GB2322008A - Switch - Google Patents

Abstract

A switch comprises a substrate 34 having a first electrical contact 35, a contact membrane 41 including a second electrical contact 43 engageable with the first contact 35 and a deformable metal structure 32 arranged for sudden deformation between an uncompressed configuration and a compressed configuration in which it presses the second contact 43 into contact with the first contact 35.

Description

KEYPAD This invention relates to a keypad, and a switch suitable for use in a keypad.

The switch could be used in other applications too.

Some driving factors behind the design of keypads are as follows: reliability, reduced cost, efficiency of manufacture and reduced component count (to address all the preceding factors).

Keypads are used in the user interfaces of devices such as calculators and radiotelephones (e.g. mobile phones). One design of keypad is shown in crosssection in figure 1. There is a flexible rubber keymat 1 which has key portions 2 surrounded and joined to the main web of the keymat by resilient deformation zones 3. Each key portion has an upper region 4 which projects through a corresponding hole 5 in an outer housing 6 of the device, and a lower region 7 which terminates in a nib 8. The upper portion can include in-moulded or printed indicia 9 to allow a user to identify the key. When a user presses the upper region 4 of the key the resilience of the deformation zone surrounding the key allows the key to travel towards a switch assembly 10 associated with the key so that the nib 8 can engage the switch. When the key portion is released the deformation zone urges it to return to its original position as illustrated in figure 1. The switch comprises an upper assembly 11, including a flexible dome 1 2 made of sheet material, and a lower assembly 1 3 comprising a circuit board 1 4 with exposed conductive tracks 1 5. When the key is pressed it urges the dome 1 2 to deform suddenly to the configuration illustrated at 1 2a or 1 2b, in which it bridges the conductive tracks 1 5 of the switch assembly and connects them together electrically. This connection is sensed by the device. When the key is released the dome returns to its natural state, disconnecting the tracks from each other. The sudden deformation of the dome provides a user with a pleasant tactile and/or aural feedback when a key is pressed.

In one design of keypad the flexible dome is made of stainless steel. The stiffness of stainless steel is such that in its depressed condition the dome adopts the configuration shown at 1 2b, in which only a small central tip of the dome can contact the circuit board. Accordingly, it is very difficult to arrange that the dome bridges two tracks on the circuit board. Instead the dome is normally connected directly to one terminal and only the other terminal is provided as a conductive region on the circuit board. In either case there is the disadvantage that the dome has to be installed accurately over the circuit board, reducing the efficiency of producing the keypad. Various designs of metal dome have been contemplated in order to overcome this problem, in all of which the conductivity of the metal dome itself has been used to bridge the tracks.

In another design of keypad the flexible dome is made of plastics material such as Mylar, coated with graphite on its lower surface. This design has the advantage that in its depressed condition it adopts the configuration shown at 1 2a, in which a relatively large area of the dome can contact the tracks. This allows flexibility of positioning the dome. However, at higher temperatures this design does not provide such a pleasant tactile feedback. This has been found to be due to instability of the plastics material of the dome in response to heat.

According to the present invention there is provided a switch comprising: a substrate having a first electrical contact region; a contact membrane including a second electrical contact region facing the first contact region and movable to make contact with the first contact region; and a deformable metal structure for sudden deformation between an uncompressed configuration and a compressed configuration in which it presses the second contact region into contact with the first contact region.

The deformable structure may provide tactile and/or aural and/or other sensory feedback to a user. The deformable structure may be a deformable member.

The deformable structure is suitably a tactile feedback device or member. The structure may be formed of metal (preferably stainless steel) sheet. The sheet may be in the shape of a dome, at least in a region which most preferably overlies the contact regions. Preferably there is an insulating layer overlying the structure. Preferably the said is deformation is sudden and/or elastic and/or involves inversion (or at least partial inversion) of the said dome (or at least of its curvature). Preferably the structure is resilient such that it is biased to adopt the first configuration. Preferably the structure has a stable state: the first configuration, and an unstable state: the second configuration. Preferably the deformable structure is made of metal (as set out above) or of another material which is relatively insensitive to heat.

Preferably the contact membrane is an sheet of electrically insulating material.

The second contact region is suitably provided on a surface of the sheet. The second contact region may comprise graphite or silver. When the deformable structure presses the second contact member into contact with the first contact member the radius of curvature of the membrane is suitably greater than a radius of curvature of the deformable structure at its point closest to the first contact member. Preferably the deformable structure makes contact with the first contact member over a larger area then that over which the deformable structure makes contact with the contact membrane.

There is preferably a spacer for spacing the contact membrane from the substrate. Preferably the spacer includes an opening in the region of the second contact region.

Preferably the second contact region is localised on the contact membrane.

The switch may be a key switch for an electrical/eiectronic device, for example a handheld device, or for another keyboard or user interface.

The present invention will now be described, by way of example, with reference to the accompanying drawings, in which: figure 1, as already described, shows a cross-section through part of a prior art keypad together with alternative modes of deformation of part of the keypad; and figure 2 shows a cross-section through part of a keypad according to the present invention.

The keypad of figure 2 is for a mobile phone. The keypad has a flexible rubber keymat 21 which has key portions 22 surrounded and joined to the main web 23 of the keymat by resilient deformation zones 24. Each key portion has an upper region 25, which projects through a corresponding hole 26 in an outer housing 27 of the mobile phone, and a lower region 28 which terminates in a nib 29. The upper portion can include in-moulded or printed indicia 30 to allow a user to identify the key. When a user presses the upper region 25 of the key the resilience of the deformation zone surrounding the key allows the key to travel towards a switch assembly 30 associated with the key so that the nib 29 can engage the switch. When the key portion is released the deformation zone urges it to return to its original position as illustrated in figure 2.

The switch comprises an upper assembly 31, including a flexible dome 32 made of stainless steel sheet, and a lower assembly 33 comprising a circuit board 34 with exposed conductive tracks 35. Between the upper assembly and the lower assembly is an intermediate assembly 37.

The dome sits in a hole 38 in insulating layer 39. The insulating layer thus locates the dome over the circuit board (the insulating layer is itself located by pegs 40). A very thin flexible insulating cap 36 is fixed to the dome by adhesive and holds the dome in the hole 38.

The intermediate assembly comprises a pair of insulating layers 41, 42.

Insulating layer 41 is a membrane having a conductive coating 43 of carbon or, preferably, silver on its lower surface. Insulating layer 42 is a spacer which spaces the membrane 41 from the circuit board 34 and keeps the conductive coating 43 from touching the tracks 35 when the membrane is in its natural, planar configuration.

When a user presses the key, the dome is pressed down and snaps suddenly from its domed, stable configuration to the buckled configuration shown at 32a, in which the centre region of the dome is inverted. In this configuration it presses on the membrane 41, pushing the conductive region 43 into contact into contact with the tracks 35 to allow the keypress to be detected by a processor in the mobile phone. The flexibility/resilience characteristics of the membrane, together with the width of the hole 44 compared to its depth mean that it makes contact with the tracks and the circuit board over a relatively large area. The fact that in its depressed condition the membrane 41 is bent with a larger radius than the dome in its depressed condition also contributes to this.

As a result, the positioning of the upper and intermediate assemblies of the keypad in relation to the lower assembly is not as critical as in the prior art. This allows a number of advantages. First, the process of assembling and testing the keypad can be less rigorous and less expensive. Second, it becomes easier to make alternative versions of the mobile phone with different layouts of keys in their keypads. In prior art keypads which required precise interrelation of the domes and the tracks even a minor alteration of the key layout necessitated a replacement circuit board with a new layout of tracks, whereas in a keyboard assembly as described above the position of the keys relative to the circuit board can easily be varied. The tactile feedback provided by the keypad assembly is also relatively insensitive to temperature.

The holes 38 and 44 and the conductive region 43 are suitably round in shape, although other shapes (e.g. square, rectangular or irregular) could be used. The conductive region 43 could be annular.

To provide better performance and to allow improved flexibility in placement of the keys the electrodes 35 are preferably provided as a set of numerous (e.g.

around 10, 20 or 30) interdigitated fingers on the circuit board substrate 33.

The dome 32 is made of sheet stainless steel stamped to a dome shape. The thickness of the sheet is suitably around 501lem. The dome could be made of other materials whose elastic properties are relatively insensitive to heat (particularly in the normal range of operating temperatures of an electronic device - e.g. 0 to 900C. For example, aluminium, brass or more stable plastics such as nylon. The dome could be replaced by a more complex multi-part arrangement and/or my apparatus of a different shape - e.g. conical. In any event the arrangement is preferably biased to a convex-up configuration.

The membrane 41 and the spacer 42 are suitably made of plastics (e.g. Mylar sheet) each with a thickness of e.g. 50calm.

The width of the hole 38 is suitably around 6mm. The width of the hole 44 is suitably around 5mm. The hole 44 is relatively wide in comparison to its depth.

The ratio of the width of the hole 44 compared to its depth could be, for example, around 100:1 or in the range 30:1 to 200:1 There may be another spacer layer between layers 31 and 41, or spacing could be provided by shoulders 45 as illustrated in figure 2.

The present invention includes any novel feature or combination of features disclosed herein either explicitly or implicitly or any generalisation thereof irrespective of whether or not it relates to the claimed invention or mitigates any or all of the problems addressed. In view of the foregoing description it will be evident to a person skilled in the art that various modifications may be made within the scope of the invention.

Claims (11)

1. A switch comprising: a substrate having a first electrical contact region; a contact membrane including a second electrical contact region facing the first contact region and movable to make contact with the first contact region; and a deformable metal structure arranged for sudden deformation between an uncompressed configuration and a compressed configuration in which it presses the second contact region into contact with the first contact region.

2. A switch as claimed in claim 1, wherein in its uncompressed configuration the deformable structure comprises a dome-shaped sheet region.

3. A switch as claimed in claim 1 or 2, wherein the contact membrane is an sheet of electrically insulating material and the second contact region is provided on a surface of the sheet.

4. A switch as claimed in any preceding claim, wherein the contact membrane is relatively inflexible in the region of the second contact region.

5. A switch as claimed in any preceding claim, wherein in its compressed configuration the tactile feedback member touches the contact membrane.

6. A switch as claimed in claim 5, wherein when the deformable structure presses the second contact member into contact with the first contact member the second contact member makes contact with the first contact member over a larger area then that over which the deformable structure makes contact with the contact membrane.

7. A switch as claimed in any preceding claim, comprising a spacer for spacing the contact membrane from the substrate.

8. A switch as claimed in claim 7, wherein the spacer includes an opening in the region of the second contact region.

9. A switch as claimed in any preceding claim wherein the second contact region is localised on the contact membrane.

1 0. A switch substantially as herein described with reference to figure 2 of the accompanying drawings.

11. A keypad comprising a key switch as claimed in any preceding claim.