Membrane switches may be categorized as user-equipment interface utilities, which are simply utilities that allow users to successfully communicate commands to electronic devices. They should not be confused with mechanical switches, which are composed of plastic parts and copper instead of a circuit and a substrate.
A membrane switch or touch switch is a multi-layered user interface device that acts as a switch to turn a device on or off. Membrane switches are printed circuits on film as opposed to mechanical switches, which are often made of copper and polymers. Silver or carbon ink is printed onto heat-stabilized polyester film to create membrane switches. As the switch’s graphic interface, an overlay is placed on the initial surface. These top substrates are known as “membranes” or “graphic overlay films” since they are constructed of thin, pliable materials.
Membrane switches have a low profile, sealing ability, and ease of cleaning. Membrane switches can be used in conjunction with other control systems such as keyboards, touch screens, and lighting, and they can be as complex as membrane keyboards and switch panels found in mobile phones and computers. Membrane switches are also known as membrane keyboards and membrane keypads, depending on the industry and application.
How Membrane Switches Work
The core of a membrane switch’s functionality lies in the meeting of electrical contacts on different layers (or membranes) of the switch. Membrane switches are normally open, meaning that the electrical circuit stays incomplete and keeps the entire switch in an “off” status until the switch is actuated. Membrane switches are generally actuated by pressure, which forces the contacts on different membranes together.
Membrane Switch Parts
At a bare minimum, membrane switches must possess at least two circuit layers in order to complete an electrical circuit. The vast majority of membrane switches, however, possess at least three to four layers. (Some more advanced versions can possess up to seven layers.) The different layers of a membrane switch can generally be broken down into a few basic categories.
The circuit layers of a membrane switch are the membranes containing electrically conductive material in order to complete the circuit. They are located at the bottom of the switch assembly and are sometimes just referred to as “bottom layers.” Circuit layers come in a variety of numbers and formats depending on the specific membrane switch they are a part of. It has been previously mentioned that membrane switches require at least two circuit layers in order to be classified as membrane switches. These two required circuit layers can be divided into the membrane layer and the static layer. The membrane layer is simply the upper, pliable circuit layer containing one pole of the electrical circuit. Similarly, the static layer is the lower circuity layer that carries the other pole of the electrical circuit – usually with some type of rigid backing.
The actual circuitry of circuit layers consists of electrically conductive patterns that are screen printed onto a polymer film using some type of metallic ink (e.g. silver-based ink). Printed circuit board (PCB) membrane switches utilize a printed circuit board (made of material such as ITO or PET) as the static or lower circuit layer. PCB-based membrane switches are able to impart structural strength and various mounting capabilities (e.g. connections for soldered parts) to the overall switch assembly. Flex circuit membrane switches, on the other hand, do not depend on rigid PCB boards. Rather, they utilize flexible film substrates (e.g. polyester or polyimide/Kapton) for both upper and lower circuit layers. Silver flex membrane switches are also created by screen printing. Other types of flex switches, such as copper flex membrane switches, are created when thin metallic sheets are laminated to films and then chemically etch the metal away, leaving trace conductive patterns.
As the most complex portion of a membrane switch, circuit layers can come in other types of forms and possess varying other components. Single-sided circuit layers have conductive material on one side of a substrate while double-sided circuit layers have conductive material on both sides of a substrate. (Circuit layers with multiple single or double-sided layers are known as multi-layer flexes.) This area of a membrane switch will also contain some type of insulation in the form of dielectric ink and/or a complete circuit spacer layer. When conductive ink is initially printed on a film substrate, dielectric ink is usually added to it in order to insulate non-contact areas of the conductive ink from unwanted electrical contact with other circuit layers. An insulating spacer layer is usually nothing more than a layer of inert gas or an adhesive bonded to the circuit board. If a membrane switch gives tactile feedback and possesses dome switches (to be discussed further below), insulating areas will be patterned with cutouts where these electronic switches can emerge.
The top layer of a membrane switch is often referred to as the graphic overlay since it contains the actual buttons and user interface for operation of the switch. Parallel to circuit layers, graphic overlays are created when switch graphics are printed on polyester or polycarbonate film substrates. (Polyester is often preferred due to better chemical corrosion resistance and superior durability.)
The graphic overlay can be made in a variety of different ways. Visible from the surface of the membrane switch, it is usually embossed or printed. It may be screen-printed with different colors and text, or it may be digitally printed. (In fact, digital printing is often preferred due to cost-effectiveness and the higher range and quality of graphic effects.) The graphic overlay may also be covered with acetate film patterned with buttons via photochemical processing. For the sake of application requirements, it can be equipped with various levels of heat resistance, impact resistance, abrasion resistance, or corrosion resistance.
Usually, lower and upper layers of a membrane switch are brought and held together with pressure-sensitive adhesive layers. Two of the most important adhesive layers are the overlay adhesive layer and the rear adhesive layer. The overlay adhesive (typically acrylic) binds the top graphic overlay to the upper circuity layer. This assists the actuation of the membrane switch, since pressure on the top graphic overlay will force the upper circuit layer into electrical contact with the lower circuit layer. As it name suggests, the rear adhesive layer secures the static, lower circuit layer(s) to rigid backing (usually consisting of plain or treated aluminum) in order to stabilize the overall membrane switch. (PCB-based membrane switches do not require rear adhesive layers since the circuit board itself provides structural stability.) It should be noted that, technically speaking, the rigid backing that the rear adhesive layer binds to is an optional component that does not actually count as part of the membrane switch itself. Membrane switches which possess rigid backing are known as membrane switch panels.
Because of the way in which they are layered, these adhesives have no spaces through which contaminants can spill. This is important, because it means rubber and plastic keypads are less likely to suffer damage caused by dirt accumulation or accidental spills. By combining keypads with metal and plastic domes, adhesive layering can genuinely create an enhanced keystroke experience.
Specific Types of Membrane Switches
Membrane switch technology ranges widely in design and function, from complex membrane keyboards used with computers to straightforward tactile switches used to control lighting. A “simple” type of membrane switch is a packaged membrane switch, which possesses a membrane and a static layer within an enclosure. Because it also possesses two pins, packaged membrane switches are often soldered to a PCB. A more advanced type of membrane switch is a transparent membrane switch, more popularly known as a “touch screen.”
Importantly, membrane switches are generally divided between tactile and non-tactile button switches. As alluded to before, the top layer of a membrane switch features actual buttons or actuators to operate the switch. The nature of these buttons determines whether the membrane switch is tactile or non-tactile.
Tactile switches are marked by small domes held in place between the layers by polyester adhesive film. These domes are typically made from either polyester or stainless steel. The domes, when pushed down adequately, give way and produce a response that confirms that the keystroke has been registered. Feedback can come in a form that is tactile (e.g. the feeling of a dome being depressed), audible (e.g. a loud signal), or visual (e.g. flashing lights). Sometimes more than one type of feedback is produced.
Tactile switches are designed to respond when pressed by a finger or an actuator. Symbols or letters are used to denote the function of each switch on an overlay over the switch. Metal domes are commonly used in the design of these switches. When the metal dome is rubbed against a conductive footprint, an action is triggered.
Non-tactile switches do not rely on dome-like buttons to actuate the switch and give direct user feedback. More so than their tactile counterparts, this category of membrane switches relies on conductive ink (especially that printed on the upper circuit layer behind the graphic overlay) to make electrical connections. To compensate for the lack of tactile feedback, non-tactile switches are often designed to give some type of visual feedback (e.g. an LED indicator).
The principle of a non-tactile switch is similar to that of a tactile switch, except it does not elicit a tactile response. Non-tactile switches are self-contained units with the bottom of the overlay attached to a conductive pad. Non-tactile switches have the advantage of being able to simply customize the active keypad sections’ shapes and sizes.
Despite their difference, tactile and non-tactile membrane switches are often successfully combined within the same membrane switch panel.
Accessories/Optional Components of Membrane Switches
Frequently, membrane switches are accompanied by backlighting. (Backlit membrane switches are particularly useful for low-lighting user applications.) Sources for such backlighting can derive from one of three methods: light emitting diodes (LED), optical fiber, or electroluminescent lamps (EL).
LEDs may be installed on a separate LED layer or they may be surface mounted to the circuit layer itself. Compared to other backlighting options, LEDs are cost-effective. However, because LEDs create bright spots, they are recommended only as selective indicator lights and not as overall panel lighting. LEDs produce bright spots and are best used as indication lights rather than as panel backlighting. Surface-mount LEDs can be attached to the circuit layer or placed on a separate LED layer. In addition to mounting LEDs to function as indicators, specialty lighting like Light Guide Film can be used to backlight graphics evenly.
Optical fiber is better suited to panel lighting than LED lighting for several reasons. It is affected by neither extreme temperatures nor humidity. In addition, it has between 10,000 and 100,000 hours of light to offer, Fiber optic backlighting is made possible by fiber optic cloth that is precisely cut into custom configurations and assembled in a layer between the graphic overlay and circuit layers. Two or more layers of woven fiber-optic cloth are utilized to make a rectangular light-emitting region in a typical design. Extremes in humidity or temperature have little effect on optical fibers.
EL lighting is less expensive and has more design flexibility than optical fiber, due to the extreme thinness of electroluminescent layers. (It is regularly applied to products such as mobile phones and automobile dashboards.) However, this lighting option is not good for long term use. Once EL layers reach their half-life (between 3,000 and 8,000 hours), their brightness begins to fade rapidly. Electroluminescent lamps are less expensive than fiber optics and provide more design freedom. Depending on the phosphors used, the color of light emitted by an EL lamp can change.
Electronic circuits and switches can suffer or be completely disabled from the occurrences of electromagnetic interference and radio frequencies in certain operating conditions. Thus, some membrane switches are equipped with some type of EMI/RFI shielding. Such shielding can be accompanied by a variety of methods, including special conductive layers, grounding tabs which connect to a backing plate or support panel, etc.
Advantages of Membrane Switches
Membrane switches tend to be valued over other types of switches due to several factors, including affordability, durability and versatility. As a fairly new technology, membrane switches require fewer materials for fabrication than other types of interface equipment – which tend to be more resource-intensive and complicated (e.g. mechanically operated keyboards.) As such, they are highly cost-effective and efficient in terms of space usage. The hallmark of membrane switches is their layered design (produced by traditional die or by precision laser cutting), which in turn imparts physical durability; both the flatness and the thinness of adhesive-bound polymer films deter moisture and make membrane switches relatively easy to clean or sanitize. A final major advantage of membrane switches is tied to their versatility. The cost effectiveness of these switches contributes to their ease of customization, while their layered design allows them to incorporate complex switch graphics and integrate other technologies in ways that other types of switches do not.
Membrane Switch Applications
The preceding advantages of membrane switches create customers for them in many industries, including aerospace, medical manufacturing, gaming and recreation, electronics, and security. Some of the oldest membrane switch applications include microwave oven panels, television remote controls, and air conditioning control panels. Today, some of their most important applications reside with keypad performance. Membrane keypads do a number of things, including ensuring the proper function of building security systems, keeping sensitive information defended, and making sure industrial and manufacturing equipment runs safely and correctly. As membrane technology advances, membrane switches gain more and more applications. They are found in cellphones, children’s toys, handheld medical devices, x-ray machines, lift bed controls, ATM machines, calculators, and even home appliances.
Membrane Switch Circuits: FPC or PET?
Membrane switches are cost-effective and reliable alternatives to mechanical switches in applications exposed to hazardous conditions. They provide complete seals and protection from liquids or debris. They are often used as keypad or control panels in the medical, aerospace, defense, industrial, and transport industries. These come into two types: tactile and non-tactile. Tactile switches are usually made with metal domes to provide tactile feedback when pressed. The force needed to activate a tactile switch varies with the type and the size of materials used for domes. On the other hand, non-tactile switches are self-contained units having the bottom of the graphic overlay connected to a conductive pad.
Membrane switches have a multi-layered structure with the top layer and the circuit mostly made of PET. PET is a low-cost material, making them preferred by some customers. However, some requirements are not achievable with PET circuits. FPC circuits offer advantages in some situations where PET designs are not suitable. To learn when to use FPC or PET for membrane switches circuits, their differences are as follows:
PET circuits use the conductive epoxy paste to attach components. Manufacturers widely use conductive glues in surface mounting components. However, the bond strength of conductive glues is not strong enough to withstand environments with excessive vibration or extreme temperature fluctuation. On the other hand, FPC circuits use a soldered connection to affix components. Soldered connections can be thrice stronger than conductive glues, enabling FPC circuits to deliver high-reliability performance.
Number and Size of Components
More components can fit in smaller areas using FPC circuits because they can achieve finer trace widths. Circuit layer count and route traces around the narrow clearance section of the layout can also be reduced with FPC circuits, thus reducing circuit complexity. A PET circuit needs to be built with two layers to exhibit similar functions to one layer of an FPC circuit. In addition, finer tracer widths allow for reduction of unique tail exit location, unlike with PET circuits where tail exit cannot be placed closer to components. Surface mount components such as LEDs, capacitors, and integrated circuits cannot be reliably affixed to PET circuits for small packages.
Circuit Element and Manufacturing Process
The circuit elements used in PET and FPC circuits are silver ink and copper, respectively. The advantages of copper over silver ink, when used as circuit elements, are lower resistance, larger current load, better oxidation and moisture resistance, firmer welding, and less space consumed. FPC circuits offer 15 times less resistance than PET circuits, making them suitable for applications requiring a maximum circuit resistance of 200 ohms or less. PET circuits are created by screen printing, while etching processes produce FPC circuits. Manufacturing and assembly of membrane switches with PET or FPC circuits have similar lead times.
Cost of Circuits
FPC circuits cost more than PET circuits because the former uses a lot of copper. The growing demand for copper drives the continuous increase of its price. Furthermore, the extent of difference in cost between the two circuits varies according to the surface area of the membrane switch. The smaller the membrane switch designs, the lower the cost impact between the two circuit designs. Other factors such as graphics, overlays, and adhesives may also affect the cost of the membrane switches.
Membrane Switch Considerations
Because of its unique design, the overall quality of a membrane switch is highly dependent upon the quality of its individual components or layers. In order to obtain the trustworthy membrane keypads, it is essential for customers to find and work with manufacturers who are equally committed to all stages of the membrane switch production process. For example, it is important for manufacturers to make sure that each element remains uncontaminated during the assembly process (e.g. by fabricating different membranes inside cleanrooms.)
As with any other industrial supplier, customers should seek out switch companies with a depth of expertise and product offerings. Carrying stock membrane switches or offering full membrane switch panels are examples of characteristics which signify a quality switch supplier. Such a manufacturer or supplier will also enable customers to correctly weigh different factors in order to choose the right type of membrane switch for a specific application. Professional experts in membrane switch design and engineering are invaluable in determining the most effective combination of switch components and materials and correctly prioritizing different aspects of membrane switch selection.
Some brief examples of switch selection can illustrate the importance of careful calculation and quality advisors. Non-tactile membrane switches are ideal for many situations since they are highly reliable, customizable, and economical. If direct user feedback is critical in a given scenario, however, a tactile switch is clearly the better choice. Even if a tactile switch is chosen, other factors still need to be taken into account. For example, graphic overlays on tactile switches need to be thick enough to provide good durability throughout many switch cycles. At the same time, they cannot be so thick that they impede the tactile feedback itself.
Membrane switch Informational Video