Abstract:
A metal dome switch assembly with a good snap ratio and having a desired click characteristic near to that of an isolated metal dome includes a carrier base layer and adhesive layer which have a plurality of holes bounded by annular sections which engage the lower outer surfaces of the domes and are larger than the keypad actuation posts, at positions corresponding to fixed contact points on a circuit board. The switch array is especially important in cell phone designs where the snap ratio, tactile feel, is desirable. The switch array can also incorporate an electroluminescent (EL) lamp while still improving the snap ratio.

Description:
RELATED APPLICATION 
   This application claims the benefit of U.S. Provisional Application No. 60/767,520 filed on May 10, 2006. 

   FIELD OF THE INVENTION 
   The present invention relates to dome-type switches and, in particular, a membrane switch assembly for low profile operating switch panels with a snap ratio and a click characteristic approaching an isolated metal dome. 
   BACKGROUND OF THE INVENTION 
   Low profile operating switch panels are used in smaller instruments, such as mobile telephones. Dome switches have been used for such applications and exhibit snap ratios and click characteristics providing desired tactile feedback to the user. Such attributes have been mitigated, however, by the need to isolate the panel circuitry from outside contaminants including humidity and particulates. A typical approach is disclosed in U.S. Pat. No. 6,917,007 wherein the metal dome switches are hermetically encased in composite carrier and adhesive layers. As a result of the composite structure, the flex characteristics of the dome, and accordingly the snap ratio are altered and tactile feedback mitigated, slightly but adversely from a user perspective. 
   The snap ratio, also referred to as tactile feel, is the response to an operator depressing a keypad and feeling a quick force drop (or click) at their fingertip. Generally, the snap ratio for a dome type switch is represented as shown below: 
   
     
       
         
           
             Snap 
             ⁢ 
             
                 
             
             ⁢ 
             ratio 
             ⁢ 
             
                 
             
             ⁢ 
             
               ( 
               % 
               ) 
             
           
           = 
           
             
               
                 ( 
                 
                   OF 
                   - 
                   RF 
                 
                 ) 
               
               OF 
             
             × 
             100 
           
         
       
     
   
   wherein, OF: (Operational Force) is the maximum value of load necessary for deforming the dome shape switch from open position dome shape to flexed state engaging fixed contacts in the closed position: and RF: (Recovery Force) is the value of resistive load at contact closed position. The desired snap ratio is reached when the result of the calculation is 50%.  FIG. 7  is a graph illustrating the snap ratio for a single isolated dome (solid lines), for a typical encased prior art dome (dashed lines), and for a metal dome type switch according to the invention (dotted lines) and as described below. The ordinate designates load, and the abscissa designates operational distance. A large force OF is needed during travel after starting to depress the apex portion. However, when the apex portion is recessed to some degree, the necessary load is reduced and a small RF is sufficient to close the fixed contacts 
   Keypads with snap ratios of 50% have excellent tactile feel and relatively long life. Keypads with snap ratios below 40% have relatively weak tactile feel, yet longer life. It can be determined from the above formula that snap ratio depends upon the variance between Operational Force (OF) and the Recovery Force (RF). This value will directly influence tactile feel, which is caused by force variation. That is, when the force drops from OF to RF, the operator can feel a force change at their fingertip immediately. There have been prior attempts to modify metal dome type switches to improve snap ratio as disclosed by example, U.S. Pat. No. 6,595,653 to Atsushi Saito wherein cut portions are provided along an outer peripheral edge of the dome shaped movable contacts. One shortcoming of this invention is that it includes cuts through the EL (electro luminescent) film to achieve an improved snap ratio. Also, under stress, the switch actuation can shear the adhesive interface between the dome contact and the cuts thereby compromising the hermetic sealing. 
   SUMMARY OF THE INVENTION 
   The present invention provides a metal dome array switch for use in low-profile operating switch panels providing a hermetically sealed unit with a good snap ratio and having a desired click characteristic near to that of an isolated metal dome. The dome array includes a carrier base layer and adhesive layer which have a plurality of holes bounded by annular sections which limitedly engage the lower peripheral surfaces of the metal domes. The holes are larger than the keypad actuation post allowing direct engagement with the domes. The switch array is especially important in cell phone designs where improved snap ratio and tactile feel are desirable. The switch array can also incorporate an electroluminescent (EL) lamp while still improving the snap ratio. The base layer of the array is adhered to the switch substrate to seal the circuitry and the domes. The limited peripheral sealing results in the needed operational requirements while having a minimal effect on the Operating Force and Recovery Force thus providing a snap ratio approaching the ideal isolated dome. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other features of the invention will become apparent upon reading the following description taken in conjunction with the accompanying drawings in which: 
       FIG. 1  is an exploded view of the membrane dome switch assembly in an operating panel according to a preferred embodiment of the invention; 
       FIG. 2  is an enlarged cross sectional view of the switch assembly of  FIG. 1  in the open position; 
       FIG. 3  is an enlarged cross sectional view of the switch assembly of  FIG. 2  is the closed position; 
       FIG. 4  is an enlarged cross sectional view of a switch assembly in accordance with another embodiment of the invention; 
       FIG. 5  is an enlarged cross sectional view of a switch assembly in accordance with another embodiment of the invention; 
       FIG. 6  is an enlarged cross sectional view of a switch assembly in accordance with another embodiment of the invention; 
       FIG. 7  is a graph illustrating the snap ratios of the present and prior switch assemblies and an isolated dome; and 
       FIG. 8  is an enlarged cross sectional view of a prior art switch assembly. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Referring to  FIGS. 1 through 3 , there is shown an operating panel  10  for an electric or electronic instrument, such as a cell phone, having a low profile dome array membrane switch assembly  12  operatively interposed between a key pad  14  and a circuit board  16 . The key pad  14  includes individual keys  18  having downwardly projecting frustoconical actuation posts  20  for selectively actuating dome switch units  22  in the switch assembly  12  to complete a circuit from the open position of  FIG. 2  to the closed position of  FIG. 3  whereat a select circuit is established between a center contact  23  and an annular outer contact  24  on the circuit board  16 . Alternatively, both contacts may be centrally located and closed by the depressed section of the dome. 
   A typical prior art membrane dome array switch assembly  100  is shown in  FIG. 6 . The switch assembly includes a base layer  102  made from a thin flexible material such as polyethylene terephthalate (PET) or similar type material, an adhesive layer  104  adhered to the base layer  102 ; a plurality of domes  106  adhered at the upper surfaces to layer  104  and located at positions corresponding to fixed contact terminals  108  of a printed circuit board  110 ; a spacer layer  112  having a plurality of holes larger than the outer diameters of the dome  106 , which is laminated to the lower surface of adhesive layer  104  and attached to positions corresponding to the metal domes  106 ; an adhesive layer  114  which is laminated to the spacer layer  112  and has a plurality of holes which are larger than the outer diameter of the dome  106 . The adhesive layer is attached to corresponding positions on the printed circuit board  110 . The board  110  includes a fixed center contact  118 ; and an annular fixed outer contact  120  which bring about an electrical connection when the metal domes  106  are depressed via a keypad  122  with center actuation post  124 . It will thus be appreciated that the layers are in composite orientation with the domes and alter the operating and recovery forces as indicated in  FIG. 7 . 
   Referring to  FIGS. 1 through 3 , the present invention removes the composite resistance at the top of the domes and allows actuation with a snap ratio approaching isolated domes. The membrane switch assembly  12  includes a carrier base layer  30  having an adhesive layer  32  on a bottom surface attached to the lower and outer peripheral surfaces of a plurality of elastically deformable domes  34  located in assembly at positions corresponding to the fixed contacts  23 ,  24  on circuit board  16 . Both layers  30 ,  32  have conformal hemispherical sectors or sleeves  36 ,  37  having center holes  38 ,  39 , respectively. The holes have a smaller diameter than the outer diameter of the metal dome  34 , and a larger diameter that than the keypad actuation post  22 . Accordingly, the sectors  36 ,  37  adhesively engage only the outer peripheral bases of the domes thereby providing hermetic sealing and presenting only minimal composite rigidity that does not significantly alter the flexing of the domes and resultant actuation forces. A spacer layer  40  is attached to the adhesive layer  32  at a top surface and to a lower adhesive layer  42  at a lower surface. The layers  40 ,  42  include through holes having a diameter larger than the base of the domes  34  and coaxial therewith. The layer  42  is adhered to the top surface of the circuit board  16  placing the domes and posts in operative registry with the associated contacts  23 ,  24 . The domes as illustrated are hemispherical about a vertical axis transverse to the circuit board; however, it will be appreciated that other conventional surfaces of revolution, parabolic or curvilinear, may be used as the dome shaped member. Suitable domes are stainless steel domes available in the P-series from Snaptron Inc., Windsor Colo. having diameters of 0.156 to 0.236 in. Dimples may be provided at the apexes, but are not required. 
   The actuation post  20  thus directly contacts the dome  34  with limited resistance at the sealed interface, which configuration results in an improved snap ratio near to that of a single isolated metal dome. 
   The base layer  30  is made from a thin flexible material such as polyethylene terephthalate (PET) or similar type material. The sections  36 ,  37  provide secure attachment of the domes  34  about their perimeter to the base layer  30  and adhesive layer  32 . Although the perimeter of the domes  34  are secure and sealed, the center is free of carrier and adhesive materials. Having the actuation post  20  directly contact the domes  34  also results in a thinner metal dome array. 
   The hole sizes  38 ,  39  are determined by the size of the metal dome  34  and associated actuation post  22 . The actuation post  20  is typically less than half the functional diameter of the metal dome  34 , so as to maximize the actuation force towards the center of the dome. A 5 mm diameter metal dome might typically have an actuation post  20  diameters of approximately 2 mm. The minimum diameter of the hole should be greater than the diameter of the actuation post  22  so as to exclude the carrier base layer material from coming between the dome  34  and actuation post  22 . A hole size of 3 mm would allow sufficient area for the actuation post diameter as well as a margin of error for tolerances associated in the manufacturing process of the metal dome array. 
   The maximum diameter of the hole should be such that it allows the dome  34  to be properly adhered to and secured to the base layer  30 . A 3 mm hole for a 5 mm diameter metal dome would allow for 1 mm of overlapping contact area about the perimeter of the dome, with which to secure the dome. It has been determined that substantially improved snap ratios may be obtained wherein the hole diameter is between 40% to 60% of the dome base diameter. A lesser ratio can increase the recovery force lowering the snap ratio. A higher ratio can provide insufficient contact to maintain mechanical positioning and hermetic sealing during actuation. 
   Metal domes as described typically have a circular profile normal to the axis of operation, however, other dome shapes such as 3 leg (tri-lobe), four leg, or other designs, which use outer segments to engage the fixed outer contact  24 . 
   Referring again to  FIG. 7 , single dome switch assemblies in accordance with the foregoing were tested with the results set forth below in Table 1. 
   
     
       
             
             
             
           
             
             
             
             
             
           
             
             
             
             
             
           
         
             
                 
               TABLE 1 
             
           
           
             
                 
                 
             
             
                 
               Travel 
               Force (g) 
             
           
        
         
             
                 
               (mm) 
               Dome 
               Prior Art 
               Invention 
             
             
                 
                 
             
           
        
         
             
                 
               0 
               0 
               0 
               0 
             
             
                 
               0.11 
               170 
               181.2 
               173.6 
             
             
                 
               0.2 
               85 
               106.4 
               90.7 
             
             
                 
                 
             
           
        
       
     
   
   It will be noted that the present invention provides a snap ratio of 47%. An isolated dome provides a switch&#39;s snap ratio is 49%. The prior art structure&#39;s snap ratio is 41%. Thus, the present invention has a good click characteristic proximate that of the single member of the metal dome. 
   Referring to  FIG. 4 , there is shown an embodiment of the invention which includes a thin flexible EL lamp  50  having an additional adhesive layer  52  adhered therebetween and the top carrier layer. The addition of the flexible EL lamp  50  and adhesive layer  52  slightly reduces the snap ratio, although still improved (higher) than in the prior art array of  FIG. 8 . It should also be noted that a liquid adhesive or other adhering agent could be used in the place of a double-sided adhesive film for the various adhesive layers, particularly adhesive layer  52  that adjoins the EL lamp  50 . A liquid adhesive has the additional advantage of providing a thinner assembly between the keypad actuator and domes, which would improve the snap ratio. 
   Referring to  FIG. 5 , there is shown a further embodiment which includes an EL lamp  60  having an additional adhesive layer  62  adhered therebetween and the top carrier layer. The adhesive layer  62  has a through hole  64  coaxial and the same diameter as the holes in the carrier layer. The holes in adhesive layer  62  may be a size similar to the holes in the carrier layer, although they can also be of a diameter larger than the metal dome. For a 5 mm dome, a hole of 5.5 mm may be used. 
   Referring to  FIG. 6 , there is shown a further embodiment in which a center hole  64  is provided in the lamp  60 . The advantage of this embodiment is that it allows direct contact between the keypad actuator and the domes, although the holes in the EL lamp may less desirable since it could impact light output of the EL lamp. 
   Having thus described a presently preferred embodiment of the present invention, it will now be appreciated that the objects of the invention have been fully achieved, and it will be understood by those skilled in the art that many changes in construction and widely differing embodiments and applications of the invention will suggest themselves without departing from the spirit and scope of the present invention. The disclosures and description herein are intended to be illustrative and are not in any sense limiting of the invention, which is defined solely in accordance with the following claims.