Keyboard switch assembly having flexible contact layer with snap initiator dome
BACKGROUND OF THE INVENTION 
     This invention is directed to keyboards employing snappable plastic domes such as shown in U.S. Pat. No. 3,860,771 and is more particularly directed to a modification of the dome structure in said patent to achieve effective snapping of the dome in cases in which the actuator or button configuration is such that a good clean snap cannot be consistently achieved. 
     It has been found in practice that in order for a plastic dome of the shape shown in U.S. Pat. No. 3,860,771 to provide consistently good snap when depressed by an actuator portion of a button, the actuator portion should be of a configuration such that a sufficient surface area of the dome is being engaged during depression to effect snap. 
     In addition, it is also important that the button does not include other surfaces which may engage the dome while it is being depressed because this too will frequently cause poor snapping of the dome. 
     The present invention provides a solution to the aforementioned difficulties by providing a snap initiator at the top of the dome which will compensate for variations in the shape of the actuator. The snap initiator will in effect act as a portion of the actuator so that the forces caused by button depression are applied across the dome in a manner in achieve consistently good snap. 
     BRIEF SUMMARY OF THE INVENTION 
     This invention comprises a keyboard having a plurality of domes which are depressible by buttons having actuator portions, the domes having a snap initiator formed at the top thereof to provide a consistently good snap action. Thus with the invention buttons having a relatively tiny actuator portion, a pair of raised surfaces which act as actuator portions or other shaped actuators can be accomodated by use of domes containing snap initiators and thus expensive modifications in the shape of the buttons can be avoided by the use of the snap initiator of this invention. 
     In the preferred embodiment the snap initiator is hollow and concave with respect to the dome under surface and is preferably bar shaped. 
     It should also be understood that in certain keyboards, all domes need not contain snap initiators if some of the actuators are shaped such that a good snap may be achieved. For example, in a calculator such as the Hewlett-Packard Company HP-22, only the larger buttons, for example, the numerical buttons are shaped such that snap initiators should be provided to achieve consistently good snap action. Whereas, the remaining smaller buttons of the HP-22 calculator are shaped so that the conventional dome as shown in U.S. Pat. No. 3,860,771 need not be modified. 
     It should also be understood that the hollow initiators may be stiffened if desired by using a filler, for example, a hard plastic such as epoxy. However, it has been found to date that this has not been necessary. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a top view of a keyboard according to the invention; 
     FIG. 2 is a side view of the keyboard of FIG. 1; 
     FIG. 3 is a top view of the bottom circuit board and circuit of the keyboard disclosed herein; 
     FIG. 4 is a bottom view of the top flexible domed sheet of the invention showing the upper circuit pattern; 
     FIG. 5 is a sectional view taken along line 5--5 in FIG. 1; 
     FIG. 6 is a sectional view taken along line 6--6 in FIG. 1 to illustrate the connection of a portion of the upper circuit pattern to the lower circuit pattern by a pin; 
     FIG. 7 is a greatly enlarged top view of the dome of the invention; 
     FIG. 8 is a sectional view taken along line 8--8 in FIG. 7; 
     FIG. 9 is a sectional view taken along line 9--9 in FIG. 8; 
     FIG. 10 is a bottom view of the key used in the keyboard of the disclosure; 
     FIG. 11 is a sectional view taken along line 11--11 in FIG. 10; 
     FIG. 12 is a sectional view taken along line 12--12 in FIG. 11; 
     FIG. 13 is a sectional view of another dome embodiment of the invention; and 
     FIG. 14 is a top view of the dome in FIG. 13 without the actuator of FIG. 13. 
    
    
     DETAILED DESCRIPTION OF THE DISCLOSURE 
     Reference should now be had to FIGS. 1 to 14 for a detailed description of the preferred embodiment of the disclosure. A keyboard 20 comprises a frame 21 preferably made of insulative or plastics material such as Lexan having windows 21-1. The keyboard includes an insulator sheet of flexible and resilient plastic 22 preferably of polyester such as Mylar, for example, of 5 mil thickness having depressible and snappable convex domes 22-1 of the configuration as shown in U.S. Pat. No. 3,860,771 and which have been modified to include a snap initiator 22-2. The domes 22-1 are convex with respect to the top of the sheet 22; that is, the portion not supporting the circuit pattern 23. 
     The sheet 22 also supports the aforementioned circuit pattern 23 which may be conductive plastic ink, etched copper. The circuit pattern includes circuit lines 23-1, depressible contactors 23-2 and a contactor 23-3 for supplying power to the circuit pattern 23. 
     The contactors 23-2 are supported on the underside of the domes 22-1 and preferably extend into the snap initiators 22-2. Positioned under the sheet 22 is a snap through layer 24 of plastic non-conductive material such as Mylar or other well known non-conductive materials the snap through layer 24 has register openings 24-1 through which the contactors 23-2 may extend to engage portions 26-1 of the underlying circuit pattern 26. 
     The underlying circuit pattern 26 comprises contacts 26-1, circuit lines 26-2 and terminals 26-3 to which external connections to the keyboard are made. The circuit pattern 26 may be etched copper or conductive plastic ink. 
     Additionally, the circuit pattern 26 includes a contact 26-5 which is coupled to contactor 23-3 through a metal pin 31 (see FIG. 6). The circuit pattern 26 is positioned on an insulator layer 25 for example made of, epoxy or polyester, etc., and the entire keyboard is held together by rods 21-2 extending from the plastic frame which is heated to form heads 21-3. The rods 21-2 may alternatively be threaded and nuts may be used to hold the assembly together. 
     Buttons 27 are pivotally positioned in the frame. Each of the buttons is made of plastics material such as ABS, Lexan (see FIGS. 5, 10, 11 and 12) and have a pin 27-1 which rotates in frame channels 21-4. The buttons 27 have portions carrying indicia which extend through the windows 21-1 so that the user may depress the button 27. 
     With the particular button 27 shown there are provided actuator bars 27-2 and 27-3 at the bottom thereof which engage the snap initiator in order to snap the domes 22-1. While the bars themselves are adequately shaped to depress the conventional dome as shown in U.S. Pat. No. 3,860,771, the particular type of button in the HP-22 calculator also has a sloping surface 27-4 which because of its height also applied a force to the conventional dome when it was being depressed. The surface 27-4 in applying a force interferred with the attaining of a good snap since it touched the dome side while the dome was being depressed by the bars 27-2 and 27-3. 
     In order to overcome the difficulty in attaining consistently good snap, the present invention has provided a bar shaped concave protrusion or initiator 22-2 at the top of the dome so that the bars 27-2 and 27-3 will engage the initiator to produce effective snap while preventing the surface 27-4 from engaging the dome 22-1 to prevent a good snap. The snap initiator 22-2 is preferably at the dome top and is preferably centered with respect to the dome 22-1 although it may be somewhat off center if necessary to accomodate a particularly pecular shaped actuator. 
     Reference should now be had to FIGS. 7 to 9 which show the dome 22-1 and initiator 22-2 in more detail to accomodate the button 27 shown in FIGS. 10 to 12. In this figure the dome has an annular opening of 312 mils, a step or pedestal 21-1A is formed with a spherical radius R 2  of about 125 mils as observed and has a height of about 4 mils, the dome cap 21-1B is formed with a spherical radius R 1  of 500 mils and has a height of 16 mils. The concave protrusion 22-2 has a height H of 10 mils, a width of 32 mils and a length of 125 mils. 
     It should be understood that these dimensions given are illustrative and not meant to be limiting and will vary obviously depending upon the actuator and the size of the dome. 
     In FIGS. 13 and 14, there is shown a plastic sheet 29 as aforementioned with a snappable dome 30 which is provided with a cylindrical concave protrusion or initiator 30-1 to accomodate a depressible button 31 having a small contact area actuator 31-1. The actuator 31-1 is used to depress the initiator surface 30-1A. 
     In cases such as this the actuator 31-1 does not depress the dome top in a manner to consistently produce a good snap since it has a tendency to cause the rolling over of a small portion of the dome 32 prior to causing it to snap. 
     To overcome this problem a substantially cylindrical initiator 30-1 is provided which is substantially greater in top surface area 30-1A than the tip of the actuator 31-1. The initiator 30-1 will cause the force provided by the actuator 31-1 to be applied over the dome in a manner to effect good snap. For example, with an actuator having a contact surface of a diameter of 60 mils a snap initiator 30-1 of a top 30-1A, diameter of 100 mils and height of 10 mils, may be provided to obtain good snap. 
     The conventional contact is shown at 34 on the circuit board 35 and the dome 30 supports the conventional contactor 36 as shown which is adhered thereto. A spacer is also shown at 37. 
     It should also be observed that the snap initiator engages the button to urge it upwardly to prevent the button from being loose in the frame. The dome with initiator, because of the material used, therefore acts as a spring to spring load the button.