 33. In other words, even when the distance 12 between the highly conductive carbon layers 36 and 37 is selected to be 0.6 mm or more as generally required for electric insulation between them, the distance 11 between the relatively lower conductive layers 32 and 33 can be selected to be less than 0.6 mm while they have to be kept separated more than 0.3 mm as generally called for in respect ofthe copper-made conductive layers.It will be easily appreciated bythoseskilled inthatartthat, in practice,thedistance 1, relativetothedistance 12can bedefinedto be an allowable minimum valuewithin the range of 0.6 mm > 11 > 0.3 mm, depending on the size of high conductive contact surface area, that is, the top surface area of the carbon layers 36 and 37 exhibiting a predetermined low contact resistance for them and being obtained under the technical conditions required for the screen printing process. Accordingly, a high wiring densityforthe circuit on the substrate can be effectively assured with the fixed contact section according to the present invention.

Referring next to FIG. 3, there is shown an embodiment of practical pattern forthe fixed contact section used in the contact unit shown in FIG. 2, wherein substantially the same component parts as those in Fig. 2 are denoted by the same reference numerals but added by 10. In the drawing, electric conductive layers 42 and 43 are shown by chain lines, one of which denoted by 42 is made cruciform at its end portion while the other layer 43 of which is formed at its end portion to surround the cruciform end portion of the layer 42 as spaced at least by the distance 11 along every opposing side edge surface.The conductive layers 42 and 43 are respectively covered attheir edgewise portions with soldering resist layer 45 over to such edgewise line as shown by a broken line, leaving their central portions uncovered, and this resist layer 45 expands overthesubstrate between the both conductive layers 42 and 43 and also outside the other layer 43. Carbon layers 46 and 47 are deposited mainly onto the uncovered central portions ofthe conductive layers 42 and 43 butto ride in particular on the peripheral edges ofthe resist layer 45 attheir edge portions 46a, 46b and 47a, 47b as shown by solid lines as hatched are respectively in a similarcontourtothe cruciform end portion of the layer 42 and the surrounding end portion of the other layer 43.In other words, each contour of the carbon layers 46 and 47 is positioned at its every part inside each contour line of the conductive layers 42 and 43 that the highly conductive carbon layers 46 and 47 are made smaller in size than the relatively lower conductive layers 42 and 43.

With the arrangement of FIGS. 2 and 3, such a button member of the movable contact section as shown in FIG. 1 carries the resilient movable contact piece 22 having a contour spreading overthe entirety ofthe carbon layer 47 and, when the button member is depressed, the movable contact piece 22 contacts across the carbon layers 36,37 or 46,47 so that an electric path will be formed between the conductive layers 32,33 or 42,43 closing a circuit connected to these conductive layers. Even in the case ofthe push button switch incorporating such a fixed contact section as shown in FIG.2 or 3, other parts of the movable contact section substantially of the same arrangement as disclosed with reference to FIG. 1 may be similarly employed.

There is shown in FIG. 4 another embodiment ofthe pattern for the fixed contact section according to the present invention, wherein a soldering resist layer 55 is provided in particular only between a pair of relatively lower conductive layers 52 and 53 and to coverthe mutually opposing edgewise portions ofthe conductive layers 52 and 53. Highly conductive carbon layers 56 and 57 are formed respectively on each ofthe conductive layers 52 and 53 so that the mutually opposing edge lines ofthecarbon layers 56 and 57will lie inside the opposing contour lines of the conductive layers 52 and 53.With this arrangement, similarly, it is possible to select the insulation distance between the carbon layers 56 and 57to be larger than 0.6 mm and that between the conductive layers 52 and 53 to be largerthan 0.3 mm butsmallerthan 0.6 mm, whereby it can be anticipated that the contact unit of FIG. 4 achieves substantially the same effect as the unit of FIGS. 2 and 3, except that the outermost edge portion of the outer positioned conductive layer 53 has no protective resist layer.

The contact unit ofthe push button switch according to the present invention can be subjected to various design modifications. For example, although the carbon layer has been shown to be provided to expand overthe edge line of the resist layer on the conductive layer, the carbon layer may be provided only in the central exposed portion of the conductive layer between the opposing edge lines ofthe soldering resist layers on the conductive layer, preferably to bethickerthan the resist layer with the top surface of the carbon layer made higher than that ofthe resist layer. It will be also readily understood that, in the embodiment of FIG. 4, the resist layer may be provided also onto the outermost edgewise portion of the outer positioned conductive layer 53 so that the outermost edgewise portion can be also prevented effectively from being exposed and oxidized.

Claims (8)

1. A contact unit of a push button switch in which an electrically high conductive contact member is provided on the bottom surface of a pushable button member, a pair of electrically relatively lowerconductive layers between which an electric path is formed through said contact member being brought into contact therewith are provided on an insulating substrate and spaced from each other by a predetermined first distance, and electrically relatively higher conductive carbon layers are provided respectively on each of said lower conductive layers, wherein an insulating covering layer is provided on said insulating substrate to cover at least from an area between the pair of said lower conductive layers to edgewise top surface portions along opposing side edges of the respective lower conductive layers, and said carbon layers are provided at least on the top surfaces of the lower conductive layers not covered with said insulating covering layer so that the carbon layers are spaced from each other at their opposing side edges by a second distance which is largerthan said first distance.

2. Acontactunitaccording to claim 1,whereinsaid insulating covering layer is of a soldering resist.

3. A contact unit according to claim 2, wherein said soldering resist layer is provided on said insulating substrate to cover all of said edgewise top surfaces, exceptthecentral portions, of said pair of lower conductive layers which consist of a copper foil.

4. A contact unit according to claim 2 or claim 3, wherein said carbon layers are provided to expand at their edges overthe top surface of said resist layer on said lower conductive layers.

5. A contact unit according to any one of claims 2 to 4, wherein said lower conductive layers of copper foil are formed by an etching process, and said resist layer and carbon layers are formed by a screen printing process.

6. Acontactunitaccordingto any preceding claim, wherein said first distance between said pair of lower conductive layers is less than 0.6m but greater than 0.3 mm.

7. Acontactunitaccordingto any preceding claim, wherein said lower conductive layers are opposed to each other at their respective ends spaced by said first distance, the end of one of said layers being cruciform and the end of the other being formed to surround said cruciform end, and said higher conductive layers having respectively a substantially similar shape to and a contou r smal ler than the lower conductive lavers.

8. A contact unit substantially as described herein with reference to Figs. to 4 of the drawings.