One-sided electrode arrangement on an intermediate spacer for a touchscreen

FIELD OF THE INVENTION

The present invention relates to two-dimensional coordinate location devices and, more particularly, to analog and digital touch-sensitive membrane switches and methods for fabricating the same.

BACKGROUND OF THE INVENTION

Touch-sensitive membrane switches have been incorporated into many electronic devices to enable operators to provide instructions to the device by selecting a corresponding horizontal and vertical coordinate location on the membrane switch. For example, membrane switches are often installed over the viewing screen of a cathode ray tube. The user of a device including such a “touchscreen” is able to operate the device by pointing to and depressing a particular location on the screen corresponding to a desired menu selection. The touchscreen then generates a voltage signal corresponding to the horizontal (“x”) and vertical (“y”) coordinates of that location. For such an application, the layers used to fabricate the membrane switch are transparent.

Other conventional applications for membrane switches are numeric and function keypads on diverse electronic items, such as microwaves, television sets, calculators, medical instrumentation, and various other devices. Membrane switches may be designed for manual finger or stylus depression for operation. The range of applications for membrane switches is ever increasing, as is the need for producing low-cost membrane switches.

One type of conventional membrane switch, often used for touch-sensitive screens, is the analog membrane switch. The membrane switch comprises a sandwich of top and bottom membranes with at least the top membrane being made from a flexible material. More typically, both membranes are made from flexible dielectric sheets. One surface of each membrane is coated with a semiconductive resistive layer, such as indium tin oxide (“ITO”), or a conductive layer such as gold.

Construction and operation of conventional membrane switches is well known in the art. One method of constructing an analog membrane switch is described in U.S. Pat. No. 5,228,562 to Burk. The membrane switch includes a flexible top membrane having a lower conductive surface; a lower membrane having an upper conductive surface; and an intermediate spacer disposed therebetween. The intermediate spacer includes a central rectangular aperture, and has upper and lower opposite surfaces. One or more first (y-axis) electrodes are formed on the upper surface of the intermediate spacer along a set of parallel edges, and one or more second (x-axis) electrodes are applied on the lower surface of the intermediate spacer along another set of parallel edges. The electrodes are applied, typically, by silk-screening with a conductive ink.

A random or fixed array of small raised dielectric projections is deposited on the upper surface of the lower membrane. Next, conductive adhesive is applied between the intermediate spacer and the top and bottom membranes to secure the intermediate spacer in place with the x- and y-axis electrodes in electrical contact with the top and bottom membranes, respectively. At this point, the raised dielectric projections are positioned within the rectangular aperture of the intermediate spacer, normally maintaining the lower conductive surface of the top membrane separated from the upper conductive surface of the lower membrane.

However, when the top membrane is depressed through the central aperture of the intermediate spacer, it contacts the bottom membrane between the projections. The x and y coordinate locations of this point of depression can be obtained by monitoring voltage drops across the electrodes. Typically, a uniform potential, such as 5 volts, is first applied across a first set of electrodes formed on the upper surface of the intermediate spacer while the voltage drop across the second set of electrodes on the lower surface of the intermediate spacer is monitored. This voltage corresponds to the horizontal, or “x” coordinate of the depression pointer. This arrangement is then switched, with a potential applied across the second set of electrodes and the voltage drop across the first set of electrodes being monitored to determine the vertical, or “y” coordinate. Monitoring of first and second sets of electrodes oscillates in this manner so that both the x and y coordinates of a depression point can be rapidly measured when such a depression occurs. Other voltage monitoring methods may be used to obtain similar results.

The method of producing a membrane switch as described above is advantageous in that many of the processing steps, such as application of electrodes, are performed on the intermediate spacer, reducing the opportunities for scratching or marring the fragile conductive coating formed on the top and lower membranes. However, the method is rather cumbersome because it requires application of electrodes on both upper and lower surfaces of the intermediate spacer. To this end, the intermediate spacer needs to be flipped over after one of its surfaces is applied with a set of electrodes, and the intermediate spacer's position needs to be carefully adjusted for precise alignment and secured before its other surface is applied with another set of electrodes. With such careful adjustment, however, application of electrodes on both sides of an intermediate spacer and possible misalignment of the electrodes often result in a final membrane switch product that is not reliable.

SUMMARY OF THE INVENTION

The present invention provides a membrane switch, and method for producing the same, which significantly reduce the amount of labor required to produce an intermediate spacer, to improve overall production efficiency of a membrane switch. Furthermore, the present invention improves the overall reliability of a membrane switch.

The membrane switch comprises a first substrate having a first electrically conductive surface; a flexible second substrate having a second electrically conductive surface; and an intermediate spacer defining a central aperture and being formed of a dielectric intermediate substrate having a third, lower surface and a fourth, upper surface. One or more first electrodes having a first contact portion are formed on the upper surface of the intermediate substrate. Further, one or more second electrodes having a second contact portion are formed on the upper surface of the intermediate substrate.

The intermediate substrate provided with the first and second electrodes is sandwiched between a first adhesive layer, which is applied to the lower surface of the intermediate substrate, and a second adhesive layer, which is applied to the upper surface of the intermediate substrate. The second adhesive layer defines one or more first apertures, which are adapted to overlie the first contact portions of the first electrodes provided on the upper surface of the intermediate substrate. The second adhesive layer further includes one or more second apertures, which are adapted to overlie the second contact portions of the second electrodes provided on the upper surface of the intermediate substrate. The second apertures are filled with conductive epoxy material so as to provide an electrical conduction path between the second contact portions and the second adhesive layer. Each of the one or more first contact portions defines a hole extending therethrough between the intermediate substrate and the first adhesive layer. The hole has a cross-sectional area that is smaller than that of the first contact portion. The first aperture through the second adhesive layer and the hole through the intermediate substrate and the first adhesive layer are then filled with conductive epoxy material so as to provide an electrical conduction path between the first contact portion and the first adhesive layer.

The intermediate spacer thus constructed is then secured between the first surface of the first substrate and the second surface of the second substrate, such that the first and second electrodes are in electrical contact with the first and second conductive surfaces, respectively. The second substrate is depressible through the central aperture of the intermediate spacer to contact the first substrate.

In one aspect of the present invention, the intermediate spacer is secured between the first substrate and the second substrate using adhesive layers in the form of films. To assist in proper placement of the various layers used in the intermediate spacer, the dielectric intermediate substrate and the adhesive layers may be assembled together prior to cutting the intermediate spacer to define the central aperture therein.

In another aspect of the present invention, the first and second conductive surfaces of the first and second substrates include conductive bus bars that are formed of material having less resistivity than the material forming the conductive surfaces. The bus bars extend over the length of the conductive surfaces, so as to enhance uniform electrical conduction across the conductive surfaces, thereby improving the linearity of the membrane switch.

In yet another aspect of the present invention, the intermediate spacer further includes one or more dielectric layers applied over the upper surface of the intermediate substrate without covering the first and/or second contact portions to insulate the first and second electrodes, depending on the space available on the intermediate substrate to accommodate the first and second electrodes.

The membrane switch of the present invention and method for producing the same result in a significant decrease in the amount of labor required to produce an intermediate spacer and, hence, a membrane switch. Because both x-axis and y-axis electrodes are applied on the same side of a third, intermediate substrate to form an intermediate spacer, the present method eliminates the labor formerly required to apply electrodes on both sides of an intermediate substrate. As a result, a significant increase in overall production efficiency in the manufacturing of a membrane switch is achieved. Furthermore, because the present method of producing a membrane switch applies all electrodes on the same side of an intermediate substrate without having to flip the substrate over, the method improves the reliability of a final membrane switch product.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing aspects and many of the attendant advantages of this invention will become more readily appreciated by reference to the following detailed description, when taken in conjunction with the accompanying drawings, wherein:

FIG. 1

provides a pictorial view of an analog membrane switch constructed in accordance with the present invention;

FIG. 2

provides an exploded view of the analog membrane switch of

FIG. 1

;

FIGS. 3A

,

4

A,

5

A,

6

A, and

7

A provide step-by-step illustrations of a method of forming an intermediate spacer used in the analog membrane switch of

FIG. 1

;

FIGS. 3B

,

4

B,

5

B,

6

B, and

7

B are enlarged cross-sectional views taken across lines

3

B—

3

B,

4

B—

4

B,

5

B—

5

B,

6

B—

6

B, and

7

B—

7

B of

FIGS. 3A

,

4

A,

5

A,

6

A, and

7

A, respectively; and

FIG. 8

provides an exploded view of an alternate embodiment of a digital membrane switch constructed in accordance with the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to

FIGS. 1 and 2

, an analog membrane switch

10

is shown, such as would be used for a touchscreen. The membrane switch

10

includes a first substrate, such as a bottom membrane

12

, and a second flexible substrate, such as a top membrane

14

. A first, upper surface

16

of the bottom membrane

12

and a second, lower surface

18

of the second membrane

14

are conductive. An intermediate spacer

20

is disposed between the bottom and top membranes

12

and

14

. The intermediate spacer

20

comprises a third, intermediate substrate

21

, made of dielectric material, which includes a frame portion

22

circumscribing a large rectangular central aperture

24

. The intermediate substrate

21

includes a third, lower surface

26

and a fourth, upper surface

28

. At least one first, or y-axis electrode

30

A,

30

B, each including at least one first contact portion

31

A,

31

B, is provided on the upper surface

28

of the intermediate substrate

21

. Further, at least one second, or x-axis electrode

34

A,

34

B, each including at least one second contact portion

38

A,

38

B, is provided on the upper surface

28

. The intermediate spacer

20

further includes a first, lower adhesive layer

43

applied to the lower surface

26

of the intermediate substrate

21

, and a second, upper adhesive layer

45

applied to the upper surface

28

of the intermediate substrate

21

. The upper adhesive layer

45

defines at least one first aperture

50

, which is sized and shaped to overlie the at least one first contact portion

31

A,

31

B of the y-axis electrodes

30

A,

30

B provided on the upper surface

28

of the intermediate substrate

21

. The upper adhesive layer

45

also defines at least one

30

second aperture

47

, which is sized and shaped to overlie the at least one second contact portion

38

A,

38

B of the x-axis electrodes

34

A,

34

B provided on the upper surface

28

of the intermediate substrate

21

. In a preferred embodiment, the intermediate substrate

21

and the lower and upper adhesive layers

43

,

45

are assembled together and, then, at least one hole

39

extending through the at least one first contact portion

31

A,

31

B of the y-axis electrodes

30

A,

30

B, through the intermediate substrate

21

and the lower adhesive layer

43

, is defined. The hole

39

has a cross-sectional area that is smaller than that of the first contact portion

31

A,

31

B. Thereafter, conductive epoxy material

41

is poured into each first aperture

50

and hole

39

so as to provide an electrical conduction path between the first contact portion

31

A,

31

B and the lower adhesive layer

43

. Conductive material is also poured into the aperture

47

that overlies the second contact portion

38

A,

38

B so as to provide an electrical conduction path between the second contact portion and the upper adhesive layer

45

.

The intermediate spacer

20

thus constructed is then secured between the upper conductive surface

16

of the bottom membrane

12

and the lower conductive surface

18

of the top membrane

14

. At this time, the lower adhesive layer

43

of the intermediate spacer

20

is associated with the upper conductive surface

16

of the bottom membrane

12

to maintain the y-axis electrodes

30

A,

30

B in electrical contact with the upper conductive surface

16

, and the upper adhesive layer

45

of the intermediate spacer

20

is associated with the lower conductive surface

18

of the top membrane

14

to maintain the x-axis electrodes

34

A,

34

B in electrical contact with the lower conductive surface

18

. In operation, a user may depress the top membrane

14

through the central aperture

24

of the intermediate spacer

20

to contact the bottom membrane

12

to activate the membrane switch

10

.

As used herein, the first substrate is referred to as the “bottom” membrane

12

, while the flexible second substrate is described as the “top” membrane

14

, with descriptions of upper and lower surfaces and other components corresponding to these labels. However, no limitation is implied by this, and it should be understood that the membrane switch

10

of the present invention can be disposed in any fashion, such as standing upright on one side. Additionally, the membrane switch

10

is described and illustrated as being rectangular in configuration with a vertical y-axis

56

and a horizontal x-axis

55

. However, membrane switches can be constructed in accordance with the present invention with other configurations, such as squares, or curvilinear shapes and, further, may have a nonplanar configuration. Also, these denotations of the x- and y-axes are provided for illustrative purposes only, and the membrane switch

10

can be disposed in other orientations.

The arrangement of the conductive surfaces and electrodes of the analog membrane switch

10

is well known in the art, and is the same as that for conventionally constructed analog membrane switches. However, in contrast to conventional analog membrane switches, in the present invention, the y-axis and x-axis electrodes

30

A,

30

B,

34

A,

34

B are formed on the same side of the intermediate substrate

21

, rather than on both sides of the intermediate substrate

21

or on the bottom and top membranes

12

,

14

.

The construction of each component of the membrane switch

10

is now described. The top membrane

14

is preferably constructed from a flexible, pliable dielectric material, such as a polyester plastic film. The bottom membrane

12

may be constructed from any dielectric material, and need not be flexible. Thus, rigid sheets of plastic or glass can be utilized. However, a second sheet of plastic film of the same type as the top membrane

14

may be utilized. If stiffening is desired, the bottom membrane

12

may be adhered to a rigid backing plate after assembly of the membrane switch

10

. The upper surface

16

of the bottom membrane

12

and the lower surface

18

of the top membrane

14

are each preferably coated with a semiconductive, resistive material, such as indium tin oxide (“ITO”). However, other conductive coatings can be utilized, such as gold.

To form the bottom and top membranes

12

and

14

, each membrane may be stamped or die-cut from a larger sheet of the conductive-coated dielectric material. An array of spaced-apart raised dielectric projections

57

is preferably deposited on the upper surface

16

of the bottom membrane

12

using conventional techniques, for normally separating the upper and lower conductive surfaces

16

,

18

. A suitable dielectric material for forming the projections is an acrylic polymer.

The conductive upper surface

16

of the bottom membrane

12

preferably includes a conductive bus bar

17

A,

17

B that extends along the length of each of the long side edges

23

A,

23

B of the upper conductive surface

16

of the bottom membrane

12

. The bus bar

17

A,

17

B is formed of material that has less resistivity than the coating forming the upper conductive surface

16

of the bottom membrane

12

. For example, the bus bar can be formed by silk-screening silver, silver-carbon blend, nickel, or gold ink to the upper conductive surface

16

formed of ITO. The bus bars

17

A,

17

B are arranged so that they directly underlie and electrically connect with the first contact portions

31

A,

31

B of the y-axis electrodes

30

A,

30

B when the bottom membrane

12

is assembled with the intermediate spacer

20

. Similarly, the lower conductive surface

18

of the top membrane

14

preferably includes a conductive bus bar

19

A,

19

B that extends along each of the short side edges

25

A,

25

B of the lower conductive surface

18

of the top membrane

14

. As before, the bus bars

19

A,

19

B are formed of material having less resistivity than the coating forming the lower conductive surface

18

of the top membrane

14

. The bus bars

19

A,

19

B are arranged so as to overlie and electrically connect with the second contact portions

38

A,

38

B of the x-axis electrodes

34

A,

34

B when the top membrane

14

is assembled with the intermediate spacer

20

. Because the bus bars

17

A,

17

B,

19

A,

19

B are less resistive than the conductive surfaces

16

,

18

, any electric potential applied to the x- or y-axis electrodes

30

A,

30

B,

34

A,

34

B is first evenly spread over the entire length of the bus bars

17

A,

17

B,

19

A,

19

B and then is applied uniformly over the conductive surfaces

16

,

18

, thereby improving the linearity of the membrane switch

10

.

Next, construction of the intermediate spacer

20

is described. The intermediate substrate

21

of the intermediate spacer

20

is preferably constructed from a polyester dielectric film. This dielectric film may be the same as that used to construct the bottom and top membranes

12

,

14

. One grade of polyester film found to be suitable is available commercially under trade name MELINEX ST 507 from ICI Films of Wilmington, Del. Other dielectric polymer films can also be utilized. The frame portion

22

of the intermediate substrate

21

comprises a narrow border that circumscribes the central aperture

24

. The intermediate spacer

20

further includes a tail portion

59

extending from one side of the frame portion

22

and projecting away from the central aperture

24

. In the preferred embodiment illustrated, the tail portion

59

extends outwardly from the center of a short side

51

B of the frame portion

22

, away from the central aperture

24

. The tail portion

59

provides a path for the y- and x-axis electrodes

30

A,

30

B and

34

A,

34

B to extend from the intermediate spacer

20

for connection to external circuitry. While in the preferred embodiment illustrated one tail portion

59

is provided, it should be apparent that other numbers and placements of tails would be possible. Thus, for example, two opposing tails can be provided with one tail carrying the x-axis electrode leads, and the other carrying the y-axis electrode leads.

The y-axis electrodes

30

A,

30

B are formed on long sides

49

A,

49

B of the intermediate substrate

21

on its upper surface

28

. The y-axis electrodes are used to determine coordinates in the vertical, or y-axis

56

, direction. The x-axis electrodes

34

A,

34

B are formed on short sides

51

A,

51

B of the intermediate substrate

21

, also on its upper surface

28

, and are used to determine the horizontal, or x-axis

55

, coordinates.

Referring to

FIGS. 3A-7B

, steps taken in constructing the intermediate spacer

20

are described in detail. Referring to

FIGS. 3A and 3B

, two y-axis electrodes

30

A,

30

B are applied on the upper surface

28

of the intermediate substrate

21

, for example, by silk-screening with conductive ink. One example of silver ink suitable for forming the electrodes in accordance with the present invention is Screen Printable Silver Conductive Ink 5007, available from Du Pont de Nemours of Wilmington, Del. However, it should be readily apparent that the electrodes could be applied by other conventional methods, e.g., by adhering copper strips onto the upper surface

28

of the intermediate substrate

21

. Each of the y-axis electrodes

30

A,

30

B includes at least one first contact portion

31

A,

31

B disposed along the long sides

49

A,

49

B of the frame portion

22

of the intermediate substrate

21

. Thus, the first contact portions

31

A,

31

B are disposed parallel to each other. In order to provide for an electrical connection between these first contact portions

31

A,

31

B and external circuitry, each y-axis electrode

30

A,

30

B further includes an electrically conductive

30

lead portion

35

A,

35

B that extends from the first contact portion

31

A,

31

B along the tail portion

59

of the intermediate spacer

20

. The first contact portions

31

A,

31

B and the lead portions

35

A,

35

B of the y-axis electrodes

30

A,

30

B form continuous strips and are applied to the intermediate substrate

21

at the same time. The different portions

31

A,

31

B and

35

A,

35

B are referred to only for the purposes of understanding the function and treatment of the electrodes.

Thereafter, in

FIGS. 4A and 4B

, optionally, a dielectric layer

32

is formed over the upper surface

28

of the intermediate substrate

21

while leaving the first contact portions

31

A,

31

B exposed. This dielectric layer may be screen printed onto the intermediate substrate

21

using dielectric ink such as acrylic polymer. Alternatively, a separate film of dielectric material, such as a polyester, may be bonded to the intermediate substrate by heat bonding or adhesion. The dielectric layer

32

is applied, for example, to insulate the lead portions

35

A,

35

B of the y-axis electrodes

30

A,

30

B from the lead portions of the x-axis electrodes, to be described below, when the frame portion

22

of the intermediate substrate

21

does not have sufficient space to provide both the y-axis and x-axis electrodes without shorting. It should be understood, therefore, that application of the dielectric layer

32

is not necessary if enough space exists on the intermediate substrate

21

.

Still referring to

FIGS. 4A and 4B

, it may also be preferable to apply the dielectric layer

32

over the upper surface

28

of the intermediate substrate

21

without covering at least one location

48

where the at least one second, x-axis electrode is to be applied. The benefit of not applying the dielectric layer over these locations

48

will be described below.

Next, referring to

FIGS. 5A and 5B

, two x-axis electrodes

34

A,

34

B are formed on the upper surface

28

of the intermediate substrate

21

, in a manner similar to the y-axis electrodes

30

A,

30

B. Each of the x-axis electrodes

34

A,

34

B includes at least one second contact portion

38

A or

38

B, and a lead portion

40

A or

40

B. Again, these portions

38

A,

38

B and

40

A,

40

B are applied at the same time, and form continuous electrode strips. The second contact portions

38

A of the x-axis electrode

34

A extend along the length of the short side

51

A of the frame portion

22

of the intermediate substrate

21

, and the lead portion

40

A of the x-axis electrode

34

A extends from the second contact portions

38

A, down along the long side

49

B of the frame portion

22

, across half of the other short side

51

B of the frame portion

22

, and across the tail portion

59

of the intermediate spacer

20

. The other x-axis electrode

34

B is formed in a general “T”-shaped configuration, and includes second contact portions

38

B that extend along the length of the short side

51

B of the frame portion

22

. The lead portion

40

B of the x-axis electrode

34

B extends from a central point along the length of the second contact portions

38

B, and across the tail portion

59

of the intermediate spacer

20

.

The x-axis electrodes

34

A,

34

B are positioned so that they do not coincide with, i.e., identically overlie, the y-axis electrodes

30

A,

30

B. As illustrated, the second contact portions

38

A,

38

B of the x-axis electrodes are preferably applied at the locations

48

(see FIGS.

4

A and

4

B), respectively, where no dielectric layer was applied. Application of the second contact portions

38

A,

38

B of the x-axis electrodes in this manner may be preferable so as to minimize the thickness of the intermediate spacer

20

. As noted above, because the top membrane

14

of the switch

10

is depressed through the central aperture

24

of the intermediate spacer

20

to contact the bottom membrane

12

, the thickness of the intermediate spacer

20

is preferably kept to a minimum.

While it has been described that the y-axis electrodes

30

A,

30

B and the x-axis electrodes

34

A,

34

B are applied in two separate steps, it should be understood that both the y-axis and x-axis electrodes may be applied simultaneously in one step. For example, if sufficient space is available on the intermediate substrate

21

to accommodate all electrodes without interfering with each other, it may be advantageous to apply all electrodes on the substrate at once.

Next, referring to

FIGS. 6A and 6B

, optionally, a second dielectric layer

36

is applied over the upper surface

28

of the intermediate substrate

21

. At this time, the dielectric layer

36

is formed while leaving both the first contact portions

31

A,

31

B of the y-axis electrodes and the second contact portions

38

A,

38

B of the x-axis electrodes exposed. As with the first dielectric layer

32

, application of the second dielectric layer

36

is optional, but may be preferable in order to ensure insulation of the lead portions

40

A,

40

B of the x-axis electrodes.

Thereafter, referring to

FIGS. 7A and 7B

, preferably, the intermediate substrate

21

heretofore constructed is sandwiched between the lower adhesive layer

43

and the upper adhesive layer

45

. Referring additionally to

FIG. 2

, the upper adhesive layer

45

preferably includes at least one predefined first aperture

50

that is adapted to overlie the first contact portion

31

A,

31

B of the y-axis electrodes

30

A,

30

B, and at least one predefined second aperture

47

that is adapted to overlie the second contact portion

38

A,

38

B of the x-axis electrode

34

A,

34

B. The lower adhesive layer

43

does not need to include any predefined holes. After the intermediate substrate

21

and the lower and upper adhesive layers

43

,

45

are assembled, at least one hole

39

is punched through each of the first contact portions

31

A,

31

B of the y-axis electrodes

30

A,

30

B, through the intermediate substrate

21

and the lower adhesive layer

43

, using any suitable punching device, preferably a precision steel tool. The hole

39

has a cross-sectional area that is smaller than that of the first contact portion so that the hole

39

is surrounded by the conductive material that forms the first contact portion. Next, conductive epoxy material

41

is poured into each first aperture

50

through the upper adhesive layer

45

and hole

39

through the intermediate substrate

21

and the lower adhesive layer

43

to provide an electrical conduction path between each of the first contact portions

31

A,

31

B (hence, the y-axis electrodes

30

A,

30

B) and the lower adhesive layer

43

. Conductive epoxy

41

is also poured into the predefined second apertures

47

defined through the upper adhesive layer

45

to provide an electrical conduction path between each of the second contact portions

38

A,

38

B (hence, the x-axis electrodes

34

A,

34

B) and the upper adhesive layer

45

.

The epoxy material

41

not only provides an electrical conduction path between the first contact portions

31

A,

31

B, of the y-axis electrodes

30

A,

30

B and the lower adhesive layer

43

, and between the second contact portions

38

A,

38

B of the x-axis electrodes

34

A,

34

B and the upper adhesive layer

45

, but advantageously also serves as a bond amongst various layers and components forming the intermediate spacer

20

. One example of the epoxy material

41

suitable for use in the present invention is Flexible Silver Conductive Epoxy 3882, available from Loctite Corporation of Hartford, Conn. Other conductive epoxy material, such as silver-carbon blend epoxy, nickel epoxy and gold epoxy may also be used.

A preferred embodiment, as described above, provides an efficient method of forming an intermediate spacer because both the y-axis and x-axis electrodes are applied on the same upper surface

28

of the intermediate substrate

21

. This embodiment is also efficient because the intermediate substrate

21

and the lower and upper adhesive layers

43

,

45

are first assembled together, and then the hole(s)

39

are defined through the intermediate substrate

21

and the lower adhesive layer

43

at once and the conductive epoxy material

41

filled into the apertures

47

and the apertures

50

and holes

39

. The conductive epoxy material

41

may be dispensed manually or, again preferably, by an automated robot.

The lower and upper adhesive layers

43

,

45

may be applied by any conventional method, such as by silk-screening onto the corresponding portions of the intermediate substrate

21

. Thus, in the embodiment illustrated in

FIG. 7B

, adhesive can be silk-screened over the lower surface

26

of the intermediate substrate

21

and over the second dielectric layer

36

.

However, in order to control placement of the adhesive, it has been found preferable to apply both the lower and upper adhesive layers

43

,

45

as films sandwiched between sheets of wax-impregnated paper transfer tape. To apply the adhesive, one paper transfer tape is peeled off and the exposed adhesive is pressed onto the corresponding portion of the intermediate substrate

21

. The second paper transfer tape is then peeled off and the intermediate spacer

20

is joined to the corresponding bottom or top membrane

12

or

14

. One suitable nonconductive adhesive film suitable for use in the present invention has been found to be the transfer tape sold under part number SCOTCH® 467 by the 3M Company of St. Paul, Minn.

To assist in proper placement of the various layers of the intermediate spacer

20

, the preferred method of fabrication of the intermediate spacer

20

is to assemble the various layers, including the intermediate substrate

21

, and the lower and upper adhesive layers

43

,

45

prior to cutting the intermediate spacer

20

from a sheet of stock material. Thus, after all the layers are applied, with the upper adhesive layer

45

including at least one first aperture

50

and second aperture

47

applied with the outer protective paper transfer tape retained thereon, and with the lower adhesive layer

43

applied again with the outer protective paper transfer tape retained thereon, the intermediate spacer

20

is cut from the stock material, such as by die-stamping. Preferably, the intermediate spacer

20

is cut to form the central aperture

24

at the same time, or immediately before or after, the holes

39

are punched through the intermediate substrate

21

and the lower adhesive layer

43

, so that the entire cutting operation may be performed at the same time. The outer sheets of paper transfer tape can then be removed from the adhesive layers, and the intermediate spacer

20

can be secured between the bottom membrane

12

and the top membrane

14

.

Referring specifically to

FIGS. 2

,

7

A, and

7

B, when the intermediate spacer

20

is secured between the bottom and top membranes

12

,

14

, the holes

39

through the lower adhesive layer

43

filled with conductive epoxy

41

contact the bus bar

17

A,

17

B provided on the upper surface

16

of the bottom membrane

12

. Similarly, the second apertures

47

through the upper adhesive layer

45

filled with conductive epoxy

41

contact the bus bar

19

A,

19

B provided on the lower surface

18

of the top membrane

14

. Thus, the first contact portions

31

A,

31

B of the y-axis electrodes

30

A,

30

B electrically connect with the upper conductive surface

16

of the bottom membrane

12

, and the second contact portions

38

A,

38

B of the x-axis electrodes

34

A,

34

B electrically connect with the lower

15

conductive surface

18

of the top membrane