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Description  |
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FIELD OF THE INVENTION
This invention generally relates to the art of electrical switches and,
particularly, to a membrane switch.
BACKGROUND OF THE INVENTION
Membrane switches have become popular for use in industrial controls, home
appliances, office equipment and automotive applications. Membrane
switches are touch activated and have some advantages over electrical
switches. For instance, mechanical switches are prone to mechanical
failure due to breakage of moving parts. This problem is not prevalent
with membrane switches. In addition, membrane switches conserve
considerable space over conventional mechanical switches due to their very
thin profile.
A typical membrane switch includes a lower insulating layer, a dielectric
spacer layer and an upper insulating layer. Typically, one of the
insulating layers is fixed and the other insulating layer is movable, with
the spacer layer separating the two insulating layers. One or more
electrically conductive contact areas are provided on the upper face of
the lower insulating layer and the lower face of the upper insulating
layer in facing relation to define the contacts of the switch. Most often,
a plurality of pairs of the facing contact areas are provided between the
juxtaposed insulating layers, and the spacer layer has a plurality of
openings through which the contact areas of each pair thereof are exposed
to each other in facing relation.
To make connection with a device to be switched, the facing contact areas
are extended to the periphery of their respective insulating layers and
are connected to external leads which, in turn, are connected to the
particular device to be switched. In use, movement of the insulating
layers toward each other, such as moving the movable insulating layer
toward the fixed insulating layer, causes a given pair of the facing
contact areas to engage and close a circuit through the external leads
connected to the device to be switched.
While membrane switches have been fairly easy and efficient to fabricate,
incorporating the switches into various apparatus has caused problems,
particularly when the fabrication involves the application of heat and/or
pressure. The insulating layers which carry the facing contact areas of
the membrane switch typically are fabricated of some type of plastic
material, such as polyester. When these layers are subjected to heat
and/or pressure, they can collapse, even when spacer layers are used in
the switch. Collapsing of the insulating layers causes the distance
between the facing contact areas to vary and can even cause the contact
areas to touch and cause permanent electrical contact.
One example of the manifestation of the problem discussed above is switches
mounted in a steering wheel for a vehicle horn in the automotive industry.
To facilitate access to the actuator for the horn switch, the actuator is
preferably disposed in the central surface of the steering wheel. The
advent of the air-bag has made it common to locate an air-bag assembly in
the center of the steering wheel thereby complicating the task of mounting
switches in steering wheels. The air-bag assembly is relatively bulky,
comprising a compacted air-bag and a gas generator. In order to locate the
horn switch assembly with the actuator disposed in the central surface of
the steering wheel, mechanical horn switches have been incorporated into
the air bag assembly. Such mechanical horn switches are attended by
complexities of increased weight and volume. As an alternative, horn
switches are sometimes placed beside the air-bag assembly, with the
actuator for the horn switch located outside the central surface of the
steering wheel. Actuators for such switches are often difficult to access
in sudden situations.
The use of a membrane switch in a steering wheel for actuating the vehicle
horn would solve the problems posed by mechanical switches, described
above. The membrane switch could provide a significantly larger area of
actuation, and the membrane switch would present considerably fewer volume
and mass problems than the mechanical switches. However, the use of a
membrane switch poses its own problems because of the application of heat
and pressure during manufacture of the steering wheel assembly, such as
during overmolding processes.
The present invention is directed to solving the above myriad of problems
by providing a method of fabricating a membrane switch, an article of
manufacture and the switch, itself, wherein the components of the switch
are protected from collapsing during manufacture, such as during processes
involving the application of heat and/or pressure.
SUMMARY OF THE INVENTION
An object, therefore, of the invention is to provide a new and improved
method of fabricating a membrane switch.
Another object of the invention is to provide a new and improved article of
manufacture in the fabrication of a membrane switch.
A further object of the invention is to provide a new and improved membrane
switch of the character described.
In the exemplary embodiment of the invention, the membrane switch is of the
type including a pair of insulating layers each having at least one
contact area formed thereon in facing relation with the contact area of
the other layer. The method of fabrication includes the steps of
juxtaposing the pair of insulating layers with the contact areas thereon
in facing relation. A protective layer is inserted between the insulating
layers at least in the vicinity of the facing contact areas to maintain
the contact areas separated during a subsequent fabrication step involving
the application of heat and/or pressure. The protective layer then is
removed after the fabrication step is performed.
As disclosed herein, the insulating layers are joined about a substantial
peripheral portion surrounding the facing contact areas, leaving a
peripheral access opening to the contact areas. The protective layer is
removed by pulling the protective layer out through the access opening.
The method may include the step of positioning a spacer layer between the
pair of insulating layers except between the facing contact areas. As
disclosed herein, the spacer layer surrounds a substantial portion of the
facing contact areas except for the peripheral access opening through
which the protective layer is pulled out from between the insulating
layers.
The invention also contemplates an article of manufacture in the
fabrication of the membrane switch. The article includes the pair of
juxtaposed insulating layers with the facing contact areas thereon, and
with the protective layer removably mounted between the insulating layers
at least in the vicinity of the facing contact areas. This article of
manufacture then can be shipped to an ultimate fabricator, such as an
automotive manufacturer, where the article of manufacture can be
incorporated in an appropriate apparatus, such as a vehicle steering
wheel.
The invention further contemplates a membrane switch which includes a pair
of juxtaposed insulating layers each having at least one conductive
contact area thereon in facing relation with the contact area of the other
layer. The insulating layers are joined about a substantial peripheral
portion surrounding the facing contact areas, leaving a peripheral opening
between the layers communicating with the facing contact areas. The
insulating layers are elongated to define two major sides and at least one
minor side, with the peripheral opening being in the minor side. A
generally U-shaped spacer layer is disposed between the pair of insulating
layers except between the facing contact areas, with an open end of the
U-shaped spacer layer being coincident with the peripheral opening between
the insulating layers. The facing contact areas are elongated and extend
between the two major sides of the elongated insulating layers.
Other objects, features and advantages of the invention will be apparent
from the following detailed description taken in connection with the
accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
The features of this invention which are believed to be novel are set forth
with particularity in the appended claims. The invention, together with
its objects and the advantages thereof, may be best understood by
reference to the following description taken in conjunction with the
accompanying drawings, in which like reference numerals identify like
elements in the figures and in which:
FIG. 1 is an exploded perspective view of the components of a membrane
switch incorporating the concepts of the invention;
FIG. 2 is a perspective view of the membrane switch in assembled condition;
FIG. 3 is a top plan view of the membrane switch;
FIG. 4 is a fragmented vertical section taken generally along line 4--4 of
FIG. 3;
FIG. 5 is a vertical section taken generally along line 5--5 of FIG. 3;
FIG. 6 is a view similar to that of FIG. 4, with the protective layer
removed; and
FIG. 7 is a view similar to that of FIG. 5, with the protective layer
removed.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
The invention herein is incorporated in a membrane switch, generally of the
type including a pair of insulating layers each having at least one
contact area formed thereon in facing relation with the contact area of
the other layer. In particular, FIGS. 1-3 show a membrane switch,
generally designated 10, which includes an upper insulating layer,
generally designated 12, and a lower insulating layer, generally
designated 14. The layers are narrow and elongated and include end tab
portions 12a and 14a. Being elongated, upper insulating layer 12 defines a
pair of opposite major sides 12b and opposite minor sides 12c and 12d.
Similarly, lower insulating layer 14 being elongated defines a pair of
opposite major sides 14b and opposite minor sides 14c and 14d. The upper
insulating layer 12 and the lower insulating layer 14 should be made from
an insulative, high-temperature material which will not melt when
subjected to overmolding temperatures. For high temperature applications,
such as overmolding that would be around 250.degree. C., a polyimide such
as KAPTON from E.I. du Pont de Nemours & Co., is the preferred material.
Other materials such as polyetherimides, polyetherketones,
polyethersulfones, polyethylene naphtaltes and meta-aramides may also be
suitable depending on the highest temperature reached during the
overmolding process.
Upper elongated insulating layer 12 has an elongated, centrally disposed
contact area, generally designated 16, on the bottom surface thereof, and
lower elongated insulating layer 14 has an elongated, centrally disposed
contact area, generally designated 18. The contact areas are imprinted or
otherwise adhered to the bottom surface of upper insulating layer 12 and
the top surface of lower insulating layer 14. Inks or adhesives used to
constitute the contact areas 16, 18 should each be made from a suitable
high-temperature material that will not melt or become sticky when
subjected to overmolding temperatures. Silver ink is preferred for high
conductivity. However, carbon/graphite ink or silver/carbon ink may be
suitable, as long as it will not become sticky when exposed to heat and/or
pressure. The preferred high temperature adhesives are acrylic pressure
sensitive adhesives, such as No. 966 from Minnesota Mining and
Manufacturing Company, but other thermoplastic adhesives may be suitable.
Contact area 16 has a rectangular peripheral conductive band 16a joined by
diagonal conductive bands 16b, and contact area 18 has a rectangular
peripheral conductive band 18a joined by a plurality of diagonal
conductive bands 18b. Diagonal conductive bands 16b define a crisscross
pattern with diagonal conductive bands 18b lengthwise of the contact areas
16 and 18, so that the contact areas will mutually engage when moved
toward each other at any point lengthwise of the pattern of conductive
bands. Contact area 16 is extended into end tab portion 12a of upper
insulating layer 12 by a conductive band extension 16c, and contact area
18 is extended into end tab portion 14a of lower insulating layer 14 by a
conductive band extension 18c. Band extensions 16c and 18c are connectible
to appropriate conductive leads to a device which is to be switched.
An elongated spacer layer, generally designated 20, may be used between
insulating layers 12 and 14 except between the elongated facing contact
areas 16 and 18. Spacer layer 20 is generally U-shaped to define a pair of
leg portions 20a joined at one end by a bight portion 20b, with an
opposite end open, as at 20c. The spacer layer is sandwiched between upper
insulating layer 12 and lower insulating layer 14. When properly
positioned, leg portions 20a of the spacer layer are sandwiched between
major sides 12b and 14b of insulating layers 12 and 14, respectively.
Bight portion 20b of the insulating layer is sandwiched between minor
sides 12c and 14c of the insulating layers 12 and 14, respectively.
Contact areas 16 and 18, thereby, are exposed facing each other between
leg portions 20a of the elongated spacer layer. Open end 20c of the spacer
layer cooperates with upper and lower insulating layers 12 and 14,
respectively, to define an access opening to the space between contact
areas 16 and 18. The access opening is in line with end tab portions 12a
and 14a of insulating layers 12 and 14, respectively. The space layer 20,
like the insulating layers 12, 14, should be made from an insulative,
high-temperature material that will not melt when subjected to overmolding
temperatures. For high temperature applications, such as overmolding that
would be around 250.degree. C., a polyimide, such as KAPTON from E.I. du
Pont de Nemours & Co. is the preferred material. Other materials such as
polyetherimides, polyetherketones, polyethersulfones, polyethylene
naphtaltes and meta-aramides may also be suitable depending on the highest
temperature reached during the overmolding process.
Generally, the invention contemplates the use of a protective layer 22
removably mounted between insulating layers 12 and 14 at least in the
vicinity of facing contact areas 16 and 18 to maintain the contact areas
separated for manufacturing purposes. Protective layer 22 is removable
from between insulating layers 12 and 14 to enable the switch for use. As
stated in the "Background", above, when membrane switches are used in
applications involving the application of heat and/or pressure, plastic
insulating layers, such as layers 12 and 14, have a tendency to collapse
in the spacing between the facing contact areas. This can cause the
membrane switch to undesirably provide permanent contact. Protective layer
22 is effective to prevent any such collapsing, and the protective layer
is readily removable for ultimate use of the switch.
More particularly, protective layer 22 has a width "W" which is slightly
less than the distance between leg portions 20a of spacer layer 20 which,
in turn, is substantially equal to the width of contact areas 16 and 18.
The protective layer has a length such that the protective layer can be
fully inserted to bight portion 21b of spacer layer 20 and minor sides 12c
and 14c of insulating layers 12 and 14, respectively, and with the
protective layer extending outwardly between end tab portions 12a and 14a
of the insulating layers. The protective layer is readily inserted and
removed from the surrounding insulating layers and spacer layer through
open end 20c of the spacer layer.
The protective layer 22 should also be made from a high-temperature
material that will not melt when subjected to overmolding temperatures and
will not adhere to the insulating layers 12, 14 or the contact areas 16,
18. The protective layer 22 is preferably made from
polytetrafluoroethylene.
In fabrication, insulating layers 12 and 14 are joined or adhered with a
high-temperature adhesive to spacer layer 20 at major sides 12b and 14b of
the insulating layers and leg portions 20a of the spacer layer, as well as
at minor sides 12c and 14c of the insulating layers and bight portion 20b
of the spacer layer. That leaves an access opening at open end 20c of the
spacer layer between the insulating layers for the insertion and removal
of protective layer 22. Actually, during fabrication, the protective layer
could be placed in position within the assembly prior to laminating the
insulating layers and spacer layer, or the protective layer can be
inserted after the insulating layers and spacer layer are joined.
FIGS. 4 and 5 show protective layer 22 located between upper and lower
insulating layers 12 and 14, respectively, and within leg portions 20a and
bight portion 20b of spacer layer 20. In this position, the protective
layer is sandwiched between contact areas 16 and 18. This article of
manufacture then can be shipped to an ultimate manufacturer whereat the
article or assembly can be incorporated in an apparatus involving the
application of heat and/or pressure. After the subsequent fabrication step
is performed, protective layer 22 can be pulled out of the assembly
through the access opening at open end 20c (FIG. 1) of the spacer layer,
leaving an open space between upper and lower insulating layers 12 and 14,
respectively in the vicinity of the facing contact areas 16 and 18. The
removal of the protective layer is shown in FIGS. 6 and 7. With the
protective layer removed, a membrane switch is provided whereby touch
pressure can be applied in the direction of arrow "A" (FIGS. 6 and 7) to
relatively move the insulating layers toward each other and cause contact
areas 16 and 18 to engage and close a circuit therethrough leading away
from the membrane switch through conductive band extensions 16c and 18c of
the contact areas.
It will be understood that the invention may be embodied in other specific
forms without departing from the spirit or central characteristics
thereof. The present examples and embodiments, therefore, are to be
considered in all respects as illustrative and not restrictive, and the
invention is not to be limited to the details given herein.
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