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Membrane switches are categorized as user-equipment interface utilities, allowing users to effectively communicate commands to electronic devices. Unlike mechanical switches, which are made of plastic parts and copper, membrane switches consist of a circuit and a substrate.

A membrane switch, also known as a touch switch, is a multi-layered device that functions as a switch to turn a device on or off. These switches feature printed circuits on film, in contrast to mechanical switches made from copper and polymers. Membrane switches are created by printing silver or carbon ink onto heat-stabilized polyester film. An overlay, serving as the switch’s graphic interface, is placed on the surface. These top substrates are called “membranes” or “graphic overlay films” due to their construction from thin, flexible materials.

Membrane switches are known for their low profile, sealing capability, and ease of cleaning. They can be integrated with other control systems like keyboards, touch screens, and lighting, and can be as intricate as the membrane keyboards and switch panels found in mobile phones and computers. Depending on the industry and application, membrane switches are also referred to as membrane keyboards or membrane keypads.

How Membrane Switches Work

The functionality of a membrane switch hinges on the electrical contacts meeting across various layers or membranes within the switch. Typically in an open state, membrane switches maintain an incomplete electrical circuit, keeping the switch in an “off” status until activated. Actuation of membrane switches commonly occurs through pressure, which brings the contacts on different membranes into contact with each other.

Membrane Switch – Douglas Corporation

Membrane Switch Parts

Membrane Switch Parts

At a minimum, membrane switches require at least two circuit layers to complete an electrical circuit. However, most membrane switches typically have three to four layers, with some advanced versions having up to seven layers. The layers of a membrane switch can generally be categorized as follows:

Circuit Layer

This layer contains electrically conductive materials necessary for completing the circuit. It is positioned at the bottom of the switch assembly, often referred to as the “bottom layer.” Depending on the specific membrane switch, circuit layers vary in number and format. At least two circuit layers are essential for a membrane switch: the membrane layer, which is the upper flexible circuit layer housing one pole of the electrical circuit, and the static layer, the lower circuit layer typically with a rigid backing carrying the other pole of the circuit.
The circuit patterns on these layers are created by screen printing electrically conductive ink, such as silver-based ink, onto a polymer film. Printed Circuit Board (PCB) membrane switches utilize a printed circuit board (made from materials like ITO or PET) as the static or lower circuit layer, providing structural strength and mounting capabilities for soldered parts. In contrast, Flex Circuit membrane switches use flexible film substrates (e.g., polyester or polyimide/Kapton) for both upper and lower circuit layers, with conductive patterns also screen printed onto them. Variants like Silver Flex membrane switches involve screen printing silver onto the films. Copper Flex membrane switches are made by laminating thin metallic sheets onto films and then chemically etching away the metal to leave conductive patterns.
Circuit layers can vary in complexity and form, including single-sided layers with conductive material on one side of a substrate, double-sided layers with conductive material on both sides, and multi-layer flexes with multiple single or double-sided layers. These layers are typically insulated using dielectric ink to prevent unintended electrical contact between different circuit layers. Additionally, an insulating spacer layer, which can be an inert gas or adhesive bonded to the circuit board, is often used. In membrane switches with tactile feedback and dome switches (discussed later), insulating areas are patterned with cutouts to accommodate these electronic components.

Graphic Overlay

The uppermost layer of a membrane switch is commonly known as the graphic overlay, as it features the buttons and user interface used to operate the switch. Similar to circuit layers, graphic overlays are produced by printing switch graphics onto polyester or polycarbonate film substrates. Polyester is often preferred for its superior chemical corrosion resistance and durability.

Graphic overlays can be manufactured in various methods. Visible on the surface of the membrane switch, they are typically embossed or printed. They may feature screen-printed colors and text, or they can be digitally printed, which is often favored for its cost-effectiveness and ability to achieve a wide range of high-quality graphic effects. Alternatively, the graphic overlay may be covered with acetate film patterned with buttons using photochemical processing. Depending on application requirements, overlays can be designed with varying levels of heat resistance, impact resistance, abrasion resistance, or corrosion resistance.

Adhesive Layers

Pressure-sensitive adhesive layers are used to bond the upper and lower layers of a membrane switch together. Two crucial adhesive layers include the overlay adhesive and the rear adhesive. The overlay adhesive, typically acrylic, adheres the top graphic overlay to the upper circuit layer. This facilitates the actuation of the membrane switch, where pressure on the graphic overlay presses the upper circuit layer into electrical contact with the lower circuit layer. The rear adhesive layer, as its name suggests, secures the static lower circuit layer(s) to a rigid backing, typically plain or treated aluminum, to stabilize the overall membrane switch. PCB-based membrane switches do not require rear adhesive layers, as the circuit board itself provides structural stability. It’s important to note that the rigid backing to which the rear adhesive layer binds is an optional component and does not count as part of the membrane switch itself. Membrane switches with a rigid backing are often referred to as membrane switch panels.

Due to their layered construction, these adhesives eliminate spaces where contaminants could infiltrate. This is crucial for protecting rubber and plastic keypads from damage caused by dirt buildup or spills. Adhesive layering enhances the keystroke experience significantly when combined with keypads with metal and plastic domes.

Specific Types of Membrane Switches

Membrane switch technology encompasses a wide spectrum of designs and functionalities, ranging from intricate membrane keyboards used in computing to basic tactile switches employed in lighting control. Packaged membrane switches represent a simpler variant, consisting of a membrane and a static layer housed within an enclosure and typically soldered onto a PCB due to their dual pin configuration.

On the other end of the spectrum, transparent membrane switches, commonly referred to as touch screens, represent a more advanced iteration of this technology.

Membrane switches can broadly be categorized into tactile and non-tactile types based on their button design. Tactile switches feature small domes secured by polyester adhesive film between layers, typically made from polyester or stainless steel. Pressing these domes provides tactile, audible, or visual feedback, confirming activation.

Conversely: non-tactile switches do not rely on dome-like structures for activation or user feedback. Instead, they utilize conductive ink, often printed on the upper circuit layer beneath a graphic overlay, to establish electrical connections. While lacking tactile response, non-tactile switches compensate with visual indicators, such as LED lights, to signal activation.

Despite their distinct characteristics, both tactile and non-tactile membrane switches are frequently integrated into the same panel, offering versatility in design and functionali

y. 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 are favored over other switch types for several reasons, including affordability, durability, and versatility. Being a relatively new technology, membrane switches use fewer materials in fabrication compared to other interface equipment, which are typically more resource-intensive and complex (like mechanically operated keyboards). This makes membrane switches highly cost-effective and efficient in terms of space utilization.

The key feature of membrane switches is their layered design, achieved through traditional die-cutting or precision laser cutting, which enhances physical durability. The flat and thin adhesive-bound polymer films also repel moisture, making membrane switches easy to clean and sanitize.

Another significant advantage of membrane switches is their versatility. Their cost-effectiveness allows for easy customization, and their layered design enables the integration of complex switch graphics and other technologies in ways that other switch types cannot match.

Membrane Switch Applications

The advantages of membrane switches have established a strong customer base across various industries such as aerospace, medical manufacturing, gaming, electronics, and security. Among the earliest applications are microwave oven panels, television remote controls, and air conditioning control panels. Today, membrane keypads play a critical role in ensuring the functionality of building security systems, safeguarding sensitive information, and ensuring safe and correct operation of industrial equipment. As membrane technology evolves, membrane switches continue to find new applications, including in cellphones, children’s toys, handheld medical devices, x-ray machines, hospital bed controls, ATMs, calculators, and household appliances.

Membrane Switch Circuits: FPC or PET?

Membrane switches offer cost-effective and reliable alternatives to mechanical switches, particularly in environments prone to hazards. They provide complete sealing and protection against liquids and debris, making them ideal for use in medical, aerospace, defense, industrial, and transportation industries. These switches are primarily used for keypad or control panel applications.

There are two main types of membrane switches: tactile and non-tactile. Tactile switches utilize metal domes to provide tactile feedback upon pressing, with the required activation force varying based on dome type and size. Non-tactile switches are self-contained units where the graphic overlay’s bottom connects to a conductive pad.

Typically constructed with a multi-layered design, membrane switches feature a top layer and circuit predominantly made from PET (polyethylene terephthalate). PET is favored for its low cost, although it may not meet all customer requirements. In situations where PET designs fall short, flexible printed circuit (FPC) circuits offer distinct advantages. Understanding the differences between FPC and PET circuits is crucial for determining the appropriate choice for membrane switch applications:

In PET circuits, components are affixed using conductive epoxy paste, a common choice in surface mounting. However, the bond strength of conductive glues used in PET circuits is insufficient for environments prone to excessive vibration or extreme temperature fluctuations. In contrast, FPC circuits utilize soldered connections, which are significantly stronger than conductive glues. This strength advantage allows FPC circuits to ensure high reliability under challenging conditions.

Number and Size of Components

FPC circuits excel in compactness, accommodating more components within smaller spaces due to their ability to achieve finer trace widths. By reducing circuit layer count and optimizing trace routing, FPC circuits simplify circuit layouts compared to PET circuits. Typically, a PET circuit requires two layers to match the functionality of a single-layer FPC circuit. Moreover, finer trace widths in FPC circuits enable more flexibility in component placement, which is restricted in PET circuits, particularly for surface mount components like LEDs, capacitors, and integrated circuits.

Circuit Element and Manufacturing Process

PET circuits utilize silver ink for circuit elements, while FPC circuits employ copper. Copper offers several advantages over silver ink, including lower resistance, higher current carrying capacity, superior resistance to oxidation and moisture, stronger solder joints, and efficient use of space. FPC circuits exhibit significantly lower resistance—15 times less than PET circuits—making them ideal for applications requiring minimal circuit resistance, typically 200 ohms or less. PET circuits are manufactured through screen printing, whereas FPC circuits are produced via etching processes. The manufacturing and assembly lead times for membrane switches using PET or FPC circuits are comparable.

Cost of Circuits

Due to their extensive use of copper, FPC circuits are generally more expensive than PET circuits. The cost difference varies depending on the size of the membrane switch, with smaller designs minimizing the impact. Factors such as graphics, overlays, and adhesives also contribute to the overall cost variation between the two circuit types.

Membrane Switch Considerations

Due to its unique design, the overall quality of a membrane switch heavily relies on the excellence of its individual components or layers. To ensure reliable membrane keypads, customers must collaborate with manufacturers committed to every stage of production. For instance, manufacturers must maintain cleanliness throughout assembly, fabricating different membranes inside controlled environments like cleanrooms.

Customers, like those dealing with any industrial supplier, should prioritize expertise and product variety when choosing to switch companies. Reliable suppliers stock membrane switches and offer complete switch panels, indicators of quality. They also assist customers in evaluating factors crucial to selecting the right membrane switch for specific applications. Expertise in membrane switch design and engineering guides decisions on component combinations and materials, ensuring optimal performance.

Careful consideration and expert advice are essential in selecting membrane switches. Non-tactile switches are often preferable for their reliability, customization options, and cost-effectiveness. However, scenarios requiring direct user feedback necessitate tactile switches for their superior suitability. Even when opting for tactile switches, considerations such as the thickness of graphic overlays impact durability and tactile feedback efficiency, requiring balanced decisions.