Capacitive Touch Screen
A device's display screen that uses finger pressure for interaction is called a capacitive touch screen. Handheld capacitive touch screen devices generally link to networks or computers using an architecture that can...
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This article provides comprehensive insights into membrane switches. Read further to learn more about:
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 are a type of user interface that is often used in various electronic devices and control panels. They consist of multiple layers of flexible materials, including a top graphic overlay with printed symbols or labels, a spacer layer, and a bottom membrane layer with conductive traces. When a user presses a button on the graphic overlay, it flexes and makes contact with the conductive traces on the bottom membrane layer, completing an electrical circuit and registering the button press. There are several types of membrane switches, including:
Non-tactile membrane switches do not provide tactile feedback or a physical "click" when pressed. They are often used in applications where a soft touch is preferred.
Tactile membrane switches are designed to provide feedback to the user when a button is pressed. They typically have a dome or a protrusion on the bottom membrane layer that collapses or clicks when the button is pressed, giving the user a tactile sensation.
Metal dome membrane switches use a small metal dome or domes as the tactile element. When the dome is compressed, it provides a distinct tactile feel and audible feedback. These are often used in applications where a crisp button press is required.
Polydome membrane switches use a polymer dome as the tactile element. These domes are made of flexible plastic and provide tactile feedback similar to metal dome switches but are less expensive.
LED backlit membrane switches incorporate LED (Light Emitting Diode) lighting beneath the graphic overlay to illuminate buttons or labels, making them visible in low-light conditions. They are commonly used in control panels and devices that require backlighting for improved visibility.
Capacitive membrane switches do not have physical moving parts like traditional membrane switches. Instead, they rely on changes in capacitance to detect when a user's finger approaches or touches a button. They are often used in applications where a touch-sensitive and sleek design is desired.
Sealed or waterproof membrane switches are designed to withstand exposure to moisture, dust, and other environmental factors. They are often used in outdoor or industrial applications where protection from the elements is necessary.
Custom membrane switches can be tailored to specific design requirements, including the number of buttons, layout, graphic design, and tactile feedback. Manufacturers can create membrane switches to meet the unique needs of various applications.
Membrane switches are extensively used in a variety of applications, whether domestic, commercial, or industrial. Other types and forms of user interfaces include touchscreens, keypads, switches, and selector knobs. However, membrane switches are preferred because of their compact profile, simple construction, reliability, resistance to harmful elements, and low cost. These advantages are further elaborated on below.
Membrane switches are renowned for their thin and compact profile, making them an ideal choice for various applications where space constraints are a concern. Typically, a membrane switch consists of multiple layers of thin plastic materials, with each layer contributing to its overall slim design. These layers typically include a graphic overlay, a spacer layer, and a printed circuit layer. The graphic overlay, often made of polyester or polycarbonate, is usually less than 0.5 millimeters thick. The spacer layer, typically made of a flexible, insulating material like silicone, adds minimal thickness, often measuring less than 0.2 millimeters. The printed circuit layer consists of conductive traces and is similarly thin, often less than 0.1 millimeters thick. When these layers are assembled, the final thickness of a membrane switch can be as thin as 1 millimeter or even less, depending on specific design requirements. This sleek and low-profile design makes membrane switches perfect for applications where aesthetics, space efficiency, and durability are essential, such as in medical devices, industrial control panels, and consumer electronics.
The preparation process for the graphic overlay is straightforward. 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. Many companies in the industry also use screen printing.
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 gasses can enter or accumulate. Membrane switches are the desired type of human-machine interface for devices with high protection ratings.
Membrane switches are typically designed with a flat, sealed surface that prevents dust, dirt, and liquid ingress, making them highly resistant to environmental hazards. Cleaning membrane switches is a straightforward process; users can simply wipe the surface with a damp cloth or a mild cleaning solution without worrying about damaging delicate components. Their durable construction ensures longevity, and they can withstand frequent cleaning without wear or degradation. This low-maintenance characteristic makes membrane switches a preferred choice for applications in industries where cleanliness and reliability are important, such as medical devices, industrial control panels, and consumer electronics.
Membrane switches are designed to provide users with a distinct and reassuring tactile sensation when pressed, ensuring that they know the input has been registered. This feedback not only enhances the user's confidence in their interactions with the device but also helps prevent accidental key presses. The tactile feedback in membrane switches is typically achieved through the use of domes or other deformable structures within the switch's membrane layers. These structures allow users to feel a distinct "click" or resistance when a key is actuated, making it easier to type accurately and comfortably on keyboards or to operate various controls.
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.
Because of its small blueprint and readily available construction materials, membrane switches are more economical than touch screens or mechanical interfaces. This cost-effectiveness stems from their simple design and efficient manufacturing process. Unlike traditional mechanical switches that involve intricate components, membrane switches consist of thin layers of flexible materials, including polyester or polyimide, with printed conductive circuits. This streamlined construction not only reduces material costs but also minimizes assembly expenses. Additionally, the ease of customization and mass production further contributes to their affordability.
Membrane switches are composed of several components in the form of layers 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.
Also known as top or graphic overlay, the overlay is the outermost layer of the 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.
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 attacks. Front side or top-surface printing, on the other hand, creates various features such as selective texture and windows.
Domes are the components that provide tactile feedback. They can be made from metal or plastic. The size of the keys of the membrane switch determines what size the dome will be, with sizes ranging from 0.24 to 0.79 inches (6 to 20 mm). Additionally, the dome’s height is closely related to the size of the dome and can be 0.010 to 0.057 inches (0.25 to 1.45 mm).
A critical aspect of using domes is the actuation force or trip force necessary to depress the dome and activate the switch, which can range from 1.41 oz to 80 oz (40 to 2250 g). Domes come in a wide assortment of shapes and sizes, including:
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. Metal domes have a very low profile and can reach life ratings of up to 10 million presses, making them ideal for many applications.
Plastic domes are typically made from polyester because of their flexibility, hence the name “poly” dome. 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 is a polyester film with dome 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.
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. A retainer layer can hold a dome in position without needing an adhesive layer.
The spacer 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 tactile-type membrane switches, 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.
This layer is where the conductive paths of the switch are applied. Conductive paths can be produced by screen printing and photochemical etching.
Depending on the type of membrane switch, the circuit can be designed and built in two ways.
The circuit tail is the part of the circuit that connects the membrane switch to the machine’s control unit. It is 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 to the termination block of the control unit. Common connector options are plain header, latching header, or solder tabs. The circuit tail can also be a ZIF (zero insertion force) style, which 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 is placed at the back of the membrane switch to create a secure bond between it and the mounting surface. They are chosen according to their bonding strength, thickness, and operating temperature. Mounting adhesives are an elastomeric compound composed of high strength or modified acrylic.
Acrylic adhesives are the industry standard due to their exceptional adhesion to metal and plastics. They allow repositioning for greater placement accuracy when bonding with plastics. The benefits of acrylic adhesives include:
The thickness of the mounting adhesive is a crucial factor in choosing the right adhesive to fit the needs of the specific membrane switch. When bonding a membrane switch to a smooth surface, an acceptable thickness is 0.079 in (2 mm). For textured surfaces, the thickness of the mounting adhesive should be 0.2 in (5 mm) to maximize the surface to which the adhesive may bond.
Before selecting a membrane switch to use or supplier to order from, it is best to understand 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. In addition, 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, other features such as coatings, backlighting, and precision cutting are worth noting.
These data provide the characteristics and performance of the electrical circuit. Some of these specifications are enumerated below.
Certifications assure that the product conforms with the general safety standards mandated by national and international organizations. Widely accepted certifications are Underwriter Laboratories (UL Listed or Recognized) and CE.
Since membrane switches operate with low voltages and currents, stray electrostatic discharges (ESD) or electromagnetic frequencies (EMF) can short the circuit or disrupt the electric current. An ESD/EMF shield is included in the membrane switch assembly by adding a layer of conductive material underneath or above the circuit. Other shield designs feature a complete wrapping of the circuit layer. An ESD/EMF shield is made of a thin layer of polyester with conductive ink printed in a grid or mesh-like pattern. Another form is copper or aluminum foil with or without polyester lamination. The shielding can be grounded by connecting it to the metal enclosure, metal backer, or grounding connection from the circuit tail.
Tactile feedback is provided by metal or plastic domes. As mentioned earlier, this feature is necessary for helping the operator know that a keypress is registered. Different dome designs have varying actuation forces. Non-tactile membrane switches, on the other hand, are used in applications where a thin profile is more valuable than feedback.
A backing layer is used to provide rigidity to the membrane switch. This layer can be omitted depending on the application since keypad support can be on the device’s panel itself. The backing layer has a pressure-sensitive adhesive applied on its underside for mounting.
Selective texturing is the application of a transparent, scratch-resistant, matte finish on specific areas of the overlay to accentuate graphic elements for improved aesthetics. This finish also helps minimize scratches that are easily developed on glossy finishes. This makes a textured finish desirable for heavy-duty use, such as interfaces for industrial equipment.
This is a common surface treatment for surfaces with low durabilities, such as plastics, paper, wood, and glass. Hard coating is done by applying a layer of ultraviolet-curable polymer resin through different surface deposition methods. The composition of the polymer and the deposition method varies from each manufacturer. The polymer used has better durability and chemical resistance than the polyester or polycarbonate film underneath.
Embossing is done to improve the aesthetics and tactile features of the overlay by raising some of its surfaces. Different types of embossing are pad, dome, and rim.
Aside from these three common embossings, raised patterns can be custom shapes such as Braille patterns, texts, and logos. The height of the embossing can be made multi-level to enhance the look and feel of the graphics.
Windows are transparent or translucent areas intended for including light crystal displays (LCD) or light-emitting diode (LED) displays. Windows are designed according to the requirement of the display to maintain readability. LCDs require clear windows with minimal filtering. For LED segment displays, optical filtering is required to maintain readability in bright light. Readability is increased by enhancing the contrast of the LED segments with their background. This is done by printing a gray or amber filter on the display window with varying degrees of transmission. The color of the filter depends on the color of the LED.
Indicators are used for pointing out activated keys, while backlighting is used to improve the interface's readability and aesthetics. The four main types of backlighting are:
Backlighting is an added feature for membrane switches that illuminates or lights up the front surface of the switch. Backlighting can be used in several ways, such as lighting one area, the entire area, or several selected areas. Backlighting is not a necessity for membrane switches, but it does add benefits such as:
Dead front is when the user interface remains unobtrusive until activated, hence the term "dead front." The switches are concealed beneath a smooth, unmarked surface, giving the panel a clean and minimalist appearance. When touched, the flexible membrane switches respond with tactile feedback, making them easy to use and providing a satisfying user experience. This technology is widely utilized in various industries, from consumer electronics to industrial control systems, where a discreet and intuitive user interface is essential. The dead front with membrane switches not only enhances the visual appeal of products but also offers a durable and reliable solution for user interaction, ensuring both form and function are in perfect harmony.
Membrane switches are designed to fit into a plastic molding or metal panel assembled with the control unit. Specific dimensional tolerances must be complied with to ensure proper mounting and sealing of the electrical components. This is influenced by the type of cutting method used. The most popular methods are steel rule die cutting and laser cutting. Steel rule die tooling is used due to its low capital cost and high cutting speed. On the other hand, laser cutting is preferred in applications where precision and burr-free cutting is necessary.
Membrane switches are used in car dashboards for functions like climate control, audio systems, and navigation. They are also used in steering wheel controls and other interior components.
Aircraft cockpit controls and instrument panels often utilize membrane switches due to their reliability and space-saving design. These switches can withstand the demanding environmental conditions of aviation.
Membrane switches are employed in military control systems, communication equipment, and vehicle consoles. They can be designed to meet stringent military specifications, including resistance to shock, vibration, and extreme temperatures.
Keyboards and control panels on telecommunication devices such as smartphones, landline phones, and network equipment often use membrane switches.
Home automation systems often incorporate membrane switches for controlling lighting, security systems, and climate control. They are also used in kitchen appliances like ovens, microwave ovens, and coffee makers.
Membrane switches are used in laboratory devices and scientific instruments to control settings and input data.
Control panels and navigation systems on boats and ships use membrane switches due to their resistance to water and harsh marine conditions.
Membrane switches can be found in cash registers, barcode scanners, and other POS equipment.
Membrane switches are commonly used in the prototyping phase of product development for their ease of customization and cost-effectiveness.
Access control panels and security systems often use membrane switches for user interaction.
Membrane switches can be customized with specific graphics, icons, and tactile feedback for specialized applications like industrial machinery, scientific instruments, and more.
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