This article takes an in-depth look at membrane keyboards. Read further and learn more about topics such as:
A membrane keyboard, also known as a membrane switch keyboard, consists of thin layers of flexible material that represent the keys. These keys are essentially mechanical switches that control electrical connections to produce signals and execute commands. Each switch controls an electrical circuit that remains inactive until pressed.
Membrane keyboard technology is commonly used in electronic gadgets and appliances due to its compact design and durability.
They are a means for inputting data into industrial, residential, and commercial devices, as well as providing commands for robotics and computer numerical controlled (CNC) machines. Computer keyboards are mainly used for typing and gaming applications.
A membrane keyboard consists of several layers. The top layer, which is where users interact with the keyboard, features the keys and is exposed to the external environment. This layer can be constructed from materials such as plastic or rubber. Beneath this, there are pressure pads made of a thin, flexible rubber or plastic membrane with conductive traces printed on their underside. The next layer, known as the spacer layer, prevents mechanical contact between the pressure pad and the bottom layer when a key is not pressed. The bottom layer contains conductive traces that are connected to the keyboard's circuitry.
When a key is pressed, the conductive trace beneath it comes into contact with the trace connected to the keyboard's circuit. This circuit includes an array of switches, each corresponding to a specific key. The contact between traces closes the switch, allowing an electrical current to pass through. This process registers the input by sending the relevant signal to the device’s processing system. Typically, the switches in the keyboard's circuit remain open until a key is pressed.
Membrane keyboards are utilized in a variety of settings, including industrial control panels, medical devices, telephone systems, computers, household electronics, point-of-sale systems, and more. Unlike traditional QWERTY keyboards, membrane keyboards can be tailored to specific applications, enhancing user interaction. They are generally lighter and more portable compared to mechanical keyboards, which rely on spring-loaded switches. In the upcoming sections, we will compare these two types of keyboards and explore the benefits of membrane keyboards in detail.
Membrane keyboards are characterized by their multiple layers and the design of their membranes, which can be flat switches or dome switches. Flat membrane switches use a pressure pad but do not provide feedback when the switch is actuated. Dome switches have a dome with icons or characters printed on them and offer tactile feedback when the dome is pressed.
When a key on a membrane keyboard is not pressed, there is no electrical current flow due to the non-conductive gap between the conductive traces and the bottom layer. Pressing a key brings the conductive traces into contact, closing the gap and allowing current to pass through.
Users often need some form of feedback when using membrane keyboards, as these keyboards can be silent without such features. Depending on the design, feedback can come in the form of sounds, lights, beeps, clicks, or other indicators to confirm that a keypress has been registered.
A membrane switch keyboard is constructed from a flexible material and covered with a clear, pliable membrane. The keyboard's graphics are printed directly on this membrane. The keyboard functions by temporarily closing and opening an electrical circuit through contact with the membrane. When a key is pressed, the membrane touches the circuit, closing it, and when the pressure is lifted, the circuit opens again. This process occurs quickly and seamlessly during use.
Membrane switch keyboards are among the simplest types of keyboards and are commonly used as keypads in various household appliances, including washing machines, air conditioners, refrigerators, and microwave ovens. They are also prevalent in commercial applications like vending machines, printers, and photocopiers.
The core components of membrane switch keyboards consist of multiple layers assembled with heat-sealed films or pressure-sensitive adhesives. These layers include graphic overlays, conductive tracks, domes, spacers, and terminals.
Circuit Terminal - The circuit terminal is a flat ribbon of conductive tracks that connect the keyboard switch to the control unit.
Membrane switch keyboards are designed to respond to light pressure, which enhances ergonomics but provides minimal physical feedback. To compensate for this lack of tactile response, some devices incorporate additional feedback mechanisms such as lights, vibrations, or sounds to confirm that a button press has been detected. For example, many microwave ovens emit a beeping sound each time a button is pressed, providing audible confirmation of the command.
Full travel membrane keyboards are commonly used in computers. These keyboards feature keys that are molded from a single piece of plastic or rubber and are integrated directly into the keypad. A flexible membrane covers a grid of electrical switches. When a key is pressed, it functions like a plunger, pushing the membrane down to make contact with the bottom layer and closing the corresponding switch. The membrane's elasticity then returns the key to its original position when pressure is released. Typically, these keyboards include a spring beneath each key to aid in this action.
Full travel membrane keyboards offer a cost-effective solution for computer input devices. However, they generally provide less physical feedback compared to mechanical keyboards.
Dome switch membrane keyboards, also known as direct-switch keyboards, blend characteristics from both membrane and mechanical switch designs. These keypads are typically constructed from rubber or silicone, with a metal or rubber dome positioned beneath each key. Each dome features a conductive trace that completes the circuit and activates the corresponding switch when the key is pressed.
Metal Domes - Metal domes, also referred to as snap domes or tactile domes, are typically made from stainless steel and come in various shapes, including circular, four-legged, triangular, and oblong. Circular metal domes are suitable for use in circuit boards with two or more layers or in precision switches. Four-legged domes are designed for single or double-sided PCBs, where both positive and negative traces are present on the same circuit.
Triangular metal domes are preferred when a tactile click is desired and are often used in small handheld devices or force control switches to prevent accidental activation. Oblong metal domes are ideal for double-sided PCBs and are well-suited for applications requiring a low profile on densely populated circuit boards or in narrow spaces.
Although stainless steel is the commonly used material for metal domes, they can be plated in gold, silver, or nickel.
Effective air venting is crucial for metal domes to ensure proper functionality. This process involves removing air from beneath the metal dome. Venting methods include using vent channels with a spacer layer, top venting through polyester fabric, or channels integrated into the circuit board.
Rubber Domes - Rubber domes are made from silicone rubber, characterized by their small size and flexibility. They are either molded with conductive carbon or have conductive ink applied post-molding. When pressure is applied, rubber domes collapse quickly to provide tactile feedback and spring back when the pressure is released.
Rubber dome switches offer several advantages, including tactile feedback, affordability, a softer feel, resistance to leakage or corrosion, and quiet operation.
Dome switch membrane keyboards are commonly used in handheld controllers, consumer electronics, mobile phones, medical devices, and personal computers. Unlike metal domes, rubber dome switches can sometimes feel too soft or mushy, potentially providing less feedback.
Scissor switch keyboards feature a rubber dome combined with a plastic scissor mechanism that connects the keycap to a plunger. When the key is pressed, the plunger compresses the rubber dome, which reduces the overall travel distance of the key. Scissor switches are known for their shorter travel distances, typically ranging from 1 mm to 2.5 mm, compared to the 2.5 mm to 4 mm travel distances found in other types of keyboards.
The “scissor legs” in these keyboards link the keycaps to the underlying mechanism, effectively shortening the key travel and reducing the distance the rubber dome and membrane must move during a keystroke. This design extends the keyboard’s lifespan by minimizing the stretching of components with each press.
While scissor switch keyboards tend to be noisier than other types due to their reduced rubber cushioning, they are favored for their durability, often rated for up to 10 million keystrokes. They also feature tightly sealed keycaps with smaller gaps, which helps to keep out dust and debris.
Scissor-switch keyboards operate quietly, even with their scissor-like mechanisms. They are commonly used in laptop keyboards and integrated keyboards. While they tend to be somewhat more expensive than other types, they also present a cleaning challenge due to the restricted key movement and multiple attachments. However, their design often features smaller gaps between the keys, which reduces the likelihood of debris getting trapped.
Chiclet keyboards, also known as island-style keyboards, are a popular choice in contemporary laptops and desktops. They feature a sleek and minimalist design with square or rectangular keys that have rounded corners and a low profile. Each key is separated by a perforated plastic plate that spans the entire keyboard, giving it a clean and uniform appearance.
Chiclet keyboards first emerged between the late 1970s and mid-1980s, notably used in personal computers like the TRS-80 Color Computer, TRS-80 MC-10, and Timex Sinclair 2068. They are named after the Chiclet gum brand due to the resemblance of their keys to the gum pieces. Initially, chiclet keyboards received negative feedback because of their perceived poor quality. However, by the 1990s, their design was revamped and gained widespread acceptance. Today, chiclet keyboards remain a popular choice among PC manufacturers.
Chiclet keyboards operate similarly to conventional membrane keyboards, with some minor variations. The key switch is closed and the keystroke is registered through one of two methods, involving the mechanical contact between the top and bottom conductive traces:
Chiclet keyboards are quieter than mechanical keyboards and offer a comfortable typing experience, making them popular in business settings. The user feedback on these keyboards is mixed. The larger gaps between the keys help reduce typographical errors, and the short, flat keystrokes are often found comfortable by some typists. However, the increased space between keys can lead to finger fatigue as users might need to stretch their fingers more. The tactile feedback provided by chiclet keyboards is generally less pronounced compared to mechanical keyboards. Additionally, they are not ideal for intensive gaming due to issues like “ghosting.” To address this, manufacturers have been working on designs to enhance the performance of chiclet keyboards for gaming applications.
Unlike membrane switches and mechanical push buttons, capacitive technology works by detecting the change in capacitance, the ratio of electric charge stored in a conductor to electrical potential, on a surface that is monitored by a microcontroller that has a base capacitance value. When a change is detected in the capacitance, the base capacitance triggers output.
The touch surface or graphic overlay of capacitive keyboards can be made of any nonconductive material with the output customized to fit the needs of an application. Capacitive touch screens are more responsive to human touch and is the reason they are used for high tech smartphones. The surface of a capacitive screen is smooth and requires only the presence of a finger to be activated without the need for the application of force.
Capacitive touch keyboards come in two main types: PCB and film. The choice between these types depends on the specific requirements of the customer and the intended application.
Film-type capacitive keyboards offer high insulation resistance and are free from dielectric loss. The different film layers used in these keyboards provide excellent resistance to variations in ambient temperature.
The following terms describe various keyboard properties. While somewhat subjective, these criteria can be crucial when selecting and evaluating keyboards. Although we might be subconsciously aware of them, they play a significant role in shaping our overall typing experience.
Key travel refers to the distance a key moves from its resting position until it reaches the actuation point, where the keystroke is registered. This distance is usually measured in millimeters. Keys with shorter travel distances often provide a lighter typing feel, while those with longer travel may offer a more deliberate keypress.
Actuation force is the amount of pressure needed to reach the actuation point of a key, where the input is registered by the keyboard. This measurement indicates how much force a user must apply to record a keystroke and is typically measured in centinewtons (cN) or gram-force (gf).
Do you notice a bump under your fingers when you press a key on your keyboard? This sensation is known as tactility. Tactility refers to the sensory feedback a user experiences when typing, indicating that the keyboard has registered the input. It is closely related to the key travel distance and the physical feedback provided by the keys.
Tactility can influence the amount of pressure a user applies to the keys. If a keyboard provides minimal feedback, users may press harder than necessary, exceeding the required actuation force, in an attempt to confirm that their input has been recorded.
As discussed in the previous chapter, tactile feedback can be categorized into audible, tactile, and visual forms. Most keyboards incorporate a combination of these feedback types to enhance the typing experience.
A light or soft touch is a desirable feature for both ergonomists and users. This characteristic is influenced by both actuation force and key travel, and it is achieved when minimal pressure is needed to press the keys. While keys that are easy to press can reduce stress on finger joints during typing, they may also decrease typing accuracy, as the reduced force required can lead to accidental key presses.
Mechanical keyboards are a major category based on their operating mechanism. Each key on a mechanical keyboard features a switch assembly consisting of a spring, a plunger, and a pair of metal contacts. When a key is pressed, the plunger compresses the spring and forces the metal contacts to meet, closing the switch and registering the input. Upon releasing the key, the spring and plunger return to their original positions, often providing a tactile bump during typing. Mechanical key switches come in three main types: linear, tactile, and clicky, each distinguished by its tactility, actuation force, and noise level.
Linear switches produce the least noise and give a smooth keystroke. The keys move up and down with little resistance. Hence, they can be actuated rapidly, but they do not provide tactile feedback. Tactile and clicky switches give a bumping sensation near or at the actuation point. Tactile switches produce moderate noise, while clicky switches are the loudest among the key switches.
Below is a comparison of the two types of keyboards based on various criteria:
Membrane keyboards are relatively quiet during typing because their membrane absorbs the sound created by keypresses. This makes them generally quieter than mechanical keyboards, which produce a distinct click sound at the actuation point. Membrane keyboards are a good choice for use in public spaces or when minimizing noise is important.
Membrane keyboards are significantly less expensive than mechanical keyboards. You can find a high-quality membrane keyboard at a lower price, making them a more budget-friendly option.
Membrane keyboards are lighter, more portable, and more compact compared to mechanical keyboards. Their simpler design includes fewer moving parts and uses a silicone, rubber, or plastic membrane for the switching mechanism. This makes them ideal for people who travel frequently with their computers. Additionally, membrane keyboards tend to have a minimalist and classic appearance.
Membrane keyboards are used in a broad range of applications beyond just computers. They can be found in household appliances, telephones, and various types of industrial, laboratory, and medical equipment.
Both membrane keyboards and mechanical keyboards with linear switches require the keys to "bottom out" to register a keystroke. In keyboards with tactile or clicky switches, the actuation point is reached before the keys bottom out, allowing the user to release their fingers partway through the key travel while still registering a keystroke.
Anti-ghosting is a feature that enables multiple keys to be pressed at the same time without losing any keystrokes, which is essential for intense gaming and fast typing scenarios. N-key rollover (NKRO) measures how many keys can be pressed simultaneously without missing any input, with a 6-KRO rating allowing for six simultaneous keystrokes. Full NKRO, which allows all keys to be pressed and registered at once, is more commonly found in mechanical keyboards due to their design's resistance to ghosting.
Membrane keyboards typically have lower NKRO because of their design, which often limits simultaneous key presses. However, full NKRO can be found in some high-end membrane keyboards.
Membrane keyboards provide a softer touch and a "mushy" feel during typing, offering less tactile feedback. However, some high-end membrane keyboards now include enhanced tactile bumps and audible feedback to improve responsiveness. Mechanical keyboards, in contrast, naturally provide more solid and tactile feedback due to their mechanical switches.
Cleaning and replacing keycaps on membrane keyboards can be challenging, and if a key or component fails, the entire keyboard may need to be replaced. Mechanical keyboards, however, allow for easy removal and cleaning of keycaps, and individual switches can be replaced if needed.
Mechanical keyboards have a lifespan ranging from 20 million to 100 million keypresses, whereas membrane keyboards typically last between 5 to 10 million keypresses. Despite this, membrane keyboards are still a cost-effective choice for light to moderate use.
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