A membrane switch can be most simply interpreted as the tactile user interface between humans and electronics. The ASTM describes a membrane switch as a momentary switching mechanism in which at least one of the contacts is made of, or adheres to a flexible substrate. Touch pads and keyboards for phones, computers, remote control devices, and microwave ovens employ membrane switches to operate their electronic functions.
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The History of Membrane Switch Technology
The typewriter was the beginning of keyboard technology. Invented in the early 1800s, Remington's Sholes and Glidden typewriter was the first commercial typewriter available, but was slow to catch on.
Between 1830 and 1870, many typewriters were designed and a number of patents granted, but few reached the commercial manufacturing stage.
By 1910, typewriters had become standardized, with a full set of upper and lower case alphabet keys, number keys, and punctuation keys. These units were typically made with a heavy cast iron body and mechanical steel keys that were stiff to operate and easily stuck together. Typing was slow and fraught with mistakes caused by the unwieldy equipment.
In 1961, IBM introduced the Selectric typewriter. It was the first electric typewriter on the commercial market. It's primary claim to fame was the development of the type ball, rather than a set of individual keys. The balls could be changed out with counterparts of different fonts at any point in the typing process, allowing for more flexibility in lettering styles. Typing speed and efficiency was drastically increased and improved models were the standard for the next two decades.
The dawn of the computer age ushered in the need for improved keyboard functionality. Mechanical switches were not meeting the need for increased data input. They were responsible for unreliable and erroneous circuit activation which greatly restricted their use for industrial purposes.
The earliest computers were not sold with keyboards as a standard part of the package. Computers were controlled with punch cards or electromagnetic teletype devices. The first keyboards were actually modified typewriters.
The first membrane switches of the early 1980s were made from polycarbonate materials. These designs, based on resistive technology, responded to pressure, but provided low tactile feedback. The polycarbonate materials were readily available and cheap, but proved to be brittle, susceptible to cracking or breaking, and were not resistant to typical wear and tear.
The mid 1980s brought polyester upgrades to membrane technology. The polyesters could be directly mounted to a custom circuit board and were more compact and functional. They were stronger, resistant to elements, and provided the tactile feedback that we are more familiar with today.
IBM introduced the Model M keyboard in 1984. The Model M touted a buckling spring system that provided distinct audible and tactile response, which equated to increased accuracy and performance.
By the late 1980s, domed keys were developed, improving the longevity and tactile response further. The rubber keypad domes proved to be lighter in weight, quieter to operate, and were cheaper to manufacture and market than the metal dome keypad.
Around the same time, lighting found its way into graphic design. LED lights were originally used, but produced a bright spot unless a secondary light diffusing element was employed. Electroluminescent (EL) films were introduced that could provide backlighting and color. Fiber optic lighting was also used. These could illuminate individual keys, sections of the keyboard, or light up the entire pad.
Advantages and Benefits of Membrane Switches
The need for more complex control over mechanical functions in a simple operational format was the basis for developing membrane switch technology. The use of the membrane keypad may allow a single operator to operate manufacturing, packaging, or shipping operations from a single point. It may allow for integration of various pieces of equipment through direct or remote electronic connections.
The brain-children of recent technology, membrane switches are increasingly popular due to ease of fabrication and lower material costs, compared to mechanical switches or more complex interface equipment. Correct manufacturing processes and good maintenance practices ensure longevity, enhancing their cost effectiveness.
The tactile response of the keys is a major design concern for the switch manufacturer. An operator needs the tactile sensation of having pushed the button without requiring undue tactile force. The correct choice of domes under the graphic layer provides acceptable tactile responses.
Switch Fabrication Process
Membrane switches are made up of layers that are glued together with pressure sensitive adhesive. The presentation layer, or graphic overlay is typically made of silicone, making it flexible and easy to clean. It provides the tactile key pattern for the graphic interface between man and machine.
Graphic overlays are either printed or embossed. They may be screen printed, complete with appropriate symbols or text, onto a silicone sheet or the button pattern may be printed on an acetate film layer, through photochemical processes. The overlays are designed to be heat, impact, and corrosion resistant.
A screen printed circuit board is glued to the overlay with an adhesive bonding layer. The circuit board uses copper polyimide flex circuits or silver-based conductive ink deposited by screen printing, on a thin sheet of polyethylene terephthalate (PET) or indium tin oxide (ITO). The adhesive spacer between the electronic circuit board and overlay is a layer of electrically insulative material.
A separation layer allows the option of a secondary circuit layer, which is attached to the base layer, or tail filler, with another layer of the electrically insulating adhesive material.
When a button on a membrane keypad or control screen is operated through touch, an electrical connection is made, activating the particular function denoted by the graphic button.
The control panel may be a custom membrane switch that is backlit, or able to be operated under any ambient lighting circumstances, by adding lights to the separation layer. There are several types of lighting used for this purpose.
Light emitting diode (LED) backlighting uses tiny LED lamps to light individual keys. They are cool burning, use very little energy, and are easy to manufacture, but create bright spots in the graphic overlay if the light is not diffused.
Optical fibers can be woven into a fabric that diffuses light from a single point, such as an LED. The light diffusing array is ideal for lighting a section of the keypad or the entire membrane switch panel. Optical fiber backlit membrane switch panels may even be used underwater.
Electroluminescent lamps (EL)are inexpensive phosphor light bulbs that are often used in cheap electronics due to their low cost and relatively short lifespan. EL lighting may be colored blue, blue/green, yellow/green, white, and orange, based on the phosphor composition.
Membrane Switch Types
- Custom Keypads
- And custom membrane switches are user-equipment interface utilities that are designed for use with specialized electronic devices.
- Flexible Circuits
- Electrical signal-bearing utilities that connect control systems with processing systems; they are characterized by their flexible construction as opposed to the rigid, often brittle construction of other circuitry varieties.
- Graphic Overlays
- Covers that are placed over control panels to indicate the location and function of buttons.
- HMI Systems
- Also known as Human Machine Interfaces, are devices that facilitate interactions between machines and humans.
- Keyboard Switches
- User-equipment interfaces that allow for the communication of commands between a user and an electronic device.
- Membrane Keyboards
- Flat surfaces with printed symbols and outlines rather than traditional moving keys.
- Membrane Keypads
- Flat surfaces, rather than moving keys, that are activated with the press of a finger.
- Membrane Switch Panels
- User-equipment interfaces that feature several control switches for the communication of commands from a user to an electronic device.
- Metal Domes
- Supply the snap and complete the electrical circuit. A dome switch is considered a tactile switch because when the button is operated, there is a noticeable feeling of having pressed a button. Metal domes are typically made of nickel, silver, or gold plated stainless steel or aluminum which makes them extremely reliable in the long run. Rubber domes are quiet and inexpensive, but do not offer the positive response of metal domes and have a short lifespan.
- Non-tactile Switches
- Membrane switches made without a snap action.
- Polyester Dome Switches
- Produced by embossing domes into one of the polyester layers in the construction, which results in a very durable tactile element with a little more movement than metal domes. Manufacturers who do this diminish the quantity of layers in the assembly.
- Proximity Switches
- Made to open or close an electrical circuit when they either make contact or come within a certain distance of an object. They detect objects in close “proximity.” Proximity switches come in four basic types, which are: infrared, acoustic, capacitive and inductive.
- Rubber Keypads
- User-equipment interface utilities that allow for the communication of commands between a user and an electronic device; they are characterized by their rubber keys as opposed to the plastic keys of other keypad varieties.
- Scissor Switch
- Places a scissor-like mechanism between the circuit board and the dome. The scissor action shortens the distance that the dome travels and adds longevity to the keyboard.
- Tactile Switches
- Assembled to supply a positive snap-action reaction to pressure. They can be attained through polyester domes made in either the graphic or circuit overlay layer or stainless steel domes embedded in the membrane switch.
- Touch Screens
- Pressure-sensitive computer display screens that act as input devices.
Where Membrane Switches are Used
Membrane switches are found in laboratory equipment, airplane dashboards, and consumer appliances, such as coffeemakers, ovens, thermostats, televisions, audio and video systems, and some lighting. Gaming devices of all kinds utilize membrane switch technology. Vending machines, ATMs, and self-checkouts rely on membrane switches.
Industrial manufacturers rely on the ease of managing operations through the use of a switch panel which may operate one machine or an entire factory full. The use of touch control allows for the management of huge industrial endeavors such as operation of dams, power plants, global communication, navigation, and transportation of every kind. Massive construction cranes, mining and excavation equipment, and metal processing plants use membrane switch controls to manage dangerous operations remotely, to ensure human safety.
From farm to table, the operations that harvest, process, transport, sell, and cook food are controlled by electronic devices that rely on membrane switches. Farm equipment, canning and bottling processes, conveyor systems, inventory tracking devices, checkout registers, and ovens are all controlled through the use of membrane switch keypads.
The Department of Defense and aerospace industries use membrane switches extensively to control and operate a wide range of equipment and machinery, from calculators to rockets. Military tanks, ships, and planes are operated by membrane switches. Medical devices must follow design practices to meet stringent FDA standards. The switch manufacturer must ensure that quality stainless steel domes are built into the membrane switch design to provide reliable function and a high tactile feel requiring low tactile force to operate the keypads. Some medical devices depend on the use of color leads to attach electrodes to appropriate body parts. The colored wire leads are designed to coordinate with the graphic overlay of the control panel.
Modern Advances in Membrane Switch Technology
The most recent designs in membrane switch technology include the capacitive touch switch, which utilizes separate conductive electrodes to generate a field of electricity behind a glass surface. By touching the surface of the glass, the voltage in the field is changed, signaling specified switches to operate.
Capacitive touch screens provide an electronic interface through a self contained, single surfaced unit. The touch screen acts as a control mechanism for other switches within complex electronic devices. Switches may be changed through touch commands, causing functions and operations to change, while the screen stays the same. The glass is easy to clean, and resistant to dirt, chemicals, and acid. Capacitive switches have no moving parts and will not wear out.
A good manufacturer will have a design team on hand to provide the best custom membrane switch for any given purpose. They will consider the scope of the operation and design the most effective switches with appropriate graphic overlays to meet the need.
The overlays that provide the outline of key patterns should be relevant to the equipment and easy for the operator to understand. Membrane switches may need to be designed with a thin or low profile. The keys may be easy to shield, operate through micro-motion, have rubber keypad overlays, flexible circuit layers, or operate through a system of metal dome arrays.
The manufacturer and design team should be able to direct you toward equipment that offers maximum service, provide maintenance procedures and schedules, and be aware of industry standards in order to meet guidelines or that would include or exclude the use of any specific materials.
Membrane Switch Terms
- Abrasion Resistance
- The degree to which a membrane switch is able to withstand surface wear.
- This is the action of working a switch apparatus.
- Actuation Force
- The pressure necessary for collapsing the walls of the dome on a polyester, rubber or metal keypad.
- The molecular attraction of one material to another. The strength of the bond is determined by the surface energy in each material.
- Backing/Rear Adhesive
- An adhesive applied to the back of a membrane switch for mounting purposes.
- Breakdown Voltage
- The minimum voltage at which the insulation between two conductors is destroyed.
- Carbon Graphite Inks
- The type of ink that consists of prepared suspensions of carbon black and is frequently printed over silver circuitry to diminish the potential of migration of silver. These are used for lessening costs when the conductivity of a metal base system is not necessary.
- A material’s ability to allow electrons to flow.
- A conductor intersection insulated by dielectric material.
- Dead Front
- A cosmetic feature of a graphic overlay in which a button is only visible when backlit.
- An insulating or non-conducting medium.
- Dielectric Inks
- Used for printing protective patterns on conductive printing to isolate selected regions from electrical contact with other conductors. This is used for crossovers and tail insulation on membrane switches.
- Dome Retainer
- An adhesive layer made to hold metal domes in the key switch.
- El Lamp
- A slender device that illuminates large areas, typically used in LCD membrane switch backlighting and control panels.
- Embedded LED
- Procedure of integrating a surface mount LED into a membrane switch assembly.
- A way to supply a raised characteristic to accentuate key surfaces through mechanical and thermoforming of graphical features. This also permits an embedding of a surface mount of an LED inside the switch.
- Gloss Level
- The extent of shininess of a substrate, commonly identified in percentages.
- Graphic Keypad
- Control keypads that use graphics for button functions for navigation on machines or process operations. Typical graphics include arrows or symbols indicative of a machine process or operation.
- Internally Vented
- Switch openings connected to one another to seal the switch from moisture and other contaminants.
- Key Height
- A measure of the distance from the highest point of a key to the base of the keypad.
- Light Emitting Diode (LED)
- Embedded in membrane switch layers to illuminate the button.
- Moisture Resistance
- A material’s ability to resist the absorption of water from the air or during complete submersion.
- The front layer of a membrane switch or control panel.
- The travel that is done by the rubber keyboard or metal dome after making contact with the circuit.
- Pillow Emboss
- Creating a raised surface in the graphic overlay over the membrane keypad area of membrane switches.
- The schematic that describes the circuit output requirements for membrane switches.
- Adhesive materials that bond after pressure without needing heat or solvents.
- Rail Emboss
- Produces a raised ridge circling the key area.
- Screen Printing
- Printing procedure that uses a stretch of mesh over a frame, permitting the use of a stencil to discriminately allow ink through. This is typically used for creating graphic overlays and membrane circuits.
- Silver Inks
- Finely-milled particles of silver suspended in various resin systems that produce conductive patterns on rigid and flexible substrates. This is a typical conductor material for membrane switches.
- A membrane switch adhesive layer that separates circuit layers to supply keyswitch openings, permitting the contact of conductors when depressed.