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.
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