Electricity: circuit makers and breakers – Multiple circuit control – Multiple switch
Reexamination Certificate
2000-04-28
2001-11-27
Scott, J. R. (Department: 2832)
Electricity: circuit makers and breakers
Multiple circuit control
Multiple switch
C200S305000, C361S212000, C361S220000
Reexamination Certificate
active
06323445
ABSTRACT:
TECHNICAL FIELD
This invention relates generally to membrane switches. More particularly, the invention concerns arrangements for protecting systems or subsystems that use membrane switches from electrostatic discharge.
BACKGROUND OF THE INVENTION
A membrane switch is a momentary switch device having at least one contact that is provided on or made of a flexible substrate. Membrane switches are commonly used in computer keyboards and myriad other applications. In a conventional membrane switch arrangement, three layers of thin polyester (e.g., Mylar™) sheets are sandwiched together. Membrane switches may be constructed with a rigid layer, e.g., a layer of glass, plastic, or printed circuit board material, as used in a touch panel membrane switch, touchpad or keypad. The outer two layers each carry on their opposing inside surfaces conductive switch circuit patterns. An intermediate layer is disposed between the outer two layers and acts to isolate the switch circuit patterns from each other. The switch circuit patterns may, however, be selectively brought into electrical contact with each other through contact apertures provided in the intermediate layer. When selective contact is made between the opposing switch circuit patterns, an electrical circuit may be completed to perform a predetermined action.
Referring to
FIG. 1
, a conventional computer keyboard
100
may utilize a conventional membrane switch structure
200
, as shown in
FIG. 2
, which is installed underneath a set of keyboard keys
101
, to convert key selections into electrical signals representing corresponding alphanumeric characters or functions. Membrane switch structure
200
generally has three layers
201
,
202
and
203
. Outer layers
201
and
202
sandwich intermediate layer
203
. The three layers are generally made out of a thin insulative sheet, for example, polyester, glass, plastic or even common printed circuit board material. Outer layers
201
and
202
each have, on respective opposing inside surfaces
210
and
211
, switch circuit patterns (
208
and
209
, respectively), which may be deposited by print transfer (e.g., silk screening), spraying or other techniques. The circuit patterns may be printed with suitable conductive inks, e.g., a polymer-based conductive ink having silver, carbon, and/or other conductive particles in suspension. Typically, each keyboard key is coupled to a flexible plunger positioned to make contact with a backside of an upper one of the two outer layers of the membrane switch. Depression of a selected keyboard key
101
causes a corresponding plunger to exert pressure on the upper outer layer of the membrane switch. The resulting pressure causes an electrical circuit printed on the inner face of the top outer layer to come resiliently into electrical contact with a corresponding circuit printed on the inner face of the bottom outer layer of the membrane switch, through contact apertures provided in the intermediate layer. The electrical contact results in generation of a signal input to an integrated circuit (IC) located within the keyboard. The IC, in turn, provides a digital output signal readable by the associated computer.
Circuit patterns
208
and
209
are appropriately laid out to provide contact points and lines of conduction for each of keyboard keys
101
within a conventional switch matrix. Thus, e.g., by key depression at location
212
, upper membrane switch circuit
208
may be brought into electrical contact with the lower membrane switch circuit
209
, through contact aperture
204
in intermediate layer
203
. The electrical contact between the designated contact points of opposing circuits
208
and
209
results in generation of an electrical signal (high or low voltage) on a particular line of the keyboard switch matrix. By recognizing the line on which the signal is generated, and the timing thereof, the keyboard mounted IC can discriminate which of keyboard keys
101
has been depressed.
Membrane switch circuitry tends to be susceptible to disturbance by electrostatic discharge (ESD). ESD events can induce noise voltages and currents on the circuits of the membrane switch resulting in device operation problems. Various schemes are known which serve to reduce the ESD susceptibility of membrane switches. Most conventionally, a metal grounding plate may be positioned under an entire switch circuit pattern for providing ESD protection. Although a metal grounding plate can shield the associated switch circuitry from ESD events, use of a metal grounding plate can significantly increase keyboard fabrication costs and product weight. A second known approach involves use of an insulative sheet with a layer of continuous conductive material printed thereon. The printed sheet emulates the presence of the metal grounding plate at a lower cost. While a continuous layer of conductive material on an insulating sheet is less expensive than a metal grounding plate, there is significant cost associated with the conductive ink required to produce the continuous conductive layer. Another known approach is to print on an insulating sheet a rectilinear conductive grid pattern
301
(see FIG.
3
), or a concentric circle grid pattern
401
(see FIG.
4
). Using these types of grid patterns can reduce the costs associated with a continuous layer of conductive ink. However, depending on the spacing and location of the gridlines relative to the lines of the switch circuit patterns, the grid patterns do not necessarily ensure that all lines of the switch circuit patterns are adequately protected from ESD. Increased ESD protection can be achieved by increasing the density of the grid lines, but only at a higher cost approaching the cost of a continuous conductive layer.
It would be desirable to have a membrane switch configuration providing optimized ESD protection at reduced costs.
SUMMARY OF THE INVENTION
In a first aspect, the invention provides a layer for use in a membrane switch. The layer has opposing first and second surfaces. A membrane switch circuit pattern is provided on the first surface, and an ESD protection circuit pattern is provided on the second surface. The protection circuit pattern includes a portion at least generally replicating at least a portion of the membrane switch circuit pattern.
In a second aspect, the invention provides a membrane switch having a first outer layer of insulating material having on its inside surface a first switch circuit pattern, and a second outer layer of insulating material having on its inside surface a second switch circuit pattern. The membrane switch firther has an intermediate insulating layer disposed between the first and second outer layers. The intermediate layer has contact apertures for selectively permitting electrical contact portions of the first and second switch circuit patterns to be brought into electrical contact with each other. An ESD protection circuit is laid down in close proximity to the first and second outer layers. At least a portion of the ESD protection circuit at least generally replicates at least a portion of a composite layout of the first and second circuit patterns.
In a third aspect, the invention provides an ESD protected keyboard having a plurality of keyboard keys and a membrane switch structure associated with the plurality of keyboard keys. The membrane switch comprises a membrane switch circuit pattern defining a key switch matrix and an ESD protection circuit pattern. At least a portion of the ESD protection circuit pattern at least generally replicates at least a portion of a layout of the membrane switch circuit pattern.
By tracking the underlying switch circuit patterns, more effective and efficient use is made of the applied conductive material, as compared to a metal grounding plate, or a printed conductive layer or grid. Advantageously, switch circuit pattern replicating ESD patterns in accordance with the present invention may be applied by silk-screen printing in the same manner as are the switch circuit patterns themselves. The advantages of ESD protection circ
Banner & Witcoff , Ltd.
Microsoft Corporation
Scott J. R.
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