Membrane switch
Encyclopedia
A membrane switch is an electrical switch for turning a circuit on and off. It differs from a mechanical switch, which is usually made of copper and plastic parts: a membrane switch is a circuit printed on PET
or ITO
. The ink used for screen printing is usually copper
/ silver
/ graphite
filled and therefore conductive.
A membrane switch typically has 4 or more layers. The top layer of a membrane switch is the graphic interface between the user and the machine. The other critical layer is a printed circuit. This can also be a flex circuit made of copper and polyimide material. The layers are normally assembled using pressure sensitive adhesives although inexpensive designs can be held together through other mechanical means such as a keyboard housing. Contact between two traces can be made through a printed shorting pad or through a metal dome that stands on legs.
The first option is using Light Emitting Diodes (LEDs) to backlight. However, LEDs create bright spots and are not suitable for overall backlighting of a panel, but rather as indicator lights. LEDs can either be surface-mounted to the circuit layer or be placed on a separate LED layer.
A second option is optical fiber
. In a typical design, two or more layers of woven fiber-optic cloth are used to form a rectangular light-emitting area. The fibers coming off one end are then bundled into a circular ferrule
and coupled to one or more LED light sources. Remote light sources offer 10,000 to 100,000 hours of life. Optical fibers are not affected by extremes in humidity (0% to 100%) or temperature (-40 to +85 deg C).
The third standard option is to use electroluminescent (EL) lamps. They are lower priced compared to fiber optics and offer additional design flexibility. The color of light emitted from an EL lamp can vary depending on the phosphors that are used. Some common colors are blue/green and yellow/green, white, blue and orange. EL lamps have a half life of approximately 3000-8000 hours depending upon the quality of the phosphor. Once they reach their half life, the brightness starts to fade rapidly. EL lamps are thus not a good choice if the lamp is on for an extended period of time. Fading or flashing could double the life of the lamp.
panel, air conditioner control panel, TV remote control
etc. Tactile feedback of keys can be provided by embossing the top PET layer or embedding metal snap domes, polyester domes or forming the graphic layer.
The benefits of membrane switches include ease of cleaning, sealing ability and their low profile.
Polyethylene terephthalate
Polyethylene terephthalate , commonly abbreviated PET, PETE, or the obsolete PETP or PET-P, is a thermoplastic polymer resin of the polyester family and is used in synthetic fibers; beverage, food and other liquid containers; thermoforming applications; and engineering resins often in combination...
or ITO
Indium tin oxide
Indium tin oxide is a solid solution of indium oxide and tin oxide , typically 90% In2O3, 10% SnO2 by weight. It is transparent and colorless in thin layers while in bulk form it is yellowish to grey...
. The ink used for screen printing is usually copper
Copper
Copper is a chemical element with the symbol Cu and atomic number 29. It is a ductile metal with very high thermal and electrical conductivity. Pure copper is soft and malleable; an exposed surface has a reddish-orange tarnish...
/ silver
Silver
Silver is a metallic chemical element with the chemical symbol Ag and atomic number 47. A soft, white, lustrous transition metal, it has the highest electrical conductivity of any element and the highest thermal conductivity of any metal...
/ graphite
Graphite
The mineral graphite is one of the allotropes of carbon. It was named by Abraham Gottlob Werner in 1789 from the Ancient Greek γράφω , "to draw/write", for its use in pencils, where it is commonly called lead . Unlike diamond , graphite is an electrical conductor, a semimetal...
filled and therefore conductive.
Construction
The ASTM defines a membrane switch as "a momentary switch device in which at least one contact is on, or made of, a flexible substrate."A membrane switch typically has 4 or more layers. The top layer of a membrane switch is the graphic interface between the user and the machine. The other critical layer is a printed circuit. This can also be a flex circuit made of copper and polyimide material. The layers are normally assembled using pressure sensitive adhesives although inexpensive designs can be held together through other mechanical means such as a keyboard housing. Contact between two traces can be made through a printed shorting pad or through a metal dome that stands on legs.
Backlighting
There are three standard methods for backlighting membrane switches.The first option is using Light Emitting Diodes (LEDs) to backlight. However, LEDs create bright spots and are not suitable for overall backlighting of a panel, but rather as indicator lights. LEDs can either be surface-mounted to the circuit layer or be placed on a separate LED layer.
A second option is optical fiber
Optical fiber
An optical fiber is a flexible, transparent fiber made of a pure glass not much wider than a human hair. It functions as a waveguide, or "light pipe", to transmit light between the two ends of the fiber. The field of applied science and engineering concerned with the design and application of...
. In a typical design, two or more layers of woven fiber-optic cloth are used to form a rectangular light-emitting area. The fibers coming off one end are then bundled into a circular ferrule
Ferrule
A ferrule is a name for types of metal objects, generally used for fastening, joining, or reinforcement...
and coupled to one or more LED light sources. Remote light sources offer 10,000 to 100,000 hours of life. Optical fibers are not affected by extremes in humidity (0% to 100%) or temperature (-40 to +85 deg C).
The third standard option is to use electroluminescent (EL) lamps. They are lower priced compared to fiber optics and offer additional design flexibility. The color of light emitted from an EL lamp can vary depending on the phosphors that are used. Some common colors are blue/green and yellow/green, white, blue and orange. EL lamps have a half life of approximately 3000-8000 hours depending upon the quality of the phosphor. Once they reach their half life, the brightness starts to fade rapidly. EL lamps are thus not a good choice if the lamp is on for an extended period of time. Fading or flashing could double the life of the lamp.
Applications
Classic applications of membrane switches include microwave ovenMicrowave oven
A microwave oven is a kitchen appliance that heats food by dielectric heating, using microwave radiation to heat polarized molecules within the food...
panel, air conditioner control panel, TV remote control
Remote control
A remote control is a component of an electronics device, most commonly a television set, used for operating the television device wirelessly from a short line-of-sight distance.The remote control is usually contracted to remote...
etc. Tactile feedback of keys can be provided by embossing the top PET layer or embedding metal snap domes, polyester domes or forming the graphic layer.
The benefits of membrane switches include ease of cleaning, sealing ability and their low profile.