Created: 10-21-21
Last Login: 10-21-21
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Membrane switches are a type of human-machine interface
characterized by being constructed from several layers of plastic films
or other flexible materials. Conductive materials and graphic inks are
printed or laminated onto the surface of these plastic films. They
function by temporarily closing or opening an electric circuit. The
compact and efficient construction of membrane switches makes them
suitable for a vast array of applications such as household appliances
and industrial equipment interfaces.
Membrane switches have been around for about five decades. In the early
1970s, the first membrane keypads were introduced. They were made up of
polycarbonate plastic films printed with copper or silver infused ink
creating the electronic circuit. These were composed of two conductive
layers with one spacer in between. The products were inexpensive but
threatened by problems such as degradation and cracking of the
polycarbonate film and missing tactile feedback.
The next iteration solved the durability and quality-of-life issue by
changing the plastic film into polyester and adding metal domes into
the design. However, membrane switches have not been readily adopted
since, during that time, the huge personal computer market was
dominated by mechanical keyboards. Mechanical keyboards were preferred
due to their more tactile feedback.
Come the 1990s, thinner keyboard keys and membrane switches were used
to make more compact and quieter keyboards. By that time, smaller
electronic devices were the future of technology. The emergence of
appliances and equipment with small electronic components further
elevated the need for membrane switches.
Today, the global
membrane switch market has a market size of approximately $4.2
billion in 2015 and is expected to grow to $13 billion by 2025.
Membrane switches are extensively used in industrial, medical, and
consumer goods applications.
Membrane switches are extensively used in a variety of applications may
it be domestic, commercial, or industrial. There are other types and
forms of user interfaces such as touchscreens, keyboards, switches, and
selector knobs. Membrane switches are preferred because of their
compact profile, simple construction, reliability, resistance to
harmful elements, and low cost. These advantages are further elaborated
below.
Thin and compact profile: Each plastic layer of a OCA membrane
switch can have a thickness of about 0.005 to 0.040 inches. They
typically have three to six layers depending on the design. Even
applying the conductive and graphic inks and installing other
components such as the metallic domes and EMF screens, the final
thickness still results in only a fraction of an inch. This makes them
suitable for household appliances and equipment controllers with small
form factors.
Simple graphic interface construction: The preparation process for the
graphic overlay is a straightforward process. The graphics design or
artwork can be made from software such as AutoCAD, SolidWorks, and
Adobe Illustrator. After creating the artwork, it is digitally printed
onto the overlay. There is no need for additional machining processes
such as embossing, engraving, or stamping. These additional processes
are only done to improve aesthetics and tactile quality. However,
digital printing is not the only method of creating graphic overlays.
Screen printing is also used by many companies in the industry.
Highly resistant against external elements: One popular advantage
of membrane switches is their sealed construction. Sealing is achieved
by pressure-sensitive adhesives or heat seals. Plastics such as
polyesters and polycarbonates provide a sufficient barrier against
moisture and chemicals, without reducing the visibility of the artwork.
There are no cavities where hazardous liquids or gases can enter or
accumulate. Membrane switches are the desired type of human-machine
interface for devices with high protection ratings.
Easy cleaning and maintenance: Since there are no cavities where water,
dust, and contaminants can accumulate, membrane switches are easy to
clean. The overlay can easily be wiped to remove any dirt. Their
complete seal allows them to be subjected to equipment washdowns
without any risk of damaging the control circuit. Moreover, because of
very few moving parts, membrane switches require almost no maintenance.
Sufficient tactile feedback: When compared to touch screen
interfaces, membrane switches have an advantage because of their
capability to provide tactile feedback. Tactile feedback is useful in
applications where there is a risk of equipment malfunction or
shutdown. This is possible when the wrong sequence of keys is pressed.
Tactile feedback helps the operator know that the key is pressed.
Shielding from environments with high electromagnetic interference:
Unwanted electromagnetic frequencies and electrostatic discharges are
potential threats to electronic devices. These can cause electronics to
malfunction, especially controllers that use low power circuits. A
layer of EMF shielding can be added to membrane switches by printing a
grid or mesh using conductive ink. The EMF shield can be made without
any discontinuity which defeats the purpose or lowers the efficiency of
the shielding.
Lower cost: Because of its small blueprint and readily available
construction materials, membrane switches are more economical than
touch screens or mechanical interfaces. They are made from lesser parts
that can be easily assembled by basic processes such as applying
pressure-sensitive adhesives or heat sealing. Its low cost makes it the
desired interface for consumer goods or household appliances.
Membrane switches are composed of several components in the form of
layers that are assembled using pressure-sensitive adhesives or heat
sealing films. Its main parts are an overlay containing the graphic
elements; a circuit that includes the conductive tracks, metal domes,
circuit tail, and terminals; and a spacer that maintains a break
between the switch contacts.
Overlay: Also known as top or graphic overlay, the overlay is the
outermost layer of the elevator program
control membrane switch. Since this layer is on the exposed side
of the membrane switch, it is made from materials that have good
flexibility, clarity, durability, chemical resistance, and barrier
properties. There are two common materials used for making the overlay,
Polyester: This is a plastic material commonly known as polyethylene
terephthalate (PET). Polyester is known for its clarity, flexibility,
and chemical resistance. Its flexibility allows it to be more durable
than other materials, especially when used on switches with tactile
feedback. To achieve good resistance to puncture and tearing, the film
is made through a process called biaxial orientation.
Polycarbonate: Polycarbonate is the desired film for industrial
applications due to its inherent flame-retarding property and abrasion
resistance even without additional surface treatments such as hard-
coating. Polycarbonate is also more economical and easier to process
than polyester. The film can be produced and processed quickly without
worrying about shrinking and warping as experienced in working with
polyester films.
Other materials that can be used as overlays are acrylic, vinyl,
and PVC.
Graphics can be printed on the reverse side or front side. Reverse side
or sub-surface printing is the more common method since it produces
longer-lasting prints. The overlay plastic film protects the graphics
from abrasion and chemical attack. Front side or top-surface printing,
on the other hand, is done to create various features such as selective
texture and windows.
Domes: Domes are the components that provide tactile feedback. They
can be made from metal or plastic.
Metal Domes: Metal domes are made from stainless steel or copper
alloys held in place by a dome retainer layer or a spacer layer. Aside
from providing tactile feedback, metal domes also function as a part of
the circuit. When pressed, the metal dome shorts the open contacts of
the switch.
Plastic or Poly Domes: Plastic domes are typically made from polyester
because of their flexibility; hence "poly" domes. Poly domes
have a layer of their own. In some designs, the poly dome layer can
also become the overlay or graphics layer. The poly dome layer can be
seen as a polyester film with domes or blister-like features. At the
concave side of the dome is a printed conductive ink that completes the
circuit when the button is pressed.
Retainer Layer: The retainer layer with the primary function of holding
the metal domes in place. This is commonly made from polyester film,
similar to the poly dome layer.
Spacer Layer: This layer is used to create a break in contact
between the two conductors of the switch. This allows the switch to
have its open position. In some designs of membrane
switch with led, it can also act as a retainer to keep the
metallic dome in place. The spacer layer has channels between the empty
cavities or the sides of the keypad for venting air. This prevents air
from being compressed in the cavity when the key is pressed.
Circuit Layer: This layer is where the conductive paths of the switch
are applied. These conductive paths can be produced through two main
methods: screen printing and photochemical etching.
Screen Printing: This method uses a stencil containing the pattern
of the circuit. Silver conductive ink is flooded on the stencil which
is placed above a substrate. The substrate used is typically a
polyester film. This method is used for thinner and more flexible
membrane keypads.
Photochemical Etching: In contrast, this method uses a copper laminated
substrate which is selectively patterned through photolithography and
chemical etching. The result can be a printed circuit board (PCB) or a
flexible printed circuit (FPC) that is thicker and more durable than
screen-printed membrane keypads.
Two-layer Circuit: In this design, the circuit layer is separated into
two: the upper circuit and the lower circuit. Each circuit layer
contains a conductive path that leads into or goes out of the switch.
The two layers are separated by the spacer layer. When a switch is
pressed, the upper circuit deflects and touches the lower circuit
completing the circuit.
Single-layer or Single-sided Circuit: As the name suggests, a single-
layer switch has only one circuit layer. A break in the circuit is
created by a discontinuity in the conductive path that is printed onto
the substrate. The circuit is completed using a metallic dome or
conductive ink printed on the reverse side of a plastic dome. When a
key is pressed, the dome flattens against the circuit layer creating a
single conductive path.
Circuit Tail: The circuit tail is the part of the circuit that
connects the membrane switch to the control unit of the machine. It is
seen as a flat, flexible ribbon composed of several conductive tracks
printed on a polyester strip. At the end of the circuit tail are
standard connectors that match with the termination block of the
control unit. Some common connector options are plain header, latching
header, or solder tabs. The circuit tail can also be a ZIF (zero
insertion force) style which basically differs on the force applied
between the circuit tail and the control unit terminals. ZIF is used
for more delicate circuits where the control unit terminals are weak
and easy to damage.
Mounting Adhesive: This is placed at the back of the
hole filling membrane switch to facilitate assembly
with the control unit. It is usually specified according to its bond
strength, thickness, and operating temperature. The material used is an
elastomeric compound which is usually composed of high strength or
modified acrylic.
Before selecting which membrane switch to use or supplier to order
from, it is best to gain an understanding of its specifications and
features. And like any other electronic or electrical device, it is
important to fully determine the characteristics of the system where
the interface will be installed. The electrical specifications of the
membrane switch must be applicable to the system to prevent any
electrical shorting or premature failure of the membrane switch or
control unit. Moreover, there are other features that are worth noting
such as coatings, backlighting, precision cutting, and so on.