Patent Publication Number: US-7712954-B2

Title: Device for assembling a touch-type crystal on a case

Description:
This application claims priority from European Patent Application No. 05012733.1 filed Jun. 14, 2005 and Swiss Patent Application No. 00507/06 filed Mar. 29, 2006, the entire disclosure of which is incorporated herein by reference. 
   FIELD OF THE INVENTION 
   The present invention concerns a device for assembling a tactile or touch type crystal on a case, wherein contact zones, located close to one edge of the crystal, for connecting electrodes structured on the inner face of the crystal, to an electronic module contained in the case, are hidden and electrically insulated. 
   The invention will be more specifically illustrated by a wristwatch having a tactile crystal extending to the outer edge of the middle part. 
   BACKGROUND OF THE INVENTION 
   The most common way of assembling a tactile crystal, generally made of sapphire, on a wristwatch case is explained hereinafter with reference to  FIG. 1  and to the schematic cross-section of  FIG. 2 , in which the contrasts and thicknesses have been greatly exaggerated for better comprehension. 
   The wristwatch shown includes, in a known manner, a case  1  formed of a middle part  3  closed on its bottom part by a back cover  5  and on its top part by a crystal  10  delimiting a compartment  7 . Compartment  7  is for housing a watch movement  8  for the display of timer data on a dial  6 . In the example shown, the display is an analogue display by means of hands for the hours  6   a , minutes  6   b  and seconds  6   c.    
   It can also be seen that the bottom face  11  of crystal  10  includes five electrodes  15  forming, for example, capacitive sensors, the electrodes  15  each being connected via conductive paths  16  to contact zones  17  made close to the edge  12  of crystal  10 . The conductive network  15 ,  16 ,  17  is made in a known manner by structuring a transparent conductive oxide, such as indium and tin oxide (ITO), deposited for example by vapour deposition and having a thickness comprised between 25 and 75 nm. Electrodes  15  and conductive paths  16  can be made practically invisible by depositing dielectric layers in the spaces comprised between the electrodes  15  and paths  16 , as disclosed for example in EP Patent No. 1 457 865. However, contact zones  17 , which are placed in contact with a connector  19  passing through an insulating connector ring  2  and connecting an electronic module  9 , capable of detecting the presence of a finger above one electrode  15  and of sending a control signal to movement  8 , are much more visible. 
   When crystal  10  is assembled on case  1  by means of a bezel  13  secured to the middle part, bezel  13  need only be given sufficient width to conceal contact zones  17 . 
   However, there exist cases, particularly for wristwatches, wherein crystal  10  extends to the outer edge of middle part  3 . Crystal  10  is assembled for example via bonding on the shoulder of the middle part  3  or on an insulating connector ring  2 . In order to conceal the adhesive joint, metallising has to be carried out by depositing at least one very thin anchorage layer, generally by chromium evaporation, and possibly other metals or alloys, close to the edge of the crystal, in order to try to harmonise the hue of the middle part shoulder seen through the crystal with that of the rest of the middle part. For tactile crystals, there is thus a significant risk of short-circuiting two neighbouring contact zones  17 , or of creating stray capacitance. 
   SUMMARY OF THE INVENTION 
   It is thus an object of the present invention to overcome the drawbacks of the aforecited prior art by providing a tactile crystal that can be assembled, for example by bonding, to the shoulder of the aperture of a case while concealing the contact zones, without any risk of causing short-circuits 
   The invention therefore concerns a device for assembling a tactile crystal, made of a transparent material, resistant to temperatures higher than 500° C., to the shoulder of the aperture of a case, such as a wristwatch case. The tactile crystal includes on its inner face, in a known manner, a conductive network comprising transparent electrodes connected by conductive paths to generally grouped contact zones to form a connection area, opposite the ends of connectors connected to an electronic module housed in a compartment of the case. The tactile crystal is characterized in that it includes, close to its edge, a deposition of enamel of which at least one part of the surface conceals the electrode connection area. 
   The transparent material resistant to temperatures higher than 500° C. can be a mono or polycrystalline material, for example quartz, spinelle or corundum, particularly sapphire. An amorphous material may also be used, such as a mineral glass, provided its softening point is higher than the temperature necessary to carry out the enamel deposition. 
   According to a first embodiment, the enamel deposition has the shape of a tape following the inner contour of the crystal and whose width is at least equal to the width of the connection area. 
   It is also desirable for the conductive network to be structured on a surface that has no discontinuities. 
   For this purpose a hollow is machined in the inner face of the crystal, the contour of which at least substantially corresponds to that of the connection area, enamel is deposited in one or several steps until a thickness, slightly greater than the depth of the hollow is achieved, then polishing is carried out to make the surface level of the enamel the same as that of the crystal in order to structure the conductive network. 
   According to a second embodiment, the enamel deposition is carried out in a groove formed in the thickness of the crystal, the depth of the groove being at least equal to the width of the connection area. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other features and advantages of the present invention will appear in the following description illustrated by a wristwatch given by way of illustrative and non-limiting example, with reference to the annexed drawings, in which: 
       FIG. 1  shows a top view of a wristwatch with a tactile crystal; 
       FIG. 2  corresponds to a semi-cross-section along the line II-II of  FIG. 1  for a wristwatch according to the prior art; 
       FIG. 3  shows a semi-cross-section along the line II-II of  FIG. 1  for a wristwatch according to the invention; 
       FIGS. 4 to 6  show the various steps of a method for making a tactile crystal according to the invention, according to a first embodiment, and 
       FIG. 7  shows a tactile crystal according to a second embodiment. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIGS. 1 and 2  have already been described for indicating the closest prior art, and only  FIG. 4  and the following Figures will be described hereinafter, in which the same references are used to designate the same elements of the wristwatch. 
     FIG. 3 , which is a semi-cross-section along line II-II of  FIG. 1  passing through two sensors  15 , a conductive path  16  and a contact zone  17 , shows that edge  12  of crystal  10  extends to the outer surface of middle part  3  and that the crystal  10  is not held by means of a bezel secured to the middle part  3 . The inner face  11  of crystal  10  includes, close to its edge, an enamel deposition  20  which extends above the entire connection area  18  (shown in dotted lines in  FIG. 1 ) where the five contact zones  17  of the five electrodes  15  are grouped. 
   It is possible to carry out this enamel deposition solely in the connection area  18  and to metallise the remaining part of the shoulder of the case where there is no risk of short-circuiting, then to assemble the crystal, for example by bonding. This method requires an additional step, which increases the costs, and it can be difficult, or impossible to find the same hue and the same tone for the enamel and the metallising, which is inconvenient when aesthetic appearance is also important. This is why it is preferable to carry out the enamel deposition in the form of a continuous tape that goes all around crystal  10  and whose width is at least equal to the width of connection area  18 , such that the same hue is observed over the entire periphery of the case. 
   Depending upon the type of case, it is also possible to mount crystal  10  by snap fitting with the insertion of a synthetic sealing gasket between the crystal and the case. 
   With reference now to  FIGS. 4 and 6 , a method will be described hereinafter for making a tactile crystal according to the invention, the material forming the crystal being sapphire by way of example. 
   In a first step shown in  FIG. 4 , a hollow  14  is made close to edge  12  of crystal  10  by means of a diamond grinding-wheel over a depth comprised between 0.1 mm and 0.5 mm, depending upon the thickness of crystal  10 . 
   In a second step shown in  FIG. 5 , an enamel deposition  20  is carried out, in one or several steps, via known soaking, spraying, buffing or silk printing techniques, each step being followed by baking, until the enamel slightly projects above the inner face of the crystal. 
   According to a step that is not shown, the entire inner surface of the crystal is polished so as to remove any discontinuities between the enamel layer and the rest of the crystal. 
   In a last step, shown in  FIG. 6 , the conductive network  15 ,  16 ,  17  is structured with a transparent conductive oxide, such as indium and tin oxide (ITO). This step will not be described further since it is well known to those skilled in the art. Crystal  10  can then be assembled on shoulder  4  of the middle part with the insertion of an adhesive joint, then pressed. 
   With reference now to  FIG. 7 , a second embodiment is shown. As can be seen, edge  12  of crystal  10  is machined to obtain a groove  25  in which the enamel layer  20  is formed. This embodiment has the advantage of making the porosities in the enamel mass invisible, facilitating the machining and polishing operations including at the aperture of groove  25 . This embodiment also allows much easier polishing of the surface where contact zones  17  have to be structured, without any risk of creating zones of discontinuity able to have a detrimental effect on conductive paths  16  connecting sensors  15  and contact zones  17 . 
   In the preceding description, the invention was illustrated by a flat tactile crystal, but it is clear that the invention also applies to any convex crystal.