Patent Publication Number: US-2010109921-A1

Title: Dome sheet and key pad

Description:
FIELD OF THE INVENTION 
     The present invention relates to a dome sheet, to a sheet for producing a dome sheet and to a key pad comprising a dome sheet. The invention further relates to a method of manufacturing a dome sheet. 
     BACKGROUND OF THE INVENTION 
     Electronic devices, such as mobile electronic devices including mobile phones, personal digital assistants and the like often comprise a key pad with one or more keys for enabling a user to enter information or control commands. In these devices, an actuation of the key is detected electrically by the closing of a contact. For this purpose, modern key pads comprise metal dome arrays, also called key dome arrays or switching arrays, which comprise plural metal domes attached to a sheet which is adhered to a printed circuit board (PCB). Contacts printed on the circuit board are covered by and in contact with the metal domes of the metal dome sheet. When the metal dome is pressed down, an inner contact is brought into contact with an outer contact, as a result of which a current can flow and the actuation of the dome can be detected. 
     A single layer metal dome array generally comprises metal dome contacts attached to a single layer film with an adhesive, e.g. a plastic film. In the film or underlying circuit board, an air hole is provided for each metal dome. When pushing in the metal dome, the air underneath the dome can escape, and the metal dome provides the operator with a “click feeling”. By feeling the click, the operator gets a feedback on whether he actuated the corresponding key. Yet as moisture and dirt may enter through the air hole, the life time of such a single layer metal dome sheet is limited. Further, the single layer may be damaged by the metal dome contact which limits the life time of such a dome sheet to a certain number of actuations. 
     Other single layer dome sheets use cuts in the adhesive layer for providing ventilation. A further problem associated with single layer metal dome sheets (MDSs) is that a precise placement of an actuator for transferring a force for an actuation of the underlying metal dome is very difficult. An imprecise placement of the actuator results in a reduced reliability for switching operations. 
     Further, double layer metal dome sheets (MDSs) are known, which use an additional sheet or layer. The actuator can more precisely be positioned between the sheets in such a double layer MDS. As two sheets are used, the metal dome sheet assembly becomes stiffer. If such a stiff double layer sheet is adhered to a PCB, adhesion is often poor and crevices can form, into which dirt and moisture may enter. To prevent this, depressions or indentations are preformed in the sheet at the positions of the metal domes. Yet the preforming has negative effects on the tactility and operation force of the resulting metal dome switches. When pressing down on such a dome, not only the metal dome, but also the two preformed layers provide a resistance. As a result, the tactile characteristics of such a double layer sheet largely diverge from the characteristics of the metal dome itself. In particular, a higher operation force is needed to actuate a dome of such a metal dome sheet. Also, the click ratio is reduced in such a double layer sheet, meaning that the “click feeling” is much weaker for an operator. 
     It is thus desirable to improve the tactile characteristics of such a double layer dome sheet. It is further desirable to provide a moisture and dust-resistant dome sheet which enables a precise actuation of the domes. Such a dome sheet should also be versatile, so that it can be used in different types of key pad assemblies, and with different types of substrates. 
     SUMMARY OF THE INVENTION 
     Accordingly, there is a need for an improved dome sheet which obviates at least some of the above-mentioned drawbacks. 
     According to a first aspect of the invention, a dome sheet for forming a switch arrangement when mounted to a contact layer is provided. The dome sheet comprises at least one elastically deformable dome of a conductive material and a bottom sheet at least partially covering the at least one dome, the bottom sheet comprising at least one opening associated with the at least one dome, the opening having at least a first portion extending in a radial direction of the at least one dome and a second portion extending in a circumferential direction of the at least one dome. The dome sheet further comprises a top sheet at least partially covering the bottom sheet. The at least one opening is adapted to form an air channel for the at least one dome when the dome sheet is mounted to the contact layer. 
     By providing the opening with a first and a second portion in the bottom sheet, the dome sheet may achieve enhanced tactile characteristics for an actuation of the dome. In particular, as the bottom sheet may comprise less material in proximity to the dome, the force for actuating the dome may be reduced. The opening may also serve as a venting channel for the dome, so that air may escape into the channel when the dome is pressed down. This may further enhance the click feeling when the dome is actuated, and may bring the tactile characteristics of the assembly closer to the characteristics of an isolated dome. Further, as the top sheet may cover the at least one opening in the bottom sheet, an air channel formed by the opening in the bottom sheet may be protected from dust or moisture. By providing the second portion of the opening, tension in the sheets may be reduced. The dome sheet may thus be less likely to come loose when adhered to a contact layer, such as a printed circuit board or a flexible printed circuit (FPC). The formation of crevices may thus be prevented, leading to a better dust and moisture resistance. As the click ratio of such a dome sheet may be increased, a better tactile and audible feedback may be provided to an operator, and the arrangement may provide a better clicking sound. 
     According to an embodiment, the second portion of the at least one opening overlaps the at least one dome. Accordingly, the opening may be connected to the dome, resulting in improved tactile characteristics. 
     According to another embodiment, the first portion of the at least one opening extends along a direction from a first of the at least two domes to a second of the at least two domes. Two domes may thus be connected by the opening. In particular, the second portion of the at least one opening may extend in a circumferential direction of the first dome, and the at least one opening may further comprise a third portion extending in a circumferential direction of the second dome. The opening may thus have a dumbbell-like shape and may connect the two domes. As the ventilation of a dome may improve in such a configuration when mounted to a circuit board, the tactile characteristics for pressing one of the domes may further be improved. 
     According to another embodiment, the dome sheet further comprises a second opening associated with the at least one dome, the second opening having at least one portion extending in a circumferential direction of the at least one dome. The second portion of the one opening and the portion of the second opening may span at least one third of the circumference of the at least one dome. They may also span a smaller or a larger fraction of the circumference, e.g. one half or one fourth. 
     The dome sheet may further comprise an actuator arranged between the top sheet and the bottom sheet, the actuator being arranged so as to transfer pressure applied to the top sheet to the at least one dome. The actuator may also be arranged between the bottom sheet and the at least one dome, so as to transfer pressure from the bottom sheet to the dome. Providing the dome sheet with such an actuator may have the advantage that a precise pressure application to the dome may be enabled, even if the pressure is applied by a key with a flat bottom. Using a top and a bottom sheet further has the advantage that the actuator may be precisely and securely positioned which is beneficial for good force transduction from a key to the dome. It may also increase the possible types of actuators that may be used. Further, the lifetime of the actuator may be prolonged, and accordingly the period of operational functionality of the corresponding dome. 
     The second portion of the at least one opening may have substantially the shape of a ring segment. The first portion of the at least one opening may have substantially the shape of a rectangle. 
     The dome sheet may comprise a plurality of domes, each dome being associated with at least one opening of the bottom sheet. A good ventilation of each dome may thus be ensured. 
     The at least one opening may be adapted to form an air channel which is sealed off to the outside when the dome sheet is mounted to the contact layer. In such a configuration, dust, dirt or moisture may be prevented from entering the sealed-off air channel, which may improve the reliability and life time of the so-formed switch arrangement. 
     The at least one opening may be adapted so as to provide a click ratio of at least 25% of the associated dome when the dome sheet is mounted to the contact layer. The opening may also be adapted to provide higher click ratios, e.g. 35%, 40%, 45%, or even 50%. By configuring the dome sheet with such click ratios, an operator using the dome sheet may be provided with a better click feeling. 
     According to another aspect of the invention, a sheet for the production of a dome sheet is provided. The sheet is adapted for an arrangement of at least one elastically deformable dome of a conductive material at a predetermined dome position on a sheet. The sheet comprises at least one opening associated with the dome position, the opening having a first portion extending in a radial direction of the dome position and a second portion extending in a circumferential direction around the dome position. The at least one opening is adapted to form an air channel for a dome at the associated dome position when the sheet is arranged in the dome sheet and the dome sheet is mounted to a contact layer. By using such a sheet in the construction of a dome sheet, the resulting dome sheet may achieve similar advantages as outlined above. 
     According to an embodiment, the material forming the sheet is polyethylene terephthalate (PET). 
     The second portion of the at least one opening of the sheet may overlap an area of the sheet which will abut a dome located at the associated dome position when the sheet is arranged in the dome sheet. The dome position may be further associated with a second opening provided in the sheet, the second opening having at least one portion extending in a circumferential direction around the associated dome position. The second portion of the opening may have substantially the shape of a ring segment. 
     According to another aspect of the invention, a key pad is provided. The key pad comprises a contact layer having at least one contact point. The key pad further comprises a dome sheet mounted to the contact layer, the dome sheet comprising at least one elastically deformable dome of a conductive material corresponding to the at least one contact point and a bottom sheet at least partially covering the at least one dome. The bottom sheet comprises at least one opening associated with the at least one dome, the opening having at least a first portion extending in a radial direction of the at least one dome and a second portion extending in a circumferential direction of the at least one dome. The dome sheet further comprises a top sheet at least partially covering the bottom sheet. The at least one opening is adapted to form an air channel with the contact layer for the at least one dome. Accordingly, a ventilation for the at least one dome may be provided, which may improve the tactile characteristics for an actuation of the dome. 
     According to an embodiment, the key pad further comprises at least one key corresponding to the at least one dome and arranged so as to actuate the dome upon an application of pressure to the key. When the key is pressed, the dome may accordingly be pressed down and may establish a contact at the contact point, which may lead to the detection of the pressing of the key. 
     According to another embodiment, the key pad further comprises an actuator arranged between the top sheet and the bottom sheet or between the bottom sheet and the dome, so that an application of pressure to the key is transferred to the dome by the actuator. Even if the key does not have a point for actuating the dome, such as a key with a flat bottom, a force applied to the key can be precisely transmitted to the dome by providing such an actuator. 
     The contact layer may be a printed circuit board or a flexible printed circuit. 
     According to another aspect of the invention, a method of adjusting an actuation force of a dome of a dome sheet is provided. The dome sheet comprises at least one elastically deformable dome of a conductive material, a bottom sheet at least partially covering the at least one dome, and a top sheet at least partially covering the bottom sheet. The method comprises the steps of providing at least one opening for the at least one dome in the bottom layer, the opening being adapted to form an air channel for ventilating the at least one dome when the dome sheet is mounted on a contact layer and when the dome is actuated, and of adjusting a force required for an actuation of the dome by adjusting a dimension and/or a shape of the opening in the bottom layer which forms the air channel when the dome sheet is mounted on a contact layer. Using such a method, the tactile characteristics and the actuation force of the dome may be tuned by adjusting the dimensions and/or the shape of the opening. By such a method, the performance of a dome sheet in particular regarding operation force and click ratio may be easily adjusted. 
     According to another aspect of the invention, a method of manufacturing a dome sheet with plural domes located at predetermined dome positions is provided. The method comprises the steps of manufacturing a bottom sheet comprising at least one opening associated with at least one of said dome positions, the opening having at least a first portion extending in a radial direction of the associated dome position and a second portion extending in a circumferential direction of the associated dome position. The bottom sheet is laminated with a top sheet at least partially covering the bottom sheet. Further, dome shaped indentations a pre-formed in the laminated bottom and top sheets at the dome positions. Elastically deformable domes of a conductive material are arranged in the indentations at the dome positions. 
     According to an embodiment, the method further comprises placing an actuator between the bottom sheet and the top sheet at the predetermined dome position before lamination. Such a method achieves a good control of the positioning of the actuator when laminating the two sheets. 
     The bottom sheet may be manufactured from polyethyleneterephthalate (PET). 
     It is to be understood that the features mentioned above and those yet to be explained below can be used not only in the respective combinations indicated, but also in other combinations or in isolation, without leaving the scope of the present invention. Features of the above-mentioned aspects and embodiments may be combined to form new embodiments. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing and other features and advantages of the invention will become further apparent from the following detailed description read in conjunction with the accompanying drawings, in which like reference numerals refer to like elements. 
         FIG. 1  is a schematic drawing of a section of a sheet for producing a dome sheet according to an embodiment of the invention. 
         FIG. 2  is a schematic drawing of a sheet for producing a dome sheet according to an embodiment of the invention comprising plural openings associated with plural dome positions. 
         FIG. 3  is a sectional side view of a key pad according to an embodiment of the invention schematically illustrating the positioning of a dome, a bottom and a top layer. 
         FIG. 4  is another sectional side view of the key pad of  FIG. 13  illustrating an air channel connecting two domes. 
         FIG. 5  is a disassembled view schematically showing a key pad according to an embodiment of the invention. 
         FIG. 6  is a flow diagram illustrating a method according to an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the following, embodiments of the present invention will be described in detail with reference to the accompanying drawings. It is to be understood that the following description of embodiments is given only for the purpose of illustration and is not to be taken in a limiting sense. The scope of the invention is not intended to be limited neither by the embodiments described hereinafter nor by the drawings, which are taken to be illustrative only, but is intended to be limited only by the appended claims and equivalents thereof. 
     It should be noted that the drawings are to be regarded as being schematic representations only, and elements in the drawings are not necessarily to scale with each other. Rather, the representation of the various elements is chosen such that their function and general purpose become apparent to a person skilled in the art. 
       FIG. 1  shows a sheet  101  according to an embodiment of the present invention for the production of a dome sheet. Sheet  101  is adapted for an arrangement of an elastically deformable dome of a conductive material, such as a metal dome, e.g. made of stainless steel, as indicated by the dashed line marking where dome  103  is to be positioned. Sheet  101  comprises an opening  106  with a first portion  107  and a second portion  108 . Portion  107  of opening  106  extends in a radial direction of the position where dome  103  is to be located. The second portion  108  of opening  106  extends in a circumferential direction around the position of dome  103 . Portion  108  overlaps the area where dome  103  will be located when assembling a dome sheet using sheet  101 , the area being indicated by the dashed line. In the present embodiment, portion  107  essentially has the shape of a rectangle, whereas portion  108  essentially has the shape of a ring segment. It should be clear that the portions may also have other shapes, as long as the opening has a portion extending in radial direction and a portion extending in the circumferential direction. When a dome sheet is assembled using sheet  101  and the dome sheet is being mounted to e.g. a PCB, opening  106  will form an air channel for a ventilation of dome  103 . 
     Sheet  101  further comprises a second opening  109  with a portion extending in a circumferential direction of the position at which dome  103  will be located, similar to the portion  108  of opening  106 . When forming a dome sheet and a corresponding switch arrangement, dome  103  will be covered by less material as sheet  101  comprises openings  106  and  109 . As a result, the force needed to actuate a switch corresponding to dome  103  will be lowered compared to an arrangement in which such openings are not provided. By using the configuration of the present embodiment for sheet  101 , the tactile feel can be improved and the characteristics for an actuation of dome  103  closely correspond to the characteristics of an isolated dome. It should be clear that sheet  101  may not only be used in the construction of a metal dome sheet, but also other types of dome sheets, which may be mounted to a printed circuit board, which may be flexible, but may also be mounted to other types of contact layers, such as a flexible printed circuit (FPC). 
     In  FIG. 1 , the position at which an actuator  104  may be placed when a dome sheet is assembled using sheet  101  is also shown with a dashed line. Actuator  104  may be located between sheet  101  and a top sheet, or between sheet  101  and dome  103 , and may be used to transfer an applied force to the dome  103 . 
       FIG. 2  shows another embodiment of a sheet  101  which may be used for producing a dome array and a corresponding switch arrangement. Plural positions at which domes  103  and actuators  104  may be located when producing a dome sheet are indicated with dashed lines. Each dome position is associated with at least one opening  106  with a first portion  107  and a second portion  108 , which are similarly configured as the corresponding portions shown in  FIG. 1 . Further, some openings such as opening  111  are located in between two dome positions at each of which a dome is to be placed. Besides portions  107  and  108 , opening  111  comprises a third portion  112 . The third portion  112  extends in a circumferential direction of the dome to be positioned at dome position  115 . Opening  111  thus has a symmetrical dumbbell-like shape in the present embodiment, and overlaps the domes to be positioned at dome position  114  and  115 . Accordingly, if sheet  101  of  FIG. 2  is used to produce a dome sheet and a switch array by mounting the dome sheet to e.g. a printed circuit board, opening  111  will form an air channel connecting the domes at position  114  and  115 . Thus, a good ventilation is provided for the domes at the respective positions. As can be seen, the dome located at position  114  is associated with further openings, so that the air channels being formed by these openings connect plural dome chambers. The air can thus escape to other dome chambers when the dome is actuated, and accordingly, the force required for actuating the dome can be reduced. A further force reduction is again achieved by providing the second opening  109  for dome position  115 , whereas for dome position  114 , a further dumbbell shaped opening is provided. Apart from the force reduction, the click ratio is increased, resulting in a positive feedback which indicates to an operator that a switch closure has occurred when actuating one of the domes. The actuation produces a tactile and audible click close to that of a single metal dome, which can not be achieved by conventional double layer metal dome sheets. 
     Sheet  101  may of course comprise more or fewer openings, and the openings may be shaped and sized differently. In particular, by adapting the dimensions, size and positioning of the openings, the click ratio, the operation force and the tactile characteristics of the domes of a resulting dome sheet can be adjusted. The click ratio may for example be adjusted to 25%, or any other value, such as 30%, 35%, 40% or even 50%. The click ratio is generally defined as the maximum load minus the minimum load normalized by the maximum load of the load profile resulting from an actuation of a dome. Higher click ratio results in a better click feeling. Accordingly, the feedback for an operator for whether a switch closure has actually occurred is better for a higher click ratio. 
     Sheet  101  may be a bottom sheet for a dome sheet and may be produced of polyethylene terephthalate (PET). The openings associated with the domes  103  are adapted to form air channels which are sealed off to the environment when the sheet  101  is mounted to a printed circuit board for constructing a switch array. As the formed air channels are sealed off, no dust or dirt can enter. The functionality of the switches formed by domes  103  can thus be ensured over long periods of time. 
     In the present embodiment, opening  109  and portion  112  of opening  111  span approximately half the circumference of a dome positioned at a position  115 . Such a choice provides good tactile characteristics for the dome, although the openings may also span a larger or a smaller portion of the circumference of the dome. Opening  109  and portion  112  are again formed in shape of a ring segment, whereas portion  107  has the shape of a rectangle. It should be clear that other shapes may also be used, portion  112  may for example have the shape of an elongated rectangle extending along the circumferential direction of the dome, or a more triangular shape with the hypotenuse of the triangle extending along the circumferential direction. Further openings  110  are provided in sheet  101 , said openings being used for mounting a dome sheet comprising sheet  101  to a circuit board or for providing illumination to a key board formed using sheet  101 , or other functionalities. In the embodiment of  FIG. 2 , domes  103  have an outline in the shape of a flattened circle. It should be clear that domes  103  may also have other shapes, such as semi-spherical shapes with a circular outline, or semi-spheroidal shapes with an elliptical outline, or the like. The domes outlined here are just an example, and any type of dome suitable for forming a dome sheet may be employed when assembling a dome sheet using sheet  101 . 
       FIG. 3  shows a key pad  200  employing a dome sheet  100  according to an embodiment of the present invention. As can be seen from the sectional side view, dome sheet  100  comprises a top layer  102 , a bottom layer  101  and a dome  103 . For the bottom sheet  101 , a sheet similar to the sheet shown in  FIG. 1  or  FIG. 2  is used. Bottom sheet  101  comprises one or more openings associated with dome  103 , such as openings  106 ,  109  or  111  mentioned above. In the sectional view of  FIG. 3 , these openings are not visible and bottom sheet  101  is shown as a continuos layer. Dome  103  of dome sheet  100  may be a metal dome, such as a stainless steel dome, yet any other type of material which is flexible and conductive may also be used. dome  103  has the shape of a spherical section, yet it may also have other shapes as mentioned above. Sheet  100  further comprises an actuator  104  placed between top sheet  102  and bottom sheet  101 . This allows a precise placement of the actuator  104  when laminating top and bottom sheet  102  and  101 . A precise placement is beneficial for a transfer of a force applied to key  202  to dome  103 . 
     Dome sheet  100  is mounted to a printed circuit board  120 , e.g. by an adhesive. Bottom sheet  101  may thus be additionally provided with an adhesive layer during fabrication of the dome sheet. Printed circuit board  120  comprises an outer ring shaped contact  121  and an inner circular contact  122 . These contacts form a contact point, or a switch contact, which is used to detect an actuation of dome  103 . When pressing down on dome  103 , the dome deforms from a convex to concave shape, which deflection causes the center of the dome to contact the inner contact  122 . As the rim of dome  103  is in contact with the outer contact  121 , an electrical circuit between contacts  121  and  122  is closed, and the actuation of the dome can be detected. 
     PCB  120  may be a single layer PCB, in which ring shaped contact  121  comprises a gap to allow a PCB track to reach inner contact  122 . On the other hand, PCB  110  can be a multi-layer PCB, in which contacts  121  and  122  can be connected via different layers. Other types of contact layers, such as a FPC, may also be used. 
     Key pad  200  further comprises a cover plate  201  used to retain one or more keys  202 . Cover plate  201  is only partially shown in  FIG. 3  and may be mounted to circuit board  120  or may be part of a housing of a device incorporating key pad  200 . Key  202  has a flat surface contacting the dome sheet  100 . Even though, actuator  104  enables a precise pressure application to dome  103  by key  202 . When key  202  is pressed, dome  103  is deflected downwards and contacts contact  122 . When the key is released, dome  103  exerts a restoring force, and key  202  returns to its original position. Actuator  104  may be formed out of hard plastic or resin for providing a good force transfer. Actuator  404  may have a diameter between 1 and 3 mm, e.g. 1.8 mm, and a height between 0.05 and 1 mm, preferably between 0.08 and 0.5 mm. The actuator provides a small pushing point, ensuring a deflection of the center of dome  103 , so that contact  122  is contacted. The precise positioning of actuator  104  enabled by the double layer construction of the dome sheet allows the use of keys with a flat bottom, yet other types of keys may also be used. Key  202  may for example have a projection on its lower surface for actuating dome  103 . 
       FIG. 4  schematically shows another sectional view of key pad  200 . Dome sheet  100  implements a bottom sheet  101  similar to the sheet shown in  FIG. 2 . The sectional side view of  FIG. 4  corresponds to a section taken along the line connecting positions  114  and  115  in  FIG. 2 . As bottom sheet  101  is laminated to top sheet  102  and mounted to PCB  120 , openings  109  and  111  provided in bottom sheet  101  form air channels. For providing ventilation, the air channel formed by opening  111  connects two dome chambers. By providing openings  109  and  111 , the force required to activate dome  103  is reduced. While a double sheet construction with a continuos bottom layer would be much stiffer, the use of a bottom layer with openings as shown in  FIG. 2  makes the construction more flexible and the characteristics of an actuation of dome  103  become closer to those of a single layer construction. In consequence, mainly one sheet contributes to the actuation force of the dome in the dome sheet of the present embodiment. 
     In the embodiment of  FIG. 4 , actuator  104  is positioned between bottom sheet  101  and top sheet  102 . Yet in another embodiment, it may be placed between bottom sheet  101  and dome  103 . The actuator may then be either adhered to bottom sheet  101  or to dome  103  before arranging the dome on the bottom sheet. This technique also achieves a precise positioning of actuator  104 . 
     Keys  202  may not only be provided as single keys, but may also be provided as a key mat, e.g. made out of silicone rubber or the like. Cover plate  201  may be manufactured out of hard plastic or a metal or another material suitable for the production of a cover plate for a housing. Top sheep  102  and bottom sheet  101  may be fabricated from PET, yet they may also be made out of another plastic or polymer material. 
       FIG. 5  schematically illustrates an exploded view of a key pad  400  according to another embodiment of the invention. Key pad  400  comprises a cover plate  201  and a plurality of keys  202 . The keys are positioned above a dome sheet  100  comprising corresponding domes  103 . Domes  103  are covered by a bottom and a top sheet with preformed indentations as illustrated in  FIG. 4 . Dome sheet  100  is mounted to printed circuit board  120  e.g. by adhesion. Domes  103  contact the outer ring shaped contacts  121  of corresponding contact portions. Air channels are formed between the bottom layer of dome sheet  100  and PCB  120 . In the present embodiment, PCB  120  is a multi layer PCB, and outer contacts  121  do not need to be provided with a gap. 
     The click sound and click feeling of keys  202 , as well as the click ratio and actuation force can again be adjusted by modifying the dimensions and the shape of the openings provided in the bottom layer of dome sheet  100 . The key pad according to the present embodiment can thus achieve a much better performance and in particular provide a better feedback to an operator compared to a conventional key pad using a conventional dome sheet. Further, such a key pad is dust and moisture resistant as the air channels formed by the bottom layer are closed off to the environment by the top layer and the PCB. A formation of crevices is also prevented due to the performed indentations in the laminated bottom and top layers. In addition to the advantageous tactile characteristics and the dust and moisture resistance, key pad  400  provides reliable and precise switching due to the use of the actuators integrated in dome sheet  100 . 
     In the flow diagram of  FIG. 6 , an embodiment of a method for producing a dome sheet is illustrated. In step  301 , a bottom sheet with openings comprising first and second portions, as illustrated with respect to  FIG. 1  or  2 , is manufactured. The sheet may for example be cut or punched from a larger sheet or molded, and the openings may also be cut or punched. Between the bottom sheet and the top sheet, which may be manufactured from the same or different material, an actuator is positioned in step  302 . In step  303 , bottom and top sheets are laminated. In the laminated sheets, indentations are formed in step  304 . The indentations are formed at positions at which keys are to be located on a corresponding key pad. In a last step  305 , domes, e.g. metal domes, are arranged in the preformed indentations. The domes may also be fixed, e.g. by adhesion, in the indentations. The method may of course comprise further steps, such as coating the bottom layer with an additional adhesive layer, while other steps are optional, such as providing an actuator in step  302 . 
     The dome sheet obtained in such a way may then be used for manufacturing a key pad, e.g. by adhering the dome sheet to a contact layer such as a printed circuit board. A key mat can then be placed on top of the dome layer so as to obtain a functional key pad. The dome sheet can be produced with a relatively low thickness, resulting in a low thickness of the total key pad assembly. The dome sheet can be produced at relatively low cost in a straight forward production process. 
     The above-mentioned key pads may be used in any type of electronic device, such as a mobile electronic device. Examples include a mobile phone, a personal data assistant (PDA), a portable music player, a camera and the like. 
     It is to be understood that the features of the various embodiments described above may be combined with each other. As an example, the actuator may also be positioned between the bottom sheet and the dome in the method as described with respect to  FIG. 6 , or the bottom sheet of  FIG. 4  may be provided with more or fewer openings as illustrated in  FIG. 1  or  FIG. 2 . Those skilled in the art will thus appreciate from the foregoing description that the teachings of the present invention can be implemented in a variety of forms. 
     Accordingly, while specific embodiments of the invention are disclosed herein, various changes and modifications can be made without departing from the scope of the invention. The present embodiments are to be considered in all respect as illustrative and non-restrictive, and all changes coming within the meaning and equivalency range of the appended claims are intended to be embraced therein.